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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..bbb4cbd --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #68670 (https://www.gutenberg.org/ebooks/68670) diff --git a/old/68670-0.txt b/old/68670-0.txt deleted file mode 100644 index 5fde5d6..0000000 --- a/old/68670-0.txt +++ /dev/null @@ -1,8223 +0,0 @@ -The Project Gutenberg eBook of The micro-organisms of the soil, by -Sir E. John Russell - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: The micro-organisms of the soil - -Authors: Sir E. John Russell - Members of the biological staff of The Rothamsted Experimental Station - -Release Date: August 2, 2022 [eBook #68670] - -Language: English - -Produced by: Charlene Taylor, Harry Lamé and the Online Distributed - Proofreading Team at https://www.pgdp.net (This file was - produced from images generously made available by The - Internet Archive/Canadian Libraries) - -*** START OF THE PROJECT GUTENBERG EBOOK THE MICRO-ORGANISMS OF THE -SOIL *** - - - - Transcriber’s Notes - - Text printed in italics has been transcribed _between underscores_; - underlined text ~between tildes~. ^x and _{x} represent a superscript - and subscript x respectively. - - Uppercase letters between square brackets (such as [A]) refer to - footnotes (to be found directly underneath the paragraph or table), - numbers between square brackets (such as [1] or [1_a_] refer to - references at the end of the chapter(s) by the same author. - - More Transcriber’s Notes may be found at the end of this text. - - - - - _~THE ROTHAMSTED MONOGRAPHS ON - AGRICULTURAL SCIENCE~_ - - EDITED BY - SIR E. J. RUSSELL, D.Sc. (LOND.), F.R.S. - - - THE MICRO-ORGANISMS OF THE SOIL - - - - -THE ROTHAMSTED MONOGRAPHS ON AGRICULTURAL SCIENCE. - -EDITED BY SIR E. JOHN RUSSELL, D.Sc., F.R.S. - - -During the past ten years there have been marked developments in -knowledge of the relations between the soil and the growing plant. The -subject involves physical, biological, and chemical considerations, and -its ramifications are now so wide that they cannot be satisfactorily -dealt with in detail in any one book. These monographs collectively -cover the whole ground. In “Soil Conditions and Plant Growth” the -general outlines are presented: in the monographs the various divisions -are fully and critically dealt with by the Heads of the Departments -concerned at Rothamsted. A homogeneous treatment is thus secured that -will, it is hoped, much facilitate the use of the series. - - SOIL CONDITIONS AND PLANT GROWTH, Fourth Edition. By SIR E. JOHN - RUSSELL, F.R.S. 16_s._ net. - - The following volumes are in preparation:-- - - MANURING OF GRASS-LANDS - FOR HAY By WINIFRED E. BRENCHLEY, D.Sc., F.Z.S. - - THE MICRO-ORGANISMS OF THE - SOIL By Sir E. JOHN RUSSELL, F.R.S., and - Members of the Biological Staff of the - Rothamsted Experimental Station. - - SOIL PHYSICS By B. A. KEEN, B.Sc. - - SOIL PROTOZOA By D. W. CUTLER, M.A., and L. M. CRUMP, - M.Sc. - - SOIL BACTERIA By H. G. THORNTON, M.A. - - SOIL FUNGI AND ALGÆ By W. B. BRIERLEY, S. T. JEWSON, B.Sc., - and B. M. ROACH (Bristol), D.Sc. - - CHEMICAL CHANGES IN - THE SOIL By H. J. PAGE, B.Sc. - - - LONGMANS, GREEN AND CO., - LONDON, NEW YORK, TORONTO, BOMBAY, CALCUTTA, AND MADRAS. - - - - - THE MICRO-ORGANISMS - OF THE SOIL - - BY - - SIR E. JOHN RUSSELL, F.R.S. - - AND - - MEMBERS OF THE BIOLOGICAL STAFF OF THE - ROTHAMSTED EXPERIMENTAL STATION - - _WITH DIAGRAMS_ - - LONGMANS, GREEN AND CO. - 39 PATERNOSTER ROW, LONDON, E.C. 4 - NEW YORK, TORONTO - BOMBAY, CALCUTTA and MADRAS - 1923 - - -_Made in Great Britain_ - - - - -INTRODUCTION. - - -The purpose of this volume is to give the broad outlines of our present -knowledge of the relationships of the population of living organisms -in the soil to one another and to the surface vegetation. It is shown -that there is a close relationship with vegetation, the soil population -being dependent almost entirely on the growing plant for energy -material, while the plant is equally dependent on the activities of -the soil population for removing the residues of previous generations -of plants and for the continued production in the soil of simple -materials, such as nitrates, which are necessary to its growth. It is -also shown, however, that the soil population takes toll of the plant -nutrients and that some of its members may directly injure the growing -plant. - -The soil population is so complex that it manifestly cannot be dealt -with as a whole in any detail by any one person, and at the same -time it plays so important a part in the soil economy that it must -be seriously studied. Team work therefore becomes indispensable, and -fortunately this has been rendered possible at Rothamsted. - -Each group of organisms is here dealt with by the person primarily -responsible for that particular section of the work. The plan of the -book has been carefully discussed by all the authors, and the subject -matter has already been presented in a course of lectures given at -University College, London, under the auspices of the Botanical -Board of Studies of the London University. The interest shown in -these lectures leads us to hope that the subject may appeal to a -wider public, and above all to some of the younger investigators in -biological science. They will find it bristling with big scientific -problems, and those who pursue it have the satisfaction, which -increases as the years pass by, of knowing that their work is not only -of interest to themselves, but of great importance in ministering to -the intellectual and material needs of the whole community. - - - - -CONTENTS. - - - CHAP. PAGE - - I. DEVELOPMENT OF THE IDEA OF A SOIL POPULATION 1 - Sir E. JOHN RUSSELL, F.R.S., Director. - - II. OCCURRENCE OF BACTERIA IN SOIL--ACTIVITIES CONNECTED WITH - THE ACQUIREMENT OF ENERGY 20 - H. G. THORNTON, B.A., Head of the Department of - Bacteriology. - - III. CONDITIONS AFFECTING BACTERIAL ACTIVITIES IN THE - SOIL--ACTIVITIES CONNECTED WITH THE INTAKE OF PROTEIN - BUILDING MATERIALS 39 - H. G. THORNTON, B.A., Head of the Department of - Bacteriology. - - IV. PROTOZOA OF THE SOIL, I. 66 - D. W. CUTLER, M.A., Head of the Department of - Protozoology. - - V. PROTOZOA OF THE SOIL, II. 77 - D. W. CUTLER, M.A., Head of the Department of - Protozoology. - - VI. SOIL ALGÆ 99 - B. MURIEL BRISTOL, D.Sc., Algologist. - - VII. SOIL FUNGI--THE OCCURRENCE OF FUNGI IN THE SOIL 118 - W. B. BRIERLEY, D.Sc., Head of the Department of - Mycology. - - VIII. SOIL FUNGI--THE LIFE OF FUNGI IN THE SOIL 131 - W. B. BRIERLEY, D.Sc., Head of the Department of - Mycology. - - IX. THE INVERTEBRATE FAUNA OF THE SOIL (OTHER THAN PROTOZOA) 147 - A. D. IMMS, D.Sc., Head of the Department of Entomology. - - X. THE CHEMICAL ACTIVITIES OF THE SOIL POPULATION AND THEIR - RELATION TO THE GROWING PLANT 164 - Sir E. JOHN RUSSELL, F.R.S., Director. - - INDEX 181 - - - - -CHAPTER I. - -THE DEVELOPMENT OF THE IDEA OF A SOIL POPULATION. - - -From the earliest times agriculturists have been familiar with the idea -that decomposition of vegetable and animal matter takes place in the -soil, and that the process is intimately connected with soil fertility. - -By the middle of the nineteenth century three different ways were known -in which the decomposition occurred. One had been since early times -specially associated with soil fertility, in that it gave rise to -humus, the black sticky substance in farmyard manure or in soil--which -was supposed up to 1840 to be the special food of plants. No good -account of the process or of the conditions in which it occurred is, -however, given by the older writers. - -A second resulted in the formation of nitrates. This process became -known as nitrification: it was described by Georgius Agricola -(1494-1555) in his book “De Re Metallica,” and it was of great -importance in the seventeenth and eighteenth centuries, because it -was used for the manufacture of gunpowder in the great wars of that -period. The conditions for the making of successful nitre beds were so -thoroughly investigated that little fresh knowledge was added to that -of 1770[A] until quite recently. This process, however, was not usually -associated with soil fertility, although both Glauber (1656) and Mayow -(1674) had insisted on the connection. - - [A] See the remarkable collection of papers entitled - “Instructions sur l’établissement des nitrières,” publié par les - Régisseurs-généraux des Poudres et Salpêtre. Paris, 1777. - -A third type of decomposition was brought into prominence by Liebig -in 1840.[7][B] Reviewing the decomposition of organic matter in the -light of the newer chemistry, he concluded that the process was a -slow chemical oxidation, to which he gave the name “Eremacausis.” -He recognised that humus was formed, but he regarded it only as an -intermediate product, and emphatically denied its importance in -soil fertility. The true fertility agents, in his view, were the -final products--CO₂, potassium and other alkaline salts, phosphates, -silicates, etc. He went on to argue brilliantly that instead of -applying farmyard or similar manures to the soil it was altogether -quicker and better to apply these mineral compounds obtained from -other sources than to wait for the slow process of liberation as the -result of decomposition. For some reason, difficult to understand, he -overlooked nitrification and the part that nitrates might play in soil -fertility. Lawes and Gilbert[6] were much attracted by this new idea; -they showed that it was incomplete because it took no account of the -necessity for supplying nitrogen compounds to the crop. When ammonium -salts were added to Liebig’s ash constituents the resulting mixture had -almost as good a fertilising effect as farmyard manure. Lawes at once -saw the enormous practical importance of this discovery, and set up -a factory for the manufacture of artificial fertilisers. He did not, -however, follow it up more closely on the scientific side. - - [B] The numbers refer to the short bibliography on p. 18. - -Both Lawes and Gilbert were in constant touch with the idea of -decomposition in the soil, and they attached so much importance to -nitrogen compounds in plant nutrition that it is not easy to understand -how they missed the connection with nitrification. But they did so, and -like other English and German workers of the day, considered that plant -roots assimilated their nitrogen as ammonia. For the first ten years of -the history of Rothamsted only few experiments with nitrates were made, -and not till thirty-five years had elapsed were they systematically -studied. - -It was by Boussingault[2] and in France that the connection between -nitrification and soil fertility was first recognised. The news came -to England, but it was not accepted, although Way, one of the most -brilliant agricultural chemists of his time, showed that nitrates -were formed in soils to which nitrogenous fertilisers were added, and -that they were comparable in their fertiliser effects with ammonium -salts.[12] “The French chemists,” he wrote in 1856, “are going further, -several of them now advocating the view that it is in the form of -nitric acid that plants make use of compounds of nitrogen. With this -view I do not at present coincide, and it is sufficient here to admit -that nitric acid in the form of nitrates has at least a very high value -as manure.” Indeed, Kuhlmann went so far as to argue that the nitrates -found in the soil were there reduced to ammonia before assimilation -by plants could take place. The water-culture work of the plant -physiologists of the ’sixties finally showed the correctness of the -French view. - -Even when the importance of nitrification was realised its mechanism -was not understood: some thought it was chemical, some physical. -Again the explanation came from France. Pasteur in 1862 had expressed -the view that nitrification would probably be a biological action, -since purely chemical oxidation of organic matter was of very limited -occurrence. “Pénétrés de ces idées,” as Schloesing tells us, he and -Müntz in a memorable investigation cleared up the whole problem, and -in 1877 opened the way to a most fruitful field of research.[10] The -formal description is given in his papers in the “Comptes Rendus,” -but a more lively account is given in his lectures before the _École -d’application des Manufacteurs de l’état_, which, though not printed, -were collected and issued in script by his distinguished son, and a -copy of this work is among the treasures of the Rothamsted Library. - -He had been asked to study the purification of sewage, and he and Müntz -showed that it was bound up with nitrification. The process was slow in -starting, then it proceeded rapidly. Why, they asked, was the delay? -There should be none if the process were physical or chemical, and the -fact that it occurred strongly suggested biological action. The process -was stopped by chloroform vapour, but could be restarted after the -removal of the vapour by the addition of a little fresh soil. - -The importance of this work in connection with soil fertility -was immediately realised by Warington, who had recently come to -Rothamsted.[11] He quickly confirmed the result, and made the valuable -discovery that two stages were involved--the conversion of ammonia to -a nitrite by one organism, and of the nitrite to nitrate by another. -He made long and persistent attempts to isolate the organisms from the -soil, using the best technique of his time, but though he found many -bacteria none of them could nitrify ammonium salts; yet the soil did -it easily. For years he continued his efforts to find the nitrifying -organism, but always failed. His health was not good, his life at -Rothamsted was not happy owing to disagreements with Gilbert, and -although his other research work was succeeding, this investigation -on which he had set his heart was not coming out; bacterial technique -was not yet sufficiently far advanced. Ten bitter, disappointing years -passed, and the crown of disappointment came when Winogradsky, a young -bacteriologist in Paris, changed the technique and succeeded at once in -isolating both the nitrite and the nitrate-forming organisms.[13] - -The numerous bacteria found by Warington in the soil suggested the -presence of a soil population, and this idea was greatly strengthened -by another line of investigation which was being followed up in France. -Boussingault had shown that soils absorb oxygen and give out carbon -dioxide; Schloesing extended this discovery, as also did Wollny. It -was concluded that oxidation was the result of the activities of the -soil organisms in decomposing the organic matter of the soil, and thus -preparing the way for the nitrifying organisms. - -A third important function of soil bacteria was revealed by -Berthelot.[1] It was known that considerable loss of nitrogen from the -soil took place as the result of the conversion of nitrogen compounds -into nitrates, which were subsequently washed out in the drainage -water. It followed inevitably that the stock of nitrogen compounds in -the soil must long ago have become exhausted had there been no addition -of nitrogen compounds to the soil. Berthelot argued that there must be -fixation of atmospheric nitrogen, and, following the prevailing trend -of thought in France, he attributed it to bacteria. He confirmed the -anticipation by exposing soil to air in such conditions that dust, -rain, etc., were excluded, and he found an increase in the percentage -of nitrogen. - -Looking back over the work, it is difficult to understand the result. -The fixation of nitrogen is a process that absorbs energy, and -should have necessitated some source of energy, which apparently was -not supplied. But in spite of this drawback the investigation was -immediately fruitful in that it gave the key to another problem which -had long puzzled agriculturists. - -It had long been known that the growth of leguminous crops, unlike that -of others, enriched the ground,[C] and Lawes and Gilbert had shown -that this was due to an increase of soil nitrogen. But no explanation -could be found till Hellriegel and Wilfarth solved the problem.[4] In -studying the nitrogen nutrition of gramineous and leguminous crops, -they discovered that the gramineous plants died in absence of nitrate, -and in its presence made growth which increased regularly with nitrate -supply; while leguminous plants sometimes died and sometimes flourished -in absence of nitrate, and behaved equally erratically with increasing -nitrate supply. When the plants flourished nodules were invariably -present on the roots, but not otherwise. They concluded, therefore, -that the nitrogen nutrition of leguminous plants differed from that -of the gramineæ, and depended on some factor which sometimes came -into their experiments and sometimes did not, and, in any case, was -associated with the nodule. Knowing that the nodules on the roots of -leguminous plants contained bacteria-like bodies, and remembering -Berthelot’s results, they explored the possibility of bacterial -fixation. They sterilised the sand and found that peas invariably -failed to develop nodules and died, but after adding a little garden -soil nodules were found and vigorous growth was obtained. - - [C] “Of the leguminous plants the bean best reinvigorates the ground - ... because the plant is of loose growth and rots easily, wherefore - the people of Macedonia and Thessaly turn over the ground when it - is in flower” (i.e. dig it into the ground if the soil is poor). - Theophrastus, “Enquiry into Plants,” bk. viii. 2, and bk. ix. I. This - book is of profound interest to agriculturists and botanists. An - excellent translation by Sir Arthur Hort is now available. (Loeb’s - Classical Library.) - -Chemical analysis showed considerable fixation of gaseous nitrogen, -which Hellriegel associated with the nodule organism. This has proved -to be correct, and the fixation of nitrogen by bacteria is now a -well-recognised process, the conditions of which are being thoroughly -worked out. Two types of organisms are known--those associated with -leguminous plants, and those living in a free and independent state in -the soil. Of the latter the Clostridium, isolated by Winogradsky, is -anaerobic, and the Azotobacter of Beijerinck is aerobic. The essential -conditions are that a source of energy must be supplied--usually -given as sugar--that the medium must not be acid, and that sufficient -phosphate must be present. - -All this brilliant work had been accomplished in the short space of the -ten years 1880 to 1890. The inspiration had in each instance come from -France, and is traceable direct to Pasteur, although coming long after -his own work on bacteriology. It is impossible for us now to realise -the thrill of wonder and astonishment with which students, teachers, -and writers of those days learned that the nutrition of plants, and -therefore the growth of crops and the feeding of themselves, was -largely the result of the activity of bacteria in the dark recesses of -the soil. It is not surprising that the ideas were pushed somewhat too -far, that the soil population was regarded as solely bacterial, and -that important chemical and physical changes were sometimes overlooked. - -Gradually there came the inevitable reaction and a somewhat changed -outlook. Continued examination showed the presence in soil of almost -every kind of bacteria for which search was made. Some of them were -almost certainly in the resting condition as spores, and the new -generation of workers had an uneasy feeling that the case for the -overwhelming importance of bacteria in the economy of the soil was -not too well founded. It was shown that the decomposition of nitrogen -compounds to form ammonia would take place without micro-organisms if, -as presumably would happen, the plant enzymes continued to act after -they got into the soil. Even the oxidation of ammonia to nitrate--the -great stronghold of the biological school--was accomplished by chemical -agents. The fixation of nitrogen in soil conditions was beyond the -power of chemists to achieve, and here it was universally agreed that -bacteria were the active agents. And finally, chemists were themselves -bringing into prominence a set of bodies--the colloids--whose -remarkable properties seemed indefinitely expansible, and were in -addition sufficiently incomprehensible to the ordinary student to -attain much of the magnificence of the unknown. - -All the time, however, a faithful body of workers was busy exploring -the ground already won, improving the technique, making counts of the -numbers of bacteria in the soil, and trying to measure the amount of -bacterial activity. Much of the value of this work was limited by the -circumstance that the bacteria were regarded as more or less constant -in numbers and activities, so that a single determination was supposed -to characterise the position in a given soil. - -This was the condition of the subject when it was seriously taken up at -Rothamsted. Before turning to agriculture, the writer had been studying -the mechanism of certain slow chemical oxidations, and one of his first -experiments in agriculture was to examine the phenomena of oxidation -in soil. The results accorded with the biological explanation of -Schloesing: when the soil was completely sterilised oxidation almost -ceased. But the striking discovery was made, as the result of an -accident to an autoclave, that partial sterilisation increased the -rate of oxidation, and therefore presumably the bacterial activity. -This remarkable phenomenon had, however, already been observed, and -it had been shown that both bacterial numbers and soil fertility -were increased thereby. A full investigation was started in 1907 by -Dr. Hutchinson and the writer.[9] From the outset the phenomena were -recognised as dynamic and not static, and the rates of change were -always determined: thus the bacterial numbers, the nitrate and ammonia -present were estimated after the several periods. Close study of the -curves showed that the chemical and bacterial changes were sufficiently -alike to justify the view that bacteria were in the main the causes -of the production of ammonia and of nitrate; although non-biological -chemical action was not excluded, there was no evidence that it played -any great part. Thus the importance of micro-organisms in the soil was -demonstrated. - -The factor causing the increased bacterial numbers after partial -sterilisation was studied by finding out what agents would, and what -would not, allow the numbers to increase, e.g. it was found that the -bacterial increases became possible when soil had been heated at 56° -C., but not at 40° C. Again, it was shown that the high numbers in -partially sterilised soils rose for a time even higher if a little -fresh untreated soil were incorporated into the partially sterilised -soil, but afterwards they fell considerably. Putting all the results -together, it appeared that some biological cause was at work depressing -the numbers of bacteria in normal soils, but not--or not so much--in -the partially sterilised soils. Studied in detail, the data suggested -protozoa as the agent keeping down bacterial numbers, and they were -found in the untreated, but not in the treated, soils. The hypothesis -was therefore put forward that bacteria are not the only members of the -soil population, but that protozoa are also present keeping them in -check, and therefore adversely affecting the production of plant food. - -This conclusion aroused considerable controversy. It was maintained -that protozoa were not normal inhabitants of the soil, but only -occasional visitants, and, in any case, they were only there as cysts; -the soil conditions, it was urged, were not suitable to large organisms -like protozoa. The objection was not to be treated lightly, but, on -the other hand, the experiments seemed quite sound. As neither Dr. -Hutchinson nor the writer were protozoologists, Dr. T. Goodey and -(after he left) Mr. Kenneth R. Lewin were invited to try and find -out, quite independently of the partial sterilisation investigation, -whether protozoa are normal inhabitants of the soil, and if so, whether -they are in a trophic condition, and what is their mode of life and -their relation to soil bacteria. Had it turned out that protozoa had -nothing to do with the matter, search would have been made for some -other organism. Goodey showed that the ciliates were not particularly -important; Lewin soon demonstrated the existence of trophic amœbæ and -flagellates. Unfortunately he was killed in the war before he had got -far with the work. After the Armistice, Mr. Cutler accepted charge of -the work: he will himself relate in Chapters IV. and V. what he has -done. - -At first sight it might be thought comparatively easy to settle a -question of this kind by examining soil under a microscope or by -sterilising it and introducing successively bacteria and known types -of protozoa. Unfortunately neither method is simple in practice. It -is impossible to look into the soil with a microscope, and methods of -teasing-out small pieces of soil on a slide under the high, or even the -low power, give no information, because the particles of soil have the -remarkable power of attracting and firmly retaining protozoa, and no -doubt bacteria as well; indeed, for protozoa (which have been the more -fully investigated) there seems to be something not unlike a saturation -capacity (see Fig. 9, p. 78). Further, complete sterilisation of soil -cannot be effected without at the same time altering its chemical and -physical properties, and changing it as a habitat for micro-organisms. -Cutler has, however, overcome the difficulties and shown that the -introduction of protozoa into soils sterilised and then reinfected with -bacteria considerably reduces the numbers of these organisms. - -The method adopted, therefore, is to take a census of population and -of production. Counting methods are elaborated, and estimates as -accurate as possible are made of the numbers of the various organisms -in a natural field soil at stated intervals. Simultaneously, wherever -possible some measure is taken of the work done. The details of the -census are finally arranged in consultation with the Statistical -Department, to ensure that the data shall possess adequate statistical -value. From the results it is possible to adduce information of great -value as to the life of the population, the influence of external -conditions, etc. - -The most important investigation of this kind carried out at Rothamsted -was organised by Mr. Cutler.[3] A team of six workers was assembled, -and for 365 days without a break they counted every day the ciliates, -the amœbæ, the flagellates, and the bacteria in a plot of arable -ground, distinguishing no less than seventeen different kinds of -protozoa. The conclusions arrived at were carefully tested by the -Statistical Department. - -Of the protozoa the flagellates were found to be the most numerous, -the amœbæ came next, and the ciliates were by far the fewest. The -numbers of each organism varied from day to day in a way that -showed conclusively the essentially trophic nature of the protozoan -population. The numbers of amœbæ--especially _Dimastigamœba_ and of -a species called α--were sharply related to the numbers of bacteria: -when the amœbae were numerous the bacteria were few, and vice versa. -Detailed examination showed that the amœbæ were probably the cause -of the fluctuations in the bacterial numbers, but Mr. Cutler has not -yet been able to find why the amœbæ fluctuated; it does not appear -that temperature, moisture content, air supply or food supply were -determining causes. The flagellates and ciliates also showed large -fluctuations, amounting in one case--_Oicomonas_--to a definite -periodicity, apparently, however, not related to bacterial numbers, or, -so far as can be seen, to external conditions of moisture, temperature -and food supply, and showing no agreement with the fluctuations of the -amœbæ. However, one cannot be certain that lack of agreement between -curves expressing protozoan numbers and physical factors implies -absence of causal relationships: the observations (though the best that -can yet be made) are admittedly not complete. If we saw only the end of -the bough of a tree, and could see no connection with a trunk, we might -have much difficulty in finding relationships between its motion and -the wind; whatever the direction of the wind it would move backwards -and forwards in much the same way, and even when the wind was blowing -along the plane of its motion it would just as often move against the -wind as with it. - -Meanwhile evidence was obtained that the twenty-four hour interval -adopted by the protozoological staff was too long for bacteria, and -accordingly the Bacteriological Department, under Mr. Thornton, refined -the method still further. Bacterial counts were made every two hours, -day and night, for several periods of sixty or eighty hours without -a break. The shape of the curve suggests that two hours is probably -close enough, and for the present counts at shorter intervals are not -contemplated. But there is at least one maximum and one minimum in the -day, although the bacterial day does not apparently correspond with -ours, nor can any relationship be traced with the diurnal temperature -curve. - -The nitrate content of the soil was simultaneously determined by Mr. -Page and found to vary from hour to hour, but the variations did not -sharply correspond with the bacterial numbers; this, however, would not -necessarily be expected. The production of nitrate involves various -stages, and any lag would throw the nitrate and bacterial curves out -of agreement. There is a suggestion of a lag, but more counts are -necessary before it can be regarded as established. - -Examination of these and other nitrate curves obtained at Rothamsted -has brought out another remarkable phenomenon. No crop is growing on -these plots, and no rain fell during the eighty hours, yet nitrate -is disappearing for a considerable part of the time. Where is it -going to? At present the simplest explanation seems to be that it is -taken up by micro-organisms. A similar conclusion had to be drawn -from a study of the nitrogen exhaustion of the soil. The whole of the -nitrate theoretically obtainable from the organic matter of the soil -is not obtained in the course of hours or even days; in one of our -experiments at Rothamsted nitrification is still going on, and is far -from complete, even after a lapse of fifty-three years. The explanation -at present offered is that part of the nitrate is constantly being -absorbed by micro-organisms and regenerated later on. - -Now what organisms could be supposed to absorb nitrates from the soil? -Certain bacteria and fungi are known to utilise nitrates, and one -naturally thinks of algæ as possible agents also. Dr. Muriel Bristol -was therefore invited to study the algæ of the soil. Her account is -given in Chapter VI. She has found them not only on the surface, but -scattered throughout the body of the soil, even in the darkness of 4 -inches, 5 inches, or 6 inches depth, where no light can ever penetrate, -and where photosynthesis as we understand it could not possibly take -place. Some modification in their mode of life is clearly necessary, -and it may well happen that they are living saprophytically. Dr. -Bristol has not yet, however, been able to count the algæ in the soil -with any certainty, although she has made some estimates of the numbers. - -The quantitative work on the soil population indicates other -possibilities which are being investigated. There is not only a daily -fluctuation in the numbers, but so far as measurements have gone, -a seasonal one also. There seems to be some considerable uplift in -numbers of bacteria, protozoa, and possibly algæ and fungi in the -spring-time, followed by a fall in summer, a rise in autumn, and a -fall again in winter. At present we are unable to account for the -phenomenon, nor can we be sure that it is general until many more data -are accumulated. - -In the cases of the protozoa and the algæ, there was a definite reason -for seeking them in the soil. - -Another section of the population, the fungi, was simply found, and -at present we have only limited views as to their function. The older -workers considered that they predominated in acid soils, while bacteria -predominated in neutral soils. Present-day workers have shown that -fungi, including actinomycetes, are normal inhabitants of all soils. -The attempts at quantitative estimations are seriously complicated -by the fact that during the manipulations a single piece of mycelium -may break into fragments, each of which would count as one, while a -single cluster of spores might be counted as thousands. Little progress -has therefore been made on the quantitative lines which have been so -fruitful with protozoa. Dr. Brierley gives, in Chapters VII. and VIII., -a critical account of the work done on fungi. - -In addition to the organisms already considered there are others of -larger size. The nematodes are almost visible to the unaided eye, -most of them are free living and probably help in the disintegration -of plant residues, though a few are parasitic on living plants and do -much injury to clover, oats, and less frequently to onions, bulbs, and -potatoes. Further, there are insects, myriapods and others, the effects -of which in the soil are not fully known. Special importance attaches -to the earthworms, not only because they are the largest in size and -in aggregate weight of the soil population, but because of the great -part they play in aerating the soil, gradually turning it over and -bringing about an intimate admixture with dead plant residues, as first -demonstrated by Darwin. Earthworms are the great distributors of energy -material to the microscopic population. Systematic quantitative work -on these larger forms is only of recent date, and Dr. Imms, in Chapter -IX., discusses our present knowledge. - -TABLE I. - -SOIL POPULATION, ROTHAMSTED, 1922. - -(The figures for algæ and fungi are first approximations only, and have -considerably less value than those for bacteria and protozoa.) - - +----------------------+-----------+--------------------------------+ - | | |Approximate Weight per Acre of--| - | | +---------+----------+-----------+ - | | Numbers | Living |Dry Matter| Nitrogen | - | | per Gram | Organ- |in Organ- | in Organ- | - | | of Soil. | isms. | isms. | isms. | - +----------------------+-----------+---------+----------+-----------+ - |_Bacteria_-- | | lb. | lb. | lb. | - | High level |45,000,000 | 50} | 2 | 0·2 | - | Low level |22,500,000 | 25} | | | - |_Protozoa_-- | | | | | - | _Ciliates_-- | | | | | - | High level | 1,000 | -- | -- | -- | - | Low level | 100 | -- | -- | -- | - | _Amœbæ_-- | | | | | - | High level | 280,000 | 320} | 12 | 1·2 | - | Low level | 150,000 | 170} | | | - | _Flagellates_-- | | | | | - | High level | 770,000 | 190} | 7 | 0·7 | - | Low level | 350,000 | 85} | | | - | _Algæ_ | | | | | - | (not blue-green)| [100,000]| 125 | 6 | 0·6 | - | Blue-green | Not known.| | Say 6 | Say 0·6 | - | _Fungi_-- | | | | | - | High level |[1,500,000]| 1700} | 60 | 6·0 | - | Low level | [700,000]| 800} | | | - | | | +----------+-----------+ - | | | | 93 | 9·3 | - | | | |= 4 parts nitrogen per| - | | | |1,000,000 of soil. | - +----------------------+-----------+---------+----------------------+ - - +-------------------------------------- - | LARGER ORGANISMS. - +-----------------+-------------------+ - | | | - | | + - | | Numbers | - | | per Acre.[D] | - | +---------+---------+ - | | | Unma- | - | | Manured.| nured. | - +-----------------+---------+---------+ - |_Oligochaeta_ | | | - | (_Limicolae_)--| | | - | Nematoda, etc. |3,609,000| 794,000| - | Myriapoda |1,781,000| 879,000| - | Insects |7,727,000|2,475,000| - | Earthworms |1,010,000| 458,000| - +-----------------+---------+---------+ - | - +-------------------------------------- - - +-----------------------------------------------------------+ - | LARGER ORGANISMS. | - +-----------------+-----------------------------------------+ - | | Approximate Weight per Acre of-- | - | +-------------+-------------+-------------+ - | | Living | Dry Matter | Nitrogen in | - | | Organisms. |in Organisms.| Organisms. | - | +------+------+------+------+------+------+ - | | Ma- | Unma-| Ma- | Unma-| Ma- |Unma- | - | |nured.|nured.|nured.|nured.|nured.|nured.| - +-----------------+------+------+------+------+------+------+ - |_Oligochaeta_ | | | | | | | - | (_Limicolae_)--| lb. | lb. | lb. | lb. | lb. | lb. | - | Nematoda, etc. | 9 | 2 | 3 | 1 | -- | -- | - | Myriapoda | 203 | 99 | 85 | 42 | 4 | 2 | - | Insects | 34 | 16 | 14 | 6 | 1 | 1 | - | Earthworms | 472 | 217 | 108 | 50 | 10 | 5 | - +-----------------+------+------+------+------+------+------+ - | Total | 210 | 99 | 15 | 9 | - +-------------------------------+------+------+------+------+ - - Total organic matter (dry weight) in this soil = 126,000 lb. per acre. - - Total nitrogen = 5700 lb. per acre. (1 lb. nitrogen per acre = 0·4 - parts per 1,000,000 of soil.) - - [D] To a depth of 9 inches. The weight of soil is approximately - 1,000,000 kilos. - -Are there any other members of the soil population that are of -importance? As already shown, the method of investigating the soil -population in use at Rothamsted is to find by chemical methods the -changes going on in the soil; to find by biological methods what -organisms are capable of bringing about these changes; and then to -complete the chain of evidence by tracing the relationships between -the numbers or activities of these organisms and the amount of change -produced. The list as we know it to-day is given in Table I. - -The method, however, does not indicate whether the account is fairly -complete, or whether there are other organisms to be found. We might, -of course, trust to empirical hunting for organisms, or to chance -discoveries such as led Goodey to find the mysterious Proteomyxan -Rhizopods, which cannot yet be cultured with certainty, so that they -are rarely found by soil workers. It is possible that there are many -such organisms, and it is even conceivable that these unknown forms far -outnumber the known. The defect of the present method is that it always -leaves us in doubt as to the completeness of the list, and so we may -have to devise another. - -Reverting to Table I., it obviously serves no purpose to add the -numbers of all the organisms together. We can add up the weights of -living organisms, of their dry matter or nitrogen, so as to form some -idea of the proportion of living to non-living organic matter, and this -helps us to visualise the different groups and place them according -to their respective masses. But a much better basis for comparing the -activities of the different groups would be afforded by the respective -amounts of energy they transform, if these could be determined. It is -proposed to attempt such measurements at Rothamsted. The results when -added would give the sum of the energy changes effected by the soil -population as we know it: the figure could be compared with the total -energy change in the soil itself as determined in a calorimeter. If -the two figures are of the same order of magnitude, we shall know that -our list is fairly well complete; if they are widely different, search -must be made for the missing energy transformers. There are, of course, -serious experimental difficulties to be overcome, but we believe the -energy relationships will afford the best basis for further work on the -soil population. - -Finally, it is necessary to refer to the physical conditions obtaining -in the soil. These make it a much better habitat for organisms than one -might expect. At first sight one thinks of the soil as a purely mineral -mass. This view is entirely incorrect. Soil contains a considerable -amount of plant residues, rich in energy, and of air and water. The -usual method of stating the composition of the soil is by weight, but -this is misleading to the biologist because the mineral matter has a -density some two and a half times that of water and three times that -of the organic matter. For biological purposes composition by volume -is much more useful, and when stated in this way the figures are very -different from those ordinarily given. Table II. gives the results for -two Broadbalk arable plots, one unmanured and the other dunged; it -includes also a pasture soil. - -The first requirement of the soil population is a supply of energy, -without which it cannot live at all. All our evidence shows that the -magnitude of the population is limited by the quantity of energy -available. The percentage by weight of the organic matter is about -two to four or five, and the percentage by volume runs about four -to twelve. Not all of this, however, is of equal value as source of -energy. About one-half is fairly easily soluble in alkalis, and may -or may not be of special value, but about one-quarter is probably too -stable to be of use to soil organisms. - -A second requirement is water with which in this country the soil is -usually tolerably well provided. Even in prolonged dry weather the soil -is moist at a depth of 3 inches below the surface. It is not uncommon -to find 10 per cent. or 20 per cent. by volume of water present, spread -in a thin film over all the particles, and completely saturating the -soil atmosphere. - -TABLE II. - -VOLUME OF AIR, WATER AND ORGANIC MATTER IN 100 VOLUMES OF ROTHAMSTED -SOIL. - - +---+-----------------+-----------+--------------+ - | | | |In Pore Space.| - | | | |Values Common-| - | | Solid Matter. | | ly Obtained. | - | +--------+--------+ +-------+------+ - | |Mineral.|Organic.|Pore Space.| Water.| Air. | - +---+--------+--------+-----------+-------+------+ - |(1)| 62 | 4 | 34 | 23 | 11 | - |(2)| 51 | 11 | 38 | 30 | 8 | - |(3)| 41 | 12 | 47 | 40 | 7 | - +---+--------+--------+-----------+-------+------+ - - (1) Arable, no manure applied to soil. - - (2) Arable, dung applied to soil. - - (3) Pasture. - -The air supply is usually adequate owing to the rapidity with which -diffusion takes place. Except when the soil is water-logged, the -atmosphere differs but little from that of the one we breathe. There -is more CO₂, but only a little less oxygen.[8] The mean temperature is -higher than one would expect, being distinctly above that of the air, -while the fluctuations in temperature are less.[5] - -The reaction in normal soils is neutral to faintly alkaline; _p_H -values of nearly 8 are not uncommon. Results from certain English soils -are shown on p. 18. - -The soil reaction is not easily altered. A considerable amount of acid -must accumulate before any marked increase in intensity of _p_H value -occurs; in other words, the soil is well buffered. The same can be said -of temperature, of water, and of energy supply. Like the reaction, they -alter but slowly, so that organisms have considerable time in which to -adapt themselves to the change. - -HYDROGEN ION CONCENTRATION AND SOIL FERTILITY. - - _p_H - Alkaline 10 --+-- Sterile: Alkali soil. - ↑ | - | 9 --+-- - | | - | 8 --+-- Fertile: Arable. - | | - Neutral 7 --+-- - | | - | 6 --+-- - | | - | 5 --+-- Potato Scab fails. - | | Nitrification hindered. - | 4 --+-- Barley fails. - ↓ | - Acid 3 --+-- Sterile: Peat. - - -A SELECTED BIBLIOGRAPHY. - - [1] Berthelot, Marcellin, “Fixation directe de l’azote atmosphérique - libre par certains terrains argileux,” Compt. Rend., 1885, ci., - 775-84. - - [2] Boussingault, J. B., and Léwy, “Sur la composition de l’air - confiné dans la terre végétale,” Ann. Chim. Phys., 1853, xxxvii., - 5-50. - - [3] Cutler, D. W., Crump, L. M., and Sandon, H., “A Quantitative - Investigation of the Bacterial and Protozoan Population of the Soil, - with an Account of the Protozoan Fauna,” Phil. Trans. Roy. Soc., - Series B, 1922, ccxi., 317-50. - - [4] Hellriegel, H., and Wilfarth, H., “Untersuchungen über die - Stickstoffnahrung der Gramineen und Leguminosen,” Zeitsch. des - Vereins f. d. Rübenzucker-Industrie, 1888. - - [5] Keen, B. A., and Russell, E. J., “The Factors determining Soil - Temperature,” Journ. Agric. Sci., 1921, xi., 211-37. - - [6] Lawes, J. B., and Gilbert, J. H., “On Agricultural Chemistry, - Especially in Relation to the Mineral Theory of Baron Liebig,” Journ. - Roy. Agric. Soc., 1851, xii., 1-40. - - [7] Liebig, Justus, “Chemistry in its Application to Agriculture - and Physiology,” 1st and 2nd editions (1840 and 1841), 3rd and 4th - editions (1843 and 1847); “Natural Laws of Husbandry,” 1863. - - [8] Russell, E. J., and Appleyard, A., “The Composition of the Soil - Atmosphere,” Journ. Agric. Sci., 1915, vii., 1-48; 1917, viii., - 385-417. - - [9] Russell, E. J., and Hutchinson, H. B., “The Effect of Partial - Sterilisation of Soil on the Production of Plant Food,” Journ. Agric. - Sci., 1909, iii., 111-14; Part II., Journ. Agric. Sci., 1913, v., - 152-221. - - [10] Schloesing, Th., and Müntz, A., “Sur la Nitrification par les - ferments organisés,” Compt. Rend., 1877, lxxxiv., 301-3; 1877, - lxxxv., 1018-20; and 1878, lxxxvi., 892-5. “Leçons de chimie - agricole,” 1883. - - [11] Warington, R., “On Nitrification,” Part I., Journ. Chem. Soc., - 1878, xxxiii., 44-51; Part II, Journ. Chem. Soc., 1879, xxxv., - 429-56; Part III., Journ. Chem. Soc., 1884, xlv., 637-72; Part IV., - Journ. Chem. Soc., 1891, lix., 484-529. - - [12] Way, J. T., “On the Composition of the Waters of Land Drainage - and of Rain,” Journ. Roy. Agric. Soc., 1856, xvii., 123-62. - - [13] Winogradsky, S., “Recherches sur les organismes de la - nitrification,” Ann. de l’Inst. Pasteur, 1890, iv., 1^e Mémoire, - 213-31; 2^e Mémoire, 257-75; 3^e Mémoire, 760-71. - - “Recherches sur l’assimilation de l’azote libre de l’atmosphère par - les microbes.” Arch. des Sci. Biolog. St. Petersburg, 1895, iii, - 297-352. - - For further details and fuller bibliography, see E. J. Russell, “Soil - Conditions and Plant Growth,” Longmans, Green & Co. - - - - -CHAPTER II. - -SOIL BACTERIA. - - -_A._ OCCURRENCE AND METHODS OF STUDY. - -To understand the development of our knowledge of soil bacteria, it -must be remembered that bacteriology is under the disadvantage that -it started as an applied science. Although bacteria were first seen -by Leeuwenhoeck about the middle of the seventeenth century, and -some of their forms described by microscopists of the eighteenth and -early nineteenth centuries, it was only with the work of Pasteur on -fermentation, and of Duvaine, Pasteur, and their contemporaries on -disease bacteria, that bacteriology may be said to have started. -From the outset, therefore, attention has been directed mainly to -the bacteria in their specialised relationship to disease or to -fermentation and similar processes. As a result, little research was -done on the pure biology of the bacteria, so that even now many of the -most fundamental and elementary problems concerning them are quite -unsolved. - -In their work on fermentations and disease bacteria, the earlier -workers were assisted by the fact that under both sets of conditions -the causative bacteria exist, as a rule, either in practically pure -culture, or else in preponderating numbers. The study and elucidation -of such a mixed micro-population as exists in the soil, became possible -only when methods had been devised for isolating the different kinds -of bacteria, and thus studying them apart from each other. It was the -development of the gelatine plate method of isolating pure cultures by -Koch[36] in 1881 that made the study of the soil bacteria practicable. -The plating method opened up two lines of research. In the first -place, it provided a simple means of isolating organisms from the mixed -population of the soil, and thus enabled a qualitative study to be made -of each organism in pure culture, and, in the second place, from it was -developed a counting technique for estimating differences in bacterial -numbers between samples of soil, from which has sprung much of our -knowledge of the quantitative side. - -The earliest studies of the soil bacteria consisted of such estimations -of numbers, and showed that the soil contained a very numerous -population of bacteria. About 20,000,000 bacteria per gram of soil -is now considered a fair average number. The number and variety of -bacteria existing in the soil is so enormous that the method of -separating out all the different forms, and of discovering their -characters and functions, has proved impracticable. In practice, -therefore, the problem has been approached from the biochemical -standpoint. That is to say, the special chemical changes that the -bacteria produce in the soil have first been investigated, and this -has been followed by the isolation and study of the various groups of -bacteria that bring about the changes under investigation. - -The method commonly employed in isolating the organisms that produce a -given chemical change in the soil is called the “elective” method. The -soil is inoculated into a culture medium that will especially favour -the group of bacteria to be isolated, to the exclusion of others. -For example, if it is desired to isolate the organisms that attack -cellulose, a medium is made up containing no other organic carbon -compounds except cellulose. Such a selective medium encourages the -growth of the group of organisms to be investigated, so that after -several transfers to fresh medium a culture is obtained containing -only two or three different types of organisms. These are separated by -plating and pure cultures obtained. - -Another difficulty which has not yet been completely overcome is -that of adequately describing an organism when it is isolated. The -morphology of bacteria is not the constant thing that is seen in -the more stable higher organisms. In many cases the appearance of a -single strain is entirely different on different media, and may be -quite altered by such conditions as changes in acidity of the medium -or temperature of incubation. Even on a single medium remarkable -changes in morphology occur, at any rate, in some bacteria. This is -well seen in a cresol-decomposing organism under investigation at -Rothamsted. In cultures a few days old this organism develops as bent -and branching rods; these rods then break up into chains of cocci and -short rods, which separate, and in old cultures all the organisms -may be in the coccoid form (Fig. 1). It is claimed by Löhnis[47_b_] -that the possession of a complex life-cycle of changing forms is a -universal character in the bacteria. The instability of shape in many -bacteria makes it necessary to standardise very carefully the cultural -conditions under which they are kept when their appearance is described. - -[Illustration: - -Culture 15 hours old. - -Culture 3 days old. - -FIG. 1.--Change in appearance, in culture, of a cresol decomposing -bacterium.] - -The inadequacy of mere morphology as a basis for describing bacteria -led to the search for diagnostic characters, based on the biochemical -changes that they produced in their culture media, and the appearance -of their growth in the mass on various media. These characters -unfortunately have also proved to be very much influenced by the exact -composition of the medium and other conditions of culture. Recently -an attempt has been made by the American Society of Bacteriologists to -standardise the diagnostic characters used in describing bacteria, and -also the media and cultural conditions under which they are grown for -the purpose of description. The need for such precautions, however, -was not sufficiently realised by the early workers, many of whose -descriptions cannot now be referred to any definite organism. - -The large number of organisms found in the soil, and the difficulty -and labour of adequately describing them, is such that even now we -have no comprehensive description of the common soil bacteria that -appear on gelatine platings. A careful study based on modern methods of -characterisation has been made of certain selected groups of bacteria, -and it is hoped that the laborious systematic work of describing the -common forms will gradually be completed. - -Several attempts have been made to classify the bacteria that appear -commonly on gelatine platings. This work was commenced by Hiltner and -Stormer in Germany, and continued by Chester, Harding, and Conn in -America. Conn[10],[14] found that the common organisms fell into the -following main groups:-- - -(1) Large spore-forming bacteria, related to _Bacillus subtilis_, which -form about 5-10 per cent. of the numbers. He adduced evidence[12],[13] -that these organisms exist in the soil mainly as spores, so that they -may not form an important part of the active soil population. - -(2) Short non-sporing organisms, related to _Pseudomonas fluorescens_, -that are rapid gelatine liquefiers. These form another 10 per cent. of -the numbers. - -(3) Short rod forms that liquefy gelatine slowly or not at all, and -develop colonies very slowly. These form 40-75 per cent. of the -numbers, and may therefore be of considerable importance in the soil. - -(4) A few micrococci also occur. - -These groups comprise the larger portion of the bacterial flora of the -soil, but, in addition to these organisms, that develop on the media -commonly used for plating, there are special and important groups -that appear only on special media, either owing to their being unable -to grow on ordinary media or because they get swamped by other forms. -Examples of such groups are the ammonia and nitrite oxidising bacteria, -the nitrogen fixing groups, the cellulose decomposing organisms, and -the sulphur bacteria. - -In order that we may apply the results of the study of a definite -organism to other localities, a knowledge of the geographical -distribution of the soil bacteria is clearly needed. We have, -unfortunately, very little knowledge of the distribution of soil -organisms. The common spore-forming groups appear to be universally -distributed. Thus Barthel, in a study of the bacterial flora of soils -from Greenland and the island of Disko, obtained soil organisms -belonging to the groups of _Bacillus subtilis_, _B. amylobacter_, _B. -fluorescens_, _B. caudatus_, and _B. Zopfii_, which are common groups -in European soil, indicating that the general constitution of the -bacterial flora of the soil in arctic regions is not widely different -from that of Western Europe. Bredemann, who made an extensive study of -the _Bacillus amylobacter_ group, obtained soil samples from widely -scattered localities, and found these organisms in soil from Germany, -Holstein, Norway, Italy, Morocco, Teneriffe, Russia, Japan, China, -the East Indies, Samoa, Illinois, Arizona, German East Africa, and -the Cameroons. Some soil organisms, on the other hand, are apparently -absent from certain districts. This may be due to the conditions, -such as climatic environment, being unfavourable to them. A study has -recently been made at Rothamsted of the distribution over Great Britain -of a group of bacteria that are capable of decomposing phenol and -cresol. One of these organisms, apparently related to the acid-fast -_B. phlœi_, has an interesting distribution. It has been found in 50 -per cent. of the soils samples examined from the drier region, where -the annual rainfall is less than 30 inches, but in only 20 per cent. -of the samples in the wetter parts of Britain. Another example of -limited distribution is found in the case of _Bacillus radicicola_, -the organism that produces tubercles on the roots of leguminous plants. -The distribution of the varieties of this organism follows that of -the host plants with which they are associated, so that when a new -leguminous crop is introduced into a country, nodules may not appear -on the roots unless the soil be specially inoculated with the right -variety of organism. In cases where a group of soil organisms is widely -distributed over the globe, it may yet be absent from many soils -owing to the soil conditions not suiting it. Thus, phenol decomposing -bacteria, though abundant in the neighbourhood of Rothamsted, are yet -absent from field plots that have been unmanured for a considerable -period. The occurrence of the nitrifying organisms and the nitrogen -fixing _Azotobacter_ is also very dependent on the soil conditions. - -Owing to the method by which our knowledge of soil bacteria has been -acquired, by studying first the chemical changes in the soil and then -the bacteria that produce them, it is natural for us to divide them -into physiological groups according to the chemical changes that they -bring about. This grouping is the more reasonable since so little is -known as to the true relationships of the different groups of bacteria -that a classification based on morphology is well-nigh impossible. In -considering the activities of bacteria in the soil, it is convenient to -group the changes which they bring about into the two divisions into -which they naturally fall in the economy of the organisms. - -In the first place, there are the changes that result in a release of -energy, which the bacteria utilise for their vital processes. - -In the second place, there are the processes by which the bacteria -build up the material of their bodies. These building up processes -involve an intake of energy for their accomplishment. - -It will be convenient to deal first with the release of energy for -their own use by bacteria, and its consequences. - - -_B._ ACTIVITIES CONNECTED WITH THE ACQUIREMENT OF ENERGY. - -Unlike the green plants, most bacteria are unable to obtain the energy -that is required for their metabolism from sunlight. They must, -therefore, make use of such chemical changes as will involve the -release of energy. - -As an example of the acquirement of energy in this way may be taken the -oxidation of methane by _B. methanicus_. This organism, described by -Söhngen, obtains its energy supply by the conversion of methane into -CO₂ and H₂O. - - CH₄ + 2O₂ = CO₂ + 2H₂O 220 Cal. - -A further example is the acetic organism that obtains its energy -through the oxidation of alcohol to acetic acid. - - C₂H₆O + O₂ = C₂H₄O₂ + H₂O 115 Cal. - -The decomposition processes brought about by micro-organisms in -obtaining energy are usually oxidations, but this is not necessarily -so, as can be seen in case of the fermentation of sugar into alcohol.[E] - - C₆H₁₂O₆ = 2C₂H₆O + 2CO₂ 50 Cal. - - [E] These examples are from Orla-Jensen (Centralblatt f. Bakt., II., - Bd. 22, p. 305). - -By far the greater part of the decomposition of organic matter is -brought about by bacteria in the process of acquiring energy. In the -soil, nearly the whole of the material utilised by bacteria as a source -of energy is derived ultimately from green plants. The energy materials -left in the soil by the plant fall into two groups, the non-nitrogenous -compounds, which are mainly carbohydrates and their derivatives, and -the nitrogenous compounds, principally derived from proteins. - - -(1) _Decomposition of Non-nitrogenous Compounds._ - -The simpler carbohydrates and starches are attacked and decomposed by -a large variety of bacteria. The addition of such substances to soil -causes a rapid increase in bacterial numbers. In nature the sugars are -in all probability among the first plant constituents to be destroyed -during the decay processes. - -A large proportion of plant tissues consist of cellulose and its -derivatives. These compounds are consequently of great importance -in the soil. Unfortunately our knowledge of the processes by which -cellulose is broken down in the soil is very inadequate. The early -experimental study of cellulose decomposition, such as that of -Tappeiner[60] and Hoppe-Seyler,[33] was mostly carried out under -conditions of inadequate aeration, and the products of decomposition -were found to include methane and CO₂, and sometimes fatty acids and -hydrogen. The bacteriology of this anaerobic decomposition was studied -by Omelianski,[54] who described two spore-bearing organisms, one of -which attacked cellulose with the production of hydrogen, and the other -with the production of methane. Both species also produce fatty acids -and CO₂. It is probable that these organisms operate in the soil under -conditions of inadequate aeration. In swamp soils, in which rice is -grown, it has been shown that methane, hydrogen, and CO₂ are evolved -in the lower layers. In these soils, however, the methane and hydrogen -are oxidised when they reach the surface layers. This oxidation is -also effected by micro-organisms. Bacteria capable of deriving energy -by the oxidation of hydrogen gas have been isolated and studied by -Kaserer,[37] and by Nabokich and Lebedeff,[52] while Söhngen[57] has -isolated an organism which he named _Bacillus methanicus_, that was -capable of oxidising methane. - -Under normal conditions in cultivated soils, however, the decomposition -of cellulose takes place in the presence of an adequate air supply, -and so follows a different course from that studied by Omelianski. Our -knowledge of this aerobic decomposition is very scanty. A number of -bacteria, capable of decomposing cellulose aerobically, are known. A -remarkable organism was investigated by Hutchinson and Clayton,[30] -who named it _Spirochæta cytophaga_. This organism, which they isolated -from Rothamsted soil, though placed among the _Spirochætoidea_, is -of doubtful affinities. During the active condition it exists for -the most part as thin flexible rods tapered at the extremities. This -form passes into a spherical cyst-like stage, at first thought to be -a distinct organism (Fig. 2). _Spirochæta cytophaga_ is very aerobic, -working actively, only at the surface of the culture medium. It is -very selective in its action. It appears unable to derive energy from -any carbohydrate other than cellulose. Indeed, many of the simple -carbohydrates, especially the reducing sugars, are toxic to the -organism in pure culture. An extensive study of aerobic cellulose -decomposition by bacteria was made by McBeth and Scales,[50] who -isolated fifteen bacteria having this power. Five of these were -spore-forming organisms. Unlike _Spirochæta cytophaga_, they are all -able to develop on ordinary media such as beef agar or gelatine, and -are thus not nearly so selective in their food requirements. - -[Illustration: FIG. 2.--_Spirochæta cytophaga._ Changes occurring in -culture. (After HUTCHINSON and CLAYTON.)] - -We are at present ignorant as to which organisms are most effective -in decomposing cellulose in the soil under field conditions, or what -are the conditions best suited to their activity. It is possible that -fungi also help in the decomposition of cellulose to a great extent. -This subject of the decomposition of cellulose offers one of the most -promising fields of research in soil bacteriology. The difficulty -of the subject is further increased by our present ignorance of the -chemical aspect of cellulose decomposition. It has been supposed that -the early decomposition products are simpler sugars, but these are -not found under conditions in which cellulose is being decomposed by -pure cultures of the bacteria mentioned above. Hutchinson and Clayton -found that their organism produced volatile acids, mucilage, and a -carotin-like pigment. The organisms isolated by McBeth and Scales also -produce acids, and in some cases yellow pigments. It is known, however, -that the decomposition products of cellulose can be utilised as energy -supply for other organisms, such as nitrogen fixing bacteria. - -When plant remains decompose in the soil there are ultimately produced -brown colloidal bodies collectively known as humus. The processes by -which this humus is produced are not yet properly understood. Humus is -of great importance in the soil, in rendering the soil suitable for -the growth of crops. It affects the physical properties of the soil -to a great extent. In the first place, it improves the texture of the -soil, making heavy clay soils more friable, and loose sandy soils more -coherent. Secondly, it has great water-retaining powers, so that soils -rich in organic matter suffer comparatively little during periods of -drought. And lastly, it exerts a strong buffering effect against soil -acids. Now, it is one of the problems of present-day farming that -soil is becoming depleted of its humus. This is due to the increasing -scarcity of farmyard manure in many districts, and the consequent use -of mineral fertilisers to supply nitrogen, potash, and phosphate to -the crop. A need has therefore arisen for a substitute for farmyard -manure, by means of which the humus content of soils may be kept up in -districts where natural manure is scarce. - -[Illustration: FIG. 3.--Cellulose decomposed by _S. cytophaga_ in media -with increasing amounts of nitrogen. (After HUTCHINSON and CLAYTON.) - - X-axis: Milligrams of nitrogen supplied as sodium-ammonium phosphate. - - Y-axis: Milligrams of cellulose decomposed in 21 days.] - -It is well known that if fresh unrotted manure or straw be added to -the soil, it often produces harmful effects on the succeeding crop. -The problem, therefore, was to develop a method by which fresh straw, -before application to the soil, could be made to rot down to a mixture -of humus compounds such as occur in well-rotted farmyard manure. The -solution of this problem came as a result of an investigation by -Hutchinson and Richards,[30_b_] at Rothamsted, into food requirements -of the cellulose decomposing bacteria. They realised that since more -than 10 per cent. of the dry weight of bacteria consists of nitrogen, -it would be necessary to supply the cellulose decomposing bacteria with -a supply of nitrogen, in order that they should attain their greatest -activity. Experiments with cultures of _Spirochæta cytophaga_ showed -that the amount of cellulose decomposed depended upon an adequate -supply of nitrogen for the organism (Fig. 3). Similarly, materials such -as straw will scarcely decompose at all if wetted with pure water. An -adequate supply of nitrogen compounds is needed to enable decomposition -to take place. Hutchinson and Richards tested the effect of ammonium -sulphate, and discovered experimentally the proportion of ammonia to -straw that produced the most rapid decomposition. They found that if -a straw heap was treated with the correct proportion of ammonia, it -decomposed into a brown substance having the appearance of well-rotted -manure. This has resulted in the development of a commercial process -for making synthetic farmyard manure from straw. The method of -manufacture is as follows: A straw stack is made and thoroughly wetted -with water. The correct amount of ammonium sulphate is then sprinkled -on the top and wetted, so that the solution percolates through the -straw. The cellulose bacteria attack the straw, breaking it down -and assimilating the ammonia. This ammonia is not wasted, as it is -converted into bacterial protoplasm that eventually decays in the soil. -Field trials of this synthetic manure show that it produces an effect -closely similar to that of natural farmyard manure. - -While cellulose and related carbohydrates are by far the most -important non-nitrogenous compounds left in the soil by plants, there -are other compounds whose destruction by bacteria is of special -interest. Such, for example, is the case of phenol. This compound is -produced by bacterial action as a decomposition product of certain -amino-acids. It occurs in appreciable amounts in cow urine. It is -probable that it forms a common decomposition product in soil and -also in farmyard manure. If this phenol were to persist in the soil, -it would eventually reach a concentration harmful to plant growth. It -does not, however, accumulate in the soil; indeed, if pure phenol or -cresol be added to ordinary arable soil, a rapid disappearance occurs. -This disappearance is of some practical importance, since it limits -the commercial use of these compounds as soil sterilising agents. -The cause of the disappearance has been to some extent elucidated at -Rothamsted,[58] where it was found to be in part a purely chemical -reaction with certain soil constituents, and partly due to the activity -of bacteria capable of decomposing it. A large number of soil bacteria -have now been isolated that can decompose phenol, meta-, para-, and -ortho-cresol, and are able to use these substances as the sole sources -of energy for their life processes. These organisms have a wide -distribution, having been found in soil samples taken from all over -Great Britain, from Norway, the Tyrol, Gough Island, Tristan da Cunha -and South Georgia. Soil bacteria have also been isolated that are able -to decompose and derive their energy from naphthalene and from toluene. -The ability of the bacteria to break up the naphthalene is very -remarkable, and all the more so since they can hardly have come across -this compound in the state of nature. The naphthalene organisms have a -distribution as world-wide as the phenol group. - - -(2) _Ammonia Production._ - -The second main group of products left in the soil by higher plants -are the nitrogen-containing compounds, such as the proteins and -amino-acids. Plant remains are not the only source of organic nitrogen -compounds available to soil bacteria. There are, in addition, the dead -bodies of other soil organisms, such as protozoa and algæ. The relative -importance of these sources of nitrogen is not known, but almost -certainly varies greatly with the state of activity of the various -groups of the soil population. Bacteria are able to utilise organic -nitrogen compounds as energy sources, as can be exemplified in the -oxidation of a simple amino-acid:-- - - H O - | // - H--C--C + 3O = 2CO₂ + H₂O + NH₃ + 152 Cal. - | \ - NH₂ OH - -It will be seen that, in the acquirement of energy from such a -compound, ammonia is released as a by-product. It is not certainly -known what is the exact course of the reactions brought about by -bacteria in soil during the breaking-down of organic nitrogen -compounds, but they result in the splitting off of most of the nitrogen -as ammonia. Herein lies the great importance of the process, for the -production of ammonia is an essential stage in the formation of nitrate -in the soil, and on the supply of nitrate the growth of most crops -largely depends. - -[Illustration: FIG. 4.--Quantities of ammonia produced by pure cultures -from 5 grams of casein in the presence of varying quantities of -dextrose. (After DORYLAND.) - - X-axis: Percentage of dextrose added. - - Y-axis: Milligrams of NH₃ produced.] - -It is very important to note that the production of this ammonia is -only a by-product in the economy of the bacteria, the benefit that they -derive from the reactions being due to the release of energy involved -in the decomposition. The common ammonia-producing bacteria in the -soil have been found equally capable of deriving their energy by the -oxidation of sugars and similar non-nitrogenous compounds. Fig. 4 -shows an experiment by Doryland,[17] in which cultures of common soil -bacteria were grown in peptone solution, to which increasing quantities -of sugar were added. One can see that, as the amount of sugar is -increased, the production of ammonia is lowered, since the bacteria -are obtaining energy from the sugar instead of from the nitrogen -compound, peptone. Consequently, if soil contains a quantity of easily -decomposible carbohydrate material, bacteria will derive their energy -from this source, and the production of ammonia and nitrate will be -lowered. Thus the addition of sugar or unrotted straw to the soil often -lowers the nitrate production, and consequently reduces the crop yield. -If the soil is sufficiently rich in carbohydrate material, the bacteria -may multiply until the supply of organic nitrogen is used up, and -then will actually assimilate some of the ammonia and nitrate already -existing. There is thus a balance of conditions in the soil due to -varying proportions of nitrogenous and non-nitrogenous energy material. -When nitrogen compounds are the predominant energy source, the bacteria -utilise them, and ammonia is released. When a non-nitrogenous energy -source predominates, this is utilised and little or no ammonia is -released, and in extreme cases ammonia may be assimilated. - -Although a large number of the common organisms in the soil produce -ammonia in culture media containing peptone, the relative importance -of these in the soil has yet to be decided. It was supposed that the -spore-forming organisms related to _Bacillus mycoides_ were of chief -importance. This supposition dates from the work of Marchal,[49] who -studied the production of ammonia by an organism of this group in -culture solution, and found it to be a very active ammonifier. As -already mentioned, however, there is some doubt as to whether the large -spore-forming organisms are very active under soil conditions.[12],[13] -The existence of rapid fluctuations in nitrate content, found to exist -in soil, may in the future indicate which are the most active of the -common bacteria in the soil itself by enabling us to observe which -types increase during periods of rapid ammonia and nitrate formation. - - -(3) _Nitrate Production._ - -The ammonia produced in the soil under normal field conditions is -rapidly oxidised successively to nitrite and to nitrate, a process -known as nitrification. The process of nitrification is more rapid than -that of ammonia production, with the consequence that no more than -traces of ammonia are able to accumulate. The rate at which nitrate is -formed in the soil is consequently set by the slower process of ammonia -production. - -The work of Schloesing and of Warington showed that the oxidation -of ammonia was the work of living organisms. It is, however, to -Winogradsky’s isolation and study of the causative organisms that we -owe our present knowledge of the biology of the process. By a new -and ingenious technique, he isolated from soil two remarkable groups -of bacteria that bring about nitrification. The first group oxidises -ammonium carbonate to nitrite, and was divided by Winogradsky into -the two genera, _Nitrosomonas_, a very short rod-like organism bearing -a single flagellum, and _Nitrosococcus_, a non-motile form found in -South America. The second group oxidises nitrites to nitrates. They are -minute pear-shaped rods to which he gave the name _Nitrobacter_. - -Winogradsky found that the first, or nitrite-producing group, would -live in a culture solution containing:-- - - 2·25 grams ammonium sulphate, - 2·0 „ sodium chloride, - 1·0 „ magnesium carbonate, - to the litre of well water. - -Nitrobacter would grow in a similar medium containing sodium nitrite -instead of ammonium sulphate. There being no organic carbon in these -media, the organisms had no source of carbon for their nutrition, -except the CO₂ of the air, or possibly that of bicarbonate in solution. -It therefore followed that the organisms must obtain their carbon -supply from one of these sources. Unlike green plants, the nitrous and -nitric organisms are able to carry on this carbon assimilation in the -dark, and must therefore obtain the energy needed for the process from -some chemical reaction. The only sources of energy in Winogradsky’s -solutions were the nitrogen compounds, and it consequently followed -that the organisms must derive their energy supply by the oxidation -of ammonia and nitrite respectively. The release of energy obtained -by these two reactions has been calculated by Orla-Jensen to be as -follows:-- - - (NH₄)₂CO₃ + 3O₂ = 2HNO₂ + CO₂ + 3H₂O + 148 Cals. - - KNO₂ + O = KNO₃ + 22 Cals. - -The exact process by which ammonium carbonate is converted into nitrite -is not at present known. The two groups of organisms are extremely -selective in their source of energy. The nitrous organisms can derive -their energy only by the oxidation of ammonia to nitrite, and the -nitric organisms only by the oxidation of nitrite to nitrate. In -culture media they are, indeed, inhibited by soluble organic compounds -such as sugars. Under natural conditions, however, they appear to -be less sensitive, since ammonium carbonate is readily nitrified -in substrata rich in organic matter. The rapid nitrification that -takes place during the purification of sewage is an example of this. -The conditions in culture, with regard to aeration and the removal -of metabolic products from the neighbourhood of the organisms, are -very different from those in the soil, and perhaps account for the -discrepancies found. - -The oxidation of ammonium carbonate by nitrosomonas results in the -formation of nitrous acid. The organisms are very sensitive to acidity, -and can only operate if the nitrous acid produced is neutralised by an -available base. In normal soils calcium carbonate supplies this base, -and in acid soils the formation of nitrite is, as a rule, increased -by the addition of lime, or of calcium or magnesium carbonate. There -is evidence that in the absence of calcium carbonate, other compounds -can be used as a base. It was found by Hopkins and Whiting[32] that -in culture solution the nitrifying organisms could use insoluble rock -phosphate as a base, producing therefrom the soluble acid phosphate. -There is evidence, however, that in ordinary soil containing calcium -carbonate very little solution of phosphate takes place in this way. -The further oxidation of nitrite to nitrate by _Nitrobacter_ does not -produce acid, and requires no further neutralising base. - -The nitrate produced in this way is the main source of nitrogen supply -to plants under normal conditions. Experiments have shown that a number -of plants are capable of utilising ammonia as a source of nitrogen, and -Hesselmann[34] has found forest soils in Sweden where no nitrification -was proceeding, and where, therefore, plants would presumably obtain -their nitrogen in this way, but such cases must be regarded as -exceptional. - -Another group of bacteria capable of deriving their energy from -an inorganic source exists in the soil. This comprises the sulphur -bacteria, which are able to derive energy by the oxidation of sulphur, -sulphides, or thiosulphates to sulphuric acid:-- - - S + 3O + H₂O = H₂SO₄ + 141 Cals. - -One organism studied by Waksman and Joffe[63] is able to live in -inorganic solution, deriving its carbon from carbon dioxide. The -sulphur bacteria have recently come into prominence in America owing to -their faculty for producing acid. Thus Thiospirillum will increase the -acidity of its medium to a reaction of P_{H} 1·0 before growth ceases. -The potato scab disease in America is now treated by composting with -sulphur. This treatment depends on the production of sulphuric acid by -the sulphur oxidising bacteria, which renders the soil too acid for the -parasite. There is some evidence also that acid thus produced can be -used to render insoluble phosphatic manures more available in the soil. - -Analogous to the sulphur organisms are certain bacteria isolated from -sheep dig tanks in South Africa by Green,[28_b_] which can derive -energy by the oxidation of sodium arsenite to arsenate. - - -(4) _Anaerobic Respiration._ - -As is seen in the examples mentioned, energy is commonly obtained by -bacteria through an oxidation process in which free oxygen is utilised. -In water-logged soil, however, or in soil overloaded with organic -matter, anaerobic bacteria may develop, which obtain their oxygen -from oxidised compounds. Thus there are soil organisms described by -Beijerinck[2] and others which can obtain oxygen by reducing sulphates -to sulphides. - -A more important source of oxygen under these conditions is nitrate, -which can supply oxygen to a larger number of bacteria. The stage to -which the reduction can be carried varies according to the organism. -A very large number of bacteria are capable of reducing nitrates to -nitrites. Many can reduce nitrate to ammonia, and some can produce -an evolution of nitrogen gas from nitrate. The effects of nitrate -reduction, therefore, appear under water-logged conditions in soils. -For example, in swamp soils in which rice is grown, it has been found -by Nagaoka,[53] in Japan, that treatment with nitrate of soda depresses -the yield, probably owing to the formation of poisonous nitrites by -reduction. - -Under normal conditions of well aerated soil, however, it is unlikely -that the reduction of nitrate is of great importance. In such soils the -activities through which bacteria acquire their energy are, as we have -seen, of vital importance to the plant, resulting in the disintegration -of plant tissues, with the ultimate formation of humus, and in the -production of nitrate. - -In their activities connected with the building up of their protoplasm, -bacteria may, on the other hand, compete with the plant. These -activities and their consequences will be reviewed in the following -chapter. - - - - -CHAPTER III. - -SOIL BACTERIA. - - -_C._ ACTIVITIES CONNECTED WITH THE BUILDING-UP OF BACTERIAL PROTOPLASM. - - -(1) _Composition of Bacteria._ - -The activities of the soil bacteria that we have yet to consider are -those connected with the building-up from simpler materials of the -protoplasm of the bacterial cell. It is important to bear in mind that -this process is one requiring an expenditure of energy on the part of -the organism. The sources of energy we have already considered. - -The bodies of bacteria contain the same elements common to other -living matter. Analyses of various bacteria have been made by a number -of workers. About 85 per cent. of their weight is made up of water. -This analysis of Pfeiffer’s Bacillus by Cramer[15] shows the typical -percentages of carbon, nitrogen, hydrogen, and ash in the dry matter:-- - -_Composition of Pfeiffer’s Bacillus (Cramer)._ - - C 50 per cent. - N 12·3 „ - H 6·6 „ - Ash 9·1 „ - -About 65-70 per cent. of the dry matter of bacteria consists of protein. - - -(2) _Sources of Carbon._ - -The biggest constituent of the dry matter of bacteria is therefore -carbon. In the soil, bacteria find an abundance of organic matter from -which they may derive their carbon supply. A special case, however, -is furnished by the nitrifying organisms, certain sulphur oxidising -bacteria, and others that derive their carbon from the CO₂ of the soil -atmosphere. The sources from which these special groups obtain the -necessary energy to accomplish this, we have already considered. - - -(3) _Assimilation of Nitrogen Compounds._ - -Of chief importance in its consequences are the means adopted by -bacteria to obtain their nitrogen supply. - -There is some reason to believe that soil bacteria do not take up -protein and peptones as such, but must first break down these bodies -into simpler compounds. When a sufficient amount of easily decomposable -organic nitrogen is present in the soil, the ammonifying bacteria use -such compounds as sources of energy, and in this case have a nitrogen -supply exceeding their requirements. - -But where there is an excess of carbohydrate or other non-nitrogenous -source of energy available in the soil, the case is different. Here -the organisms have a supply of energy which enables them to multiply -rapidly until the organic nitrogen is insufficient for their needs. -Hence they turn to the ammonia and nitrate present in the soil, and -build up their proteins from this source. Doryland[17] has shown that -many common soil ammonifiers assimilate ammonia and nitrate when -supplied with carbohydrate. There may thus be a temporary loss of -nitrate from soil when sugar, starch, straw, or such materials are -added to it. - - -(4) _Fixation of Free Nitrogen._ - -The bacteria that we have so far considered take up their nitrogen -directly from compounds containing this element. There remain, however, -a comparatively small but very important group of bacteria possessing -the power of causing elemental nitrogen to combine, and of building it -up into their proteins. This fixation of nitrogen by micro-organisms -is a vital step in the economy of nature. Losses of nitrogen from the -land are continually occurring through the washing-out of nitrates -by rain, and through the evolution of gaseous nitrogen during the -processes of decay. To maintain the supply of combined nitrogen -which is essential to living organisms, there must therefore be a -compensating process by which the supply of nitrogen compounds in the -soil is kept up. - -It was discovered in the middle of the nineteenth century that if soil -were kept moist and exposed to the air, there was an increase in the -amount of nitrogen compounds present. Berthelot, in 1893, studied the -nitrogen relationships of soil, and recognised that this fixation of -nitrogen in soil was the work of micro-organisms. - -Winogradsky followed up his work and isolated from soil a large -anaerobic spore-forming organism, capable of fixing nitrogen, to which -he gave the name _Clostridium pasteurianum_. In 1901 the investigations -of Beyerinck, in Holland, led to the important discovery of a group of -large aerobic organisms, which he named _Azotobacter_. These were found -to be very active in fixing free nitrogen. More recently, a number of -other nitrogen-fixing bacteria have been described, and the property -has been found to exist to a small extent in several previously -well-known organisms. - -It becomes important to determine which are the groups of bacteria -whose nitrogen-fixing powers are of chief importance in the soil. - -On account of its energetic fixation of nitrogen in culture media, -_Azotobacter_ has attracted the greatest attention of workers. The -evidence seems to be consistent with the view that _Azotobacter_ is of -importance in the soil. Thus the distribution of _Azotobacter_ would -appear to be world-wide. It is found all over Western Europe and the -United States. Lipman and Burgess[45] found it in soils collected from -Italy and Spain, Smyrna, Cairo, the Fayum, the Deccan in India, Tahiti, -Hawaii, Mexico, Guatemala, and Canada. C. M. Hutchinson[29] found it -to be distributed throughout India. It was found by Omelianski[55] -to be widely distributed in European and Asiatic Russia, and by -Groenewege[28] in Java. Ashby[1] at Rothamsted, isolated it from soils -from the Transvaal, East Africa, and Egypt. Also, an association has -sometimes been found between the ability of a soil to fix nitrogen and -the occurrence and vigour of its _Azotobacter_ flora. Thus Lipman and -Waynick[46] found that if soil from Kansas were removed to California, -its power to produce a growth of _Azotobacter_, when inoculated into a -suitable medium, was lost, and, at the same time, its nitrogen-fixing -power was greatly reduced. Moreover, it is known that conditions -favourable to the fixation of nitrogen by _Azotobacter_ in cultures on -the whole favour nitrogen fixation in soils. The conditions that favour -other aerobic nitrogen-fixing bacteria are, however, not sufficiently -distinct to make such evidence of great value. - -It is usually found that nitrogen fixation is most active in -well-aerated soil. Thus Ashby,[1] at Rothamsted, found the -nitrogen-fixing power of a soil to decrease rapidly with depth. Similar -results were obtained in Utah by Greaves. This suggests, at first -sight, that anaerobic nitrogen fixers are unimportant under normal -soil conditions. It is, however, quite possible that they may assume -an importance when acting in conjunction with aerobic organisms. -Thus Omelianski and Salunskov[55] found that beneficial association, -or symbiosis, could occur between _Azotobacter_ and _Clostridium -pasteurianum_, the former absorbing oxygen from the surroundings, and -thus creating a suitable anaerobic environment for the _Clostridium_. - -The question of symbiosis of nitrogen-fixing bacteria with each other -and with other organisms offers an inviting field for research. -There is evidence that this factor may have considerable importance. -Beijerinck and Van Delden[3] early recognised that _Azotobacter_ -in mixed cultures fixed more nitrogen than in pure cultures. -_Granulobacter_, an organism which they found to be commonly associated -with _Azotobacter_ in crude cultures, appears to increase its -nitrogen-fixing powers (Krzeminiewski).[41] It was also found by -Hanzawa[31] that a greater fixation of nitrogen was obtained when two -strains of _Azotobacter_ were grown together. A symbiosis between -_Azotobacter_ and green algæ has been described, and will be further -discussed by Dr. Bristol. It is likely that this association may be of -importance under suitable conditions on the soil surface where the algæ -are exposed to light. - -The combination of elemental nitrogen is an endothermic process which -requires a very considerable amount of energy for its accomplishment. -This fact is well illustrated by the various commercial processes -in use for fixation of atmospheric nitrogen. The nitrogen-fixing -bacteria obtain this energy from the carbon compounds in the soil. -A number of compounds were compared as sources of energy by Löhnis -and Pillai,[47] who tested their effect on the amounts of nitrogen -fixed by _Azotobacter_ in culture. It was found that mannitol and the -simpler sugars give the best results as sources of energy, but that -other organic compounds can also be used. Mockeridge[51] has adduced -evidence that ethylene glycol, methyl-, ethyl-, and propyl-alcohol, -lactic, malic, succinic, and glycocollic acids could also be utilised. -Since so large a part of the organic matter added to soil is in the -form of celluloses, it is of great importance to ascertain how far -these compounds and their decomposition products can be utilised in -nitrogen fixation. Stubble, corn-stalks and roots, oak leaves, lupine -and lucerne tops, maple leaves, and pine needles may all serve as -useful sources of energy to nitrogen-fixing organisms in the soil. Pure -cellulose cannot apparently be used as a source of energy, but when -acted upon by cellulose decomposing organisms, it becomes available as -a source of energy. Hutchinson and Clayton, at Rothamsted, found that -a fixation of nitrogen could be brought about by mixed cultures of -_Azotobacter_, and of the cellulose attacking _Spirochæta cytophaga_, -when grown in cultures containing pure cellulose. It is not known -how far cellulose decomposition must proceed to produce an effective -source of energy, nor what are the substances thus produced that are -utilised. This point will not be decided until something more is known -of the course of changes in the breaking-down of cellulose in the soil. - -The amount of nitrogen fixed per unit of energy material decomposed -varies greatly, according to the organism and the conditions. -Winogradsky found that his _Clostridium_ assimilated 2-3 mgs. of -nitrogen per gram of sugar consumed. Lipman found that _Azotobacter_ -fixed 15-20 mgs. of nitrogen per gram of mannite consumed. - -[Illustration: FIG. 5. - - Caption: Azotobacter. Decrease in efficiency in N fixation with age - of culture. (Koch & Seydel.) - - X-axis: Days. - - Y-axis: Milligrams of Nitrogen fixed per gram of dextrose consumed.] - -It is found, however, that in liquid culture, the ratio of nitrogen -fixed to carbohydrates oxidised varies according to the age of the -culture, falling off rapidly as the age increases[42] (Fig. 5). This -decreasing efficiency in cultures may be due to the accumulation of -metabolic products such as would not occur under soil conditions. -Indeed, the efficiency of _Azotobacter_ in a sand culture has been -found by Krainskii[39] to be considerably greater than in solution. It -is thus probable that in soil the nitrogen-fixing organisms are less -wasteful of energy material than under the usual laboratory conditions. -It is to be hoped that future research will indicate what are the -conditions that produce the greatest economy of energy material in -nitrogen fixation. - -The fixation of nitrogen in soil is depressed by the presence of -considerable amounts of nitrates. This is, in all probability, due -to the fact that nitrogen-fixing organisms are able to utilise -compounds of nitrogen where these are available. The energy needed -to build up amino-acids and proteins from nitrate or ammonia is, of -course, far less than that required to build up these substances -from elemental nitrogen. It is, therefore, not surprising that where -nitrate is available, _Azotobacter_ will use it in preference to fixing -atmospheric nitrogen.[5] - -TABLE III.--ASSIMILATION OF NITRATES. - -BY AZOTOBACTER IN PURE CULTURE--(_Bonazzi_). - - +---------------------------+--------+-----------+------+ - | | Nitrate| Organic |Total | - | | and | Nitrogen |Fixed | - | | Nitrite|and Ammonia| or | - | |Present.| Present. |Lost. | - +---------------------------+--------+-----------+------+ - | | mgs. | mgs. | mgs. | - |_Culture with nitrate_-- | | | | - | At beginning | 8·55 | 0·76 | -- | - | After growth | 0·2 | 8·71 | -0·4 | - |_Culture without nitrate_--| | | | - | At beginning | -- | 0·76 | -- | - | After growth | -- | 4·50 | +3.74| - +---------------------------+--------+-----------+------+ - - (Growth period--24 days at 25° C.) - -The chemical process by which nitrogen is fixed is quite unknown, -although a number of speculative suggestions have been made. The -appearance of considerable amounts of amino acids in young cultures of -_Azotobacter_ suggests that these may be a step in the process, but at -present the data are too inconclusive to form a basis for theorising. - -_Azotobacter_ is very rich in phosphorus, an analysis of the surface -growth in _Azotobacter_ cultures, made by Stoklasa, giving about 60 -per cent. of phosphoric acid in the ash. In cultures it has been found -that a considerable amount of phosphate is needed to produce full -development. As would be expected, therefore, nitrogen fixation in soil -is often greatly stimulated by the addition of phosphates. Christensen -has, indeed, found soils where lack of phosphate was the limiting -factor for _Azotobacter_ growth. - -_Azotobacter_ is very intolerant of an acid medium, and is very -dependent on the presence of an available base. In cultures this -is usually provided in the form of calcium or magnesium carbonate. -Gainey[21] found that _Azotobacter_ occurred in soils having an acidity -not greater than P_{H} 6·0, and Christensen,[7],[9] in Denmark, has -found a close association between the occurrence of _Azotobacter_ in -soils and the presence of an adequate supply of calcium carbonate. So -close was this association that he devised a technique based on this -fact for detecting a deficiency of lime in a soil sample. - -In addition to the groups already discussed, there is a remarkable and -important group of nitrogen-fixing bacteria that inhabit and can carry -on their functions within the root tissues of higher plants. It has -been known at least from classical times that certain leguminous plants -would, under suitable conditions, render the soil more productive. On -the roots of leguminosæ small tubercles are commonly found. These were -noted and figured by Malpighi in the seventeenth century, and for a -long time were regarded as root-galls. As was described in Chapter I., -the true nature of these tubercles was finally elucidated by Hellriegel -and Wilfarth in 1886. As the result of a series of pot experiments, -they made the very brilliant deduction that the ability to fix -nitrogen, possessed by the legumes, was due to bacteria associated with -them in the tubercles. - -These bacteria were finally isolated and studied in pure culture -by Beijerinck. Since then a very great deal of literature has -accumulated on the subject of the nodule-producing bacteria, which -it is impossible to deal with in a small space. The nodule organism, -_Bacillus radicicola_, when grown on suitable media, passes through a -number of different changes in morphology. The most connected account -of these changes is given in a paper by Bewley and Hutchinson.[4] -In a vigorous culture the commonest type is a rod-shaped bacillus -which may or may not be motile. As these get older they often become -branched, or irregular in shape, the formation of these branched forms -being perhaps due to conditions in the medium. These irregular forms, -known as “bacteroids,” are a characteristic type in the nodules. Their -production in culture media has been found to be stimulated by sugars -and organic acids such as would occur in their environment within the -host plant. In the older rods and bacteroids the staining material -becomes condensed into granules, and finally the rods disintegrate or -break up into coccoid forms. By suitable culture conditions, Bewley and -Hutchinson obtained cultures consisting almost entirely of this stage. -If such a culture be inoculated into a fresh medium rich in sugar, -the swarmer stage appears in great numbers. These swarmers are very -minute coccoid rods, ·9 × ·18 in size, that are actively motile. They -apparently develop later into the rod stage. - -[Illustration: FIG. 6.--_Bacillus radicicola._ Stages in the life -cycle. (After HUTCHINSON and BEWLEY.) - - Motile Rods - - Vacuolated Stage - - “Swarmers” - - “Bacteroids” - - “Pre-swarmers”] - -Very little is known as to the life of the organism in the soil. -It is able, however, to fix nitrogen in cultures, and it has been -claimed[35],[48] that it can do so in the soil outside the plant, so -that it is possible that we must take it into consideration in this -connection. More knowledge is needed as to the optimum conditions for -the growth of the organism in the soil. It seems to be more tolerant of -acid soil conditions than _Azotobacter_. The limiting degree of acidity -has been found to vary among different varieties of the organism from -P_{H} 3·15 to P_{H} 4·9. - -A long controversy has been held as to whether the nodule organisms -found in different host-plants all belong to one species, or whether -there are a number of separate species, each capable of infecting a -small group of host-plants. As the term “species” has at present no -exact meaning when applied to bacteria, the discussion in this form -is unlikely to reach a conclusion. The evidence seems to show that -the nodule organisms form a group that is in a state of divergent -specialisation to life in different host-plants, and that this -specialisation has reached different degrees with different hosts. -Thus the organisms from the nodule of the pea (_Pisum sativum_) will -also produce nodules on vicia, Lathyrus, and Lens, but seem to have -lost the ability normally to infect other legumes. On the other hand, -the bacteria from the nodules of the Soy Bean (_Glycine hispida_) -have become so specialised that they do not infect any other genus -of host-plant, and soy beans are resistant to infection by other -varieties of the nodule organism. Burrill and Hansen,[6] after an -extensive study, divided the nodule bacteria into eleven groups, within -each of which the host-plants are interchangeable. The existence of -different groups of nodule organisms has been confirmed by the separate -evidence of serological tests (Zipfel, Klimmer, and Kruger).[40] The -results of cross-inoculation tests have sometimes been conflicting. It -seems, indeed, that the host-plant has a variable power of resisting -infection, so that when its resistance is lowered it may be capable of -infection by a strange variety of the nodule organism. The question -that has thus arisen of the ability of the legume to resist infection -is of fundamental importance, and its elucidation should throw light on -the relation of plants to bacterial infection as a whole. - -The stage of the organism that infects the plant is not at present -known. It may be supposed that it is the motile “swarmer.” The entry is -normally effected through the root-hairs. The hair is attacked close -to the tip, and an enzyme is apparently produced which causes the tip -to bend over in a characteristic manner. The organisms multiply within -the root hair and pass down it, producing a characteristic gelatinous -thread filled with bacteria, in the rod form. This “infection thread” -passes down into the cells of the root tissue, where it branches -profusely. In young stages of nodule formation the branches can be seen -penetrating cells in the pericycle layer. Rapid cell division of these -root cells is induced. In the course of this cell division abnormal -mitotic figures are sometimes found, such as occur in pathological -growths. The cells push outward the root cortical layer, and so form a -nodule. - -Certain of the cells in the centre of the nodule become greatly -enlarged, and in the fully grown nodule are seen to be filled with -bacteria. Differences have been described in the morphology of the -organisms in different parts of the nodule.[62] Whether the different -stages of the organism are equally capable of fixing nitrogen, or what -is the significance of these stages within the nodule, is not certainly -known. It has been held that it is the irregular bacteroid forms that -are chiefly concerned with nitrogen fixation. In older nodules the -organisms become irregular and stain faintly, and the bacteroidal -tissue breaks down, the nodule finally decaying. In the fixation of -nitrogen that occurs in the nodules, the bacteria without doubt derive -the necessary energy from the carbohydrates of the host-plant. There -is evidence that the plant assists the process of fixation by removing -soluble metabolic products from the neighbourhood of the bacteria. -Golding[22] was able to obtain a greatly increased fixation of nitrogen -in artificial cultures by arranging a filtering device so as to remove -the products of metabolism. - -The great practical importance of leguminous crops in agriculture has -led to numerous attempts being made to increase their growth, and the -fixation of nitrogen in them, by inoculating the seed or the soil with -suitable nodule-bacteria. This inoculation can be effected either with -soil in which the host-plant has been successfully grown, and which -should consequently contain the organism in fair numbers, or else pure -cultures of the organisms isolated from nodules may be used. Very -varying results have been obtained with inoculation trials. - -In farm practice a leguminous crop has often been introduced into a -new area where it has never previously grown. In such soil it is very -probable that varieties of the nodule organism capable of infecting the -roots may not exist. In such cases inoculation with the right organism -or with infected soil often produces good results. - -The more difficult case, however, is that in which the legume crop -has been grown for a long time in the locality, and where the soil -is already infected with right organisms. This, the more fundamental -problem, applies especially to this country. Here it would seem that -inoculation with a culture of the organism will benefit the plant only -(1) if the naturally occurring organisms are present in very small -numbers; or (2) if the organisms in the culture added are more virulent -than those already in the soil. The problem of successful inoculation -would therefore seem to be bound up with that of grading up the -infective virulence of the organism to a higher level. - -Successful nodule development in a legume crop is also dependent to a -large degree on the soil conditions. The effects of soil conditions on -nodule development have been studied by numerous workers. Moisture has -been found very greatly to affect the nodule development. Certain salts -have a very definite effect on nodule formation.[64] Their effect on -the number of nodules developing has been studied, but the reason for -this effect is unusually difficult to decide. The action is usually a -complex one. Thus phosphates are known to stimulate nodule formation. -They probably act in several ways. In the first place, they may cause -the nodule organisms to multiply in the soil; in the second place, they -produce a greater root development in the plant, thus increasing the -chances of infection; and in the third place, Bewley and Hutchinson[4] -have found that phosphates cause the appearance of the motile stage -of the organism in cultures. A real understanding of the influence of -environment on nodule production will produce great improvements in our -methods of legume cropping. - - -_D._ THE RELATION OF BACTERIAL ACTIVITIES TO SOIL FERTILITY. - -The various activities of the soil bacteria have a vital importance to -the growth of higher plants, which are dependent for their existence -on certain of these processes. In the first place, as we have seen, -bacteria decompose the tissues of higher plants and produce humus -materials, which are essential to the maintenance of good physical -properties in the soil. Then the nitrate supply on which most higher -plants depend is produced by the decomposition of organic nitrogen -compounds by bacteria in their search for energy. The depletion of the -total nitrogen content of the soil through rain and through the removal -of nitrogen in the crops, is to some extent compensated by the fixation -of atmospheric nitrogen by certain bacteria. On the other hand, in the -assimilation of nitrogen compounds to build up protein, the bacteria -are competing with higher plants for one of their essential food -constituents, and their action may, under certain conditions, cause a -temporary nitrogen starvation. One must remember, however, that large -quantities of nitrate are lost from field soils by washing-out through -rain action, especially in winter. The assimilation of nitrate and -ammonia by micro-organisms keeps some of this nitrogen in the soil, and -at certain periods may thus be beneficial. - -There is another important respect in which soil bacteria influence -plant growth. Their activities result in the release of inorganic -salts, such as potash and phosphates, in a form available for the use -of plants. The release of phosphorus and potassium compounds takes -place in two ways. In the first place, organic matter containing -phosphorus and potassium, in an insoluble form, is attacked by -bacteria, resulting in these elements being set free as inorganic -salts available to the higher plant. Secondly, much of the phosphorus -supplied to the soil from rock minerals is present as insoluble -phosphates, such as apatite and iron phosphate. Much of the potassium, -too, is derived from insoluble silicate minerals. In both cases the -conversion of the insoluble minerals into soluble phosphates and -potassium compounds is brought about to a large extent by solution in -water containing carbonic and other acids. These acids are largely -produced by micro-organisms, which, in addition to carbonic acid, -produce organic acids, and in specialised cases, sulphuric and nitrous -acids. It has been found, for example, that in a compost of soil -with sulphur and insoluble phosphate, sufficient sulphuric acid may -be produced by the oxidation of the sulphur by bacteria to convert -an appreciable amount of phosphate into a soluble form. When we -consider the functions performed by soil bacteria, therefore, it is -not surprising to find that high bacterial activity in the soil is -associated, as a rule, with fertility. - - -_E._ CHANGES IN BACTERIAL NUMBERS AND ACTIVITIES, AND THEIR RELATION TO -EXTERNAL FACTORS. - -The object of soil bacteriologists is to discover means of favouring -the activity of soil bacteria, especially those activities that are -useful to the higher plant. Knowledge is therefore needed of the -changes in numbers and activities of the soil bacteria, and of the -influence of soil conditions on them. The necessity of studying these -changes has required the development of a quantitative technique by -which the numbers of bacteria in the soil and their activities can be -estimated. - -The method commonly used in counting bacteria in soil is a modification -of the plating method of Koch. In counting bacteria two difficulties -have to be overcome--their immense numbers and their small size. The -numbers of bacteria in soil are so large that the bacterial population -of a gram of soil could not, of course, be counted directly. The method -adopted, therefore, is to make a suspension of soil in sterile salt -solution, and to dilute this suspension to a convenient and known -extent, which will depend on the numbers of bacteria expected. In -ordinary field soils it is found convenient, for example, to dilute the -soil suspension so that one cubic centimeter of the diluted suspension -will contain 1/250,000th of a gram of soil. Such a volume will commonly -contain a number of bacteria sufficiently small to count. The second -difficulty is that the organisms are microscopic, and yet cannot be -readily counted under the microscope owing to the presence of soil -particles in the suspension. Hence recourse is had to plating. One -cubic centimeter of diluted suspension is placed in a petri dish and -mixed with a suitable nutrient agar medium, melted, and cooled to about -40° C. The medium sets, and after a few days’ incubation the organisms -multiply and produce colonies visible to the naked eye. By counting -these colonies we obtain an estimate of the number of bacteria in -the one cubic centimeter of suspension, it being assumed that every -organism has developed into one colony, and by multiplying this number -by the degree of dilution we obtain the numbers per gram of soil. -In practice a number of parallel platings are made from one cubic -centimeter portions of the diluted suspension and the mean number of -colonies per plate is taken. By this means the error due to the random -distribution of bacteria in the suspension is reduced, because of the -greater number of organisms counted. - -In drawing conclusions from bacterial count data, it is necessary to -distinguish between the indication which the method gives of the -absolute numbers of bacteria in the soil and the accuracy with which -it enables the numbers of two soil samples to be compared. The method -cannot be used for the former purpose at present. We do not know how -far the figures obtained by this counting method fall short of the -actual number of bacteria in the soil. One reason for this is the -difficulty of effecting a complete separation of the clumps of bacteria -into discrete individuals in the suspension. Then again, there is no -known medium upon which all the physiological groups of bacteria will -develop and produce colonies. And even on a suitable medium some of the -individuals may fail to multiply. - -In comparing the bacterial numbers in two soil samples, however, -the case is different. Within each bacterial group investigated the -plate method should give counts proportional to the bacterial numbers -in the soil. Thus, by the method one should be able to tell whether -the bacterial numbers are increasing or decreasing over a period of -time, or whether a certain soil treatment produces an increase or -a decrease. With this end in view the technique of the method has -been improved by recent workers. It was found that, when carefully -standardised, the process of dilution of the soil could be carried -out without significant variation in result (Table IV.), and that the -accuracy of the method is limited mainly by the variation in colony -numbers on parallel platings, due in part to random distribution of -bacteria throughout the final suspension, and partly to the uneven -development of colonies on the medium. The question of the medium was -therefore taken up with a view to improving the uniformity of results -obtained with it. Lipman, Conn, and others effected an improvement by -using chemical compounds as nutrient ingredients, thus making their -media more closely reproducible. On most agar media, an important -disturbing factor is the growth of spreading colonies, which prevent -the development of some of the other colonies. A medium has been -devised at Rothamsted on which these spreading organisms are largely -restricted.[61] A statistical examination[19] has shown that on this -medium errors due to the uneven development of colonies, except in -special cases, are prevented, so that in fact the variation in colony -numbers between parallel plates is found to be that produced merely by -random distribution of bacteria in the diluted suspension (see Table -IV.). In this case the accuracy of the counts of the bacteria in the -diluted suspension depend directly on the number of colonies counted, -and can be known with precision. - -TABLE IV.--BACTERIAL COUNTS OF A SOIL SAMPLE. - -PARALLEL PLATE COUNTS FROM FOUR SETS OF DILUTIONS MADE BY DIFFERENT -WORKERS. - - +------------------------------------------+ - | Counts of Colonies on each Plate. | - +------+--------+--------+--------+--------+ - |Plate.| Set I. | Set II.|Set III.| Set IV.| - +------+--------+--------+--------+--------+ - | 1 | 72 | 74 | 78 | 69 | - | 2 | 69 | 72 | 74 | 67 | - | 3 | 63 | 70 | 70 | 66 | - | 4 | 59 | 69 | 58 | 64 | - | 5 | 59 | 66 | 58 | 62 | - | 6 | 53 | 58 | 56 | 58 | - | 7 | 51 | 52 | 56 | 54 | - +------+--------+--------+--------+--------+ - | Mean | 60·86 | 65·86 | 64·28 | 62·86 | - +------+--------+--------+--------+--------+ - - Standard deviation between the four sets = 5·62. - - Standard deviation between plates within the sets = 7·76. - -The knowledge obtained from counts of soil bacteria is subject to -another serious limitation. We do not know which of the bacteria -counted are the most effective in bringing about the various changes -that take place in the soil. It is not even known which of them are -active in the soil and which are in a resting condition. It is thus -possible to have two soils containing equal numbers of bacteria but -showing widely different biochemical activity, if one soil contains -organisms of a higher efficiency. Moreover, as has been pointed -out, many important groups of soil bacteria do not develop on the -plating media, and so are not counted. These considerations led to -the development of supplementary methods by which it was hoped to -estimate the actual biochemical activity of the soil microflora. The -first of these methods was developed by Remy, who attempted to study -the biochemical activity of a soil by placing weighed amounts into -sterile solutions of suitable and known composition, keeping them -under standard conditions for a definite time and then estimating the -amount of the chemical change that was being studied. Thus, to test the -activity of the organisms that produce ammonia from organic nitrogen -compounds, he inoculated soil into 1 per cent. peptone solution and -measured the amount of ammonia produced in a given time. By similar -methods the power of a soil to oxidise ammonia to nitrate, to reduce -nitrate, or to fix atmospheric nitrogen, is tested. This method has -been extensively used and developed by more recent workers. It suffers, -however, from the same serious disadvantage that it was designed to -avoid, for we cannot be certain that those bacteria that develop in -the nutrient solution are the types that are active in the soil, and, -moreover, even where the same types do function in the two conditions, -we do not know that the degree of their activity is the same in soil -and in solution cultures. For instance, _Nitrosomonas_ appears to show -very different degrees of activity in soil and in culture. - -Another method, therefore, of studying the activity of soil -micro-organisms is the obvious one of estimating the chemical changes -that they produce in the soil itself. This method has obvious -advantages over the unnatural methods developed from Remy’s, but it has -a number of limitations that make its actual application difficult. -In the first place, we cannot always tell whether changes found to -occur in soil are due to the activity of micro-organisms, or are -purely chemical reactions unassisted by biological agencies. Then, if -we succeed in showing that the changes are due to micro-organisms, it -is very difficult to determine which organisms are effecting them. -This cannot be definitely tested by isolating suspected organisms and -testing their activity in sterile soil, because in sterilising soil its -nature and composition is altered. In spite of these difficulties, -however, the study of the chemical changes that take place in the soil -has produced valuable knowledge, when it has been combined with a -study of the changes in the number and variety of the micro-organisms -that accompany these reactions. This method of investigation is well -illustrated by the work of Russell and Hutchinson on the effects of -heat and volatile antiseptics on soil, where a study of the chemical -changes such as ammonia production, that occurred in these treated -soils, combined with a study of the changes in bacterial numbers, led -to the realisation that the soil micro-population was a complex one, -containing active protozoa. - -A great difficulty in applying quantitative methods to bacteria in the -field is the great variation in the density of the bacterial population -over a plot of field soil, which may be so great that a bacterial count -from a single sample is quite valueless. For example, the distribution -of bacterial numbers over a plot of arable soil near Northampton was -studied by taking sixteen samples distributed over an area about -12 feet square. The result showed that in some cases the bacterial -numbers in samples taken 6 inches apart differed by nearly 100 per -cent. Fortunately, under favourable conditions, a remarkably uniform -distribution of bacterial numbers over a plot of soil can be found. - -On such a plot it is possible to investigate the rapidity with which -the numbers of the soil micro-organisms alter in point of time. For -example, on the dunged plot of Barnfield, Rothamsted, which has been -cropped with mangolds for forty-seven successive years, the area -distribution of bacteria has been found to be so uniform that if a -number of samples of soil are taken from the plot at the same time, the -difference in bacterial numbers between the samples cannot be detected -by means of the counting technique (see Table V.). The work of Cutler, -Crump, and Sandon[16] on this plot showed that the bacterial numbers -vary very greatly from one day to the next, and that these fluctuations -took place over the whole plot, since two series of samples, taken in -two rows 6 feet apart, showed similar fluctuations (see Fig. 7). The -discovery of these big daily fluctuations in numbers led to an inquiry -as to how quickly bacterial numbers change, and samples from Barnfield, -taken at two-hourly intervals, showed that significant changes in -numbers took place even at such short intervals. - -TABLE V.--BACTERIAL COUNTS OF FOUR SOIL SAMPLES. - -FROM BARNFIELD, TAKEN SIMULTANEOUSLY. - - +------------------------------------------------------+ - | Counts of Colonies on each Plate. | - +------+-----------+-----------+-----------+-----------+ - |Plate.| Sample I. | Sample II.|Sample III.| Sample IV.| - +------+-----------+-----------+-----------+-----------+ - | 1 | 38 | 45 | 43 | 27 | - | 2 | 32 | 40 | 34 | 41 | - | 3 | 52 | 45 | 52 | 35 | - | 4 | 32 | 31 | 55 | 36 | - | 5 | 40 | 43 | 38 | 45 | - |Mean | 38·8 | 40·8 | 44·4 | 36·8 | - +------+-----------+-----------+-----------+-----------+ - - Standard deviation between the four samples = 7·25. - - Standard deviation between parallel plates within the sets = 7·55. - -[Illustration: FIG. 7. - - X-axis (top): Days. - - Y-axis (left): (Series A) Bacteria--millions per gramme of soil. - - Y-axis (right): (Series B) Bacteria--millions per gramme. - - Caption: Daily changes in bacterial numbers in field soil. - - Counts from two series of soil samples taken 6 feet apart. - - (After Cutler.)] - -Since the bacteria involved in this fluctuation are of great importance -to the crops, being for the most part ammonia producing types, further -knowledge as to the cause of this fluctuation and of its effect on the -ammonia and nitrate in the soil is of fundamental importance. There is -evidence, which will be discussed later, that the cause is connected -with the changing activities of certain soil protozoa, since the daily -changes in the numbers of active amœbæ in the soil have been found -to be in the reverse direction to those of the bacterial numbers. It -appears, therefore, that we are dealing with an equilibrium between -the various members of the soil population, the point of equilibrium -changing at frequent intervals. - -In addition to daily changes, it is possible to detect changes in the -numbers and activity of the soil population related to the season. -There is a well-marked increase in the spring and autumn (see Figs. -15, 16, pp. 89, 90). This is well seen when the fortnightly averages -of the daily bacterial and protozoal counts from Barnfield soil -are plotted. These spring and autumn increases comprise both the -bacterial and the protozoal population, and therefore differ from the -short time fluctuations in being due, not to a disturbance of the -bacteria-protozoa equilibrium, but to a general rise in activity of -both groups of organisms. - -When we consider the action of external conditions on the soil -bacteria, the existence of a complex soil population and the -interdependence of its members must be borne in mind. Changes in -external conditions may affect the different components of the -population in different ways or to different degrees, thus upsetting -the equilibrium between the various groups. For example, the addition -of a mild aromatic antiseptic to the soil apparently affects the -protozoa in such a way as to disturb the bacteria-protozoa equilibrium -in favour of the bacteria, while in some cases the aromatic compound -affords a food supply to special bacteria, causing these to increase, -upsetting the equilibrium between the different bacterial groups. When -our knowledge of the effect of external factors on the soil population -becomes sufficient, it will probably be found that in nearly all -cases a change in the soil conditions produces some disturbance in -the equilibrium between the components of the soil population, though -at present there are only certain examples where this disturbance is a -probable explanation of the facts. - -Since bacteria are dependent on adequate supplies of energy and food, -it is to be expected that additions of organic matter or of inorganic -food materials will greatly benefit their activities. The effect of -added farmyard manure in increasing bacterial activities has been much -studied.[27] Some of the increased bacterial numbers and activities -in this case may be due to the addition of bacteria with the manure, -but it is thought that this factor is of less importance than the -added energy and food supply which the general soil flora obtain from -it. Nutritive salts such as phosphates and salts of potassium usually -increase the bacterial activities. - -The effect of alkali salts on soil bacteria has been especially studied -in the Western United States, where the existence of alkali in the -soil is a serious problem.[23] Soil bacteria are usually stimulated by -small doses of alkali salts that are toxic in higher concentration. -As a rule, chlorides are the most toxic salts, the electronegative -ion playing an important part in the effect of the salt. Salts -affect bacteria both owing to the changes in osmotic pressure which -they produce, and through their specific action on the bacterial -protoplasm.[26] When equal weights of various salts are added to soil, -their toxic action on bacteria shows so little association with their -respective osmotic pressures that we must conclude that this factor is -the less important. There is reason to suppose that the toxic action -of salts on bacteria is often connected with an effect of the specific -ions on the permeability of the bacterial cell-wall. This conclusion -is based on the changes in electrical conductivity of bacterial -suspensions in the presence of various salts.[59] - -A definite antagonism between various salts has been found to exist. -It is possible that future work in this line may indicate what are -the proportions of common electrolytes which will produce a properly -“balanced” soil solution so that the harmful excess of one salt may be -antagonised. - -Certain salts, such as those of arsenic[24] and manganese, seem to -exercise a stimulating action on bacterial activities; the causes of -this action are not at present understood. - -The acidity of the soil has an important effect on the bacterial -processes. The acidity of soils may increase to such a point that the -decomposition of plant tissues by bacteria is hindered, a peat layer -being thus produced. The degree of acidity that is toxic varies very -greatly with different soil bacteria, some of them, like Azotobacter -and Nitrosomonas being very intolerant of acidity. - -The conditions of aeration, water content, and temperature are -inter-related in field soil. Ammonifying organisms are not greatly -dependent on aeration, but this factor is sometimes a limiting one in -the case of the very aerobic nitrifying bacteria. Hence efficient soil -cultivation is beneficial to nitrification. - -Many attempts have been made to correlate the temperature and moisture -of field soils with the bacterial numbers and activities. These -attempts have given very discordant results. It is generally agreed -that a plentiful moisture supply is beneficial. Thus Greaves, in Utah, -found the optimum water content for ammonia and nitrate production -to be about 60 per cent. of the water-holding capacity. On the other -hand, Prescott[56] found that the summer desiccation of soil in -Egypt was followed by increased bacterial activities. Fabricius and -Feilitzen,[18] using moor soil, found a direct relationship between -soil temperature and bacterial numbers, showing that temperature can -be a limiting factor under certain conditions. With normal arable -soils, however, no such direct effect of temperature or moisture can -be found[16] (see Fig. 8). It has even been found by Conn[11] that -freezing of the soil may cause a marked increase in bacterial numbers. -The erratic effects of temperature and moisture on the soil bacteria -probably afford instances of a disturbance of the equilibrium between -the bacteria and other components of the soil micro-population. Thus -desiccation and freezing, though they harmfully affect the bacteria, -may inhibit other micro-organisms to a greater degree, thus freeing -the bacteria from competition. It is in the investigation of this -equilibrium, and of the factors that can control it to our benefit, -that the great advances in soil biology in the future are to be -expected. - -[Illustration: FIG. 8.--Effect of frost on the bacterial numbers in the -soil. (After CONN.) - - X-axis: Nov.-May - - Y-axis (bottom): Temperature--Degrees C. - - Y-axis (top): Bacteria--Millions per Gramme of Soil.] - - -REFERENCES TO CHAPTERS II. AND III. - - [1] Ashby, S. F., Journ. Agric. Sci., 1907, vol. ii., p. 35. - - [2] Beijerinck, M. W., Centr. f. Bakt., 1900, Abt. II., Bd. 6, p. 1. - - [3] Beijerinck, M. W., and Van Delden, A., Centr. f. Bakt., 1902, - Abt. II., Bd. 9, p. 3. - - [4] Bewley, W. F., and Hutchinson, H. B., Journ. Agric. Sci., 1920, - vol. x., p. 144. - - [5] Bonazzi, E., Journ. Bact., 1921, vol. vi., p. 331. - - [6] Burrill, T. J., and Hansen, R., Illin. Exp. Sta., 1917, Bulletin - 202. - - [7] Christensen, H. R., Centralblatt. f. Bakt., 1915, Abt. II., Bd. - 43, p. 1. - - [8] Christensen, H. R., Centralblatt. f. Bakt., 1907, Abt. II., Bd. - 17, pp. 109, 161. - - [9] Christensen, H. R., and Larsen, O. H., Centralblatt. f. Bakt., - 1911, Abt. II., Bd. 29, p. 347. - - [10] Conn, H. J., Centralblatt. f. Bakt., 1910, Abt. II., Bd. 28, p. - 422. - - [11] Conn, H. J., Centralblatt. f. Bakt., 1914, Abt. II., Bd. 42, p. - 510. - - [12] Conn, H. J., Journ. Bact., 1916, vol. i., p. 187. - - [13] Conn, H. J., Journ. Bact., 1917, vol. ii., p. 137. - - [14] Conn, H. J., Journ. Bact., 1917, vol. ii., p. 35. - - [15] Cramer, E., Arch. f. Hyg., 1893, Bd. 16, p. 151. - - [16] Cutler, W., Crump, L. M., and Sandon, H., Phil. Trans. Roy. - Soc., 1923, Series B, vol. ccxi., p. 317. - - [17] Doryland, C. J. T., N. Dakota Agr. Exp. Sta., 1916, Bulletin 116. - - [18] Fabricius, O., and Feilitzen, H., Centr. f. Bakt., 1905, Abt. - II., Bd. 14, p. 161. - - [19] Fisher, R. A., Thornton, H. G., and Mackenzie, W. A., Ann. Appl. - Biol., 1922, vol. ix., p. 325. - - [20] Fred, E. B., and Hart, E. B., Wisconsin Agr. Exp. Sta. Research, - 1915, Bulletin 35. - - [21] Gainey, P. L., Journ. Agric. Research, 1918, vol. xiv., p. 265. - - [22] Golding, J., Journ. Agric. Sci., 1905, vol. i., p. 59. - - [23] Greaves, J. E., Soil Sci., 1916, vol. ii., p. 443. - - [24] Greaves, J. E., Journ. Agric. Res., 1916, vol. vi, p. 389. - - [25] Greaves, J. E., Soil Sci., 1920, vol. x., p. 77. - - [26] Greaves, J. E., and Lund, Y., Soil Sci., 1921, vol. xii., p. 163. - - [27] Greaves, J. E., and Carter, E. G., Journ. Agric. Research, 1916, - vol. vi., p. 889. - - [28] Groenewege, J., Arch. Suikerindust., 1913, Bd. 21, p. 790. - - [28_b_] Green, H. H., Union of S. Africa Dept. Agr., Rept. of - Director Vet. Res., 1918, p. 592. - - [29] Hutchinson, C. M., Rept. Agr. Res. Inst. and Col. of Pusa, 1912, - p. 85. - - [30] Hutchinson, H. B., and Clayton, J., Journ. Agric. Sci., 1919, - vol. ix., p. 143. - - [30_b_] Hutchinson, H. B., and Richards, H. H., Journ. Min. Agric., - 1921, vol. xxviii., p. 398. - - [31] Hanzawa, J., Centr. f. Bakt., 1914, Abt. II., Bd. 41, p. 573. - - [32] Hopkins, C. G., and Whiting, A. L., Ill. Agr. Exp. Sta., 1916, - Bulletin 190, p. 395. - - [33] Hoppe-Seyler, G., Ztschr. Phys. Chem., 1886, vol. x, pp. 201, - 401; 1887, vol. xi., p. 561. - - [34] Hesselmann, H., Skogsvårdsför. Tidskr., 1917, No. 4, p. 321. - - [35] Joshi, N. V., Mem. Dept. Agr. in India, Bact. Ser., 1920, vol. - i., No. 9. - - [36] Koch, R., Mitt. Kais. Gesundh., 1881, vol. i., p. 1. - - [37] Kaserer, H., Centr. f. Bakt., 1906, Abt. II., Bd. 16, p. 681. - - [38] Kaserer, H., Centr. f. Bakt., 1905, Abt. II., Bd. 15, p. 573. - - [39] Krainskii, A. V., Centr. f. Bakt., 1910, Abt. II., Bd. 26, p. - 231. - - [40] Klimmer, M., and Kruger, R., Centr. f. Bakt., 1914, Abt. II., - Bd. 40, p. 257. - - [41] Krzeminiewski, S., Centr. f. Bakt., 1909, Abt. II., Bd. 23, p. - 161. - - [42] Koch, A., and Seydel, S., Centr. f. Bakt., 1912, Abt. II., Bd. - 31, P. 570. - - [43] Lipman, C. B., Bot. Gaz., 1909, vol. xlviii., p. 106. - - [44] Lipman, C. B., and Burgess, P. S., Centr. f. Bakt., 1914, Abt. - II., Bd. 41, p. 430. - - [45] Lipman, C. B., and Burgess, P. S., Centr. f. Bakt., 1915, Abt. - II., Bd. 44, p. 481. - - [46] Lipman, C. B., and Waynick, D. O., Soil Sci., 1916, vol. i., p. - 5. - - [47] Löhnis, F., and Pillai, N. K., Centr. f. Bakt., 1908, Abt. II., - Bd. 20, p. 781. - - [47_b_] Löhnis, F., and Smith, T., Journ. Agric. Res., 1914, vol. - vi., p. 675. - - [48] Mackenna, J., Rept. Prog. Agric., India, 1917, p. 101. - - [49] Marchal, E., Bull. Acad. Roy. Belgique, 1893, vol. xxv., p. 727. - - [50] McBeth, I. G., and Scales, F. M., U.S. Dept. Ag., Bureau Plant - Indus., 1913, Bulletin 266. - - [51] Mockeridge, J., Biochem. Journ., 1915, vol. ix., p. 272. - - [52] Nabokich, A. J., and Lebedeff, A. F., Centr. f. Bakt., 1906, - Abt. II., Bd. 17, p. 350. - - [53] Nagaoka, M., Bull. Coll. Agr., Tokyo, 1900, vol. vi., No. 3. - - [54] Omelianski, W. L., Comptes Rendus Acad. Sci., 1895, vol. cxxi., - p. 653; 1897, vol. cxxv., pp. 907, 1131; Arch. Sci. Bio., (St. - Petersburg), 1899, vol. vii., p. 411. - - [55] Omelianski, W. L., and Sohmskov, M., Arch. Sci. Biol., Publ. - Inst. Imp. Med. Exp. (Petrograd), 1916, vol. xviii., pp. 327, 338, - 459; vol. xix., p. 162. - - [56] Prescott, J. A., Journ. Agr. Sci., 1920, vol. x., p. 177. - - [57] Söhngen, N. L., Centr. f. Bakt., 1905, Abt. II., Bd. 15, p. 513. - - [58] Sen Gupta, N., Journ. Agr. Sci., 1921, vol. xi., p. 136. - - [59] Shearer, C., Journ. Hyg., 1919, vol. xviii., p. 337. - - [60] Tappeiner, Ber. Deut. Chem. Gesell., 1883, vol. xvi., p. 1734; - Zeitsch. Biol., 1884, vol. xx., p. 52. - - [61] Thornton, H. G., Ann. Appl. Biol., 1922, vol. ix., p. 241. - - [62] Wallin, I. E., Journ. Bact., 1922, vol. vii., p. 471. - - [63] Waksman, S. A., and Joffe, J. S., Journ. Bact., 1922, vol. vii., - p. 239. - - [64] Wilson, J. K., Cornell Agric. Exp. Sta., 1917, Bulletin 386. - - - - -CHAPTER IV. - -PROTOZOA OF THE SOIL, I. - - -That protozoa could be isolated from the soil was a matter of common -knowledge to the biologists of the nineteenth century, but not until -the early part of the present century was it suggested that these -organisms might be playing some part in the general economy of the soil -micro-population. Of recent years a great deal of our knowledge of the -cytology of the different groups of protozoa, especially the Amœbæ, has -been obtained from the study of representatives normally living in the -soil; but unfortunately little or no knowledge has been gained of the -biology of these animals in their natural habitat. - -The view that the presence of these organisms in excessive numbers may -lead to “soil sickness” was first put forward by Russell and Hutchinson -in 1909, and elaborated in their further papers dealing with “Partial -Sterilisation of the Soil.” - -It is unnecessary to discuss in detail this important branch of -agriculture, but to obtain a clear idea of the development of -the study of soil protozoa it is necessary to give as briefly as -possible the conclusions deduced by Russell and Hutchinson from their -extensive experiments on soils treated with steam and various volatile -antiseptics[21],[22]:-- - -“(1) Partial sterilisation of the soil causes first a fall, then a -rise, in bacterial numbers, which goes on till the numbers considerably -exceed those present in the original soil. - -“(2) Simultaneously there is a marked increase in the rate of -accumulation of ammonia which is formed from organic nitrogen -compounds. - -“(3) The increase in bacterial numbers is the result of improvement in -the soil as a medium for bacterial growth, and not an improvement in -the bacterial flora. - -“(4) The improvement in the soil brought about by partial sterilisation -is permanent, the high bacterial numbers being kept up even for 200 -days or more. It is evident from (3) and (4) that the factor limiting -bacterial numbers in ordinary soil is not bacterial, nor is it any -product of bacterial activity, nor does it arise spontaneously in soils. - -“(5) But if some of the untreated soil is introduced into partially -sterilised soil, the bacterial numbers, after the initial rise, begin -to fall. Thus the limiting factor can be reintroduced from untreated -soils. - -“(6) Evidence of the limiting factor in untreated soils is obtained -by studying the effect of temperature on bacterial numbers. Untreated -soils were maintained at 10°, 20°, 30° C. in a well-moistened aerated -condition, and periodical counts were made of the numbers of bacteria -per gram. Rise in temperature rarely caused any increase in bacterial -numbers. But after the soil was partially sterilised the bacterial -numbers showed the normal increase with increasing temperatures. - -TABLE VI. - - +--------+------------------------+------------------------+ - | | | Soil Treated | - | | Untreated Soil. | with Toluene. | - |Tempera-+------+-----+-----+-----+------+-----+-----+-----+ - | ture of| |After|After|After| |After|After|After| - |Storage.| At | 13 | 25 | 70 | At | 13 | 25 | 70 | - | °C. |Start.|Days.|Days.|Days.|Start.|Days.|Days.|Days.| - +--------+------+-----+-----+-----+------+-----+-----+-----+ - | 5°-12° | 65 | 63 | 41 | 32 | 8·5 | 73 | 101 | 137 | - | 20° | 65 | 41 | 22 | 23 | 8·5 | 187 | 128 | 182 | - | 30° | 65 | 27 | 50 | 16 | 8·5 | 197 | 145 | 51 | - | 40° | 65 | 14 | 9 | 33 | 8·5 | 148 | 52 | 100 | - +--------+------+-----+-----+-----+------+-----+-----+-----+ - -“(7) It is evident, therefore, that the limiting factor in the -untreated soils is not the lack of anything, but the presence of -something active. The properties of the limiting factor are:-- - - “(_a_) It is active and not a lack of something. - - “(_b_) It is not bacterial. - - “(_c_) It is extinguished by heat or poisons. - - “(_d_) It can be re-introduced into soils from which it has been - extinguished by the addition of a little untreated soil. - - “(_e_) It develops more slowly than bacteria. - - “(_f_) It is favoured by conditions favourable to trophic life in the - soil, and finally becomes so active that the bacteria become unduly - depressed. - -“It is difficult to see what agent other than a living organism can -fulfil these conditions. Search was therefore made for a larger -organism capable of destroying bacteria, and considerable numbers of -protozoa were found. The ciliates and amœbæ are killed by partial -sterilisation. Whenever they are killed the detrimental factor is found -to be put out of action; the bacterial numbers rise and maintain a high -level. Whenever the detrimental factor is not put out of action, the -protozoa are not killed. To these rules we have found no exception.” - -From such premises as the above Russell and Hutchinson founded the -“protozoa theory of partial sterilisation,” and at Rothamsted there was -commenced the serious study of these soil organisms. - -Goodey was one of the early workers on this new subject, and added -considerably to our knowledge of the species living in normal soils, -and of the chemical constitution of the cyst wall of ciliates. He -also made investigations on the effects of various chemicals on the -micro-population of soils, but was unable to draw very definite -conclusions.[11] - -One of the first criticisms raised against the protozoa theory of -partial sterilisation was that the protozoa were not normal inhabitants -of the soil, and were present only in small numbers, all of them in the -cystic, quiescent condition. It was further held that these cysts were -carried by the wind from dried-up ponds and streams. It is difficult to -trace the origin of this view, since early observers, viz., Ehrenberg -and Dujardin, in 1841, were of the opinion that the protozoa were -living in the trophic active condition in the soil, and it was not -until 1878 that Stein showed that free living protozoa can encyst. To -Martin and Lewin, however, must be ascribed the distinction of first -proving that the soil possesses an active protozoan population, for -by a series of ingenious experiments these observers isolated several -flagellates and amœbæ in a trophic condition from certain of the -Rothamsted soils.[18] The more recent work in this country has been -in the direction of devising new quantitative methods of research, -since by this means alone is it possible to elucidate many fundamental -questions. - -In America and elsewhere experiments have been devised for testing the -conclusions of Russell and Hutchinson. Cunningham and Löhnis,[2] in -America, Truffaut and Bezssonoff,[24] in France, supply evidence in -favour of the theory, but most of the American work is in opposition to -it. - -Sherman[23] is perhaps the most prominent in opposing the phagocytic -action of protozoa on soil bacteria in spite of the fact that certain -of his experimental results apparently show enormous decreases in -bacterial numbers in the presence of protozoa. In many of his soil -inoculation experiments, however, it was not demonstrated that his -active cultures remained alive after entering the soil. - -The experimental difficulties of dealing with soil protozoa are -considerable, and without a thoroughly sound technique investigators -may easily go astray. - - -CLASSIFICATION. - -The animal kingdom is divided into two main groups or sub-kingdoms--the -Protozoa and the Metozoa. In the latter the characteristic feature -is that the body is composed of several units, called cells, and -consequently such animals are often spoken of as multicellular. The -Protozoa, on the other hand, are usually designated as uni-cellular, -since their bodies are regarded as being homologous to a single unit or -cell of the metozoan body. For various reasons exception has been taken -by Dobell[9] and others to the use of the term uni-cellular, for, as -Dobell says, “If we regard the whole organism as an individual unit, -then the whole protozoan is strictly comparable with a whole metozoon, -and not with a part of it. But the body of a protozoan, though it shows -great complexity of structure, is not differentiated internally into -cells, like the body of a metozoon. Consequently it differs from the -latter not in the number of its cellular constituents, but in lacking -these altogether. We therefore define the sub-kingdom of the protozoa -as the group which contains _all non-cellular animals_.” - -It should be pointed out that this view does not find favour with many -zoologists, but it is useful in bringing into prominence the fact -that each protozoan is comparable as regards its functions with the -multi-cellular animals. - -The protozoa are again further divided into four main classes:-- - - I. Rhizopoda. - II. Mastigophora. - III. Ciliophora. - IV. Sporozoa. - -Of the above classes, representatives of each of the first three are -found living in the soil, but up to the present there is no evidence -that any sporozoon is capable of living an active life in the soil, -though the cysts of such organisms may be present. - -The class _RHIZOPODA_ consists of those protozoa whose organs of -locomotion and food capture are _pseudopodia_, that is, temporary -extensions of the living protoplasm. The body is typically naked, that -is to say, without any cuticular membrane, though in some forms, ex. -_Amœbæ terricola_, the external layer of protoplasm is thickened to -form a pellicle. A skeleton or shell may be present. - -The class is further sub-divided into various sub-classes, only -two of which concern the soil protozoologist, viz., the _Amœbæ_ -and the _Mycetozoa_, of which the most important representative is -_Plasmodiophora brassicæ_, which attacks the roots of many cruciferous -plants, causing the disease familiarly known as “Fingers and Toes.” - -The _Amœbæ_ are again divided into two orders:-- - - (_a_) _Nuda_, without shell or skeleton; - - (_b_) _Testacea_, with shells often termed _Thecamœbæ_. - -Representatives of the “naked” amœbæ commonly found in soils are -_Nægleria (Dimastigamœba) gruberi_, _Amœba diploidea_ (possessing -two nuclei) and _A. terricola_, the last two forms possessing a -comparatively thick skin or pellicles. _Trinema enchelys_, _Difflugia -constricta_ and _Chlamydophrys stercorea_ are examples of soil -Thecamœbæ. - -The class _MASTIGOPHORA_ consists of those protozoa whose typical modes -of progression are by means of flagella, whip-like filaments which, by -their continual lashing motion, cause movement of the animal. - -The body may be naked or corticate. The only organisms which concern -the soil biologist belong to the _Flagellata_ order. - -The Flagellates differ considerably among themselves, both as regards -their mode of feeding, and the number of flagella, thus making their -classification difficult and outside the scope of this book. Suffice -it to say that in the soil such organisms occur possessing one, two, -three or four flagella, ex. _Oicomonas termo_, _Heteromita globosus_, -_Dallengeria_ and _Tetramitus spiralis_. Further, their mode of feeding -may be _saprophytic_ in which nourishment is absorbed by diffusion -through the body surface in the form of soluble organic substances, -_holozoic_ where solid food particles are taken in, or _holophytic_ in -which food is synthesised by the energy of sunlight. This last group -is commonly spoken of as the _Phyto flagellates_, which are to all -intents and purposes unicellular algæ, and as such will be dealt with -in Chapter VI. - -The class _CILIOPHORA_ consists of those protozoa whose typical organs -of locomotion are threads or cilia. These organisms can in one sense -be regarded as the highest of the protozoa, since in no other division -does the body attain so great a complexity of structure. Moreover, they -are typically characterised by a complicated nuclear apparatus with -the vegetative and generative portions separated into distinct bodies, -the macro-nucleus and the micro-nucleus. Their mode of nutrition is -_holozoic_, though recently Peters has brought forward evidence that -certain species can obtain their nourishment saprophytically. - -The sub-class Ciliata comprises four orders, all of which are -represented in the soil. - -I. _Holotricha._ The cilia are equal in length and uniformly -distributed over the whole body in the primitive forms, though -restricted to special regions in the specialised forms. Typical soil -forms are _Colpoda cucullus_, _Colpidium colpoda_. - -II. _Heterotricha._ There is a uniform covering of cilia, and a -conspicuous spiral zone of larger cilia forming a vibratile membrane -and leading to the mouth. - -III. _Hypotricha._ The body is flattened dorso-ventrally and the cilia -are often fused to form larger appendages or cirri confined to the -ventral surface. Movement is typically a creeping one. Typical soil -forms are _Pleurotricha_, _Gastrostylis_, _Oxytricha_. - -IV. _Peritricha._ Typically of a sedentary habit and the cilia are -reduced to a zone round the adoral region of the body. A typical soil -form is _Vorticella microstomum_. - -The above classification is far from complete, but should be sufficient -to give an idea of the general grouping of the organisms. For a more -detailed account reference must be made to the numerous text books on -protozoa. - - -LIFE HISTORIES. - -The life history of each species has its own characteristic features as -regards nuclear division, etc., and in many forms, notably the amœbæ, -it is impossible to identify them with certainty unless the chief -stages of the life history are known. In general, however, the soil -protozoa pass through very similar phases and develop in a perfectly -straightforward way. Broadly speaking, there are two main phases of -the life history--a period of activity often mistermed vegetative, and -a period of rest. In the former the animal moves, feeds and reproduces, -while in the latter there is secreted round the body a thick wall, -capable of resisting adverse external influences. This condition is -termed the cystic stage, and by means of it the animals are distributed -from place to place by air, water, etc. Indeed, so resistant are the -cysts that many of them are capable of withstanding the action of the -digestive juices of the intestines of animals, through which they pass -to be deposited by the fæces on fresh ground. - -This cystic stage of the life history is found in practically all -free-living protozoa, though it is not formed in exactly the same -manner in every case. In the majority of instances the cyst is the -product of a single organism, round which is formed a delicate -gelatinous substance which soon hardens and gradually acquires the -peculiar characters of the wall. Concerning the chemical nature of this -wall there is little known, but Goodey,[11] working on the cysts of -_Colpoda cucullus_, found it to be formed of a carbohydrate, different -from all carbohydrates previously described, to which the name “Cytose” -was given. When in this state the animals are able to remain dormant -for considerable periods until favourable conditions once more obtain -when the wall is ruptured and the animal again resumes the active phase -of its life history. This simple process is characteristic of such -species as _Heteromita globosus_, _Cercomonas spp._, and many others. -It will be noted that no increase of numbers, i.e. reproduction, -occurs. A more complex condition is, however, sometimes found, as, for -example, in the ciliate _Colpoda steinii_, where actual reproduction -into small animals takes place within the cyst. - -Finally there is the less common type of cyst formation, such as is -found in the flagellate _Oicomonas termo_ described by Martin.[19] This -flagellate, in common with all other forms, reproduces by dividing into -two; the division of the nucleus initiating the process. At certain -undetermined periods of the life history, however, conjugation occurs -between two similar animals forming a large biflagellate body known as -the zygote. After swimming about for varying periods of time, during -which the size increases and a large vacuole appears, the zygote -secretes a thick wall, loses its flagella, and becomes a cyst. While in -this condition the two gamete nuclei fuse to form one, and eventually a -single _Oicomonas_ emerges from its cyst. - -Similarly in _A. diploidea_ the cysts are formed after two individuals -have come together. In the young cysts two amœbæ are found in close -association, and according to Hartmann and Nägler[12] a sexual process -occurs inside the cyst involving a “reductive” division of the nuclei. -This requires confirmation, but it is certain that only one individual -comes out of the cysts, which originally contained two amœbæ. - -Such cysts have been termed by some writers “reproductive,” evidently a -misleading term, since no increase in numbers, but rather a decrease, -results from the process. A better term is, perhaps, conjugation cyst. - -In soil protozoa, then, three different modes of cyst formation obtain, -and failure to make the distinction inevitably leads to confusion. - -Before leaving the question of life histories, reference must be made -to a peculiar and characteristic feature of _Nægleria gruberi_. This -amœba under certain circumstances assumes a free-swimming biflagellate -stage. After variable periods of time the flagella are lost and the -ordinary amœboid condition resumed. What are the factors concerned in -the production of flagellates is unknown, but flooding the coverslips -with distilled water is an effective method for causing their -appearance. - - -DISTRIBUTION OF SOIL PROTOZOA. - -For both the bacteria and algæ observations have been made regarding -their distribution through successive depths of the soil; little can -be said, however, about the protozoa in this connection. It is certain -that they occur throughout the first six inches of the Rothamsted -soils, though their relative frequencies in the successive inches has -not been determined, but probably they are most abundant in the 2nd to -the 4th inch. - -In this country experiments have not been made to determine whether -sub-soil normally contains protozoa; but from some South African -soil, taken under sterile conditions 4 ft. down and examined in this -laboratory, large numbers of protozoa were cultivated. - -This soil, however, could not, for various reasons, be regarded as a -typical sub-soil. - -Kofoid records the presence of _Nægleria gruberi_ in clay and rock -talus taken from the sides of excavations of over 20 ft. depth, but the -possibility of external infection does not appear to have been excluded. - -The presence of protozoa is not peculiar to British soil since they -have been found by various workers in Germany, France, the United -States, and elsewhere. In view of their probable importance in the soil -economy there has been instituted a survey of the protozoan species of -soil from all parts of the world. - -This work is in charge of Mr. Sandon, to whom I am indebted for the -following summary of his as yet unpublished research. - -“The majority of soil protozoa (like the fresh-water forms) appear to -be quite cosmopolitan, for the species found in such widely separated -localities as England, Spitsbergen, Africa, West Indies, Gough Island -(in the South Atlantic) and Nauru (in the Pacific) are, with few -exceptions, identical. This distribution indicates an ability to -withstand an extremely wide range of conditions, for the same species -occurring in Arctic soils, which are frozen for the greater part of -the year, are found also in soils exposed to the direct rays of the -tropical sun. Even sand from the Egyptian desert contains protozoa, -though it seems probable that in such cases they must be present only -in the encysted condition for the greater part of the time. - -“Not every sample of soil, however, contains all the species capable -of living in soil, but the local conditions determining the presence -or absence of any species are at present unknown. In general the -numbers, both of species and of individuals present, follow the number -of bacteria. They are consequently most numerous in rich moist soils. -The statement sometimes made that protozoa are most numerous in peaty -soils is based solely on the number of Rhizopod shells found in such -localities; but as most of these shells are empty, their abundance is -probably due simply to the slowness with which they disintegrate in -these soils where bacterial activity is low, they do not indicate a -great protozoal activity. Active protozoa do occur even in extremely -acid soils, but their numbers in such cases are low. The common soil -protozoa, in fact, appear to be as tolerant of differences in soil -acidity as they are of differences in climate, for many of the same -forms which occur in acid soils are found also in soils containing -high percentages of chalk. It is possible that some of the less common -species may be confined within closer limits of external conditions but -the information available on this point is inadequate. All the species, -however, which in Rothamsted soils occur in the highest numbers (e.g. -_Oicomonas termo_, _Heteromita spp._, _Cercomonas crassicauda_, -_Nægleria gruberi_, _Colpoda cucullus_, _C. steinii_) occur in -practically every soil which is capable of supporting vegetation, -though, of course, in very varying numbers.” - -It is evident, therefore, that the protozoa must be regarded as -constituting part of the normal micro-organic population of soils, -and as such are probably playing an important rôle. Unfortunately our -knowledge of the physiology of these organisms is extremely scant, and -much of future research must be directed towards elucidating their -functions and their responses to varying environmental conditions. - - - - -CHAPTER V. - -PROTOZOA OF THE SOIL, II. - - -In the preceding chapter an outline has been given of the development -of the study of soil protozoa, with especial reference to its -qualitative aspects. - -Here it is proposed to deal with the quantitative methods which have -been devised for studying these organisms and the results obtained. - -From the beginning great difficulty has been encountered in finding -means for counting protozoa; and most of the early results have been -obtained by the use of one of the following methods: (1) direct counts -in a known volume of soil suspension by means of a microscope; (2) -dilution method as used for counting bacteria, and suggested by Rahn, -who made dilutions of the soil and determined, by examination at -periodic intervals, the one above which protozoa did not grow; (3) Agar -plating as used by Killer; (4) counting per standard loop of suspension -as devised by Müller. Of these the two last have been little used, -and for various reasons are now discarded by most workers. Direct -methods have been used extensively in the United States by Koch[13] -and others,[16] who claim to have got satisfactory results; they are, -however, highly inaccurate and should be discontinued. The present -writer[3] has shown that there exists a surface energy relationship -between the soil particles and the protozoa, so that the two are always -in intimate contact; thus rendering it impossible to count under the -microscope the number of organisms in a given weight of soil suspension -(Fig. 9). Further, in a clay soil, such as is found at Rothamsted, the -clay particles alone make it very difficult to use such methods. - -The demonstration of this surface energy relationship affords an -effective rejoinder to the criticism made against Russell and -Hutchinson’s hypothesis, viz., that soil protozoa must be very few in -numbers, since it was impossible to see them on examining soil under -the microscope. - -[Illustration: FIG. 9.--Showing the number of amœbæ and flagellates -withdrawn from suspensions of varying strengths by different types of -solid matter. A = clay: B = partially sterilized soil: C = ignited -soil: D = fine sand: E = waste sand. Since complete withdrawal occurs -when the numbers of organisms added are less than the capacity of the -solid matter, the first part of each of the above curves is coincident -with the ordinate. The numbers of organisms are given in thousands. -(From Journ. Agric. Soc., vol. ix.) - - X-axis: Number of Organisms per c.c. left in Solution. - - Y-axis: Number of Organisms per c.c. taken up by Solid Matter.] - -The second or dilution method is the one, therefore, that has been most -extensively developed. - -Cunningham obtained concordant results in this way, and his method, -modified by L. M. Crump, was as follows: 10 grams of soil were added -to 125 c.c. of sterile tap-water and shaken for three minutes. This -gives a 1 in 12·5 dilution. From it further dilutions were made -until a sufficiently high one was obtained. Petri dishes, containing -nutrient agar, were inoculated with 1 c.c. of each of the dilutions and -incubated. At intervals covering 28 days the plates were examined and -the presence or absence of protozoa on each recorded. In this way the -approximate number of organisms per gram of soil could be found. - -By methods essentially similar to this numerous counts have been made -of the bacteria and protozoa in field soil and in partially sterilized -soils. They were, however, inconclusive; thus, on the one hand, -Goodey and several American observers, found no correlation between -the numbers of protozoa and bacteria, while Miss Crump and Cunningham -obtained evidence pointing to the reverse conclusion. - -Such divergence of opinion was probably mainly due to two causes: -firstly, that the time elapsing between the successive counts was too -long, for it has been shown recently that the number of bacteria and -protozoa fluctuate very rapidly; and secondly, the method was not -completely satisfactory since only the total numbers of protozoa were -considered, no means having been found of differentiating between the -cystic and active forms. This was a particularly serious source of -error for it is possible for soil to contain large numbers of bacteria -and protozoa, of which a high percentage of the latter are in the form -of cysts. A count made on such a soil would give results apparently -opposed to the theory that protozoa act as depressors of bacteria. - -This difficulty has, however, been overcome by a further modification -of the dilution method, and it is now possible in any soil sample -to count both the numbers of cysts and active forms. Also a further -advance in technique has made it possible to recognise and enumerate -the common species of protozoa, instead of simply grouping them as -Ciliates, Flagellates, and Amœbæ, as was done in the past.[7] - -Briefly the method consists in dividing the soil sample into equal -portions (usually 10 grams each) one of which is counted, thus giving -the total numbers of protozoa (active + cystic) present. The second -portion is treated over-night with 2 per cent. hydrochloric acid, the -HCl used being B.P. pure 31·8 per cent. Previous experiments have shown -that such acid kills all the active protozoa, leaving viable the cysts. -The number of cysts is therefore found by counting this treated sample, -and the number obtained subtracted from the total gives the active -number.[F] - - [F] The proof of the accuracy of this method will be found in the - following papers:-- - - (1) Cutler, D. W. (1920), Journ. Agric. Sci., vol. x., 136-143. - - (2) Cutler, D. W., and Crump, L. M. (1920), Ann. App. Biol., vol. - vii., 11-24. - -The discovery of this method at once puts into the hands of the -investigator a much more efficient instrument for studying the -activities of the soil micro-population, especially since at a slightly -later date Thornton’s method for counting bacteria was devised. - -Early in 1920 Cutler and Crump[6] decided to make a preliminary survey -of the protozoon and bacterial populations of one of the Rothamsted -field soils (Broadbalk dunged plot). The investigation was continued -for 28 days, daily soil samples being taken. The results so obtained -showed that an extended investigation of the micro-population of field -soil would yield interesting and important results, especially as -it was evident that certain views held by soil biologists required -modification. - -In July of the same year, therefore, it was decided to start an -extended investigation of the soil protozoa and bacteria. The method -adopted was to make counts of the numbers of bacteria and of six[G] -species of protozoa in soil samples taken daily direct from the field -(Barnfield dunged plot) and by statistical methods to correlate these -counts one with another and with the data for external conditions. -Observations at shorter periods than 24 hours could not be made, but it -was found possible to continue the research for 365 days.[7] - - [G] Actual counts were made of six species, though, as stated on p. - 10, observations were made on seventeen. - -[Illustration: FIG. 10.--Daily numbers of active amœbæ (Dimastigamœba -and Species α) and bacteria in 1 gram of field soil, from August 29 to -October 8, 1920. (From Phil. Trans. Roy. Soc., vol. ccxi.) - - X-axis: August September October - - Y-axis (left): Amoebae Active numbers per gramme of soil - - Y-axis (right): Bacteria in millions per gramme of soil - - Legend: Dimastigamoeba - - Species α - - Bacteria] - -The number of all the organisms showed large fluctuations of two kinds, -daily and seasonal. The size of the changes that took place within -so short a period as 24 hours was, perhaps, the most surprising fact -that the experiment revealed. Thus three consecutive samples gave -58·0, 14·25 and 26·25 millions of bacteria per gram respectively; and -the changes exhibited by any of the species of protozoa were at times -even larger. This fact is of extreme importance, since in the past it -has always been assumed that the number of bacteria remained fairly -constant from day to day, and investigators have not hesitated to -separate the taking of soil samples by long periods. It is now obvious -that such a procedure is of little use for comparative purposes (Fig. -10). - -It has usually been assumed that the changes in the external conditions -markedly affect the density of the soil population. To test this the -environmental conditions--temperature, moisture content and rainfall -were examined; but contrary to all expectation no connection could be -traced between any of these and the daily changes in numbers of any -of the organisms investigated, and moreover the species of protozoa -appeared in the main to be living independently of one another. - -It is difficult to believe that external conditions are as inoperative -as appears from the above; and in view of the known complexity of -the soil it is possible that further research will show that certain -combinations of external conditions are important agents in effecting -the changes. - -[Illustration: FIG. 11.--Numbers of active amœbæ (Dimastigamœba and -Species α) and bacteria to 1 gram of field soil for typical periods in -February and April, 1921. (From Phil. Trans. Roy. Soc., vol. ccxi.) - - X-axis: Feby. Feby. April - - Y-axis (left): Amoebae Active numbers per gramme of soil - - Y-axis (right): Bacteria millions - - Legend: Dimastigamoeba - - Species α - - Bacteria] - -In the case of the bacteria, however, the agent causing the -fluctuations is mainly the active amœbæ. This was well shown during the -year’s count, for with only 14 per cent. of exceptions, 10 per cent. -of which can be explained as due to rapid excystation or encystation, -a definite inverse relationship was established between the active -numbers of amœbæ and the number of bacteria (Figs. 11 and 12). Thus a -rise from one day to the next in the amœbic population was correlated -with a fall in the numbers of bacteria and vice versa. It must not be -supposed that the flagellates are of no account in this process; some -species, known to eat bacteria, undoubtedly induce slight depressions, -but, owing to their small size, any effect is masked by the greater -one of the amœbæ. - -[Illustration: FIG. 12.--Numbers of active amœbæ (Dimastigamœba and -Species α) and bacteria in 1 gram of field soil for typical periods in -September, October, and November, 1920. - - X-axis: August September October - - Y-axis (left): Amoebae, thousands - - Y-axis (right): Millions, Bacteria - - Legend: Dimastigamoeba - - Species α - - Bacteria] - -These experiments seem to admit of no doubt that in field soil the -active protozoa are instrumental in keeping down, below the level -they might otherwise have attained, the numbers of bacteria; but a -further proof of this contention ought to be obtained by inoculation -experiments. It should be possible, by inoculating sterile soil -with bacteria alone and with bacteria plus protozoa, to demonstrate -fluctuations in bacterial numbers in the latter, while those of the -former remained constant. This admittedly crucial test has often been -tried, but owing to difficulties in technique, etc., has always failed. -Recently, however, by using new methods confirmatory results have been -obtained.[5] - -Ordinary field soil was sterilised by heat at 100° C. for 1 hour on -four successive days; it was then divided into equal portions, one of -which was inoculated with three known species of bacteria, and the -other inoculated with the same number of bacteria plus the cysts of the -common soil amœba _Nægleria gruberi_. The numbers of bacteria in each -soil were counted daily for the first eight days and then daily from -the 15th to the 21st day after the experiment started. The results are -given in Table VII. and Fig. 13. - -TABLE VII. - - +------------+---------+--------+ - | Numbers of | Control | Control| - | Days after |(Bacteria|Bacteria| - |Inoculation.| alone).|+ Amœbæ.| - +------------+---------+--------+ - | 0 | 13·0 | 12·2 | - | 1 | 48·6 | 35·4 | - | 2 | 97·6 | 117·2 | - | 3 | 127·0 | 178·4 | - | 4 | 154·8 | 154·4 | - | 5 | 196·8 | 177·0 | - | 6 | 214·4 | 151·8 | - | 7 | 193·4 | 75·6 | - | 8 | 165·2 | 65·8 | - | 15 | 169·2 | 72·8 | - | 16 | 174·8 | 30·2 | - | 17 | 175·6 | 53·2 | - | 18 | 168·4 | 82·8 | - | 19 | 160·4 | 43·8 | - | 20 | 171·2 | 70·8 | - | 21 | 176·2 | 28·2 | - +------------+---------+--------+ - - The numbers of bacteria are given in millions per gram of soil. - -[Illustration: FIG. 13.--Numbers of bacteria counted daily in soils -containing - - A. Bacteria alone. - B. Same Bacteria as in A + Amœbæ. - C. Same Bacteria as in A + Flagellates. - -(From Ann. Appl. Biol., vol. x.)] - -It will be noted that the numbers of bacteria in each soil rose -steadily until a maximum was reached 6-8 days after inoculation. This -is in accordance with expectation, since the reproductive rate of -bacteria is much greater than that of the amœbæ, which, until their -active forms are numerous, will not exert any appreciable influence on -the bacterial population. Further, since the protozoa were inoculated -as cysts an appreciable time would elapse before excystation took -place. The last seven days of the experiment are of particular -interest. During this period the amœbæ were known to be active in the -soil, and were depressing the bacterial numbers, for in the control -(protozoa-free) soil the variation in numbers was within experimental -error, while in the other soil the variations were considerable and -well outside experimental error. In fact the variations were comparable -with those found from day to day in untreated field soils. Finally, -the experiment shows that the bacteria in protozoa-free soil are able -to maintain high numbers for a longer period than those living in -association with protozoa. - - -SEASONAL CHANGES. - -Superimposed on the daily variations in numbers there are seasonal -changes, as is clearly shown when fourteen day averages are made of -the numbers for each species. Bacteria have long been known to show -autumn and spring rises, but recent research has demonstrated that the -protozoan population also rises to a maximum at the end of November, -with a less marked spring rise at the end of March and beginning of -April (Figs. 14 and 15). - -It has sometimes been claimed that the numbers of soil organisms are -closely linked with the soil moisture, but no support for this view -was found during the course of the experiment. Similarly, as in the -case of the daily variations, no connection could be traced between the -seasonal changes and any of the external conditions considered. - -It is interesting to note, however, that the seasonal variations in the -numbers of soil organisms is very similar to those recorded for many -aquatic organisms. Miss Delf,[8] for instance, found that in ponds at -Hampstead the algæ are most numerous in spring and again in the autumn, -and like changes are recorded in British lakes by West and West[25] and -in the Illinois river by Kofoid.[14] - -[Illustration: FIG. 14.--Fortnightly averages of total numbers of -Oicomonas, Species γ, and Species α, and of bacteria, moisture, and -temperature. (From Phil. Trans. Roy. Soc., vol. ccxi.) - - X-axis: Fortnight beginning 1920. July. Aug. Sep^t. Oct. Nov. Dec. - Jan 1921. Feb^y. Mch. April. May. June. - - Y-axis (bottom left): Percentage of moisture - - Y-axis (top left): Logarithms of numbers of active protozoa per - gramme of soil - - Y-axis (bottom right): Temperature F - - Y-axis (top right): Bacteria in millions per gramme - - Legend: Oicomonas - - Species γ - - Species α - - Bacteria - - Temperature - - Moisture] - -It is difficult to resist the conclusion that these annual variations -are produced by similar causes, from which it follows that the increase -in the numbers of protozoa in the soil is not wholly conditioned by an -increased food supply--the bacteria--for the algæ are not dependent on -such a form of nourishment. This is substantiated by the fact that the -numbers of protozoa, except those of _Oicomonas_, rose during March, -whereas the corresponding increase in the bacteria was delayed till the -early part of April. - -[Illustration: FIG. 15.--Fortnightly averages of total numbers of -Heteromita, Cercomonas, and Dimastigamœba and of bacteria, moisture, -and temperature. (From Phil. Trans. Roy. Soc., vol. ccxi.) - - X-axis: Fortnight beginning July 1920. Aug. Sep^t. Oct. Nov. Dec. - Jan. 1921. Feb. Mar. April May June - - Y-axis (bottom left): Percentage of moisture. - - Y-axis (top left): Logarithms of numbers of active protozoa per - gramme of soil. - - Y-axis (bottom right): Temperature F - - Y-axis (top right): Bacteria in millions per gramme - - Legend: Heteromita - - Cercomonas - - Dimastigamoeba - - Bacteria - - Temperature - - Moisture] - -Owing to the variations in the numbers of both protozoa and bacteria, -little reliance can be placed on figures obtained from an isolated -count, since on one day the total numbers of flagellates may be nearly -2,000,000 per gram and drop by more than half this figure in 24 days. -It is certain, however, that the numbers recorded in the past are much -too low, since the total flagellate and amœbæ species were lumped -together in two groups. Some idea of the size of the soil population -can be obtained, nevertheless, by using the fourteen-day averages -mentioned above. In Table VIII. are tabulated the average total numbers -of flagellates, and amœbæ for the two periods of the year when the -population was at its maximum and minimum respectively. An endeavour -has also been made to strike a rough balance sheet as to the amount of -protoplasm represented by protozoa and bacteria in a ton of soil. For -this purpose it has been assumed that the organisms have a specific -gravity of 1·0 and are spheres of diameters, 6µ for the flagellates, -10µ for the amœbæ, and 1µ for the bacteria; and that they are uniformly -distributed through the top nine inches of soil. The top nine inches of -soil is taken as weighing 1000 tons. - -TABLE VIII. - - +-----------+---------------------------+---------------------------+ - | | Maximum Period. | Minimum Period. | - | + ____________/\___________ + ____________/\___________ + - | |( )|( )| - | | No. | Weight | Weight| No. per | Weight | Weight| - | | per |in Gram |in Tons| Gram. |in Gram |in Tons| - | | Gram. | per | per | | per | per | - | | | Gram. | Acre. | | Gram. | Acre. | - +-----------+----------+--------+-------+----------+--------+-------+ - |Flagellates| 770,000|0·000087| 0·087 | 350,000|0·000039| 0·039 | - |Amœbæ | 280,000|0·000147| 0·147 | 150,000|0·000078| 0·078 | - |Bacteria |40,000,000|0·000020| 0·02 |22,500,000|0·000012| 0·012 | - +-----------+----------+--------+-------+----------+--------+-------+ - -It must be remembered that the above figures are minimum ones, as many -species of bacteria and protozoa, known to occur in the soil, are not -included in the statement owing to their not appearing on the media -used for counting purposes. - -[Illustration: FIG. 16.--Daily variations in the numbers of active -individuals of a species of flagellate, _Oicomonas termo_ (Ehrenb.) -during March, 1921. (From Phil. Trans. Roy. Soc., vol. ccxi.) - - X-axis: March - - Y-axis: Active numbers per gramme of soil] - -Before leaving the discussion of daily variations in numbers of -protozoa, reference must be made to the flagellate species. As already -mentioned, their active numbers fluctuate rapidly, and for the most -part entirely irregularly. One species, however, _Oicomonas termo_, is -characterised by possessing a periodic change; high active numbers on -one day being succeeded by low, which are again followed by high on the -third day. This rhythm was maintained, with few exceptions, for 365 -days (Fig. 16), and has been shown to take place in artificial culture -kept under controlled laboratory conditions (Fig. 17). - -[Illustration: FIG. 17.--Daily variations in the numbers of active -individuals of _Oicomonas termo_ (Ehrenb.) in artificial culture media -kept at a constant temperature of 20° C. A, in hay infusion; B, in egg -albumen. - - X-axis: Days - - Y-axis: Thousands] - -It was thought that an explanation of this phenomenon might be found -in alternate excystation and encystation, since the latter is a -constituent part of the animals’ life history (see p. 73). This, -however, does not hold, for the cyst curve is not the inverse of that -of the active; and, moreover, statistical treatment demonstrated that -cyst formation is wholly unperiodic in character. - -An explanation must therefore be sought in the changes in the -organisms during the active period of their life, and the deduction -can be drawn that, increased active numbers tend to be followed by -death, conjugation, or both, while decreases in the active numbers -are followed by rises in total numbers, i.e., reproduction, and this -rhythmically. - -This somewhat surprising conclusion appears to hold, in a lesser -degree, for other soil protozoa, and is of sufficient importance -to warrant further research. The direction in which this is being -pursued is by a study of the reproductive rates of pure cultures of -certain ciliates and flagellates under varying external conditions. -Space does not admit of adequate discussion of this problem, but the -results already obtained justify the view that such lines of work will -elucidate some of the baffling problems of soil micro-biology. - - -SOIL REACTION. - -The development of the artificial fertiliser industry has in many -ways revolutionised farm practice, with the inevitable result that -new problems have arisen, not the least of which are biological in -character. - -If, as seems to be indubitable, the micro-organisms of the soil are of -importance to soil fertility, it is necessary for us to know in what -way this population is affected by the application of fertilisers, -and a start has been made by investigating the effects of hydrogen -ion concentration on soil protozoa. Much has already been written -concerning this question, but almost entirely on results obtained -in artificial cultures. It is always dangerous to argue from the -artificial to the natural environment of organisms and particularly -so in respect to the soil. Also, as Collett has shown, the toxic -effects of acids are probably not entirely a function of the hydrogen -ion concentration, but that the molecules of certain acids are in -themselves toxic, an action which can, however, be diminished by the -antagonistic powers of many substances such as NaCl. - -In this laboratory S. M. Nasir, by unpublished work, has shown that -the limiting value on the acid side for _Colpoda cucullus_ was P_{H} -3·3; for a flagellate (_Heteromita globosus_), 3·5; and for an amœba -(_Nægleria gruberi_), 3·9. - -Also Mlle. Perey, investigating the numbers of protozoa in one of the -Rothamsted grass plots of P_{H} 3·65, found a total of 13,600 protozoa, -of which 90 per cent. were active. - -The tolerance, therefore, of these organisms to varying external -conditions is greater than has formerly been supposed, a conclusion -which is becoming more evident from the researches mentioned in Chapter -IV. on soils from different parts of the world. - - -PROTOZOA AND THE NITROGEN CYCLE. - -In partially-sterilised soil from which protozoa were absent Russell -and Hutchinson obtained an increased ammonia production, a result -also obtained by Cunningham. Hill, on the other hand, concluded that -protozoa have no effect on ammonification, but his technique is open to -criticism. - -Lipman, Blair, Owen and McLean’s work[17] contains many figures -obtained by adding dried blood, tankage, soluble blood flour, -cottonseed meal, soy-bean meal, wheat flour, corn meal, etc., to soil. -It is difficult to understand how accurate results could be expected -when, to an already little understood complex substance, such as soil, -is added a series of substances whose effects are practically unknown. - -Free nitrogen-fixation in soils is an important process, more -especially in soils of a light sandy nature, from which crops are -taken year after year without any application of manure. The effect of -protozoa on the organisms causing this process has in the past received -little attention. Recently, however, Nasir[20] has studied the -influence of protozoa on Azotobacter, both in artificial culture and in -sand. From a total of 36 experiments done in duplicate or triplicate, -31 showed a decided gain in nitrogen fixation over the control, while -only 5 gave negative results. - -[Illustration: FIG. 18.--Showing the highest fixations of nitrogen -above the control recorded for Azotobacter in the presence of different -species of Protozoa. (From Ann. Appl. Biol., vol. ii.) - - X-axis (left): Artificial Media C A F AF AC ACF - - X-axis (right): Sand Cultures C A AF AC - - Legend: C represents CILIATES. - - A -do.- AMOEBAE. - - F -do.- FLAGELLATES.] - -As might be expected, the fixation figures varied from culture to -culture, the highest recorded being 36·04 per cent. above the control -and this in a sand culture (Fig. 18). Reference to the details of the -experiments shows that the criticisms made against similar work done in -the past do not hold here, and one must conclude that Azotobacter is -capable of fixing more atmospheric nitrogen in the presence of protozoa -than in their absence. - -At present it is impossible to say how this occurs, but it is highly -improbable that the protozoa are themselves capable of fixing nitrogen. -A more likely explanation is that the protozoa, by consuming the -Azotobacter, kept down the numbers, and transfer the nitrogen to their -own bodies. This will tend to prevent the bacteria from reaching a -maximum density, and reproduction, involving high metabolism, will be -maintained for a longer period than would have otherwise occurred. This -and other possible explanations, are being tested. - -Little has been said regarding the application of protozoology to the -question of soil partial sterilisation. As already pointed out, in -the past much work has been done, but the results were conflicting. -In view, however, of our recently acquired knowledge of the life of -protozoa in ordinary field soil, most of the early experiments require -repeating. A beginning has already been made, but the work is not -sufficiently advanced to warrant discussion. - -What is urgently needed, however, is to increase our knowledge of the -general physiology of these unicellular animals. Until we know what -are the inter-relationships between the members of the micro-organic -population of normal soil it is almost impossible to hope that means -will be devised by which they can be controlled. - -At present we are almost entirely ignorant of the simplest of -physiological reactions, such as the exact effect of various inorganic -salts found in the soil. - -Also some experiments in Germany and the States indicate that amœbæ are -selective as regards the bacteria they ingest. If this is substantiated -it may prove of importance to economic biology. - -It has been shown that the flagellates occur in the soil in large -numbers, and many of them feed on bacteria. It is probable, however, -that certain of them feed saprophytically and must therefore exert some -influence on the soil solution, though what this may be is entirely -unknown. - -Finally, as Nasir has shown, the protozoa play a part in the -complicated nitrogen cycle, and work of this type needs extending. - -Such, then, are a few of the outstanding problems that confront the -soil protozoologist; but he must always remember that the organisms he -studies are but a small fraction of the total, and that any influence -affecting one part of the complex will be reflected in another. As -Prof. Arthur Thomson said in his Gifford Lectures, “No creature lives -or dies to itself, there is no insulation. Long nutritive chains often -bind a series of organisms together in the very fundamental relation -that one kind eats the others.” Such nutritive chains obtain in the -soil as markedly as in other haunts of living creatures. - - -SELECTED BIBLIOGRAPHY. - -* _Papers giving extensive bibliographies._ - - [1] Cunningham, A., Journ. Agric. Sci., 1915, vol. xvii., p. 49. - - [2] Cunningham, A., and Löhnis, F., Centrlb. f. Bakt. Abt. II., 1914, - vol. xxxix., p. 596. - - [3] Cutler, D. W., Journ. Agric. Sci., 1919, vol. ix., p. 430. - - [4] Cutler, D. W., Journ. Agric. Sci., 1920, vol. x., p. 136. - - [5] Cutler, D. W., Ann. App. Biol., 1923, vol. x., p. 137. - - [6] Cutler, D. W., and Crump, Ann. App. Biol., 1920, vol. vii., p. 11. - - [7] * Cutler, D. W., Crump and Sandon, Phil. Trans. Roy. Soc. B., - 1922, vol. ccxi., p. 317. - - [8] Delf, E. M., New Phytologist, 1915, vol. xiv., p. 63. - - [9] Dobell, C. C., Arch. f. Protisenk., 1911, vol. xxiii., p. 269. - - [10] * Goodey, T., Roy. Soc. Proc. B., 1916, vol. lxxxix., p. 279. - - [11] Goodey, T., Roy. Soc. Proc. B., 1913, vol. lxxxvi, p. 427. - - [12] Hartmann, M., and Nägler, K., Sitz-Ber. Gesellsch. Naturf. - Freunde, 1908, Berlin, No. 4. - - [13] Koch, G. P., Journ. Agric. Res., 1916, vol. li., p. 477. - - [14] Kofoid, C. A., Bull. Illinois State Lab. Nat. Hist., 1903 and - 1908. - - [15] * Kopeloff, N., Lint, H. C., and Coleman, D. A., Centrlb. f. - Bakt. Abt. II., 1916, vol. xlvi., p. 28. - - [16] Kopeloff, N., Lint, H. C., and Coleman, D. A., Centrlb. f. Bakt. - Abt. II., 1916, vol. xlv., p. 230. - - [17] Lipman, J. G., Blair, A. W., Owen, L. L., and McLean, H. C., - N.J. Agric. Exp. Sta. 1912, Bull., No. 248. - - [18] Martin, C. H. and Lewin, K. R., Journ. Agric. Soc., 1915, vol. - vii., p. 106. - - [19] Martin, C. H., Roy. Soc. Proc. B., 1912, vol. lxxxv., p. 393. - - [20] * Nasir, S. M., Ann. App. Biol., 1923, vol. x., p. 122. - - [21] Russell, E. J., Roy. Soc. Proc. B., 1915, vol. lxxxix., p. 76. - - [22] * Russell, E. J., “Soil Conditions and Plant Growth,” 1921, 4th - ed. - - [23] * Sherman, J. M., Journ. Bact., 1916, vol. i., p. 35, and vol. - ii., p. 165. - - [24] Truffaut, G., and Bezssonoff, H., Compt. Rend. Acad. Sci., 1920, - vol. clxx., p. 1278. - - [25] West, W., and West, G. S., Journ. Linn. Soc. Bot., 1912, vol. - xl., p. 395. - - - - -CHAPTER VI. - -ALGÆ. - - -I. GENERAL AND HISTORICAL INTRODUCTION. - -Speaking broadly, the organisms of the soil may be classified into -several distinct groups differing conspicuously in their general -characters and physiological functions and therefore in their economic -significance; among such groups may be mentioned the bacteria, -protozoa, algæ and fungi. It is found, however, that though typical -members of these groups are conspicuously different from one another, -yet there exist a number of unicellular forms which have characters -in common with more than one of these big groups, and the lines of -demarcation between them become difficult to define. It becomes -advisable, therefore, to depart a little from the systematist’s rigid -definitions and to adopt a somewhat more logical grouping of the soil -organisms based on their mode of life. - -To give but a single example: _Euglena viridis_ occurs quite commonly -in soil. Through its single flagellum, its lack of a definite cellulose -wall, its changeable shape and its ability to multiply by simple -fission in the motile state it definitely belongs systematically to the -group of protozoa. But its possession of chlorophyll, in enabling it to -synthesise complex organic substances from CO₂ and water in a manner -entirely typical of plants, connects it physiologically so closely with -the lower green algæ that in studying the biology of the soil it seems -best to include it and other nearly related forms with the algæ. - -On this physiological basis “soil-algæ” may be defined as those -micro-organisms of the soil which have the power, under suitable -conditions, to produce chlorophyll. Such a definition has the -advantage that it is wide enough to include the filamentous protonema -of mosses, which, though alga-like in form and in physiological -action, is nevertheless separated from the true algæ by a wide gulf. -A more accurate name for such a group of organisms would be the -“chlorophyll-bearing protophyta” of the soil; they may be classified -briefly as follows (Table IX.):-- - -TABLE IX. - - +----+------------------------------------+-----------+--------------+ - | | Group. | Colour. | Pigments. | - +----+------------------+-----------------+-----------+--------------+ - | I.|_Flagellatæ._ |Euglenaceæ. |Green. |_Chlorophyll._| - | | |Cryptomonadineæ. | | | - | | | | | | - | II.|_Algæ_-- | | | | - | | 1. Myxophyceæ. |Mostly filamen- |Blue-green |Phycocyanin. | - | | |tous, chiefly |to violet |_Chlorophyll._| - | | |Oscillatoriaceæ |or brown. |Carotin. | - | | |and Nostocaceæ. | | | - | | 2. Bacillariaceæ.|Mostly pennate, |Golden- |Carotin. | - | | |chiefly Navi- |brown. |Xanthophyll. | - | | |culoideæ. | |_Chlorophyll._| - | | 3. Chlorophyceæ. | (i) Protococ- |Green. |_Chlorophyll._| - | | | cales, Ulo- | | | - | | | trichales, | | | - | | | Conjugatæ, | | | - | | | etc. | | | - | | |(ii) Heterokontæ.|Yellow- |_Chlorophyll._| - | | | |green. |Xanthophyll. | - | | | | | | - |III.|_Bryophyta._ |Filamentous moss |Green. |_Chlorophyll._| - | | |protonema. | | | - +----+------------------+-----------------+-----------+--------------+ - -The importance of the lower algæ from a biological standpoint has -long been recognised, since their extremely primitive organisation, -coupled with their ability to synthesise organic compounds from simple -inorganic substances, singles them out as being not very distantly -removed from the group of organisms in which life originated upon the -earth. But the possibility of their having a very much wider economic -significance was completely overlooked until about a quarter of a -century ago, when Hensen demonstrated their importance in marine -plankton as the producers of the organic substance upon which the -whole of the animal life of the ocean is ultimately dependent. In -consequence, it has been generally assumed that the growth of algæ, -since they contain chlorophyll, is entirely dependent on the action of -light. Hence the recent idea of the existence of algæ which actually -inhabit the soil has been received with a certain amount of scepticism, -though the results of modern physiological research on a number of the -lower algæ show that there is very good reason to believe that such a -soil flora is entirely possible. - -In considering the alga-flora of a soil it is necessary to distinguish -between two very different sets of conditions under which the organisms -may be growing. In the first place, they may grow on the surface of the -soil, being subjected directly to insolation, rain, the deposition of -dew, the drying action of wind, relatively quick changes of temperature -and other effects of climate. Certain combinations of these conditions -present so favourable an environment for the growth of algæ that at -times there appears on the surface of the soil a conspicuous green -stratum, sometimes so dark in colour as to appear almost black. Strata -of this nature are well known, and in systematic works there are -constant references to species growing “on damp soil”; for instance, of -the 51 well-defined species of _Nostoc_ recognised by Forte, no less -than 31 are characterised as terrestrial. Such appearances, however, -seem to have been regarded as sporadic and more or less accidental, -rather than as the unusually luxuriant development of an endemic -population, and have been frequently attributed to an excessively moist -condition of the soil due to defective drainage. - -In the second place, the algæ may be living within the soil itself, -away from the action of sunlight and under somewhat more uniform -conditions of moisture and temperature. - -Up to the present time the greater number of the investigations carried -out in this subject have been of a systematic nature, and extremely -little direct evidence has been obtained which can throw any light on -the subject of the economic significance of the soil algæ. - -The earliest systematic work was carried out by Esmarch, in 1910-11, -who investigated by means of cultures the blue-green algæ of a number -of soils from the German African Colonies, the samples being taken from -the surface and also from the lower layers of the soil. He obtained a -considerable number of species and observed that in cultivated soils -they were not confined to the surface but occurred regularly to a depth -of 10-25 cms. and occasionally as low as 40-50 cms. He attributed -their existence in the lower layers to the presence of resting spores -carried down in the processes of cultivation, since his samples from -uncultivated soils were unproductive. - -Later, Esmarch extended his investigations to a far larger -number of samples, 395 in all, of soils of different types from -Schleswig-Holstein. He found that blue-green algæ were very widely -distributed in soils of certain types, though they occurred rarely -in uncultivated soils of low water-content, and he described no less -than 45 species of which 34 belonged to the _Oscillatoriaceæ_ and -_Nostocaceæ_. Certain of the commoner species were obtained from soils -of widely different types, as shown in Table X., while other forms -occurred only rarely and with a much more limited distribution. - -TABLE X.--FREQUENCY OF OCCURRENCE OF CERTAIN COMMON SPECIES IN -ESMARCH’S SOIL SAMPLES. - - +-------------------------+----------------------------------------+ - | | Percentage of Samples | - | | containing given Alga. | - | +--------------------+-------------------+ - | | Uncultivated | Cultivated | - | | Damp Sandy Soil. | Soils. | - | +------+------+------+------+-----+------+ - | |Shores|Shores| | | | | - | | of | of | Sea- | | |Marsh-| - | Species. | Elbe.|Lakes.|shore.|Sandy.|Clay.| land.| - +-------------------------+------+------+------+------+-----+------+ - |Anabæna variabilis | 46 | 43 | 9 | 10·3 | 60 | 46 | - |Anabæna torulosa | 31 | 14·3 | 63·6 | 27·6 | 34·3| 56·4 | - |Cylindrospermum muscicola| 23 | 28·6 | 0 | 24 | 48·6| 59 | - |Cylindrospermum majus | 0 | 14·3 | 0 | 38 | 40 | 33·3 | - |Nostoc Sp. III. | 7·7 | 0 | 0 | 38 | 37 | 48·7 | - +-------------------------+------+------+------+------+-----+------+ - -Taking the number of samples containing blue-green algæ as a rough -measure of their relative abundance, Esmarch obtained the following -interesting figures (Table XI.):-- - -TABLE XI. - - +------------------------------+----------------+---------+ - | | Percentage | | - | | of Samples |Number of| - | | Containing | Samples | - | Kind of Soil. |Blue-green Algæ.|Examined.| - +------------------------------+----------------+---------+ - | | | | - |Cultivated marshland | 95 | 40 | - |Cultivated clay soil | 94·6 | 37 | - |Uncultivated moist sandy soils| 88·6 | 35 | - |Cultivated sandy soil | 64·4 | 45 | - | {Woodland | 12·5 | 40 | - |Uncultivated{Sandy heathland | 9 | 34 | - | {Moorland | 0 | 35 | - +------------------------------+----------------+---------+ - -In noting that the soils fell into two groups, those relatively rich -and those poor in blue-green algæ, Esmarch concluded that the two -chief factors governing the distribution of the _Cyanophyceæ_ on the -surface of soils are, (1) the moisture content of the soil, (2) the -availability of mineral salts, cultivated soils being especially -favoured in both of these respects. He further distinguished between -cultivated land of two kinds, viz. arable land and grass land, and -found that on all types of soil grassland was richer in species than -was arable land. - -Esmarch examined, in addition, 129 samples taken from the lower layers -of the soil immediately beneath certain of his surface samples, 107 at -10-25 cms. and the rest at 30-50 cms. depth. - -In cultivated soils, whether grassland or arable land, he found that -blue-green algæ occurred almost invariably in the lower layers in those -places bearing algæ on the surface and that, with rare exceptions, the -algæ found in the lower layers corresponded exactly to those on the -surface, except that with increasing depth there was a progressive -reduction in the number of species. - -In uncultivated, moist, sandy soils the agreement was far less -complete, for though algæ were rarely absent from the lower layers -their vertical distribution was frequently disturbed by the action -of wind and rain. Other uncultivated soils not subject to periodic -disturbance were found to be uniformly lacking in algæ in the lower -layers, but as the limited number of samples examined came completely -from places where there were no algæ on the surface this means very -little. - -By direct microscopic examination of soil Esmarch claims to have -found living filaments of blue-green algæ at various depths below the -surface. He realised, however, that there was no indication of the -length of time that such filaments had been buried, and therefore -conducted a series of experiments from which he concluded that the -period during which the algæ investigated could continue vegetatively -in the soil after burial varied with different species from 5-12 weeks, -but that during the later part of the period the algæ gradually assumed -a yellowish-green colour. - -It is unfortunate that Esmarch’s investigations were directed only -towards the blue-green algæ since observations made in this country -indicate that such a series of records gives but a very incomplete -picture of the soil flora as a whole. - -Petersen, in his “Danske Aërofile Alger” (1915) added considerably to -our knowledge of soil algæ, especially of diatoms. Unfortunately he -confined his investigations of the green algæ to forms growing visibly -on the surface of the ground. He observed, however, that acid soils -possessed a different flora from that commonly found on alkaline or -neutral soils, the former being dominated by _Mesotænium violascens_, -_Zygnema ericetorum_, and 2 spp. of _Coccomyxa_, while the latter were -characterised by _Mesotænium macrococcum var._, _Hormidium_, 2 spp., -and _Vaucheria_, 3 spp. - -Of diatoms he obtained no less than 24 species and varieties from -arable and garden soils, and five characteristic of marshy soils, while -from forest soils and dry heathland they appeared to be often absent. -He omitted all reference to blue-green algæ. - -Meanwhile Robbins, examining a number of Colorado soils that contained -unprecedented quantities of nitrate, obtained from them 18 species of -blue-green algæ, 2 species of green algæ, and one diatom. Moore and -Karrer have demonstrated the existence of a subterranean alga-flora of -which _Protoderma viride_, the most constantly occurring species, was -shown to multiply when buried to a depth of one metre. - -In this country attention was first called to the subject by Goodey and -Hutchinson of Rothamsted who, in examining certain old stored soils -for protozoa, obtained also a number of blue-green forms which were -submitted to Professor West for identification. This ability of certain -algal spores to retain their vitality for a long resting period was so -very striking that an investigation was begun at Birmingham in 1915 to -ascertain whether other forms were equally resistant. The investigation -was carried out on a large number of freshly collected samples of -arable and garden soils which were first aseptically air-dried for at -least a month and then grown in culture. No less than 20 species or -varieties of diatoms, 24 species of blue-green and 20 species of green -algæ were obtained from these cultures (Table XII.). In the majority -of the samples there was found a central group of algæ, including -_Hantzschia amphioxys_, _Trochiscia aspera_, _Chlorococcum humicola_, -_Bumilleria exilis_ and rather less frequently _Ulothrix subtilis -var. variabilis_, while moss protonema was universally present. These -species were thought to form the basis of an extensive ecological plant -formation in which, by the inclusion of other typically terrestrial -but less widely distributed species smaller plant-associations were -recognised. - -In certain of the soils, associations consisting very largely of -diatoms were present, and it is to be noted that the majority of the -forms that have been described are of exceedingly small size. It is -doubtless this characteristic which enables them to withstand the -conditions of drought to which the organisms of the soil are liable to -be subjected, small organisms having been shown to be better able to -resist desiccation than are larger ones. Since the soil diatoms belong -to the pennate type, they are further adapted to their mode of life by -their power of locomotion, which enables them in times of drought to -retire to the moister layers of the soil. - -In the soils examined in this work blue-green algæ were less -universally present than were diatoms or green algæ, and the species -found appeared to be more local in occurrence. There seemed to be, -however, an association between the three species, _Phormidium tenue_, -_Ph. autumnale_, and _Plectonema Battersii_, at least two of the three -species having been found together in no less than 16 of the samples, -while all three occurred in 7 of them. - -TABLE XII.--ALGÆ IN DESICCATED ENGLISH SOILS. (BRISTOL.) - - +---------------+-----------+------------------------------+ - | | | Number of Species. | - | | Number of +-----------+-----------+------+ - | | Samples | Maximum | Average | | - | Group. |Productive.|per Sample.|per Sample.|Total.| - +---------------+-----------+-----------+-----------+------+ - | | per cent. | | | | - |Diatoms | 95·5 | 9 | 3·7 | 20 | - |Blue-green algæ| 77·3 | 7 | 2·5 | 24 | - |Green algæ | 100 | 7 | 4·3 | 20 | - |Moss protonema | 100 | -- | -- | -- | - +---------------+-----------+-----------+-----------+------+ - | Total | -- | 20 | 10·5 | -- | - +---------------+-----------+-----------+-----------+------+ - -It was generally noticeable that those soils found to be rich in -blue-green algæ contained only a few species of diatoms, and vice -versa. Diatoms appeared most frequently in soils from old gardens, -whereas blue-green algæ were more characteristic of arable soils. The -green algæ and moss protonema, on the other hand, were distributed -universally. - -The majority of green algæ typically found in soils are unicellular, -but a few filamentous forms occur. With the exception of _Vaucheria_ -spp. these are characterised, however, by an ability to break down -in certain circumstances into unicellular or few-celled fragments, in -which condition identification is often very difficult. - -It was also found by cultural examination of a number of old stored -soils from Rothamsted that germination of the resting forms of a -number of algæ could take place after an exceedingly long period of -quiescence. No less than nine species of blue-green algæ, four species -of green algæ, and one species of diatom were obtained from soils that -had been stored for periods of about forty years, the species with the -greatest power to retain their vitality being _Nostoc muscorum_ and -_Nodularia Harveyana_. - - -II. THE SOIL AS A SUITABLE MEDIUM FOR ALGAL GROWTH. - -Were it not for the recent advances that have been made in our -knowledge of the mode of nutrition of many of the lower algæ, it would -be very difficult to account for the widespread occurrence of algæ in -the soil, for it is undoubtedly true of some of the more highly evolved -algæ that their mode of nutrition is entirely typical of that of green -plants in general. The application of bacteriological technique to the -algæ, however, by Beijerinck, by Artari, and by Chodat and his pupils, -and the introduction of pure-culture methods have led to a study of -the physiology of some of the lower algæ, in the hope of getting to -understand some of the fundamental problems underlying the nutrition -of organisms containing chlorophyll. It is impossible here to do more -than mention the names of a few of the more important of those who -have worked along these lines, such as Chodat, Artari, Grintzesco, -Pringsheim, Kufferath, Nakano, Boresch, Magnus and Schindler, and to -condense into a few sentences some of their more important conclusions. - -It is now established that although in the light the algæ are able to -build up their substance from CO₂ and water containing dilute mineral -salts, yet in such conditions growth is sometimes very slow, and with -some species at any rate it is greatly accelerated by the addition of -a small quantity of certain organic compounds. The ability of the lower -algæ to use organic food materials varies specifically, quite closely -related forms often reacting very differently to the same substance, -but there have been shown to be a considerable number of forms which -can make use of organic compounds to such an extent that they can -grow entirely independently of light. In such cases the nutrition -of the organism becomes wholly saprophytic, and the chlorophyll may -be completely lost; it has frequently been observed, however, that -on suitable nutrient media, even in complete darkness, certain algæ -continue to grow and retain their green colour, provided that a -sufficient supply of a suitable nitrogenous compound is present. - -_Chlorella vulgaris_, an alga frequently found in soil, has been shown -to be extremely plastic in its relations to food substances. Given only -a dilute mineral-salts solution as food source, it absorbs CO₂ from -the air, and grows in sunlight with moderate rapidity. The addition -of glucose to the medium in the light greatly increases the rate and -amount of growth and the size of the cells, while in the dark the -colonies not only remain green but have been shown to develop more -vigorously than in full daylight. The organism is also able to use -peptone as a source of nitrogen in place of nitrates. - -_Stichococcus bacillaris_ and _Scenedesmus spp._, also occurring -in soils, have been shown to be almost equally adaptable, though -in these cases the organisms grow more slowly in the dark than on -the corresponding medium in the light. Liquefaction of gelatine by -the secretion of proteolytic enzymes has been shown to be a further -property of certain species, resulting in the formation of amino acids -such as glycocoll, phenylalanine, dipeptides, etc. This property is, -however, possessed by only a limited number of species and in varying -degree. - -Up to the present very little work of this kind has been done upon algæ -actually taken from the soil, and our knowledge is therefore very -scanty. Of the species so far examined all show considerable increase -in growth on the addition to the medium of glucose and other sugars, -and tend to be partially saprophytic; a few have been shown to liquefy -gelatine to some extent. - -Servettaz, Von Ubisch, and Robbins have also demonstrated that the -protonema of some mosses can make use of certain organic substances, -especially the sugars, and grow vigorously in the dark. It has been -shown, however, that light is essential for the development of the moss -plant. - -It was thought at Rothamsted that some light might be thrown upon the -activities of the soil-algæ by making counts of the numbers present -in samples of soil taken periodically within a circumscribed area. -A dilution method similar to that in use in the protozoological -laboratory was adopted and applied to samples of arable soil taken -from the surface, and at depths of 2, 4, 6 and 12 inches vertically -beneath. A considerable number of samples were examined in this way -from two plots on Broadbalk wheat-field, viz.: the unmanured plot and -that receiving a heavy annual dressing of farmyard manure. The numbers -in the unmanured soil were observed to fall far short of those in that -containing a large amount of organic matter, while in both plots the -numbers varied considerably at different times of the year. The chief -species in both plots were identical, and their vertical distribution -was fairly uniform, but it was observed that the numbers of individuals -varied according to the depth of the sample. The 6th and 12th inch -samples contained very few individuals of comparatively few species, -but the 4th inch samples yielded numbers that were not significantly -less than those in the top inch. The 2nd inch sample was usually much -poorer in individuals than either the top or the 4th inch. - -It is unfortunate that this method of counting is not really -satisfactory for the algæ, chiefly because it takes no account of the -blue-green forms. The gelatinous envelope which encloses the filaments -of these algæ prevents their breaking up into measurable units. -Assuming, as appears to be the case for the two plots investigated, -that the blue-green algæ are at least as numerous as the green forms, -the total numbers should probably be at least twice as great as those -calculated. Taking 100,000 as a rough estimate of the number of algæ -per gram of manured soil in a given sample, and assuming the cells to -be spherical and of average diameter 10µ, it has been calculated that -the volume of algal protoplasm present was at least 3 times that of -the bacteria though only one-third of that of the protozoa. This is -probably only a minimum figure for this sample. - -A soil population of this magnitude can not be without effect on the -fertility of the soil. When growing on the surface of the ground -exposed to sunlight the algæ must, by photosynthesis, add considerably -to the organic matter of the soil, but when they live within the soil -itself their nutrition must be wholly saprophytic, and they can be -adding nothing either to the energy or to the food-content of the soil. -How these organisms fit into the general scheme of life in the soil is -at present undetermined, and there is a wide field for research in this -direction. - - -III. RELATION OF ALGÆ TO THE NITROGEN CYCLE - -Probably the most important limiting factor in British agriculture is -the supply of nitrogen available for the growing crop, and it seems -likely that the soil-algæ are intimately connected with this question -in several ways. - -Periodic efforts have been made during the last half century to -establish the fact that a number of the lower organisms, including -the green algæ, have the power of fixing atmospheric nitrogen and -converting it into compounds which are then available for higher -plants. This property has been definitely established for certain -bacteria, and rather doubtfully for some of the fungi, but until -recently no authentic proof had been produced that algæ by themselves -could fix nitrogen. The subject is too wide to be discussed in much -detail here. - -Schramm in America, working with pure cultures of algæ, tried for -ten years to establish the fact of nitrogen fixation, and failed -completely; more recently Wann has extended Schramm’s work, and claims -to have proved indisputably that, given media containing nitrates as -a source of nitrogen and a small amount of glucose, the seven species -of algæ tested by him fixed atmospheric nitrogen to the extent of 4-54 -per cent. of the original nitrogen content of the medium. So important -a result needed corroboration, and Wann’s experiment, with some slight -improvements, was therefore repeated at Rothamsted last summer. - -This work has not yet been published, but in the whole series of -ninety-six cultures, with four different species, each growing on -six different media, there is no evidence that nitrogen fixation has -taken place; but there has been a total recovery at the end of the -experiment of 98·93 per cent. of the original nitrogen supplied. On -the other hand, a flaw has been detected in Wann’s method of analysing -those media containing nitrates, sufficiently great to account for the -differences he obtained between the initial and final nitrogen content -of his cultures. Hence, though one hesitates to say that the algæ are -unable, given suitable conditions, to fix atmospheric nitrogen, one -must admit that no one has yet proved that they can do so. - -It is far more likely, however, that the experiments of Kossowitsch -and others throw more light on the relation of soil algæ to -nitrogen fixation. They affirm that greater fixation of nitrogen is -effected by mixtures of bacteria and certain gelatinous algæ than -by nitrogen-fixing bacteria alone, and that the addition of algæ to -cultures of bacteria produces a stimulating effect only slightly less -than that of sugar. It is probable, therefore, that the algæ, in their -gelatinous sheaths, provide easily available carbohydrates from which -the bacteria derive the energy essential to their work, and that -nitrogen fixation in nature is due to the combined working of a number -of different organisms rather than to the individual action of single -species. - -Russell and Richards have shown that the rate of loss of nitrogen by -leaching from uncropped soils is far less than would be expected from -a purely chemical standpoint, and suggest that certain organisms are -present in the soil which, by absorbing nitrates and ammonium salts -as they are formed, remove them from the soil solution and so help to -conserve the nitrogen of the soil. It is probable that the soil algæ -act in this manner, though to what extent has not yet been determined. - - -IV. RELATION OF ALGÆ TO SOIL MOISTURE AND TO THE FORMATION OF HUMUS -SUBSTANCES. - -In warmer countries than our own, especially those with an adequate -rainfall, the significance of soil algæ is perhaps more obvious to -a casual observer. Treub states that after the complete destruction -of the island of Krakatoa by volcanic eruption in 1883, the first -colonists to take possession of the island were six species of -blue-green algæ, viz., _Tolypothrix_ sp., _Anabæna_ sp., _Symploca_ -sp., _Lyngbya_ 3 spp. Three years after the eruption these organisms -were observed to form an almost continuous gelatinous and hygroscopic -layer over the surface of the cinders and stones constituting the soil, -and by their death and decay they rapidly prepared it for the growth -of seeds brought to the island by visiting birds. Hence the new flora -which soon established itself upon the island can be said to have -had its origin in the alga-flora which preceded it. Fritsch has also -emphasised the importance of algæ in the colonisation of new ground in -Ceylon. - -Welwitsch ascribes the characteristic colour from which the “pedras -negras” in Angola derive their name to the growth of a thick stratum of -_Scytonema myochrous_, a blue-green alga, which gradually becomes black -and completely covers the soil. At the close of the rainy season this -gelatinous stratum dries up very slowly, enabling the underlying soil -to retain its moisture for a longer period than would otherwise be the -case. - -The gelatinous soil algæ are probably very important in this respect, -for their slow rate of loss of water is coupled with a capacity for -rapid absorption, and they are therefore able to take full advantage of -the dew that may be deposited upon them and increase the power of the -soil to retain moisture. - - -V. RELATION OF ALGÆ TO GASEOUS INTERCHANGES IN THE SOIL. - -In the cultivation of rice the algæ of the paddy field have been found -to be of extreme importance. Brizi in Italy has shown that although -rice is grown under swamp conditions yet the roots of the rice plant -are typical of those of ordinary terrestrial plants and have none -of the structural adaptations to aquatic life so characteristic of -ordinary marsh plants. Hence the plants are entirely dependent for -healthy growth upon an adequate supply of oxygen to their roots from -the medium in which they are growing. A serious disease of the rice -plant, characterised by the browning and dying off of the leaves, -which was thought at first to be due to the attacks of fungi, was -found to be the effect of the inadequate aeration of the roots, while -the entry of the fungi was shown to be subsequent to the appearance -of the physiological disease. The presence of algæ in the swamp water -was found to prevent the appearance of this disease, in that they -unite with other organisms to form a more or less continuous stratum -over the surface of the ground, and add to the gases which accumulate -there large quantities of oxygen evolved during photosynthesis. The -concentration of dissolved oxygen in the water percolating through the -soil is thereby raised to a maximum, and the healthy growth of the crop -ensured. - -This work has been corroborated by Harrison and Aiyer in India, and a -sufficient supply of algæ in the swamp water is now regarded as one of -the essentials for the production of a good rice crop. - -From what has been said, it appears that, although our knowledge of -the soil algæ is extremely limited, and our conception of the part -they play is largely based on speculation, yet the subject is one of -enormous interest and worthy of investigation in many directions. In -its present undeveloped state, it is a little difficult to foresee -which lines of study are likely to prove most profitable, but there -is little doubt that eventually the soil algæ will be shown to play a -significant part in the economy of the soil. - - -SELECTED BIBLIOGRAPHY. - -* _Papers giving extensive bibliographies._ - - -I. GENERAL. - - [1] Bristol, B. M., “On the Retention of Vitality by Algæ from Old - Stored Soils,” New Phyt., 1919, xviii., Nos. 3 and 4. - - [2] Bristol, B. M., “On the Alga-Flora of some Desiccated English - Soils: an Important Factor in Soil Biology,” Annals of Botany, 1920, - vol. xxxiv., No. 133. - - [3] Brizi, U., “Ricerche sulla Malattia del Riso detta ‘Brusone,’ - Sect. IV. Influenza che le alghe verdi esercitano in risaia,” - Annuario dell Instituzione Agraria Dott. A. Ponti, Milan, 1905, vol. - vi., pp. 84-89. - - [4] Esmarch, F., “Beitrag zur Cyanophyceen-Flora unserer Kolonien,” - Jahrb. der Hamburgischen wissensch. Anstalten, 1910, xxviii., 3. - Beiheft, S. 62-82. - - [5] Esmarch, F., “Untersuchungen über die Verbreitung der - Cyanophyceen auf und in verschiedenen Boden,” Hedwigia, 1914, Band - lv., Heft 4-5. - - [6] Fritsch, F. E., “The Rôle of Algal Growth in the Colonisation of - New Ground and in the Determination of Scenery,” Geog. Journal, 1907. - - [7] Harrison, W. H., and Aiyer, P. A. Subramania, “The Gases of - Swamp Rice Soils,” Mem. Dept. Agr. in India, Chem. Ser. (I.) “Their - Composition and Relationship to the Crop,” 1913, vol. iii., No. - 3; (II.) “Their Utilisation for the Aeration of the Roots of the - Crop,” 1914, vol. iv., No. 1; (IV.) “The Source of the Gaseous Soil - Nitrogen,” 1916, vol. v., No. 1. - - [8_a_] Hensen, V., “Ueber die Bestimmung des Planktons oder des im - Meere treibenden Materials am Pflanzen und Thieren.” Fünfter Ber. - Komm. wiss. Unters. deutschen Meere, 1887. - - [8] Moore, G. T., and Karrer, J. L., “A Subterranean Alga Flora,” - Ann. Miss. Bot. Gard., 1919, vi., pp. 281-307. - - [9] Nadson, G., “Die perforierenden (kalkbohrende) Algen und ihre - Bedeutung in der Natur,” Scripta bot. hort. Univ. Imp. Petrop., 1901, - Bd. 17. - - [10] Petersen, J. B., “Danske Aërofile Alger,” D. Kgl. Danske - Vidensk. Selsk. Skrifter, 7 Raekke, Naturv. og mathem., 1915, Bd. - xii., 7, Copenhagen. - - [11] Robbins, W. W., “Algæ in some Colorado Soils,” Agric. Exp. Sta., - Colorado, 1912, Bulletin 184. - - [12] Treub, “Notice sur la nouvelle Flora de Krakatau,” Ann. Jard. - Bot. Buitenzorg, 1888, vol. vii., pp. 221-223. - - -II. RELATION OF ALGÆ TO LIGHT AND CARBON. - - [13] Artari, A., “Zur Ernährungsphysiologie der grünen Algen,” Ber. - der D. bot. Ges., 1901, Bd. xix., S. 7. - - [14] Artari, A., “Zur Physiologie der Chlamydomonaden (Chlam. - Ehrenbergii);” (I.) Jahrb. f. Wiss. Bot., 1913, Bd. lii., S. 410; - (II.) _Ibid._, 1914, Bd. liii., S. 527. - - [15] Adjarof, M., “Recherches expérimentales sur la Physiologie de - quelques Algues vertes,” Université de Genève--Institut Botanique, - Prof. R. Chodat--1905, 6 serie, vii. fascicule, Genève. - - [16] Beijerinck, M. W., “Berichte über meine Kulturen niederer Algen - auf Nährgelatine,” Centr. f. Bakt. u. Paras., 1893, Abt. I., Bd. - xiii., S. 368, Jena. - - [17] Boresch, K., “Die Färbung von Cyanophyceen und Chlorophyceen in - ihrer Abhängigkeit vom Stickstoffgehalt des Substrates,” Jahrbücher - für Wiss. Botanik., 1913, lii., pp. 145-85. - - [18] Chodat, R., “Étude critique et expérimentale sur le - polymorphisme des Algues,” Genève, 1909. - - [19] Chodat, R., “La crésol-tyrosinase, réactif des peptides et - des polypeptides, des protéides et de la protéolyse,” Archiv. des - Sciences physiques et naturelles, 1912. - - [20] Chodat, R., “Monographie d’Algues en Culture pure: Matériaux - pour la Flore Cryptogamique Suisse,” 1913, vol. iv., fasc. 2, Berne. - - [21] Dangeard, P. A., “Observations sur une Algue cultivée à - l’obscurité depuis huit ans,” Compt. Rend. Acad. Sci. (Paris), 1921, - vol. clxxii., No. 5, pp. 254-60. - - [22] Étard et Bouilhac, “Sur la présence de la chlorophyll dans un - Nostoc cultivé à l’abri de la lumière,” Compt. Rend., t. cxxvii, 1898. - - [23] Grintzesco, J., “Recherches expérimentales sur la morphologie et - la physiologie expérimentale de _Scenedesmus acutus_,” Meyen. Bull. - herb. Boiss., 1902, Bd. ii., pp. 219-64 and 406-29. - - [24] Grintzesco, J., “Contribution à l’étude des Protococcoidées: - _Chlorella vulgaris_ Beyerinck,” Revue générale de Botanique, 1903, - xv., pp. 5-19, 67-82. - - [25] * Kufferath, H., “Contribution à la physiologie d’une - protococcacée nouvelle, _Chlorella luteo-viridis_ Chod. n. sp. var., - _lutescens_ Chod. n. var.,” Recueil de l’institut bot. Léo Errera, - 1913, t. ix, p. 113. - - [26] Kufferath, H., “Recherches physiologiques sur les algues vertes - cultivées en culture pure,” Bull. Soc. Roy. Bot. Belgique, 1921, - liv., pp. 49-77. - - [27] Magnus, W., and Schindler, B., “Ueber den Einflusz der Nährsalze - auf die Färbung der Oscillarien,” Ber. der D. Bot. Gesellschaft, - 1912-13, xxx., p. 314. - - [28] * Nakano, H., “Untersuchungen über die Entwicklungs- und - Ernährungsphysiologie einiger Chlorophyceen,” Journ. College of Sci. - Imp. Univ. Tokyo, 1917, vol. xl., Art. 2. - - [29] Pringsheim, E., “Kulturversuche mit chlorophyll-führenden - Mikroorganismen,” Cohns Beiträge Z. Biol. d. Pflanzen. (I.) - Die Kultur von Algen in Agar, 1912, Bd. xi., S. 249; (II.) Zur - Physiologie der _Euglena gracilis_, 1913, Bd. xii., S. 1.; (III.) Zur - Physiologie der Schizophyceen, 1913, Bd. xii., S. 99. - - [30] Radais, “Sur la culture pure d’une algue verte; formation de - chlorophylle à l’obscurité,” Comptes Rendus, 1900, cxxx., p. 793. - - [31] Richter, O., “Zur Physiologie der Diatomeen.” (I.) Sitzber. d. - kais. Akad. d. W. in Wien, math, naturw. Kl., 1906, Bd. cxv., Abt. - I., S. 27; (II.) Denkschrift d. math. naturw. Kl. d. kais. Akad. - d. W. in Wien, 1909, Bd. lxxxiv., S. 666; (III.) Sitzber. d. Kais. - Akad., etc., 1909, Bd. cxviii., Abt. I., S. 1337. - - [32] Richter, O., “Ernährung der Algen,” 1911. - - [33] Robbins, W. J., “Direct Assimilation of Organic Carbon by - _Ceratodon purpureus_,” Bot. Gaz., 1918, lxv., pp. 543-51. - - [34] Schindler, B., “Ueber den Farbenwechsel der Oscillarien,” - Zeitsch. f. Bot., 1913, v., pp. 497-575. - - [35] Ternetz, Charlotte, “Beiträge zur Morphologie und Physiologie - der _Euglena gracilis_,” Jahrb. f. Wiss. Bot., 1912, Bd. 51, S. 435. - - -III. RELATION OF ALGÆ TO NITROGEN. - - [36] Berthelot, “Recherches nouvelles sur les microorganismes - fixateurs de l’azote,” Comptes Rend., 1893, cxvi., pp. 842-49. - - [37] Bouilhac, R., “Sur la fixation de l’azote atmosphérique par - l’association des algues et des bactéries,” Comptes Rend., 1896, - cxxiii., pp. 828-30. - - [38] Bouilhac and Giustiniani, “Sur une culture de sarrasin en - présence d’un mélange d’algues et de bactéries,” Comptes Rendus, - 1903, cxxxvii., pp. 1274-76. - - [39] Charpentier, P. G., “Alimentation azotée d’une algue: Le - Cystococcus humicola,” Ann. Inst. Pasteur, 1903, 17, pp. 321-34. - - [40] Fischer, Hugo, “Über Symbiose von Azotobacter mit Oscillarien,” - Centr. f. Bakt., 1904, xii. - - [41] Frank, B., “Uber den experimentellen Nachweis der Assimilation - freien Stickstoffs durch Erdbewohnende Algen,” Ber. der D. Bot. - Gesellsch., 1889, vol. vii., pp. 34-42. - - [42] Frank, B., “Ueber den gegenwärtigen Stand unserer Kenntnisse der - Assimilation elementaren Stickstoffs durch die Pflanze,” Ber. der. D. - Bot. Ges., 1889, vii., 234-47. - - [43] Frank, B., and Otto, R., “Untersuchungen über Stickstoff - Assimilation in der Pflanze,” Ber. der D. Bot. Ges., 1890, viii., - 331-342. - - [44] Gautier and Drouin, “Recherches sur la fixation de l’azote par - le sol et les végétaux,” Compt. Rend., 1888, cvi., pp. 1174-76; - General Conclusions, p. 1232. - - [45] Kossowitsch, P., “Untersuchungen über die Frage, ob die Algen - freien Stickstoff fixiren,” Bot. Zeit., 1894, Heft 5, S. 98-116. - - [46] Krüger, W., und Schneidewind, “Sind niedere chlorophyllgrüne - Algen imstande, den freien Stickstoff der Atmosphäre zu assimilieren - und Boden an Stickstoff zu bereichern?” Landwirtschaftliche Jahrb., - 1900, Bd. 29, S. 771-804. - - [47] Moore, Benjamin, and T. Arthur Webster, “Studies of the - photosynthesis in f.w.a.” (I.) “The fixation of both C and N from - atmosphere to form organic tissue by green plant cell”; (II.) - “Nutrition and growth produced by high gaseous dilutions of simple - organic compounds, such as formaldehyde and methylic alcohol”; (III.) - “Nutrition and growth by means of high dilution of CO₂ and oxides of - N without access to atmosphere,” Proc. Roy. Soc., London, 1920, B. - xci., pp. 201-15. - - [47_a_] Moore, B., Whiteley, Webster, T. A., Proc. Roy. Soc., London, - B., 1921; xcii., pp. 51-60. - - [48] Reinke, J., “Symbiose von Volvox und Azotobacter,” Ber. der d. - Bot. Ges., 1903, Bd. xxi., S. 481. - - [49] Russell, E. J., and Richards, E. H., “The washing out of - Nitrates by Drainage Water from Uncropped and Unmanured Land,” Journ. - Agric. Sci., 1920, vol. x., Part I. - - [50] Schloesing, fils, and Laurent, E., “Recherches sur la fixation - de l’azote libre par les plantes,” Ann. de l’Institut Pasteur, 1892, - vi., pp. 65-115. - - [51] Schramm, J. R., “The Relation of Certain Grass Green Algæ to - Elementary Nitrogen,” Ann. Mo. Bot. Gard., 1914, i., No. 2. - - [52] Wann, F. B., “The Fixation of Nitrogen by Green Plants,” Amer. - Journ. Bot., 1921, viii., pp. 1-29. - - - - -CHAPTER VII. - -THE OCCURRENCE OF FUNGI IN THE SOIL. - - - NOTE.--I am indebted to my late colleague Miss Sibyl S. Jewson, - M.Sc., for permission to include unpublished data from our - investigations on the soil fungi. - -In 1886 Adametz,[1] investigating the biochemical changes occurring -in soils, isolated several species of fungi. It was, however, only -with the work of Oudemans and Koning,[17] in 1902 when forty-five -species were isolated and described, the majority as new to science, -that the real study of the fungus flora of the soil commenced. There -is now no doubt that fungi form a large and very important section -of the permanent soil population, and certain forms are found only -in the soil. Indeed, Takahashi[22] has reversed the earlier ideas by -suggesting that fungus spores in the air are derived from soil forms. -The majority of investigations on this subject fall, perhaps, into -one or more of three classes: (_a_) purely systematic studies such as -those of Oudemans and Koning,[17] Dale,[5] Jensen,[9] Waksman,[25a] -Hagem,[8c] Lendner,[12] and others, which consist in the isolation -and identification of species from various soils: (_b_) physiological -researches, such as those of Hagem[8c] on the Mucorineæ of Norway, or -the many investigations on the biochemical changes in soils produced by -fungi, such as those of Muntz and Coudon,[15] McLean and Wilson,[15] -Kopeloff,[11] Goddard,[7] McBeth and Scales,[14] and others: (_c_) -quantitative studies, such as those of Remy,[20] Fischer,[6] -Ramann,[18] Waksman,[25c] and Takahashi,[22] which involve numerical -estimates of the fungus flora in soils. - - -QUALITATIVE STUDY. - -With very rare exceptions soil fungi cannot be examined in situ, and -the necessary basis of any qualitative research is the isolation of -the organisms in pure culture. Most soil forms belong to the _Fungi -imperfecti_, and often show considerable plasticity on artificial -media. This makes it very difficult to determine them by comparison -with type herbarium specimens or published morphological diagnoses. -In consequence many soil fungi have not infrequently been given new -specific names, as _humicola_, _terricola_, and so forth, which is -very unsatisfactory, and means that the determinations have little -significance. - -Furthermore, most artificial media are slight variations on a few -common and simple themes, and are very selective, permitting the growth -of a moiety only of the fungi present. In addition, many fungi grow -so slowly that they are overwhelmed by the more rapidly germinating -or spreading forms, or on the other hand, they may be eliminated by -the metabolic products of different adjacent colonies. The extremely -selective nature of the technique commonly used is shown if one -tabulates systematically all the fungi which have been recorded or -described in soil investigations. Of _Phycomycetes_ there are fifty-six -species of eleven genera; of _Ascomycetes_ twelve species of eight -genera; and of _Fungi imperfecti_, including _Actinomycetes_ but not -sterile _Mycelia_, 197 species of sixty-two genera. Rusts and Smuts -one might not expect, but that of the multitudes of _Basidiomycetes_ -growing in wood and meadow not one should have been recorded is indeed -startling. It was at first thought that many imperfect fungi might -be conidial stages of _Basidiomycetes_, but much search among forms -isolated at Rothamsted has, up to the present, failed to reveal clamp -connections in the hyphæ. - -Since various species of soil fungi have different optimum temperature, -humidity and other conditions[3] one would not expect to find an even -geographic distribution. Very little is yet known of this aspect, but -_Rhizopus nigricans_, _Mucor racemosus_, _Zygorrhynchus vuilleminii_, -_Aspergillus niger_, _Trichoderma koningi_, _Cladosporium herbarum_, -and many species of _Aspergillus_, _Penicillium_, _Fusarium_, -_Alternaria_, and _Cephalosporium_ have been commonly found throughout -North America and Europe wherever soils have been examined. Species of -_Aspergillus_, however, would appear to be more common in the soils -of south temperate regions and species of _Penicillium_, _Mucor_, -_Trichoderma_, and _Fusarium_ more abundant in northern soils. - -It is well known that in many plant and animal communities there -occurs a definite rhythm, various species following each other in a -regular sequence as dominants in the population. Although it is not yet -possible to make any definite statement there would seem indications -that this may also be true of the soil fungi. - -Much work has been done on the distribution of species at different -depths in the soil, but the results are still confusing. Thus, -examining eighteen species, Goddard[7] found no difference in relative -distribution down to 5½ inches. Werkenthin[26] found identical species -from 1-4 inches, and then an absence of fungi from 5-7 inches, which -latter was the greatest depth he examined. Waksman[25] found little -difference in the first six inches, but very few species below 8 inches -except _Zygorrhynchus vuilleminii_, which extended down to 30 inches -and was often the only species occurring below 12 inches. Taylor[23] -has reported species of _Fusarium_ at practically every depth to 24 -inches. Rathbun[19] found _Aspergillus niger_, _Rhizopus nigricans_, -and species of Fusarium and Mucor down to 34 inches, and _Oospora -lactis_, _Trichoderma koningi_, _Zygorrhynchus vuilleminii_ and species -of _Penicillium_, _Spicaria_ and _Saccharomyces_ as deep as 44 inches. -Eleven species were isolated from the alimentary canal of grubs and -worms, and Rathbun concluded that soil fungi may be spread by these -organisms. - -On an unmanured grass plot at Rothamsted twenty species were isolated -from a depth of 1 inch, nineteen from 6 inches, and eleven from -12 inches, whereas on the unmanured plot of Broadbalk wheat field -twenty-six species were obtained from 1 inch, seven from 6 inches, and -five from 12 inches. There appeared to be no conspicuous differences -between the floras of the two plots save that in the Broadbalk plot -there were fewer Mucorales, and _Zygorrhynchus mœlleri_ and _Absidia -cylindrospora_ were absent. In the grass plot samples about one-half -the forms occurring at the lower levels were isolated also from the -upper levels, but in the Broadbalk sample the five forms isolated from -12 inches, and five out of seven of those at 6 inches occurred only at -those levels, i.e. each of the three levels appeared to have a specific -flora. The difference in depth distribution in these two cases may -be due to the fact that in the Broadbalk plot the stiff clay subsoil -occurs at 5-7 inches, whereas in the grass plot the depth of soil is -greater than 12 inches. Much further work needs to be done on this -aspect before any definite conclusion can be reached. - -Much scattered information is available concerning the effect of -soil type, manuring, treatment, cropping, and so forth upon the -fungus content, but no clear issue as yet emerges from the results. -Hagem[8] found that cultivated soils vary greatly from forest soils -in the species of _Mucor_ present, and that certain species seem -to be associated in similar environments. Thus in pinewoods _Mucor -ramannianus_ is usually found, together with _M. strictus_, _M. -flavus_, and _M. sylvaticus_, and with this “_M. Ramannianus Society_,” -_M. racemosus_, _M. hiemalis_, and _Absidia orchidis_, are frequently -associated. The differences found by Hagem between the species of -_Mucor_ from forest and cultivated land could not, however, be -confirmed by Werkenthin.[26] - -Dale,[5] examining sandy, chalky, peaty and black earth soils, found -specific differences, although many of the species were common to all. -A soil which had been manured continuously for thirty-eight years -with ammonium sulphate alone, contained twenty-two species, whereas -the same soil with the addition of lime only had thirteen species. -Both Goddard[7] and Werkenthin,[26] in their investigations, found -a constant and characteristic fungus flora regardless of soil type, -tillage, or manuring. Waksman’s[25] studies of forest soils showed -few species of _Mucor_ but many of _Penicillium_ and _Trichoderma_[2]; -orchard soil contained no species of _Trichoderma_, very few of -_Penicillium_, but a large number of species of _Mucor_; species of -_Trichoderma_ were common in acid soils, whilst cultivated garden -soil contained all forms. The examination of very differently manured -plots on the Broadbalk wheat field at Rothamsted has not shown any -striking differences in the fungus flora, all the more important groups -of species being represented in every plot, but significant minor -differences are present. Thus, plot 13, manured with double ammonium -salts, superphosphate and sulphate of potash, is especially rich in -“species” of Trichoderma, whereas the unmanured plot contains large -numbers of species of green _Penicillium_, _Trichoderma_, and a species -of _Botrytis_ (pyramidalis?). - -The effect of the crop upon the fungus flora is seen in cases where -the same crop is grown year after year as in certain flax areas, where -species of _Fusarium_ accumulate in the soil and tend to produce “flax -sickness.”[13] - - -QUANTITATIVE STUDY. - -As it is not possible to count the soil fungi _in situ_, any estimation -of the numbers present in a soil must be arrived at by indirect means. -The method adopted is to make as fine a suspension as possible of -a known quantity of soil sample in a known amount of water, dilute -this to 1/5000, 1/10000, and so forth by regular gradations, incubate -cubic centimetres of the final dilution on artificial media in petri -dishes, and count the colonies of fungi developing in each plate. -Using the average figures from a series of duplicate plates, the -number of “individual” fungi in a gram of the original soil sample may -then be calculated. The very few students who have made quantitative -estimations have obtained very unsatisfactory results. In bacterial or -protozoal estimations, the shaking of the soil suspension separates the -unicellular individuals, so that in the final platings each individual -from the soil theoretically gives rise to one colony on the medium. -In the case of fungi, the organisms may be in the form of unicellular -or multicellular spores or larger or smaller masses of unicellular or -multicellular mycelium differing for each particular species or phase -of development within the single species. The organisms may be sterile -in the soil or form fruiting bodies, consisting of few or myriads -of locally or widely distributed spores. In the process of shaking -the soil-suspension fungi of different organisation or of differing -developmental stages may be broken up and moieties fragmented in -totally different ways or to very different degrees. With protozoa and -bacteria the relation of soil individual to plate colony is direct; -with fungi we do not know what is the soil “individual” nor whether it -is the same for different fungi; nor can we yet profitably discuss any -significant numerical relationship of plate colonies to soil organisms. -Thus Conn[4] has pointed out that the plate count of a fungus indicates -only the ability to produce reproductive bodies and found that the -spores of one colony of _Aspergillus_, if distributed evenly through -a kilogram of soil, could produce the average plate counts obtained -by Waksman. Abundant vegetative growth may, in some species, reduce -or inhibit spore formation, so that of two species the one giving a -lower count might really be much the more important and plentiful -in the soil. Further, the colonies developing in the final plates -represent only a selected few of the fungi present in the soil sample, -the _Basidiomycetes_, and no doubt many other forms, being absent. -In addition, different media differ among themselves in the average -number of colonies developing on the plates, each medium giving, -as it were, its own point of view. Thus, in one experiment carried -out at Rothamsted by Miss Jewson, using the same soil suspension, -twenty plates of Coon’s Agar gave 357 colonies, of Cook’s Agar 246, -of Czapek’s Agar 215, and of Prune Agar 366. Thus if one only used -Coon’s Agar and Prune Agar one would obtain a total of 723 colonies, -whereas the same suspension on Cook’s Agar and Czapek’s Agar would -give only 461, and the calculated numbers of fungi per gram of soil -would be totally different. Further, if a single medium be taken, it -is found that slight alterations in the degree of acidity may make -very considerable differences in the final numbers. Thus Coon’s Agar -acidified to a hydrogen ion concentration of 5·0 gave as the results of -four series the following average numbers of colonies per plate, 17, -23·75, 18, 23. When, however, the medium was acidified to a PH of 4·0 -to 4·3, corresponding averages from three series were 38, 46·3, and -44·8; i.e. the final estimations of numbers of fungi in the soil was -about twice as great. Again, the degree of dilution of soil suspension -used in plating may also be a very serious factor. Thus, if a series of -dilutions be made of 1/80,000, 1/40,000, 1/20,000, 1/10,000, 1/5,000 -and 1/2,500, the average plate numbers should be in the proportions -of 1, 2, 4, 8, 16, and 32 respectively. In an actual experiment, the -following average plate numbers were obtained, 15·4, 32·8, 59·1, 104·0, -150, 224·5, which show a very decided reduction in the higher numbers. -If, however, dilutions of a suspension of spores of a single species be -made, this reduction does not occur. - -These are but three of the very numerous factors involved in the -technique of quantitative estimation, and every single factor may be -the source of errors of similar magnitude, minute fluctuations in the -operations leading to the final platings having very considerable -effect upon the numbers of colonies that develop. - -By critically evaluating each particular factor in the method, and -making statistical correction, it has, however, been found possible to -obtain series of duplicate plates comparing very favourably and thus to -extract certain figures which, whilst not possessing any final value, -have yet a certain general and comparative worth. Thus, 20·0, 18·2, -and 16·8 were obtained as the averages of six plates each, of a soil -suspension divided into three parts, and the individual plate numbers -in all three series were within the range of normal distribution. The -meaning of these numerical estimates in relation to fungi per gram -of soil sample is, however, entirely hypothetical, and to have value -quantitative comparison should only be made between single species -or groups of species closely related physiologically, and where the -technique is standardised. - -[Illustration: FIG. 19.--Monthly Counts of Numbers of Fungi per gramme -of Dry Soil. Broadbalk Plot 2 (Farmyard Manure), Rothamsted. - - X-axis: ~Apr.~ 1921 May Jun. ~Jul.~ Aug. Sep. ~Oct.~ Nov. ~Dec.~ Jan. - 1922 ~Feb.~ Mar. Apr. May ~Jun.~ Jul. Aug. ~Sep.~ ~Oct.~ - - Y-axis: 10.000 per Gramme of Soil] - -No comparative estimations have been made of the number of fungi in -the soils of different regions. There are, however, certain figures -which show that decided seasonal differences exist. Thus, correcting -and averaging certain of Waksman’s results[25] the following numbers -of fungi per gram of soil at 4 inches deep are obtained; September, -768,000; October, 522,000; November, 310,000; January, 182,000. At -Rothamsted results have been obtained which would appear to mark -a clear seasonal rhythm, corresponding in the time of its maxima -in Autumn and Spring with the periodicities known for many other -ecological communities (Fig. 19). - -The numbers of fungi at various depths in the soil show very clearly -marked differences. The distribution in the top 4-6 inches depending -probably upon the depth of soil, is more or less equal, but there is a -very rapid falling off in numbers, especially between 5-9 inches, until -at 20-30 inches fungi are either very few in number or absent. Thus -Takahashi[22] found 590,000 fungi per gram at a depth of 2 cms. and -only 160,000 at 8 cms. - -TABLE XIII.--INFLUENCE OF SOIL TREATMENT UPON THE NUMBERS OF FUNGI AS -DETERMINED BY THE PLATE METHOD--(AFTER WAKSMAN). - - +--------------------------+---------+-----------------+ - | | |Numbers of Fungi | - | Soil Fertilisation. |Reaction.|per Gram of Soil.| - +--------------------------+---------+-----------------+ - | | P.H. | | - |Minerals only | 5·6 | 37,300 | - |Heavily manured | 5·8 | 73,000 | - |Sodium nitrate | 5·8 | 46,000 | - |Ammonium sulphate | 4·0 | 110,000 | - |Minerals and lime | 6·6 | 26,200 | - |Ammonium sulphate and lime| 6·2 | 39,100 | - +--------------------------+---------+-----------------+ - -The type of soil and its treatment exercise a great influence over -the number of fungi present. Fischer[6] found that farmyard manure -increased the number of fungi in uncultivated “Hochmoor,” cultivated -“Grunlandmoor,” and a clay soil by two, three, and five times -respectively. Waksman’s results[25] indicate that the more fertile -soils contain more fungi, both in number and species, than the less -fertile ones, and if one averages his results, the following figures -are obtained: garden soil, 525,000 per gram; orchard soil, 250,000; -meadow soil, 750,000; and forest soil, 151,000. Recently Waksman[25_e_] -has found that manure and acid fertilisers increase the numbers of -fungi in the soil, whereas the addition of lime decreases them (Table -XIII.). - -Jones and Murdock[10] examined surface and sub-surface samples of -forty-six soils representing seventeen soil types in eastern Ontario. -Molds were fairly uniform in numbers in all soils except a sandy clay -loam and sandy clay shale, in which they were absent. - -It has also frequently been pointed out that acid and water-logged -soils are richer in fungus content than normal agricultural soils. -On the other hand, Brown and Halversen[2] found, examining six plots -receiving different treatment and studied through a complete year, that -the numbers of fungi were unaffected by moisture, temperature, or soil -treatment. Against this, however, must be set the work of Coleman[3] -who studied the activities of fungi in sterile soils and found such -factors as temperature, aeration and food supply to exercise a deciding -control. - -Investigations at Rothamsted show that Broadbalk plot 13, receiving -double ammonium salts, superphosphate and sulphate of potash and -yielding 31 bushels per acre, and plot 2, receiving farmyard manure and -yielding 35·2 bushels, contain approximately equal numbers of fungi. -This figure is about half as high again as that for plot 3, which -is unmanured and yields 12·6 bushels, plot 10, with double ammonium -salts alone and yielding 20 bushels, and plot 11, with double ammonium -salts and superphosphate and yielding 22·9 bushels per acre. A primary -factor, however, in all considerations such as these is the equality -of distribution of fungi laterally in any particular soil. There are -probably few soils so homogeneous as the Broadbalk plots at Rothamsted, -and on plot 2 (farmyard manure since 1852) samples taken from the lower -and upper ends and the middle region gave average numbers of colonies -per plate of 24, 23, and 25 respectively. On the other hand, soil -samples taken only a few yards apart in the middle region of the plot -gave average plate counts of 33·7 and 56·8. - - -CONCLUSION. - -Surveying generally the field covered in this chapter, one can only -be impressed with the fragmentary character of our knowledge and with -the fact that, owing to the selective nature of the technique, the -data we possess, if assumed to be representative, give an entirely -partial and erroneous picture of the soil fungi. From the qualitative -aspect, the chief impediment is the impossibility of obtaining reliable -specific determinations of very many of the soil fungi. Lists of -doubtfully-named forms from particular soils or geographic regions -are useless or a positive evil, and there is imperative need for the -systematising of selected genera by physiological criteria, such as has -been partially done for _Penicillium_, _Fusarium_, and _Aspergillus_. -Furthermore, until a standardised and non-selective technique has been -devised, or a number of standardised selective methods for particular -groups, comparative investigations into specific distribution can -give little of value. This latter criticism is also very applicable -if regard be paid to the quantitative aspect of soil work, for -progress here largely depends upon the elaboration of a standardised -fractionation technique. Every single factor in these methods needs -exact analysis, for each gives opportunity for great error, and each -error is magnified many thousand times in the final results. Much -has been done in this direction at Rothamsted, but more remains to -do. Finally, working with single species in sterilised soil under -standardised conditions, there is fundamental work to be done on the -relation of plate colony to soil “individual.” - - [1] Adametz, I., “Untersuchungen über die niederen Pilze der - Ackerkrume,” Inaug. Diss., Leipzig, 1886. - - [2] Brown, P. E., and Halversen, W. V., “Effect of Seasonal - Conditions and Soil Treatment on Bacteria and Molds in Soil,” Iowa - Agric. Expt. Sta. 1921, Res. Bull., 56. - - [3] Coleman, D. A., “Environmental Factors Influencing the Activity - of Soil Fungi,” Soil Sci., 1916, v., 2. - - [4] Conn, H. J., “The Microscopic Study of Bacteria and Fungi in - Soil,” N.Y. Agric. Expt. Sta., 1918, Bull. 64. - - [5] Dale, E., (_a_) “On the Fungi of the Soil,” Ann. Mycol., 1912, - 10; (_b_) “On the Fungi of the Soil,” Ann. Mycol., 1914, 12. - - [6] Fischer, H., “Bakteriologisch-chemische Untersuchungen; - Bakteriologischen Teil,” Landw. Jahrb., 1909, 38. - - [7] Goddard, H. M., “Can Fungi living in Agricultural Soil Assimilate - Free Nitrogen?” Bot. Gaz., 1913, 56. - - [8] Hagem, O., (_a_) “Untersuchungen über Norwegische Mucorineen - I., Vidensk. Selsk, I.,” Math. Naturw. Klasse, 1907, 7; (_b_) - “Untersuchungen über Norwegische Mucorineen II., Vidensk. Selsk. I.,” - Math. Naturw. Klasse, 1910, 10. - - [9] Jensen, C. N., “Fungus Flora of the Soil,” N.Y. (Cornell) Agric. - Expt. Sta., 1912, Bull. 315. - - [10] Jones, D. H., and Murdock, F. G., “Quantitative and Qualitative - Bacterial Analysis of Soil Samples taken in Fall of 1918,” Soil Sci., - 1919, 8. - - [11] Kopeloff, N., “The Effect of Soil Reaction on Ammonification by - Certain Soil Fungi,” Soil Sci., 1916, 1. - - [12] Lendner, A., “Les Mucorinées de la Suisse,” 1908. - - [13] Manns, S. F., “Fungi of Flax-sick Soil and Flax Seed,” Thesis, - N. Dak. Agric. Expt. Sta., 1903. - - [14] McBeth, I. G., and Scales, F. M., “The Destruction of Cellulose - by Bacteria and Filamentous Fungi,” U.S. Dept. Agric. Bur. Plant - Indust., 1913, Bull. 266. - - [15] McLean, H. C., and Wilson, G. W., “Ammonification Studies with - Soil Fungi,” N.J. Agric. Expt. Sta., 1914, Bull. 270. - - [16] Muntz, A., and Coudon, H., “La fermentation ammoniaque de la - terre,” Compt. Rend. Acad. Sci. (Paris), 1893, 116. - - [17] Oudemans, A. C., and Koning, C.J., “Prodrome d’une flore - mycologique, obtenue par la culture sur gelatin préparée de la terre - humeuse du Spanderswoud, près de Bussum,” Arch. Néerland. Sci. Exact - et Nat., 1902, s. ii., 7. - - [18] Ramann, E., “Bodenkunde,” Berlin, 1905. - - [19] Rathbun, A. E., “The Fungus Flora of Pine Seed Beds,” - Phytopath., 1918, 8. - - [20] Remy, T., “Bodenbakteriologischen Studien,” Centr. f. Bakt., - 1902, ii., 8. - - [21] Sherbakoff, C. D., “Fusaria of Potatoes,” N.Y. (Cornell) Agric. - Expt. Sta., 1915, Mem. 6. - - [22] Takahashi, T., “On the Fungus Flora of the Soil,” Anns. - Phytopath. Soc., Japan, 1919, 1. - - [23] Taylor, M. W., “The Vertical Distribution of _Fusarium_,” - Phytopath., 1917, 7. - - [24] Thom, Ch., “Cultural Studies of Species of Penicillium,” U.S. - Dept. Agric. Bur. Animal Indus., 1910, Bull. 118. - - [25] Waksman, S. A., (_a_) “Soil Fungi and their Activities,” Soil - Sci., 1916, 2; (_b_) “Do Fungi Actually Live in the Soil and Produce - Mycelium?” Science, 1916, 44; (_c_) “Is there any Fungus Flora of - the Soil?” Soil Sci., 1917, 3; (_d_) “The Importance of Mold Action - in the Soil,” Soil Sci., 1918, 6; (_e_) “The Growth of Fungi in the - Soil,” Soil Sci., 1922, xiv. - - [26] Werkenthin, F. C., “Fungus Flora of Texas Soils,” Phytopath., - 1916, 6. - - - - -CHAPTER VIII. - -THE LIFE OF FUNGI IN THE SOIL. - - -In the last chapter fungi were considered as so many specific but -functionless units in the soil. Unless, however, they are regarded -merely as inert spore contaminations from the air, a view which is -now no longer tenable, their very presence implies the existence -of innumerable vital relationships between the organisms and their -environment. From this point of view the studies treated in the -previous chapter are but the necessary first steps to an understanding -of the relation of soil fungi to living plants and of the part played -by them in the soil economy. - - -RELATION OF SOIL FUNGI TO LIVING PLANTS. - -Older classifications of fungi frequently divided these organisms -into four categories--parasites, saprophytes, facultative parasites, -and facultative saprophytes, but the further mycological studies are -carried the more clearly it is seen that these groups are entirely -artificial. There are probably few fungi that cannot, under particular -conditions, invade living tissues, and it only seems a question of -time before at all events the vast majority of fungi will be grown on -synthetic media in the laboratory. From our present point of view the -importance of this lies in the fact that fungi living saprophytically -in the soil may, given the right conditions or the presence of some -particular host plant, become parasites or symbionts, and conversely -well-known pathogens may live a saprophytic existence. Thus Cucumber -Leaf Spot is caused by _Colletotrichum oligochætum_, and Bewley[3] has -repeatedly isolated this fungus from glasshouse manure and refuse of -various kinds. In his early studies, Butler[13] isolated many parasitic -species of _Pythium_ from Indian soils, and the presence of _P. de -Baryanum_ as a soil saprophyte has been confirmed by Bussey, Peters, -and Ulrich.[11] De Bruyn[17] has recently found that most species of -_Phytophthora_, including _P. erythroseptica_ and _P. infestans_ may -live as saprophytes in the soil, whilst Pratt[53] has isolated from -virgin lands and desert soils various fungi, which cause disease in -potatoes. In 1912 Jensen[29] gave a list of twenty-three “facultative -parasites” isolated from soil, and these are but a moiety of those -which could be listed to-day. - -Furthermore, it was shown by Frank[24] many decades ago that forest -humus is not merely a mass of the remains of animals and plants, -but that a considerable part of its organic substance is made up of -fungus hyphæ, which ramify and penetrate in all directions. Evidence -is rapidly accumulating that this is also true of most other soils -containing organic matter. It is well known that many of the higher -plants live in symbiotic or commensal relationship with these humus -fungi, which are present in the host tissues as mycorrhiza, and further -studies only serve to show the widespread and fundamental nature -of this relationship. Thus many _Basidiomycetes_[50] (species of -_Tricholoma_, _Russula_, _Cortinarius_, _Boletus_, _Elaphomyces_, etc.) -possess a mycorrhizal relationship with various broad leaved trees, -such as beech, hazel, and birch[57] and with various conifers and -certain Ericales. Other Ericales show this relationship with species of -the genus _Phoma_,[62] many orchids, with species of _Rhizoctonia_[2] -(or _Orcheomyces_[10]), whilst _Gastrodia elata_ contains _Armillaria -mellea_.[36] Certain species of _Pteridophyta_ and _Bryophyta_ are -also known to certain mycorrhizal fungi. Of the numerous fungi taking -part in these mycorrhizal relationships, only a small number have yet -been identified, but there is little doubt that perhaps the majority -of these organisms must be regarded as true soil forms.[14],[45] The -mycological flora of the soil thus plays an important part in the life -of many higher forms of vegetation, and this relationship is a very -fruitful field for study. - - -RELATION OF FUNGI TO SOIL PROCESSES. - -The great cycle of changes occurring in the soil whereby organic matter -is gradually transformed and again made available as plant food is -entirely dependent upon micro-organisms. Until a decade ago it was -thought that bacteria were by far the most important group concerned -in the bringing about of these changes, but recent studies have shown -that, in at all events certain arcs of this great organic cycle, -the fungi have, perhaps, an equal part to play. The life of fungi -in the soil may, for our purposes, be considered from three points -of view--their part in the decomposition of carbon compounds, their -nitrogen relationships, and their work in the mineral transformations -of the soil. - - -CARBON RELATIONSHIPS. - -Of primary importance in the carbon relationships of soil fungi is -the part played in the decomposition of the celluloses, which compose -almost all the structural remains of plant tissues. Our first real -knowledge of this subject was given by Van Iterson[28] in 1904 when he -showed the wide extent of cellulose destruction by fungi, and devised -methods whereby fifteen cellulose-decomposing forms, many of which have -since proved to be common soil fungi, were isolated. Three years later -Appel[1] published his account of the genus _Fusarium_, and showed -that many of the species could destroy filter paper. A difficulty was -introduced in 1908 by Schellenberg,[60] who, working with common soil -forms, found that only hemicelluloses and not pure cellulose were -destroyed. This has recently been supported by Otto,[48] but from the -practical point of view the discussion is academic for the amount of -pure cellulose in plants is insignificant. - -In 1913 McBeth and Scales[43] showed that a considerable number -of common soil fungi were most active cellulose destroyers, pure -precipitated cellulose and cotton being readily attacked. This was -supported by McBeth in 1916,[42] whilst Scales[59] has found that -most species of _Penicillium_ and _Aspergillus_ decompose cellulose, -especially where ammonium sulphate is the source of nitrogen. -Waksman[65] tested twenty-two soil fungi and found that eleven -decomposed cellulose rapidly and four slowly, whilst Dascewska,[16] -Waksman,[66],[67] and others have concluded that soil fungi play -a more important part in the decomposition of cellulose and in -“humification” than soil bacteria. Schmitz[61] has recently shown that -cellulose-destroying bacteria play no important part in the decay of -wood under natural conditions. - -In addition to the celluloses, practically all simple and complex -organic carbon compounds are attacked by soil fungi, and in many cases -the decomposition is very rapid.[26] Many _Actinomycetes_, _Aspergilli_ -and _Penicillia_ are active starch splitters, and it is of interest to -note that some of the strongest cellulose decomposers (_Melanconium -sp._, _Trichoderma sp._, and _Fusaria_) secrete little diastase.[66] -The _Mucorales_ apparently do not attack cellulose, but can only -utilise pectin bodies, monosaccharides, and partly disaccharides.[26] -Dox and Neidig[19] have shown that various species of _Aspergillus_ -and _Penicillium_ are able to attack the soil pentosans. Roussy,[58] -Kohshi,[24] Verkade and Söhngen,[64] and many other workers have found -that fats and fatty acids are readily used as food by soil fungi, and -Koch and Oelsner[33] have recently shown that tannins are readily -assimilated. Klöcker,[32] Ritter,[56] and others have shown that the -utilisation of many carbon compounds is to a large extent determined by -the source of nitrogen and its concentration in the pabulum. - -There would seem, therefore, no doubt that the decomposition of -celluloses and other carbon compounds is of primary importance in the -life-activities of soil fungi. - - -NITROGEN RELATIONSHIPS. - -In this section we shall consider the problems of nitrogen fixation and -nitrification, of ammonification, and of the utilisation of nitrogenous -compounds by soil fungi. - -As soil fungi form so large a part of the soil population, the question -of whether they can make use of the free nitrogen of the air is of -primary importance. During the last two decades many investigators have -attempted to solve the problem, often studying allied or identical -species; but if one consults some thirty researches published during -this period, opinion is found to be about equally divided. Even, -however, in those studies where nitrogen fixation has been recorded the -amounts are very slight, usually being below 5 mgrms. per 50 c.c. of -solution, and often being obviously within the limits of experimental -error. Latham,[37] however, working on _Aspergillus niger_, recorded -variations ranging from a nitrogen loss of 42·5 mgrms. to a nitrogen -fixation of 205·1 mgrms. per 50 c.c. of medium. Ternetz[63] found -that different strains of _Phoma radicis_ may fix from 2·5 mgrms. of -nitrogen in the lowest case, to 15·7 mgrms. in the highest per 50 c.c. -of nutrient solution. Duggar and Davis[20] report that _Phoma betæ_ -may fix nitrogen in quantities of 7·75 mgrms. per 50 c.c. of medium. -The latter authors, in a very able critique of the problem, indicate -certain possible sources of error in previous work, and if one examines -the studies in which nitrogen fixation has been recorded in the light -of these criticisms, it is difficult not to think that, with the -exception of the genus _Phoma_, good evidence for nitrogen fixation -by fungi is lacking. _Phoma betæ_ is a common pathogen attacking -beets, whilst _P. radicis_ is a mycorrhizal form inhabiting various -Ericales. Apart from these exact quantitative studies, which have -given a negative verdict, there is a considerable amount of positive -but indirect evidence for nitrogen fixation by mycorrhizal fungi,[55] -and it is very unfortunate that more of these forms have not been -investigated quantitatively. As the evidence stands to-day, one must -conclude that the fungus flora does not play any part in the direct -nitrogen enrichment of the soil. - -Equally obscure is the question of nitrification and denitrification -by soil fungi, but this is the result of a lack of study rather than -of a plethora of indeterminate researches. Direct nitrification or -denitrification has not been established, but the work of Laurent[38] -and a few other workers appears to show that soil fungi can reduce -nitrates to nitrites. - -The second primary nitrogen relationship that we have to consider is -the process of ammonification. The ammonifying power of soil fungi was -first demonstrated by Muntz and Coudon,[46] and by Marchal[40] in 1893, -the former showing that _Mucor racemosus_ and _Fusarium Muntzii_ gave a -larger accumulation of ammonia in soil than any of the bacteria tested; -and the latter that _Aspergillus terricola_, _Cephalothecium roseum_ -and other soil fungi were active ammonifiers, especially in acid -soils. Shibata,[62] Perotti,[49] Hagem,[26] Kappen,[31] Löhnis,[39] -and others, have observed that urea, dicyanamide and cyanamide are -decomposed with the liberation of ammonia; and Hagem[26] has recorded -the same process for peptones, amino acids, and other organic nitrogen -compounds in plant and animal remains in the soil. The latter author -considers soil fungi more important ammonifying agents in the soil than -bacteria, a conclusion in which McLean and Wilson,[44] and perhaps most -later workers concur. McLean and Wilson[44] found large differences in -the ammonifying powers of various soil fungi, the _Moniliaceæ_ being -the strongest ammonifiers, the _Aspergillaceæ_ the weakest. Generic and -specific differences have been confirmed by Coleman,[15] Waksman,[67] -and other authors. Waksman and Cook[70] suggested that such variations -may be due, not to innate differences in the metabolic activities of -the several organisms, but to differences in reproductive times, and -that there might be some relationship between sporogeny and the ability -to accumulate nitrogen. Kopeloff[35] has carried out experiments on -the inoculation of sterilised soil with known quantities of spores -and found that, although the amount of ammonia accumulated increased -with the number of spores the proportion was not direct but modified -by the food supply. After the first five days’ growth, the rate of -ammonia production varied markedly in a two-day rhythm which seemed -to be due to the metabolism of the fungus rather than to recurrent -stages of spore formation and germination in the life history. The -amount of ammonia liberated has been shown by recent work[66] to depend -upon the available sources of carbon and nitrogen. In the absence of -a carbohydrate supply the protein is attacked both for carbon and -nitrogen, and since more of the former is required much ammonia is -liberated. In addition, however, to the carbon and nitrogen control, -the process of ammonification by soil fungi is intimately related to -physical conditions. Working with pure cultures, McLean and Wilson,[44] -Coleman,[15] Kopeloff,[35] Waksman and Cook,[70] and other students, -have shown that the amount of ammonia accumulated depends upon such -factors as the presence of phosphates, the period of incubation of the -fungi, aeration, the moisture in the soil, the temperature, the degree -of soil acidity, the type of soil, and so forth. - -That fungi take a very important place as ammonifying agents in the -soil can no longer be doubted, but the question yet remains to be -considered of the balance of profit or loss resulting from their -activities. It has usually been considered that a part of the ammonia -freed is used by the fungi themselves, but that the greater part -is liberated, and so rendered available to nitrifying organisms. -Both Neller[47] and Potter and Snyder[51] found that typical soil -fungi inoculated into sterile soil grew with a vigour approximately -equal to the growth induced by an inoculation of the entire soil -flora. This is largely to be accounted for by the fact that when -soils are sterilised by heat or by certain chemicals, breaking-down -changes occur, and substances are liberated which are peculiarly -favourable to fungus growth. This fact must be borne in mind when -interpreting ammonification and other studies where the method is that -of inoculation of fungi into sterilised soil. In many cases it tends -to nullify any application of the results to normal soils, whilst in -others the conclusions must be accepted with some reserve. In all -cases Potter and Snyder[51] found that fungi caused a diminution in -the amount of nitrates, that the ammonia was not much changed in -amount, and that there was a decrease in the quantities of soluble -non-protein nitrogen. The range of organic and inorganic nitrogenous -compounds utilisable by soil fungi is very great. Ritter[56] has shown -that certain forms can use the nitrogen of “free” nitric acid in the -medium; Ritter,[56] Hagem,[26] and others, that soil fungi can use -ammonia nitrogen equally with nitrate nitrogen, and Ehrenberg[21] -concluded that soil fungi play a more important part in the building -of albuminoids from ammonia than bacteria do. Ehrlich[22] has shown -that various heterocyclic nitrogen compounds and alkaloids can serve as -sources of nitrogen to soil fungi, whilst Ehrlich and Jacobsen[23] have -found that soil fungi can form oxy-acids from amino-acids. Hagem,[26] -Povah,[52] Bokorny,[6],[8] and others, state that for many soil forms -organic nitrogen sources are better than inorganic sources, and that -peptones, amino-acids, urea, and uric acids, etc., are very quickly -utilised by species of _Mucor_, yeasts, and so forth. Butkevitch,[12] -and Dox[18] have recently found that it depends on circumstances which -compounds of protein molecule can be utilised by particular fungi, and -that soil fungi can utilise both amino and amido complexes for the -formation of ammonia. In 1919 Boas[4] showed for _Aspergillus niger_ -that if a number of nitrogenous compounds are available the fungus -absorbs the most highly dissociated. - -In the welter of scattered observations on the utilisation of -nitrogenous compounds, it is difficult to trace any clear issue. That -proteins, amino-acids, and other complex organic compounds are readily -broken down to ammonia by soil fungi is clear, and, on the other hand, -it is also clear that soil fungi utilise extensively ammonia and -nitrates as sources of nitrogen. On which side the balance lies it is -yet impossible to say. - - -MINERAL RELATIONSHIPS. - -Heinze[27] and Hagem[26] have stated that soil fungi make the insoluble -calcium, phosphorus, and magnesium compounds in soil soluble and -available for plant food; and Butkevitch[12] has used _Aspergillus -niger_ in determining the availability of the mineral constituents, -but practically no work has yet been carried out on these problems. -A further matter on which sound evidence is greatly to be desired is -the part played by soil fungi in the oxidation processes of iron and -sulphur. - -A point which may be mentioned here, as it is of some considerable -practical importance, is the large quantity of oxalic, citric, and -other acids formed by certain common soil fungi. Acid formation -is partly dependent upon the species of fungus--even more the -physiological race within the species--and partly upon the substratum, -particularly the source of carbon.[5],[54] It is interesting that as -a group _Actinomycetes_ do not form acids from the carbon source but -alkaline substances from the nitrogen sources.[69] - - -CONTROL OF SOIL FUNGI. - -In the preceding sections an attempt has been made to sketch rapidly -the chief outlines of the widespread relationships of soil fungi and -of the fundamental part that they play in the biochemical changes -occurring in the soil. It will be evident, even from this survey, -that their occurrence is of the utmost agricultural importance, both -when helpful as in mycorrhizal relationships or as agents in making -complex organic materials available as plant food, or when harmful -as when causal agents of disease in plants. It is clear that could -the soil fungi be controlled to human ends by the encouragement of -the useful forms and the elimination of the harmful, a valuable power -would be placed in the hands of the grower of plants. Certain aspects -of this control, the cruder and more destructive perhaps, are already -practicable, whilst the finer and more constructive aspects remain -possibilities of to-morrow. - -Theoretically, the technique of control is selective in that it aims -to determine one or more particular fungi, leaving the remaining flora -untouched. Its highest expression is seen, perhaps, in the utilisation -of pure cultures of mycorrhizal fungi for horticultural purposes, such -as orchid cultivation, but there is no reason why this should not be -done for other purposes on a field scale similar to the way in which -cultures of special strains of the root nodule organisms of legumes -are employed. A second aspect is the direct encouragement of special -components of the fungus flora for particular purposes by selective -feeding. Thus, in a laboratory experiment, McBeth and Scales[43] record -an increase of 2000 times in cellulose-destroying and other soil fungi -by this method. It has been pointed out that soil fungus activities -such as ammonification, proteolysis and carbohydrate decomposition are -controlled by factorial equilibria, and for special purposes it would -seem feasible to weight the balance so that particular activities may -be favoured. A further step in this direction is the controlling of -particular physical conditions so that the activities of certain fungi -may be restricted. Professor L. R. Jones[30] and his colleagues at -Madison have shown the primary importance of the control of the soil -temperature in certain parasitic relationships; the work of Gillespie -and Hurst[25] and later workers has demonstrated that the parasitism -of certain species and strains of _Actinomyces_ upon the potato -is conditioned by definite ranges of soil acidity; and many other -relationships of similar nature are known. Data along such lines are -rapidly accumulating, and in certain cases are already susceptible of -practical application. In other cases, particular soil fungi are less -open to persuasive influences, and more drastic treatment needs to be -adopted. Certain chemicals mixed intimately with the soil increase or -diminish the numbers of particular fungi or groups of fungi; whilst -these organisms may be totally eliminated from the soil by wet or -dry heat for definite periods or by treatment with potent fungicides -such as formaldehyde. Although soil sterilisation and crude treatment -in other ways has been practised for decades, the possibility of a -more delicate control of soil fungi is only now being realised. Its -concrete expression will depend upon the progress that is made in exact -knowledge of the activities of soil fungi under natural and controlled -conditions, of the balance of factors in the environment which controls -any particular function and of the genetic nature of the soil fungi -which occur. Each of these aspects is a fruitful field of study. - - -RELATION TO SOIL FERTILITY. - -From a general survey of the researches that have been carried out on -soil fungi during the past two decades certain issues emerge. It would -seem clear that fungi occupy, perhaps, a primary place as factors in -the decomposition of celluloses, and thus may be the chief agents in -the transformation of plant remains to humus and to soluble compounds -which can be used as food by the nitrogen-fixing bacteria. Furthermore, -soil fungi are very important ammonifiers, but whether the balance -of ammonia freed is utilised by the fungi themselves, or whether it -is made available to nitrifying bacteria is not yet clear. If the -latter is the case, soil fungi play a valuable indirect rôle in the -accumulation of available plant food in the soil. On the other hand, by -utilising nitrates as sources of nitrogen, fungi may play an important -part in the depletion of the nitrogenous food in the soil available to -crop plants. Thirdly, soil fungi apparently take no part in the direct -nitrogen enrichment of the soil. Thus, soil fungi would seem to be the -most important factor in the first half of that great cycle whereby -organic remains become again available as organic food. - -The impression left on one’s mind by the study of the life of fungi -in the soil is of an infinitely complex series of moving equilibria, -the living activities being determined by both biological and -physico-chemical conditions. All these factors play an integral part in -the life of the soil fungi and must be considered if a true picture is -to be drawn. The principal factors may be classified into the following -groups: Most evident, perhaps, are the natures and specificities of -the fungi and the relative composition of the fungus flora. Equally -important, however, are the quantity and quality of the foods available -and the non-biological environment which results from the complex -series of physical and chemical changes occurring in the soil causally -independent of the organisms present, which interacts with the equally -vast series of changes resulting from fungus activities. Finally, one -must consider the interacting biological environment of surface animals -and plants and the microscopic fauna and flora. The complexities are -such that only the application of Baconian principles can unravel -them. A beginning has been made in the study of pure cultures of soil -fungi on synthetic media, and much valuable data have accrued, but -it is obviously not possible to apply directly to soil the results -obtained in such work. They remain possibilities; in certain cases -probabilities, but nothing more. A further step, one already taken -and of great promise, is the investigation of the changes occurring -in sterilised soils inoculated with known quantities of one or more -pure cultures of particular soil fungi. Such intensive study of single -factors in a standardised natural or artificial soil, to which has been -added a pedigreed fungus, is, perhaps, the most fruitful avenue of -progress. In all such work, however, one must bear acutely in mind the -fact that a sterilised soil and, still more, an artificial soil, is a -very different complex from a normal soil, and that results obtained -from the inoculation of such soils are not applicable directly in the -elucidation of ordinary soil processes. At present there is no method -known of completely sterilising a soil which does not destroy the -original physico-chemical balance. It is evident that the complexities -are such that chemist, physicist, and biologist must all co-operate -if the significance of the processes is to be understood, and a solid -foundation laid for future progress and for practical application. - - [1] Appel, O., “Untersuchungen über die Gattung _Fusarium_,” Mitt. - Biol. Reichanst. Land- u. Forstw., 1907, 4. - - [2] Bernard, N., “L’évolution dans la symbiose. Les Orchidées et - leurs Champignons commensaux,” Ann. Sci. Nat. (Bot.), Ser. 9, 1909, 9. - - [3] Bewley, W. F., “Anthracnose of the cucumber under glass,” Journ. - Min. Agric., 1922, xxix. - - [4] Boas, F., “Die Bildung löslicher Stärke im elektiven - Stickstoff-Stoffwechsel,” Ber. deut. bot. Ges., 1919, 37. - - [5] Boas, F., und Leberle, H., “Untersuchungen über Säurenbildung bei - Pilzen und Hefen II.,” Biochem. Ztschr., 1918, 92. - - [6] Bokorny, T., “Benzene derivatives as sources of nourishment,” - Zentr. Physiol., 1917, 32. - - [7] Bokorny, T., “Sugar fermentation and assimilation,” Allg. Brau. - Hopfen Zeit., 1917, 57. - - [8] Bokorny, T., “Verhaltung einiger organischer Verbindungen in der - lebenden Zelle,” Pflügers Archiv., 1917, 168. - - [9] Brown, P. E., “Mould action in soils,” Science, 1917, 46. - - [10] Burgeff, H., “Die Wurzelpilze der Orchideen,” Jena. 1909. - - [11] Bussey, W., Peters, L., and Ulrich, P., “Ueber das Vorkommen - von Wurzelbranderregern im Boden,” Arb. Kais. Biol. Anst. Land- u. - Forstw., 1911, 8. - - [12] Butkevitch, V. S., “Ammonia as a product of protein - transformations caused by mould fungi, and the conditions of its - formation,” Recueil d’articles dedié au Prof. C. Timiriazeff, 1916. - - [13] Butler, E. J., “An account of the genus _Pythium_ and some - _Chytridiaceæ_,” Mem. Dept. Agr. India, 1907, Bot. Ser. 5, 1. - - [14] Christoph, H., “Untersuchungen über die mykotrophen Verhältnisse - der Ericales und die Keimung von Pirolaceen,” Beihefte Bot. Centr., - 1921, 28. - - [15] Coleman, D. A., “Environmental factors influencing the activity - of soil fungi,” Soil Sci., 1916, 2. - - [16] Dascewska, W., “Étude sur la désagrégation de la cellulose dans - la terre de bruyère et la tourbe,” Univ. Genève, Inst. Bot., 1913, S. - 8. - - [17] De Bruyn, H. L. G., “The saprophytic life of _Phytophthora_ in - the soil,” Meded. v. d. Landbouwhoogeschool Wageningen, 1922, xxiv. - - [18] Dox, A. W., “Amino acids and micro-organisms,” Proc. Iowa Acad. - Sci., 1917, 24. - - [19] Dox, A. W., and Neidig, R. E., “Pentosans in lower fungi,” - Journ. Biol. Chem., 1911, 9. - - [20] Duggar, B. M., and Davis, A. R., “Studies in the physiology of - the fungi. (I.) Nitrogen fixation,” Ann. Mo. Bot. Gard., 1916, 3. - - [21] Ehrenberg, P., “Die Bewegung des Ammoniakstickstoffs in der - Natur,” Mitt. Landw. Inst., Breslau, 1907, 4. - - [22] Ehrlich, F., “Yeasts, moulds, and heterocyclic nitrogen - compounds and alkaloids,” Biochem. Ztschr., 1917, 79. - - [23] Ehrlich, F., and Jacobsen, K. A., “Über die Umwandlung von - Aminosäuren in Oxysäuren durch Schimmelpilze,” Ber. Deut. Chem. - Gesell., 1911, 44. - - [24] Frank, B., “Ueber die auf Wurzelsymbiose beruhende Ernährung - gewisser Bäume durch unterirdische Pilze,” Ber. d. Deut. Bot. - Gesell., 1885, 3. - - [25] Gillespie, L. J., and Hurst, L. A., “Hydrogen-ion - concentration--soil type--common potato scab,” Soil Sci., 1918, 6. - - [26] Hagem, O., “Untersuchungen über Norwegische Mucorineen,” - Vidensk. Selsk. I., Math. Naturw. Klasse, 1910, 7. - - [27] Heinze, B. H., “Sind Pilze imstande den elementaren Stickstoff - der Luft zu verarbeiten und den Boden an Gesamtstickstoff - anzureichen,” Ann. Mycol., 1906, 4. - - [28] Van Iterson, C., “Die Zersetzung von Cellulose durch Aërobe - Mikroorganismen,” Centr. f. Bakt., 1904, ii, 11. - - [29] Jensen, C. N., “Fungous flora of the soil,” Agric. Expt. Sta. - Cornell, Bull. 1912, 315. - - [30] Jones, L. R., “Experimental work on the relation of soil - temperature to disease in plants,” Trans. Wisc. Acad. Sci., 1922, 20. - - [31] Kappen, H., “Ueber die Zersetzung des Cyanamids durch Pilze,” - Centr. f. Bakt., 1910, ii, 26. - - [32] Klöcker, A., “Contribution à la connaissance de la faculté - assimilatrice de douze espèces de levure vis-à-vis de quatre Sucres,” - Compt. Rend. Trav. Lab., Carlsberg, 1919, 14. - - [33] Koch, A., und Oelsner, A., “Einfluss von Fichtenharz und Tannin - auf den Stickstoffhaushalt des Bodens und seiner physikalischen - Eigenschaften,” Centr. f. Bakt., 1916, ii, 45. - - [34] Kohshi, O., “Ueber die fettzehrenden Wirkungen der Schimmelpilze - nebst dem Verhalten des Organfettes gegen Fäulnis,” Biochem. Ztschr., - 1911, 31. - - [35] Kopeloff, N., “The inoculation and incubation of soil fungi,” - Soil Sci., 1916, 1. - - [36] Kusano, S., “_Gastrodia elata_ and its symbiotic association - with _Armillaria mellea_,” Journ. Coll. Agric., Imp. Univ., Tokyo, - 1911, iv. - - [37] Latham, M. E., “Nitrogen assimilation of _Sterigmatocystis - niger_ and the effect of chemical stimulation,” Torrey Bot. Club, - Bull. 1909, 36. - - [38] Laurent, “Les reduction des nitrates en nitrites par les graines - et les tubercles,” Bull. Acad. Roy. Sci. Belg., 1890, 20. - - [39] Löhnis, F., “Ammonification of cyanamid,” Ztschr. f. - Gärungsphysiol., 1914, v. - - [40] Marchal, E., “Sur la production de l’ammoniaque dans le sol par - les microbes,” Bull. Acad. Roy. Sci. Belg., 1893, 25. - - [41] Mazé, P., Vila et Lemoigne, “Transformation de la cyanamide en - urée par les microbes du sol,” Compt. Rend. Acad. Sci., Paris, 1919, - 169. - - [42] McBeth, I. G., “Studies on the decomposition of cellulose in - soils,” Soil Sci., 1916, I. - - [43] McBeth, I. G., and Scales, F. M., “The destruction of cellulose - by bacteria and filamentous fungi,” U.S. Dept. Agric, Bur. Pl. Ind., - 1913, Bull. 266. - - [44] McLean, H. C, and Wilson, G. W., “Ammonification studies with - soil fungi,” New Jersey Agric. Expt. Sta., 1914, Bull. 270. - - [45] Melin, E., “Ueber die mykorrhizenpilze von _Pinus silvestris_ - (L.) und _Picea abies_ (L.), Karst.” Svensk. Botan. Tidskr., 1921, xv. - - [46] Muntz, A., and Coudon, H., “La fermentation ammoniaque de la - terre,” Compt. Rend. Acad. Sci., Paris, 1893, 116. - - [47] Neller, J. R., “Studies on the Correlation between the - production of carbon dioxide and the accumulation of ammonia by soil - organisms,” Soil Sci., 1918, 5. - - [48] Otto, H, “Untersuchungen über die Auflösung von Zellulosen und - Zellwänden durch Pilze,” Dissert., Berlin, 1916. - - [49] Perotti, B., “Uber das physiologische Verhalten des Dicyanamides - mit Rücksicht auf seinen Wert als Düngemittel,” Centr. f. Bakt., - 1907, ii, 18. - - [50] Peyronel, B., “Nuovi casa di rapporti micorizici tra - Basidiomiceti e Fanerogame arboree,” Bull. Soc. Bot. Ital., 1922. - - [51] Potter, R. S., and Snyder, R. S., “The production of carbon - dioxide by moulds inoculated into sterile soil,” Soil Sci., 1918, 5. - - [52] Povah, A. H. W., “A critical study of certain species of - _Mucor_,” Bull. Torrey Bot. Club, 1917, 44. - - [53] Pratt, O. A., “Soil fungi in relation to diseases of the Irish - potato in Southern Idaho,” Journ. Agric. Res., 1918, 13. - - [54] Raistrick, H., and Clark, A. B., “On the mechanism of oxalic - acid formation by _Aspergillus niger_,” Biochem. Journ., 1919, 13. - - [55] Rayner, M. C., “Nitrogen fixation in Ericaceae,” Bot. Gaz., - 1922, 73. - - [56] Ritter, G. E., “Contributions to the physiology of mould fungi,” - Voronege, 1916. - - [57] Rosseels, E., “L’influence des microorganismes sur la croissance - des végétaux supérieurs,” Bull. Soc. Centrale Forest. Belg., 1916, 23. - - [58] Roussy, A., “Sur la vie des champignons en milieux Gras,” Compt. - Rend. Acad. Sci., Paris, 1909, 149. - - [59] Scales, F. M., “The Enzymes of _Aspergillus terricola_,” Journ. - Biol. Chem., 1914, 19. - - [60] Schellenberg, H. C., “Untersuchungen über das Verhalten einiger - Pilze gegen Hemizellulosen,” Flora, 1908, 98. - - [61] Schmitz, H., “The relation of bacteria to cellulose fermentation - induced by fungi with special reference to the decay of wood,” Ann. - Mo. Bot. Gard., 1919, vi. - - [62] Shibata, K., “Uber das Vorkommen vom Amide spaltenden Enzymen - bei Pilzen,” Beitr. Chem. Physiol. u. Path., 1904, 5. - - [63] Ternetz, C., “Über die Assimilation des atmosphärischen - Stickstoffs durch Pilze,” Jahrb. f. wiss. Bot., 1907, 44. - - [64] Verkade, P. E., and Söhngen, N. L., “Attackability of cis- and - trans-isomeric unsaturated acids by moulds,” Centr. f. Bakt., 1920, - ii, 50. - - [65] Waksman, S. A., “Soil fungi and their activities,” Soil Sci., - 1916, 2. - - [66] Waksman, S. A., “The influence of available carbohydrate upon - ammonia accumulation by micro-organisms,” Journ. Amer. Chem. Soc., - 1917, 39. - - [67] Waksman, S. A., “Proteolytic enzymes of soil fungi and - _Actinomycetes_,” Journ. Bact., 1918, 3. - - [68] Waksman, S. A., “On the metabolism of _Actinomycetes_,” Proc. - Soc. Amer. Bact. Abstract Bact., 1919, 3. - - [69] Waksman, S. A., “The influence of soil reaction upon the growth - of _Actinomycetes_ causing potato scab,” Soil Sci., 1922, xiv. - - [70] Waksman, S. A., and Cook, R. C., “Incubation studies with soil - fungi,” Soil Sci., 1916, 1. - - - - -CHAPTER IX. - -THE INVERTEBRATE FAUNA OF THE SOIL (OTHER THAN PROTOZOA). - - -The micro-organisms of the soil have been fully discussed in the -preceding chapters of this volume. There now remains to be considered -the fauna of invertebrate animals, other than protozoa, which inhabit -that same medium. In the first place, it is necessary to define what -groups of invertebrate animals are to be regarded as coming under the -category of soil organisms. The latter expression has rather a wide -application and, for the present purpose, is held to mean any organism -of its kind which, in some stage or stages of its life-cycle, lives -on or below the surface of the soil. It will be obvious that, with so -comprehensive a definition, the intimacy of the association of these -animals with the soil will vary within very wide limits. Some animals -pass their whole life-cycle in the soil; others are only present during -a limited phase, and not necessarily in a trophic condition, but since -their occurrence is constant, they cannot be entirely omitted from -consideration. - -Unlike the groups of organisms which have been dealt with in the -foregoing pages, the invertebrates of the soil do not admit, as a rule, -of investigation in culture media. It is, in consequence, much more -difficult to achieve in the laboratory the same control over their -environmental conditions. This fact in itself largely explains why -the interpretations of field observations in animal ecology have not -usually been subjected to the test of laboratory experimentation. The -study of animal ecology, in so far as the denizens of the soil are -concerned, is of very recent birth. It has not, as yet, passed the -preliminary stage of cataloguing empirical data, and much spade work -will be necessary before the various factors controlling the phenomena -actually observed are understood. - -Owing to the paucity of information available, this chapter is -essentially based upon observations conducted at Rothamsted. Its object -is not so much to attempt to evaluate the invertebrate fauna of the -soil, as to suggest a line of ecological work demanding investigation -on land of many different types. - - -METHOD OF INVESTIGATING THE SOIL FAUNA. - -The method adopted at Rothamsted consists in taking weekly soil samples -from a given area for a period of twelve months. Each sample is a cube -of soil, with a side dimension of nine inches, and a total content -of 729 cubic inches. The samples are taken by means of an apparatus -consisting of four iron plates, which are driven into the ground down -to the required depth so as to form a kind of box, which encloses a -cube of soil (_vide_ Morris, 1922 A). The latter is then removed in -layers, each layer being transferred to a separate bag for the purpose. -When the complete sample has been extracted, there are five bags -containing layers of soil taken from the surface to a depth of 1″, from -1″ to 3″, from 3″ to 5″, from 5″ to 7″, and 7″ to 9″ respectively. -Below a depth of 9″ no samples have been taken. - -The sample obtained in this manner may be gradually worked into small -fragments by hand, and examined whenever necessary under a binocular -microscope for the smaller organisms present. This procedure, however, -is very tedious and has been replaced by the use of an apparatus -consisting of a series of three sieves, with meshes of decreasing size -(_vide_ Morris, 1922). The soil is washed through these sieves by means -of a stream of water, and the meshes of the final strainer are small -enough to retain all except the most minute organisms present, while at -the same time they allow the finest soil particles to be carried away. -When desirable, the effluent can be passed through a bag or sieve of -bolting silk, in order to collect such organisms that may have passed -through the third sieve. - -In addition to the actual taking and examination of the samples, a -botanical survey of the area under investigation is made; chemical -and mechanical analyses of the soil are also required. It is further -necessary to take soil temperature readings, to determine the moisture -content of the samples taken, and the amount of organic matter which -they contain. - - -GROUPS OF INVERTEBRATA REPRESENTED IN THE SOIL. - -The various groups of invertebrates represented in the soil may be -briefly referred to in zoological order. - -_Nematoda._--The Nematoda or thread-worms are chiefly animal parasites, -nevertheless they usually lead an independent existence in the soil -in certain stages of their development. The numerous small species -belonging to the family _Anguillididæ_, or eel-worms, form a definite -constituent of the soil fauna; they are generally free-living and -non-parasitic. Certain members of this family, however, are enemies of -cultivated plants. - -_Annelida._--Terrestrial Annelida are almost entirely confined to the -order _Oligochæta_, the majority of which are earthworms (_Terricolæ_), -whose whole life-cycle is passed within the confines of the soil. -The small white worms of the family _Enchytræidæ_ belong to the -aquatic section (_Limicolæ_) of the order, but they have various -representatives which are abundant in damp soil containing organic -matter. - -_Mollusca._--The terrestrial Mollusca are included in the sub-order -_Pulmonata_ of the _Gastropoda_. These organisms, which include the -snails (_Helicidæ_) and slugs (_Limacidæ_), regularly deposit their -eggs in moist earth. Slugs adopt the soil as a frequent habitat, only -leaving it for feeding purposes in the presence of sufficient moisture. -They are frequent consumers of vegetation, with the exception of -_Testacella_, which is carnivorous. - -_Crustacea._--The few species of Crustacea inhabiting the soil belong -to the order _Isopoda_, family _Oniscidæ_, which are popularly referred -to as “woodlice,” “slaters,” etc. - -_Myriapoda._--The _Diplopoda_ or millipedes include enemies of -various crops and are common denizens of the soil. The _Chilopoda_ or -centipedes are usually less abundant and are carnivorous. The minute -_Symphyla_ are often evident but are of minor importance. - -_Insecta._--Insects form the dominant element in the invertebrate -fauna. Phytophagous species devour the subterranean parts of plants, -and notable examples are afforded by the larvæ of _Melolontha_, -_Agriotes_ and _Tipula_. Saprophagous forms are abundantly represented -by the _Collembola_, and by numerous larval _Diptera_ and _Coleoptera_. -Predaceous species preying upon other members of the soil fauna -are exemplified by the _Carabidæ_ and many larval _Diptera_. -Parasitic species pass their larval stages on or within the bodies -of other organisms. The groups of _Hymenoptera_, and the dipterous -family _Tachinidæ_, which exhibit this habit, constitute, along -with predaceous forms, one of the most important natural agencies -controlling the multiplication of insect life. There are also insects -(ants, and other of the aculeate _Hymenoptera_) which utilize the -soil as a suitable medium wherein to construct their habitations or -brood chambers, without necessarily deriving their food from the soil. -Lastly, there are many insects, notably _Lepidoptera_, which only -resort to the soil for the purpose of undergoing pupation. The insect -fauna is, therefore, a closely inter-connected biological complex; for -a discussion and an enumeration of its representatives reference may be -made to papers by Cameron (1913, 1917), and Morris (1921, 1922 a). - -_Arachnida._--The two principal classes represented in the soil are the -_Areinida_, or spiders, and the _Acarina_, or mites, and ticks. The -_Areinida_, which are well-known to be carnivorous, are an unimportant -constituent of the fauna. _Acarina_, on the other hand, are abundant, -and exhibit a wide range of feeding habits; most of the soil forms are -probably carnivorous, and either free-living or parasitic. - - -NUMBER OF ORGANISMS PRESENT AND THEIR DISTRIBUTION IN DEPTH. - -In computing the number of invertebrates normally present in a given -type of soil, the method adopted consists of making individual counts -of all such organisms as occur in each sample of a series taken over -a period of twelve months. This method considerably reduces errors -due to season and to the possible deviation of one or more samples -from the average. If the total number of these organisms is known for -the samples taken, it becomes a simple procedure to arrive at their -approximate numbers per acre. - -TABLE XIV. - -(Based on Morris, 1922 A.) - - +------------------------------+---------------+-------------+ - | |Unmanured Plot.|Manured Plot.| - +------------------------------+---------------+-------------+ - |Insects | 2,474,700 | 7,727,300 | - +------------------------------+---------------+-------------+ - |Larger Nematoda and Oligochæta| | | - | Limicolæ | 794,600 | 3,600,400 | - +------------------------------+---------------+-------------+ - |Myriapoda-- | | | - | Diplopoda | 596,000 | 1,367,000 | - | Chilopoda | 215,400 | 208,700 | - | Symphyla | 64,000 | 215,500 | - | +---------------+-------------+ - | Total | 875,400 | 1,791,200 | - +------------------------------+---------------+-------------+ - |Oligochæta (Terricolæ) | 457,900 | 1,010,100 | - +------------------------------+---------------+-------------+ - |Arachnida-- | | | - | Acarina | 215,400 | 531,900 | - | Areinida | 20,200 | 20,200 | - | +---------------+-------------+ - | Total | 235,600 | 552,100 | - +------------------------------+---------------+-------------+ - |Crustacea (Isopoda) | 33,700 | 80,800 | - +------------------------------+---------------+-------------+ - |Mollusca (Pulmonata) | 13,500 | 33,700 | - +------------------------------+---------------+-------------+ - | Total Invertebrata | 4,885,400 | 14,795,600 | - +------------------------------+---------------+-------------+ - -[Illustration: FIG. 20.--Distribution in depth of the more important -groups of soil invertebrates in the manured and unmanured (or control) -plots at Rothamsted. (From Morris, “Annals of Applied Biology,” vol. -ix., nos. 3 and 4, Cambridge University Press.)] - -Table XIV. represents a numerical estimate of the invertebrate fauna of -two plots of arable land at Rothamsted. The soil is clay with flints -overlying chalk, and the land in question has been devoted for eighty -years to continuous cropping with wheat; one plot (No. 3) receives an -annual dressing of farmyard manure at the rate of 14 tons per acre, and -the other plot (No. 2) receives no natural or artificial fertilizer. -The significant feature in a comparison of the fauna of the two plots -is the great numerical increase in organisms due to the addition of -manure. From the point of view of distribution in depth, Fig. 20 -clearly demonstrates that the bulk of the fauna is concentrated in the -first three inches of the soil. With the exception of the _Acarina_ it -is evident that the limits of vertical distribution extend below the -depth of nine inches investigated, although the numbers of organisms -likely to be present are inconsiderable. The _Oligochæta_, or true -earthworms, occur in Rothamsted soil in numbers very much in excess of -the figures given by Darwin, who quoted observations by Hensen. The -latter authority calculated that there were 53,767 earthworms in an -acre of garden soil, and estimated that about half that number would -be present in an acre of corn field. In the Rothamsted investigations -their numbers exceeded Hensen’s estimate over 16 times in unmanured -land, and over 36 times in manured land. - -In an area of pasture-land in Cheshire few insects occurred below -a depth of 2 inches, and they reached the limit of their vertical -distribution at or near 6 inches. Their number (3,586,000 per acre) -is considerably in excess of that present in unmanured arable land at -Rothamsted. - -[Illustration: - - 1, _Collembola_; 2, _Thysanura_; 3, _Orthoptera_; 4, _Thysanoptera_; - 5, _Hemiptera_; 6, _Lepidoptera_; 7, _Coleoptera_; 8, _Diptera_; 9, - _Hymenoptera_. - -FIG. 21.--Number of individuals in the different orders of insects in -manured and unmanured arable land at Rothamsted. (From Morris, “Annals -of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University -Press.)] - - -DOMINANCE OF CERTAIN SPECIES AND GROUPS. - -In Fig. 21 a numerical analysis is given of the different orders -of insects represented in Rothamsted soil. The ascendency of the -_Hymenoptera_ and _Collembola_ is almost entirely due to the occurrence -of three species in large numbers, viz., the ant _Myrmica lævinodis_ -and the _Collembola_, _Onychiurus ambulans_ and _O. fimetarius_. In -the unmanured plot these two _Collembola_ constituted 13 per cent. of -the insects and the species of ant accounted for nearly 28 per cent. -In the manured plot they amounted respectively to 27 per cent. and 36 -per cent. of the insects present. Next in order of numerical ascendency -are larval _Diptera_, mainly belonging to the families _Cecidomyidæ_, -_Chironomidæ_, and _Mycetophilidæ_. The _Diptera_ are followed by -the _Coleoptera_, whose most abundant representatives are larval -_Elateridæ_ (wireworms). - -[Illustration: - - 1, _Collembola_; 2, _Thysanura_; 3, _Orthoptera_; 4, _Thysanoptera_; - 5, _Hemiptera_; 6, _Lepidoptera_; 7, _Coleoptera_; 8, _Diptera_; 9, - _Hymenoptera_; 10, _Diplopoda_; 11, _Chilopoda_; 12, _Areinida_; 13, - _Acarina_. - -FIG. 22.--Number of species of different orders of invertebrates -present in the manured and unmanured (or control) plots at Rothamsted. -(From Morris, “Annals of Applied Biology,” vol. ix., nos. 3 and 4, -Cambridge University Press.)] - -In point of view of number of species present (Fig. 22), _Coleoptera_ -take the front rank; in the unmanured plot they are very closely -approached by _Collembola_ and _Diptera_. - -Passing from the insects, the next assemblage of organisms in point of -number of individuals are the smaller worms. The difficulties attending -the specific identification of these organisms are great, and, in the -present survey, the _Nematodes_ and all the smaller _Oligochætes_ have -not been separated. - -The abundance of the _Myriapoda_ is mainly due to the prevalence of -_Diplopoda_, which are represented by four species. The _Chilopoda_ -almost entirely consist of a single species _Geophilus longicornis_. - -The dominant group of the _Arachnida_ is the _Acarine_ family -_Gamascidæ_, which are represented by about a dozen species. - - +---------+-------------+-------------+------------+--------------+ - | |Phytophagous.|Saprophagous.|Carnivorous.|Heterophagous.| - +---------+-------------+-------------+------------+--------------+ - |Unmanured| | | | | - |plot | 14 | 48 | 13 | 20 | - |Manured | | | | | - |plot | 13 | 58 | 9 | 20 | - +---------+-------------+-------------+------------+--------------+ - - -CLASSIFICATION OF SOIL INVERTEBRATES ACCORDING TO FEEDING HABITS. - -From the point of view of the fauna as a whole, the zoological -classification of the soil invertebrates is only significant when the -various groups are analysed according to the feeding habits of their -members. All animals are directly or indirectly dependent upon plant -life for their nutrition. For the present purpose they are divided -into four categories, and the position of each class of animals in the -scheme is based upon the habits of its chief representatives in the -soil. Definite information on this subject, however, is not always -forthcoming, and it is only possible to achieve approximate estimates. -In the table above the percentages in number of individuals present in -the two plots investigated at Rothamsted are given under each type of -feeding habit. - -It must be borne in mind that these estimates only apply to average -conditions; the outbreak of a plant pest in any one year must naturally -materially alter the proportions given. The phytophagous organisms are -represented by a certain number of the _Insecta_ together with the -pulmonate _Mollusca_. Carnivorous forms which are mainly beneficial -from the agricultural standpoint, include _Insecta_, together with -the _Chilopoda_, many _Acarina_ and the _Areinida_. Saprophagous -forms constitute the dominant element of the soil fauna. More than -30 per cent. of the _Insecta_ exhibit this habit, which is also the -dominant one in the _Oligochæta_, _Symphyla_, and in many of the soil -_Nematodes_. Heterophagous species include all those of somewhat -plastic habits; for the most part they are saprophagous, but, on the -other hand, a considerable proportion of the species attack growing -plants or exhibit both habits. Under this category are included a -certain number of the _Insecta_, the _Diplopoda_, _Isopoda_, and some -_Acarina_. - - -THE INFLUENCE OF ENVIRONMENTAL FACTORS UPON THE INVERTEBRATES OF THE -SOIL. - -Since animals are endowed with powers of independent locomotion: they -are not necessarily tied to their environment to the same extent -that plants are. The investigation of the influence of environmental -factors sooner or later involves a study of the tropisms of the -animals concerned. Until these are adequately understood it is -scarcely possible to arrive at any exact conclusions relative to their -behaviour in the soil. Insects, for example, respond to the stimuli of -various, and often apparently insignificant forces, acting upon their -sensory organs. Such responses are known as chemotropism, phototropism, -hydrotropism, thermotropism, and so forth according to the nature of -the stimuli. Tropisms are automatic and, so far as they distinguish -sensations, are independent of any choice, and consequently of psychic -phenomena. Animal automatism, however, does not present the rigidity -of mechanical automatism. Differential sensibility, vital rhythms, or -periodicity, etc., are other important aspects of animal behaviour. - -The environmental factors, affecting more especially the insect -population of the soil, have been discussed by Cameron (1917) and -Hamilton (1917), and certain broader aspects of animal ecology by -Adams (1915) and Shelford (1912). These factors are so numerous and -so inter-connected, that it is only possible to refer to them briefly -in the space available. As might be expected, soils that are of a -light and open texture are the ones most frequented by soil insects, -nutritional and other factors being equal. Furthermore, it has already -been shown that in arable land insects and other animals penetrate to -a greater depth than in pastures. This fact is primarily due to the -greater looseness of the soil occasioned by agricultural operations, -which ensure at the same time better drainage aeration, and greater -facilities for penetration. Hamilton found that soil insect larvæ are -very sensitive to evaporation, and especially so if the temperature -is 20° C. or over. In their natural habitat the relative humidity of -the air, in moist or wet soil, is not far below saturation, and the -temperature of the soil rarely goes above 20°-23°C., and then only in -exposed, dry, hard soil in which these larvæ do not occur. - -The significance of the rate of evaporation as an environmental -factor was first emphasised by Shelford. According to him the best -and more accurate index of the varying physical conditions affecting -land animals, wholly or in part exposed to the atmosphere, is the -evaporating power of air. By means of a porous cup-atmometer, as -devised by Livingston, Shelford has carried out an important series -of experiments on the reactions of various animals to atmospheres of -different evaporation capacities, and reference should be made to his -text-book. - -The importance of the organic matter present in the soil is well -illustrated in the table on p. 152. The great increase in the number of -insects and other animals is partly due to their direct introduction -along with the manure, and partly to their entry into the soil in -response to chemotropic stimuli exerted by fermentation. Organic matter -influences the fauna in other ways also; it increases the moisture -content of the soil, and it provides many species with an abundance of -food material. Also, the amount of carbon dioxide present in the soil -is partly dependent upon decaying organic matter. Hamilton conducted -experiments on the behaviour of certain soil insects in relation to -varying amounts of carbon dioxide. Although his work is of too limited -a nature to be accepted without reserve, it lends support to the -conclusions of Adams who says: “The animals which thrive in the soil -are likely to be those which tolerate a large amount of carbon dioxide, -and are able to use a relatively small amount of oxygen, at least for -considerable intervals, as when the soil is wet during prolonged rains. -The optimum soil habitat is therefore determined, to a very important -degree, by the proper ratio or balance between the amount of available -oxygen and the amount of carbon dioxide which can be endured without -injury.” - -Little is known concerning the occurrence of ammonia in the soil -atmosphere, but its presence in minute quantities is probably an -important chemotropic factor in relation to saprophagous organisms -which are the largest constituent of the fauna. A great increase in -Dipterous larvæ occurs on the addition of farmyard manure, and this -is noteworthy in the light of Richardson’s experiments (1916), which -indicate that ammonia exercises a marked attraction for _Diptera_, -which spend some part of their existence in animal excrement in some -form or another. - -The nature of the vegetation supported by the soil is of paramount -importance in relation to phytophagous organisms, and examples need -scarcely be instanced of certain species of soil insects being -dependent upon the presence of their specific food plants. - - -THE RELATION OF SOIL INVERTEBRATES TO AGRICULTURE. - -The relation of these organisms to agriculture may be considered from -three points of view: (_a_) their influence upon the soil itself; (_b_) -their relation to the nitrogen cycle; and (_c_), their direct influence -upon economic plants. - -(_a_) The behaviour of earthworms as a factor inducing soil fertility -is discussed by Darwin in his well-known work on the subject, and their -action may be briefly summarised as follows. In feeding habits they -are very largely saprophagous, and consume decaying vegetable matter -including humus, which they swallow, together with large quantities of -soil. Earthworms come to the surface to discharge their fæces (“worm -casts”), and in this process they are continually bringing up some of -the deeper soil to the air. Darwin estimated that earthworms annually -brought to the surface of the soil in their “casts” sufficient earth -to form a layer ·2 inch in depth, or 10 tons per acre. Their action, -along with the atmosphere, are the chief agencies which produce the -uniformity and looseness of texture of the surface soil. By means of -their burrows earthworms facilitate the penetration of air and water -into the soil, while their habit of dragging leaves and other vegetable -material into these burrows increases the organic matter present -below the surface. These facts are generally agreed upon, but it is a -disputed point whether earthworms, by devouring organic matter, aid the -conversion of the latter into plant food more rapidly than takes place -solely through the activities of micro-organisms. - -Soil insects and other arthropods, by their burrowing activities, -are also instrumental in loosening the soil texture and thereby -facilitating soil aeration and the percolation of water. The action of -termites in warmer countries is discussed by Drummond in his “Tropical -Africa,” who compares the rôle of subterranean termites to that of -earthworms. The great abundance of ants renders them also significant -in this same respect, and very few species are direct enemies of the -agriculturist. - -[Illustration: FIG. 23.--Diagram showing the Relation of the Soil -Invertebrata (other than Protozoa) to the Nitrogen Cycle.] - -(_b_) In their relation to the nitrogen cycle (_vide_ p. 174), the -activities of the soil invertebrates may be expressed diagrammatically, -as a side-chain in the process (Fig. 23). The proteins, elaborated by -plants, are utilised as nitrogenous food by the phytophagous animals -present. The waste products of the latter, which contain the nitrogen -not used for growth or the replacement of loss by wear and tear, are -returned to the soil. Here they disintegrate, and are ultimately -converted into ammonium salts, mainly by bacterial action. The dead -bodies of these animals are also broken down by various means, becoming -eventually chemically dissociated and available as plant food. Animal -(and plant) residues serve, however, as food for the large number of -saprophagous invertebrates present in the soil. In this event the -nitrogen contained in such residues becomes “locked up,” as it were, -for the time being in their bodies. Both saprophagous and phytophagous -animals are preyed upon by carnivorous species, but ultimately the -nitrogen is returned to the soil upon the death of those organisms. -The amount present in the bodies of the whole invertebrate fauna has -been calculated by Morris (1922) upon analyses furnished by chemists -at Rothamsted. It is estimated that the fauna of manured land contains -about 7349 grm., or 16·2 lb. of nitrogen per acre, and that of -untreated land, 3490 grm., or 7·5 lb. per acre. These amounts are equal -respectively to the nitrogen content of 103·6 lb. and 48 lb. of nitrate -of soda. - -The primary question affecting agriculture is, whether any notable -loss of nitrogen is occasioned by the presence of these organisms in -the soil. It has been mentioned that their nitrogenous waste material, -and their dead bodies, ultimately undergo disintegration; any loss, -if any, takes place during the latter process. With the more complex -compounds it probably consists in the production of amino-acids and -their subsequent hydrolysis or oxidation. During this process an -appreciable loss of nitrogen in the gaseous form occurs. This loss, -which is discussed on p. 173 would represent the net deficit occasioned -by the incidence of invertebrates in the soil. Against this loss must -be placed the beneficial action of such organisms as earthworms, which, -in all probability, more than counterbalances it. - -(_c_) Many soil insects, on account of their phytophagous habits, are -well-known to be some of the most serious enemies of agriculture. -Certain of these, and also other classes of invertebrates, which are -likewise directly injurious, have been instanced in the earlier pages -of this chapter. Detailed information on this subject will be found in -textbooks of economic zoology, notably the volume by Reh (1913). - - -LITERATURE REFERRED TO. - - ADAMS, C. C., “An Ecological Study of Prairie and Forest - Invertebrates,” Bull. Illin. St. Lab. Nat. Hist., 1915, xi. - - CAMERON, A. E., “General Survey of the Insect Fauna of the Soil,” - Journ. Econ. Biol., 1913, viii. “Insect Association of a Local - Environmental Complex in the District of Holmes Chapel, Cheshire,” - Trans. Roy. Soc. Edin., 1917, lii. - - DARWIN, C., “Vegetable Mould and Earthworms,” London, 1881. - - HAMILTON, C. C., “The Behaviour of some Soil Insects in Gradients of - Evaporating Power of Air, etc.,” Biol. Bull., 1917, xxxii. - - MORRIS, H. M., “Observations on the Insect Fauna of Permanent Pasture - in Cheshire,” Ann. App. Biol., 1921, vii. “On a Method of Separating - Insects and other Arthropods from Soil,” Bull. Entom. Res., 1922, - xiii. “The Insect and Other Invertebrate Fauna of Arable Land at - Rothamsted,” Ann. App. Biol., 1922 A, ix. - - REH, L., In Sorauer’s “Pflanzenkrankheiten,” 1913, iii. - - RICHARDSON, C. H., “The Attraction of Diptera to Ammonia,” Ann. Ent. - Soc. Amer., 1916, ix. - - RUSSELL, E. J., “The Effect of Earthworms on Soil Productiveness,” - Journ. Agric. Sci., 1910, iii. - - SHELFORD, V. E., “Animal Communities in Temperate America,” Chicago. - 1914, “The Importance of the Measure of Evaporation in Economic - Studies of Insects,” Journ. Econ. Entom., 1912, vii. - - - - -CHAPTER X. - -THE CHEMICAL ACTIVITIES OF THE SOIL POPULATION AND THEIR RELATION TO -THE GROWING PLANT. - - -In the preceding chapters it is shown that the soil is normally -inhabited by a very mixed population of organisms, varying in size from -the smallest bacteria up to nematodes and others just visible to the -unaided eye, on to larger animals, and finally earthworms, which can -be readily seen and handled. These organisms all live in the soil, and -therefore must find in it the conditions necessary for their growth. -We have dealt in the first chapter with the supplies of water, air, -and heat, without which life is clearly impossible. Equally necessary -is the source of energy, for the organism requires energy material as -surely as the motor engine requires petrol, and it ceases to function -unless an adequate supply is forthcoming. - -All the energy comes in the first instance from the sun, if we exclude -the unknown but probably small fraction coming from radio-active -elements. But this radiant energy is not utilisable by the soil -population, excepting surface algæ; it has to be transformed into -another kind. So far, chlorophyll is the only known transformer; -it fixes the energy of sunlight and stores it up in bodies like -hemicellulose, sugar, starch, protein, etc. The transformation is -imperfect; even the heaviest yielding crops grown under glass, in -conditions made as favourable as our knowledge permits, utilise only -about 4 per cent. of the total energy available during their period of -growth; in natural conditions not more than 0·4 per cent. is utilised. -Such as it is, however, the energy fixed in the plant represents all, -indeed more than all, that the soil organisms can obtain. - -In the state of Nature, vegetation dies and is left on the soil. Two -things may then happen. It may become drawn into the soil by earthworms -and other agents; the energy supply is thus distributed in the soil -to serve the needs of the varied soil population. This is the normal -case, associated with the normal soil population and the normal flora. -If, however, the mingling agents are absent, the dead vegetation lies -like a mat on the surface of the soil, only partially decomposing, -unsuitable for the growth of most seedlings, and effectually preventing -most of the vegetation below from pushing a way through: thus there -comes to be no vegetation at all, or only a very restricted and special -flora. The soil population becomes also specialised. Peats and acid -grassland afford examples. - -On the neutral grass plots at Rothamsted, the dead vegetation does not -accumulate on the surface but is rapidly decomposed or drawn into the -soil, leaving the surface of the earth bare and free for the growth -of seedlings. On the acid plots dead vegetation remains long on the -surface, blotting out all new growth excepting two or three grasses -which form underground runners capable of penetrating the mat, and -sorrel, the seedling roots of which seem to have the power of boring -through a fibrous layer of this sort. It is possible to remove the -mat entirely by bacterial action alone, if sufficient lime be added -periodically to make the reaction neutral, but failing these repeated -additions the mat persists. - -We shall confine ourselves to the normal case where earthworms bring -the source of energy into the soil. - -Directly the energy is available, it begins to be utilised. Two laws -govern the change. The first is well-known to biologists: it states -that the total energy of the system remains constant and can neither -be increased nor diminished except from outside; in other words, that -energy can be neither created nor destroyed. The second law is less -familiar: it is that energy once transformed to heat by one organism -cannot be used again by another. It is not destroyed; it remains -intact, but is useless to the organism. One cannot have an indefinite -chain of organisms living on each other’s excretory products; there was -a certain quantity of energy in the food eaten by the first, and no -more than this quantity can be got out whether one organism obtains the -whole or whether others share it. - -The outside value for the amount of energy fixed in the soil is -obtainable by combustion of the soil in a calorimeter, but much of -this is not available to the soil organisms. The normal sedimentary -soils of England still contain decomposition products of the débris -of plants and animals originally deposited with them, but in the long -course of ages much of the extractable energy has been utilised. The -soil population is thus dependent on recently grown vegetation, and it -is therefore largely confined to the layer, usually in this country -about 6 inches thick, through which the recently dead vegetation is -distributed. Below this level there may be sufficient air, water, -temperature, etc., but there is insufficient source of energy for any -large population. - -Unfortunately there is no ready means for distinguishing between the -total and the actually available quantity of energy in the soil. But -it is not difficult, by adopting the Rothamsted analytical method, to -ascertain the approximate amount of energy that has been transformed in -a given period. The Rothamsted plots are periodically analysed and a -balance sheet is drawn up showing how much of each constituent has been -added to and removed from the soil in the intervening period. For two -of the Broadbalk plots the results are shown in Tables XV., XVI. - -The dunged plot receives 14 tons farmyard manure per annum, a quantity -in excess of what would usually be given; the unmanured plot, on the -other hand, has received no manure for many years and is abnormally -poor. Normal soils lie somewhere between these limits, but tending -rather to the value for the dunged than for the unmanured plot. It will -be seen that each acre of the dunged land loses on an average 41,000 -calories per day, while each acre of the unmanured land loses on an -average 2700 calories per day. - -TABLE XV.--MATERIAL BALANCE SHEET: BROADBALK SOIL, ROTHAMSTED. - -(LB. PER ACRE PER ANNUM.) - - +-------------------------+-------------+---------------+ - | | Farmyard | No Manure | - | |Manure Added.| Added. | - | +------+------+-------+-------+ - | | C. | N. | C. | N. | - +-------------------------+------+------+-------+-------+ - |Added in farmyard manure| 3600 | 200 | nil | nil | - |Added in stubble | 300 | 3 | 100 | 1 | - +-------------------------+------+------+-------+-------+ - | Total added | 3900 | 203 | 100 | 1 | - |Taken from soil | nil | nil | 200 | nil | - |Stored in soil | 200 | 30 | nil | nil | - +-------------------------+------+------+-------+-------+ - | Lost from soil | 3700 | 170 | 300 | nil[H]| - | Per cent. | 95 | 84 | 100 | nil | - +-------------------------+------+------+-------+-------+ - - Initial C : N ratio in farmyard manure, 18 : 1 - - Final C : N ratio in soil, 10 : 1. - - [H] Gain of 6 lb. See p. 173. - -TABLE XVI.--ANNUAL ENERGY CHANGES IN SOIL: BROADBALK. APPROXIMATE -VALUES ONLY. - -MILLIONS OF KILO CALORIES PER ACRE PER ANNUM. - - +---------------------------------+-------------+---------+ - | | Farmyard |No Manure| - | |Manure Added.| Added. | - +---------------------------------+-------------+---------+ - |Added in manure | 14 | nil | - |Added in stubble | 2 | 0·3 | - | +-------------+---------+ - | Total added | 16 | 0·3 | - |Taken from soil | nil | 0·5-1 | - |Stored in soil | 0·5-1 | nil | - | +-------------+---------+ - | Dissipated per annum | 15 | 1 | - +---------------------------------+-------------+---------+ - |Per day: calories | 41,000 | 2700 | - |Equivalent to | 12 men. | ¾ man. | - |The human food grown provides for| 2 men. | ½ man. | - +---------------------------------+-------------+---------+ - -These numbers are interesting when we reflect that the human food -produced on the dunged land yields only 7000 calories per day, from -which it is clear that our agricultural efforts so far provide more -energy for the soil population, for which it was not intended, than for -ourselves. - -The account is not complete; we have omitted all reference to the -oxidation of ammonia and of elements other than carbon. Nature -seems to be in an unexpectedly economical mood in the soil, and all -compounds which can be oxidised with liberation of energy seem to -have corresponding organisms capable of utilising them. Even phenol, -benzene, hydrogen, and marsh gas can all be oxidised and utilised as -energy sources by some of the soil population. - -Even with this remarkable power the soil population has insufficient -energy to satisfy all its possibilities; our present knowledge -indicates that energy supply is, in this country at any rate, the -factor limiting the numbers of the population. Increases in the water -supply or the temperature of the soil produce no consistent effect on -the population, but directly the energy supply is increased the numbers -at once rise. - - -MATERIAL CHANGES. - -These transformations of energy involve transformations of matter. -The original plant residues may be divided roughly into substances -forming the structure of the plant, such as the hemicelluloses, the -pentosans, gums, and the contents of the cell--the protoplasm and the -storage products, protein; in addition, there are smaller quantities of -fats and waxes and other constituents. Some of the easily-decomposable -carbohydrates never reach the soil at all, being broken down by -intracellular respiration or attack of micro-organisms. But much of the -structure material--hemicelluloses, pentosans, etc.--remains. - -Once the plant residues pass through the earthworm bodies they become -completely disintegrated and lose all signs of structure. - -The only visible product so far known is humus, the black sticky -substance characteristic of soil and of manure. Two modes of formation -have been suggested. Carbohydrates, sugars, pentosans, etc., are -known to yield furfuraldehyde or hydroxymethylfurfuraldehyde on -decomposition, and it has been shown at Rothamsted that this readily -condenses to form a humus-like body, if not humus itself. In the -laboratory the reaction is effected in presence of acid, but even -amino-acids suffice. All the necessary conditions occur in the soil, -and humus formation may proceed in this way. - -Some of the structure material--the lignin--contains aromatic ring -groupings. Fischer and Schrader have shown that in alkaline conditions -these ring substances absorb oxygen and form something very like humus. -It is quite possible that humus formation also proceeds in the soil -in this way. Whether the two products are chemically identical is not -known. - -The scheme can be represented thus:-- - - Cell structure material - | - +----------------+----------------+ - | | - Aliphatic Aromatic - (Hemicelluloses, (Lignin, etc., - Pentosans, etc.) in presence of - | oxygen and under - +-------+------------+ aerobic conditions) - | | | - Fatty acids Furfuraldehyde | - | or | - | Hydroxymethylfurfuraldehyde | - | (in presence of acid) | - | · | - | · | - | · | - | · | - | · | - | · | - | · | - | · | - | · | - ↓ ↘ ↓ - Calcium carbonate. Humus. - -The disintegration of the cell and the first stages in the -decomposition of the structure material are almost certainly brought -about by micro-organisms. Whether they complete the process is not -known: purely chemical agencies could easily account for part. - -The decomposition of protein in the soil has not been studied in any -detail. From what is known of the acid hydrolysis and the putrefactive -decompositions, however, it is not difficult to draw up a scheme which, -at any rate, accords with the facts at present known. It is probable -that the protein gives rise to amino-acids, which then break down by -one of the known general reactions. - -Two types of non-nitrogenous products may be expected: The aliphatic -amino-acids give rise to ammonia and fatty acids; these form -calcium salts which break down to calcium carbonate. The aromatic -amino-acids--tyrosin, phenylalanine, etc.--which would account for -about 6 per cent. of the nitrogen of vegetable proteins, would be -expected to give ammonia and phenolic substances. Now phenols are -poisonous to plants and if no method existed for their removal the -accumulation would ultimately render the soil sterile. Matters would be -even worse on cultivated soils, since cows’ urine, which enters into -the composition of farmyard manure and is the chief constituent of -liquid manure, contains, according to Mooser, no less than 0·25 to 0·77 -grams of _p_-cresol per litre,[I] a quantity three to ten times that -present in human urine. Fortunately this contingency never arises, for -the soil contains a remarkable set of organisms capable of decomposing -the phenols and leaving the soil entirely suitable for plant growth. -This affords an interesting case of an organism--in this case the -plant--growing well in a medium in spite of some adverse condition, not -because it is specially adapted to meet this condition, but because -some wholly different agent removes it. - - [I] Mooser, Zeitschrift physiol. Chem., 1909, lxiii., 176. No phenol - was found. It is possible that the _p_-cresol is not entirely derived - from the protein, but that some comes from the glucosides in the - animals’ food. - -Other ring compounds, e.g. pyrrol, arise in smaller quantity in the -decomposition of protein, but their fate in the soil is not known. - -We may summarise the probable changes of the protein as follows:-- - - Protein. - +-------------------+------------------+ - | | | - Aliphatic Aromatic Other - amino-acids amino-acids compounds - | | (Pyrrol, etc.) - +--------+--------+ +--------+--------+ - | | | | - Fatty acids and Ammonia Phenolic - hydroxy acids | compounds - | Nitrite | - | | | - | | | - ↓ ↓ ↓ - Calcium Nitrate CO₂ - carbonate - -It must be admitted that the evidence is indirect. The rate of -oxidation of ammonia by bacteria in the soil is more rapid than the -rate of formation, so that ammonia is practically never found in the -soil in more than minimal amounts (1 or 2 parts per 1,000,000); indeed, -the only evidence of its formation was for a long time the fact that -no compound other than ammonia could be oxidised by the nitrifying -organism. It has, however, since been shown at Rothamsted that ammonia -accumulates in soils in which the nitrifying organism has been killed. - -Nothing is known of the mechanism of the oxidation of ammonia beyond -the fact that it is biological; the reaction is not easily effected -chemically at ordinary temperatures. Possibly the organism assimilates -ammonia at one end of a chain of metabolic processes and excretes -nitrates at the other. Or, the reaction may be simply a straight -oxidation for energy purposes, the ammonia changing to hydroxylamine -and then to nitrous and nitric acids. - -The nitrate does not remain long in the soil. Some is taken up by the -plant and some is washed out from the soil. Part, however, either of -the nitrate itself or of one of its precursors is converted into an -insoluble form: probably it is changed into protein by the action of -micro-organisms; it then goes through the whole process once more. - -These are the general outlines; they present no particular chemical -difficulties. When we come to details, however, there is much that -cannot be understood. - -First of all, there is the slow rate at which complex nitrogen -compounds disappear from the soil in comparison with the rate of -oxidation of the carbon. Thus, in the original plant residues, there -is some forty times as much carbon as nitrogen: before they have been -long in the soil there is only ten times as much carbon as nitrogen; -this seems to be the stable position. What is the reason for this -preferential oxidation of the carbon? No explanation can yet be given. - -[Illustration: FIG. 24. - - X-axis: 1887-8 1890-1 1900-1 1910-11 - - Y-axis: ℔ per acre] - -An equally difficult problem arises in connection with the length of -time the process will continue. Decomposition of the nitrogen compounds -never seems to be complete in the soil; it dribbles on interminably. -In the year 1870 Lawes and Gilbert cut off a block of soil from its -surroundings and undermined it so that the drainage water could be -collected and analysed. The soil has been kept free from vegetation -or addition of nitrogen compounds from that time till now; yet it has -never failed to yield nitrates, and the annual yield falls off only -very slowly (Fig. 24). This same peculiarity is seen in the yield of -crops on unmanured land: it decreases, but very gradually; even after -eighty years the process is far from complete, and there is no sign -that it will ever come to an end. - -TABLE XVII.--APPROXIMATE LOSS OF NITROGEN FROM CULTIVATED SOILS: -BROADBALK WHEAT FIELD, ROTHAMSTED, FORTY-NINE YEARS (1865-1914.) - - +------------------------+---------------------+---------------------+ - | | Rich Soil: Plot 2. | Poor Soil: Plot 3. | - | | Lb. per Acre. | Lb. per Acre. | - +------------------------+---------------------+---------------------+ - |Nitrogen in soil in 1865|·175 per cent. = 4340|·105 per cent. = 2720| - |Nitrogen added in | | | - |manure, rain (5 lb. per | | | - |annum), and seed (2 lb. | | | - |per annum) | 10,140 | 340 | - +------------------------+---------------------+---------------------+ - |Nitrogen expected in | | | - |1914 | 14,480 | 3060 | - |Nitrogen found in 1914 |·259 per cent. = 5950|·095 per cent. = 2590| - +------------------------+---------------------+---------------------+ - |Loss from soil | 8530 | 470 | - |Nitrogen accounted for | | | - |in crops | 2500 | 750 | - +------------------------+---------------------+---------------------+ - |Balance, being dead loss| 6030 | -280[J] | - |Annual dead loss | 123 | - 6[J] | - +------------------------+---------------------+---------------------+ - - [J] Gains. Possibly the result of bacterial action. - -A further remarkable fact connected with the decomposition of the -nitrogen compounds is that it seems invariably to be accompanied by -an evolution of gaseous nitrogen. Apparently there are two cases. -Under anaerobic conditions many of the soil organisms have the power -of obtaining their necessary oxygen from nitrates, thereby causing -a change in the molecule which leads in some cases to liberation of -gaseous nitrogen; but the same result seems to be attained in aerobic -conditions, especially when carbon is being rapidly oxidised. - -It is possible that the reaction is the same, and that in spite of the -general aerobic conditions there is locally an anaerobic atmosphere. -But it is also possible that some direct oxidation of protein or -amino-acids may yield gaseous nitrogen. However it is brought about -it affects a considerable proportion of the entire stock of nitrogen, -and it becomes more serious as cultivation is intensified. Thus, on -the Broadbalk plot receiving farmyard manure the loss is particularly -heavy; on the unmanured plot it cannot be detected. The nitrogen -balance-sheet is shown in Table XVII. - -The oxidation of carbonaceous matter, however, is not invariably -accompanied by a net loss of nitrogen; in other circumstances there is -a net gain. In natural conditions there seems always to have been some -leguminous vegetation growing; the gain may, therefore, be ascribed -to the activity of the nodule organism. In pot experiments, however, -it has been found possible, by adding sugar to the soil, to obtain -gains of nitrogen where there is no leguminous vegetation, and this is -attributed to the activity of Azotobacter. - -The nitrogen cycle as observed in the soil is as follows:-- - - +--------------→ Protein ←----------------+ - | · · | - | · · | - | · · | - By certain| · · | - organisms | | | |By Azotobacter, - and by | ↓ | |Clostridium, - growing | Ammonia | Mechanism |nodule organisms, - plants | | | uncertain |etc. - | ↓ | | - | Nitrite | | - | | ↓ | - | ↓ Gaseous | - +------- Nitrate ------→ Nitrogen --------+ - - By denitrifying organisms - -There has been but little study of the process of decomposition of the -other compounds in plants. Part, if not all, of the sulphur is known -to appear as sulphate, and some of the phosphorus as phosphate. It is -certain that the plant constituents decompose, for there is no sign of -their accumulation in the soil. They may exert transitory effects, but -there is nothing to show permanent continuance. The toxic conditions -which cause trouble in working with pure cultures of organisms in -specific cultures media do not, so far as is known, arise in the soil. -All attempts to find bacterio-toxins or plant toxins in normal soils -have failed. The product toxic to one organism seems to be a useful -nutrient to another, and so the mixed population keeps the soil healthy -for all its members. - -There is little precise knowledge as to the part played by the -different members of the soil population in bringing about these -changes. - -We know in a general way that earthworms effect the distribution of -the plant residues in the soil, and serve to disintegrate them; there -is no evidence, however, that they play any indispensable part in the -decomposition. Many root and other fragments do not go through this -process; observation shows that fungi can force a way in, and they may -be followed by nematodes which continue the disintegration. Possibly -some of the flagellates help, and certainly the bacteria do. After that -nothing is certain. We cannot, with certainty, assign any particular -reaction in the decomposition to any specific organism, with the -exception of the oxidation of the phenolic substances, the conversion -of ammonia to nitrite and nitrate, and the fixation of nitrogen. With -these exceptions many organisms seem capable of bringing about the -reactions, and indeed some of the reactions may be purely chemical and -independent of biological agencies. - -The relationships between the soil population and soil fertility are -readily stated in general outline, but they are by no means clear cut -when one comes to details; fertility is a complex property, and some -of its factors are independent of soil micro-organisms. - -The general relationship between plants and soil organisms is one of -complete mutual interdependence. The growing plant fixes the sun’s -energy and converts it into a form utilisable by the soil organisms; -without the plant they could not exist. The plant is equally dependent -on the soil organisms in at least two directions: their scavenging -action removes the dead vegetation which would, if accumulated on the -surface of the soil, effectively prevent most plants from growing. -Further, the plant is dependent on the soil population for supplies -of nitrates. Nothing is known about the relative efficiencies of the -various soil organisms as scavengers. Numerous fungi and bacteria are -effective producers of ammonia, the precursor of nitrates; it is not -known, however, whether flagellates and such higher forms as nematodes -act in this way. - -This widespread power of producing ammonia makes it impossible in our -present knowledge to regard any particular group of organisms as _par -excellence_ promoters of fertility. Indeed, it is safest not to attempt -to do so. The primary purpose of the activities of a soil organism -is to obtain energy and cell material for itself; any benefit to the -plant is purely incidental. For cell material it must have nitrogen -and phosphorus; here it competes with the plant. If it produces more -ammonia than it utilises--in other words, if it is driven to nitrogen -compounds for its energy, then the plant benefits. If, on the other -hand, it absorbs more ammonia than it produces, as happens when it -derives its energy from non-nitrogenous substances, the plant suffers. -Thus, addition of peptone to the soil or an increase in bacterial -numbers effected without addition of external energy (e.g. by partial -sterilisation) leads to increased ammonia supply, and, therefore, -to increased fertility. But addition of sugar to the soil causes so -great an increase of numbers of bacteria and other organisms that -considerable absorption of ammonia and nitrate occurs, and fertility -is for a time depressed. - -Both actions proceed in soils partially sterilised by organic -substances, such as phenol, which are utilised by some of the soil -organisms; there is first a great rise in numbers of these particular -organisms with a depression of ammonia and nitrate, then a drop to the -new level, higher than the old one, and an increased production of -ammonia and nitrate resulting from the partial sterilisation effects. - -We must then regard the soil population as concerned entirely to -maintain itself, and only incidently benefiting the plant, sometimes, -indeed, injuring it; always essential, yet always taking its toll, and -sometimes a heavy toll, of the plant nutrients it produces. - -This effect makes it difficult to deduce simple quantitative -relationships between bacterial activity and soil fertility, and the -difficulty is increased by the fact that bacteria and plants may both -be injured or benefited by the same causes, so that high bacterial -numbers in a fertile soil would not necessarily be the cause, but might -be simply the result of fertility. - -The circumstance that certain soil organisms--bacteria, algæ, and -fungi--themselves assimilate ammonia and nitrate may account for -the remarkable slowness of nitrate accumulation, to which reference -has already been made. The protein formed from the assimilated -nitrogen remains in the bodies of the organisms, living or dead, till -decomposition sets in. It is not difficult to picture a cycle of events -in which much of the nitrate formed is at once reabsorbed by other -organisms, and only little is actually thrown off into the soil. Such a -process might continue almost interminably so long as any carbonaceous -material remained. - -Finally, we come to the very interesting problem--is it possible to -control the population of the soil? - -The problem may seem superfluous in view of the difficulties just -mentioned. Some aspects of it, however, are fairly clearly defined. - -In the first instance, some organisms appear to be wholly harmful to -the plant; among them are parasitic eelworms and fungi, and bacteria -causing disease. - -Control of these organisms can be brought about by partial -sterilisation, and of all methods heat is the most effective, but it -is costly, and attempts are now being made to replace it by chemical -treatment. The results are promising, but the investigation is -laborious; the organisms show specific relationships, and in finding -a sufficiently potent and convenient poison it is necessary in each -case to make an investigation into the relationship between chemical -constitution and toxicity to the particular organism concerned. -Formaldehyde is usually potent against fungi, and the cresols, and -particularly their chlor- and chloronitro-derivatives, are potent -against animals (eelworms, etc.). - -One group of organisms is wholly beneficial, those associated with -leguminous plants. Attempts have been made to increase their activities -by inoculating the soil with more vigorous strains. The practical -difficulties still remain very considerable, but there is hope that -they may be overcome. - -It is also possible to shift the balance of the soil population in -certain directions. Special groups of soil organisms can be caused -to multiply temporarily, if not permanently, by satisfying their -particular requirements. Thus, when a soil has been heated above 100° -C. it becomes specially suited to the growth of fungi, and quite -unsuited to certain bacteria such as the nitrifying organisms and -others; if this heated soil is infected with a normal soil population -the fungi develop to a remarkable extent. The nodule organisms appear -to be stimulated by addition of farmyard manure and of phosphates, and -the phenol-destroying organisms by successive small additions of phenol. - -Finally, quite apart from the control of disease organisms, it is -possible to alter the soil population considerably by partial -sterilisation, using a temperature of only about 60° C., or a poison -like toluene that favours few of the soil organisms. This problem has -already been discussed in Chapter I. - -The control of the soil population is still only in its infancy, but -it already promises useful developments. It cannot, however, be too -strongly insisted that the only sure basis of control is knowledge, and -we cannot hope to push control further till we have learned much more -about the soil population than we know at present. - - - - -AUTHOR INDEX. - - - ADAMETZ, 118. - - Adams, 158. - - Aiyer, 113. - - Appel, 133. - - Artari, 107. - - Ashby, 42. - - - BARTHEL, 24. - - Beijerinck, 6, 37, 41, 42, 46, 107. - - Berthelot, 5, 6, 41. - - Bewley, 47, 51, 132. - - Bezssonoff, 69. - - Boas, 138. - - Bokorny, 138. - - Bonazzi, 45. - - Boresch, 107. - - Boussingault, 3. - - Bredemann, 24. - - Bristol, 106. - - Brizi, 113. - - Brown and Halversen, 127. - - Burgess, 41. - - Burrill, 48. - - Bussey, Peters and Ulrich, 132. - - Butkevitch, 138, 139. - - Butler, 132. - - - CAMERON, 150, 158. - - Chodat, 107. - - Christensen, 46. - - Clayton, 28, 43. - - Coleman, 127, 136. - - Conn, 23, 54, 61, 123. - - Cramer, 39. - - Crump, 57, 79, 80. - - Cunningham, 69. - - Cutler, 57, 58, 78, 80. - - - DALE, 118, 121. - - Darwin, 153. - - Dascewska, 134. - - De Bruyn, 132. - - van Delden, 42. - - Delf, 87. - - Doryland, 33, 40. - - Dox, 138. - - Dox and Neidig, 134. - - Drummond, 160. - - Duggar and Davis, 135. - - Duvaine, 20. - - - EHRENBERG, 138. - - Ehrlich and Jacobsen, 138. - - Esmarch, 102, 103. - - - FABRICIUS, 61. - - Feilitzen, 61. - - Fischer, 118, 126. - - Forte, 101. - - Frank, 132. - - Fritsch, 112. - - - GAINEY, 46. - - Gillespie and Hurst, 140. - - Goddard, 118, 120, 121. - - Golding, 49. - - Goodey, 68, 73, 79, 105. - - Greaves, 42, 61. - - Green, 37. - - Grintzesco, 107. - - Groenewege, 42. - - - HAGEM, 118, 121, 136, 138, 139. - - Hamilton, 158, 159. - - Hansen, 48. - - Hanzawa, 43. - - Harrison, 113. - - Heinze, 139. - - Hellriegel, 5, 6, 46. - - Hensen, 100. - - Hesselmann, 36. - - Hill, 94. - - Hiltner, 23. - - Hopkins, 36. - - Hutchinson, C. M., 42. - - Hutchinson, H. B., 27, 43, 47, 51, 57, 105. - - - VAN ITERSON, 133. - - - JENSEN, 118, 132. - - Jewson, 123. - - Joffe, 37. - - Jones, D. H. and Murdock, 127. - - Jones, L. R., 140. - - - KAPPEN, 136. - - Karrer, 105. - - Kaserer, 27. - - Klöcker, 134. - - Koch, A., 44, 134. - - Koch, R., 20, 53. - - Kofoid, 88. - - Kohshi, 134. - - Kopeloff, 118, 136. - - Kossowitsch, 111. - - Krainskii, 44. - - Krzeminiewski, 43. - - Kufferath, 107. - - - LATHAM, 135. - - Laurent, 136. - - Lawes and Gilbert, 5. - - Lebedeff, 27. - - Leeuwenhoeck, 20. - - Lendner, 118. - - Lipman, C. B., 41, 42, 44, 54. - - Lipman, J. G., Blair, Owen, and McLean, 94. - - Löhnis, 22, 43, 69, 136. - - - MAGNUS, 107. - - Malpighi, 46. - - Marchal, 34, 136. - - Martin, 73. - - Martin and Lewin, 69. - - McBeth, 28, 134. - - McBeth and Scales, 118, 140. - - McLean and Wilson, 118, 136. - - Mockeridge, 43. - - Moore, G. T., 105. - - Morris, 150, 151, 162. - - Muntz and Coudon, 118, 136. - - - NABOKICH, 27. - - Nagaoka, 38. - - Nakano, 107. - - Nasir, 94, 95. - - Neller, 137. - - - OMELIANSKI, 27, 42. - - Orla-Jensen, 26, 35. - - Otto, 133. - - Oudemans and Koning, 118. - - - PASTEUR, 3, 20. - - Perey, 94. - - Perotti, 136. - - Petersen, 104. - - Pillai, 43. - - Potter and Snyder, 137, 138. - - Povah, 138. - - Pratt, 132. - - Prescott, 61. - - Pringsheim, 107. - - - RAMANN, 118. - - Rathbun, 120. - - Reh, 162. - - Remy, 118. - - Richards, 112. - - Richardson, 159. - - Ritter, 138. - - Robbins, W. J., 109. - - Robbins, W. W., 105. - - Roussy, 134. - - Russell, 112. - - Russell and Hutchinson, 57, 66, 94. - - - SALUNSKOV, 42. - - Sandon, 57, 75. - - Scales, 28, 134. - - Schellenberg, 133. - - Schindler, 107. - - Schloesing, 3, 4, 34. - - Schmitz, 134. - - Schramm, 111. - - Servettaz, 109. - - Seydel, 44. - - Sherman, 69. - - Shibata, 136. - - Söhngen, 26, 27, 134. - - - TAKAHASHI, 118. - - Taylor, 120. - - Ternetz, 135. - - Treub, 112. - - Truffaut, 69. - - - VERKADE and Söhngen, 134. - - Von Ubisch, 109. - - - WAKSMAN, 37, 118, 120, 121, 123, 125, 126, 134, 136. - - Waksman and Cook, 136. - - Wann, 111. - - Warington, 4, 34. - - Waynick, 42. - - Welwitsch, 112. - - Werkenthin, 120, 121. - - West, 88, 105. - - Whiting, 36. - - Wilfarth, 46. - - Winogradsky, 4, 6, 34, 41, 44. - - - - -SUBJECT INDEX. - - - _Absidia_, 121. - - _Acarina_, 150, 151, 157. - - Acid formation by Fungi, 139. - - Acidity of soil, 17; effect on Actinomyces, 140; relation to - nitrification, 36. - - _Actinomycetes_, 119, 134, 139. - - Aeration of soil, effect on bacteria of, 61. - - _Agriotes_, 150. - - Air supply in soil, 17. - - Algæ, agents causing disappearance of nitrate from soil, 12; - associations of, in soil, 105, 106; blue green, 102 _sqq._ (see also - Cyanophyceæ and Myxophyceæ); colonisation of new ground by, 112; - conditions of growth for, 101, 104, 107, 108; distribution of, 102, - 104, 106, 109; economic significance of, 100, 102; filamentous, - 106; flora of soil, 101, 112; formation of humus substances, 112; - fragmentation of filaments, 107, 110; frequency of occurrence, 102 - _sqq._; glucose, effect of, on growth, 108, 109; green, 104 _sqq._ - (see also Chlorophyceæ); importance in cultivation of rice, 113; - numbers in soil of, 109, 110; nutrition of, 107, 108, 110; producers, - of organic substance, 100; pure cultures of, 107, 111; relation to - gaseous interchange in soil, 113; relation to soil moisture, 112; - seasonal changes in numbers of, 88; subterranean, 105. - - Alkaloids, as source of nitrogen for fungi, 138. - - _Alternaria_, 119. - - Amino-acids, formation of, by algæ, 108. - - Amino-compounds, decomposition of, by fungi, 136, 138. - - Ammonia, assimilation of, by bacteria, 33, 40, 45; effect of partial - sterilisation on soil content of, 66; formation in soil, 170; - formation in soil by bacteria, 32 _sqq._; formation in soil by fungi, - 135 _sqq._, 141; influence of physical conditions on formation of, - 137; property of attracting Diptera, 159; utilisation by higher - plants, 36. - - Ammonium sulphate, effect on fungi, 121, 126, 127. - - _Anabæna_, 102, 112. - - _Annelida_, 149. - - Antagonism of salts in soil, 60. - - Ants, 153. - - _Arachnida_, 150, 151. - - Arctic soil, bacterial flora of, 24. - - _Areinida_, 150, 151, 157. - - _Armillaria_, 132. - - _Ascomycetes_, 119. - - _Aspergillaceæ_, 136. - - _Aspergillus_, 119, 120, 135, 136, 138, 139. - - Azotobacter, 6, 41, 95, 96; assimilation of nitrates by, 45; - decreasing efficiency in liquid culture, 44; indicator of soil - acidity, 44. - - - BACILLARIACEÆ, 100 (see also Diatom). - - _Bacillus amylobacter_, distribution of, 24. - - _Bacillus radicicola_, 24, 46 _sqq._; inoculation of soil with, 50; - life cycle of, 47. - - Bacteria, association with algæ in nitrogen fixation, 111; anærobic - respiration of, 37; effect of arsenic on, 61; cellulose destroying, - 134; changes in morphology in culture, 22, 47; classification of main - groups, 23, 25; composition of cells of, 39; inverse relationship - with protozoa, 10, 79, 82 _sqq._; isolation from soil, 21; methods - of describing, 21; method of estimating numbers of, 53 _sqq._, 80; - nitrogen fixation by, 110, 111; numbers in relation to algæ, 110; - numbers in soil, 52 _sqq._; oxidation of hydrogen by, 27, 37; effect - of partial sterilisation on, 8, 9, 66, 67; part played in soil - fertility by, 7; pure cultures, isolation by plating, 20; seasonal - changes in numbers of, 59, 87 _sqq._; effect of salts on, 60; short - time changes in numbers of, 11, 57, 58; effect of temperature on, 67; - uneven distribution of, 57. - - _Basidiomycetes_, 119, 123, 132. - - Beets, attacked by _Phoma betæ_, 135. - - _Boletus_, 132. - - _Botrytis_, 122. - - Bryophyta, 100, 132. - - _Bumilleria_, 105. - - - CALCIUM compounds in soil and fungi, 139. - - _Carabidæ_, 150. - - Carbohydrates, decomposition by bacteria, 26 _sqq._; decomposition by - fungi, 140; decomposition in soil, 168; effect on ammonia production - in soil, 33; presence in algal sheath and bacteria, 111. - - Carbon, changes in amount in soil, 167; relationships of bacteria, - 27; relationships of fungi, 133; source of, for soil bacteria, 39; - sources of, for soil fungi, 139. - - Carbon dioxide, assimilation by algæ, 99, 107, 108; assimilation by - soil bacteria, 35, 36, 40. - - Carotin, in algæ, 100; formed by _Spirochæta cytophaga_, 29. - - _Cecidomyidæ_, 155. - - Cellulose, decomposition by bacteria, 27, _sqq._; decomposition by - fungi, 133, 134, 141; relation of nitrogen supply to decomposition - of, 30; decomposition in soil, 168; as source of energy for nitrogen - fixation, 43. - - Centipedes, see _Chilopoda_. - - _Cephalosporium_, 120. - - _Cephalothecium_, 136. - - _Chilopoda_, 157. - - _Chironomidæ_, 155. - - _Chlorella_, 108. - - _Chlorococcum_, 105. - - _Chlorophyceæ_, 100. - - Chlorophyll, loss of, from algæ, 108. - - Ciliates, classification of, 72; cyst wall of, 73. - - Citric acid, formation of, by fungi, 139. - - _Cladosporium_, 119. - - Clamp connections in fungi, 119. - - Classification, of algæ, 100; of bacteria, 23, 25; of fungi, 131; of - protozoa, 69 _sqq._ - - Climate, effect of, on algæ, 101. - - _Clostridium_, 41, 44; as fixer of nitrogen, 6. - - _Coccomyxa_, 104. - - _Coleoptera_, 150, 154, 155. - - _Collembola_, 150, 153, 154. - - _Colletotrichum_, 131. - - Commensals, 132. - - _Conjugatæ_, 100. - - _Cortinarius_, 132. - - Cotton, destroyed by fungi, 134. - - Counting, of algæ, 109; of bacteria, 53 _sqq._; of fungi, 122; of - protozoa, 77, 79, 80. - - Cresol, decomposition of, by bacteria, 22, 24, 31. - - Criteria, physiological, of fungi, 128. - - Crop growth, effect on fungi, 122. - - _Cryptomonadineæ_, 100. - - Cucumber leaf spot, 131. - - Cyanamide, decomposition of, by fungi, 136. - - _Cyanophyceæ_, 103 (see also _Myxophyceæ_ and blue-green algæ). - - _Cylindrospermum_, 102. - - Cysts, 68, 73, 74. - - - DENITRIFICATION, by bacteria, 37; by fungi, 136. - - Desiccation, resistance to, by algæ, 106. - - Dew, relation to algæ, 101, 113. - - Diatoms, 104 _sqq._ (see also _Bacillariaceæ_). - - Dicyanamide, decomposition of, by fungi, 136. - - Dipeptides, formation of, by algæ, 108. - - _Diplopoda_, 157. - - _Diptera_, 150, 154, 155, 159. - - Disaccharides and fungi, 134. - - - EARTHWORMS, abundance of, in soil, 153; effect of, in soil, 13, 160, - 175. - - Eel-worms, 149 (see also _Nematoda_). - - _Elaphomyces_, 132. - - _Enchytræidæ_, 149. - - Energy, laws of, 165; relationships of soils, 166; requirements of - soil organisms, 15, 16. - - Energy supply, relation of bacterial activities to, 25 _sqq._, 40, - 44; sources of, for soil bacteria, 26 _sqq._, 40, 43; supplies of, - for soil organisms, 111, 164, 167, 168. - - Environmental conditions in soil, 16. - - Eremacausis, 2. - - Ericales, 132, 135. - - _Euglena_, 99. - - _Euglenaceæ_, 100. - - Experimental error, in bacterial counts, 54; in fungal counts, 124. - - - FARMYARD manure, see Manure. - - Fats, used by fungi, 134. - - Fatty acids used by fungi, 134. - - Fertility of soil, views on, 2; effect of decomposition of plant - residues on, 1, 165; effect of organisms on, 175. - - Filter paper, destruction of, by fungi, 133; destruction of, by - _Spirochæta cytophaga_, 28. - - Fixation of nitrogen, discovery of, by Berthelot, 5; by bacteria, 40 - _sqq._; by algæ, 110, 111; by mixtures of bacteria and algæ, 111; by - fungi, 135 _sqq._ (see also Nitrogen Fixation). - - _Flagellatæ_, 100. - - Flax sickness and fungi, 122. - - Formaldehyde, as agent for destroying fungi, 141. - - Fungi, control of, in soil, 139 _sqq._; counting of, 122; - distribution of, in soil, 119 _sqq._, 127; fertilisers, effect of, on - numbers of in soil, 126; as facultative parasites, 131, 132; fruiting - bodies of, 123; destruction of hemicelluloses by, 133; individual, - 122, 123; action on monosaccharides of, 134; mineral relationships - of, 139; mycorrhizal, 132, 135, 139, 140; heterocyclic nitrogen - compounds and, 138; occurrence in soil, 118; qualitative study of, - 118; selective feeding of, 140; specific determination of, 119. - - _Fungi imperfecti_, 119. - - _Fusaria_, 134. - - _Fusarium_, 119, 120, 122, 128, 133, 136. - - - _Gamascidæ_, 156. - - Gases of swamp water (Paddy soils), 113. - - _Gastrodia_, 132. - - Gelatinous envelope of algæ, 109, 111. - - Geographical distribution of azotobacter, 41; of soil bacteria, 24; - of protozoa, 75, 76; of soil fungi, 119, 125. - - Germination, of algal spores, 107. - - Glucose, use of, by algae, 108, 109, 111; use of, by moss protonema, - 109. - - Glycocoll, formation of, by algæ, 108. - - _Granulobacter_, 42. - - Greenland, bacteria in soil from, 24. - - “Grunlandmoor,” fungi in, 126. - - - _Hantzschia_, 105. - - _Hemiptera_, 154. - - _Heterokontæ_, 100. - - “Hochmoor,” fungi in, 126. - - _Hormidium_, 104. - - Humus, the food of plants, 1; formation of, by fungi, 134, 141; - formation of, in soil, 168; forest, 132; fungal hyphæ as constituent - of forest humus, 132. - - Hydrogen ion concentration, in soil, 17; effect on fungi of, 124. - - _Hymenoptera_, 150, 154. - - - _Insecta_, 150, 157. - - Insects, numbers present in soil, 154. - - Invertebrata, definition of, 147; method of investigating, 148; - groups represented, 149; distribution in the soil, 151; dominant - species and groups, 153; environmental factors of, 157; feeding - habits, 156; relation to agriculture, 160; relation to nitrogen - cycle, 161. - - Iron compounds, oxidation by fungi, 139. - - _Isopoda_, 150, 151. - - - _Leguminosæ_, association with bacteria, 46 _sqq._; enrichment of - ground by, 5. - - _Lepidoptera_, 150, 154. - - Life cycles, of bacteria, 22, 47; of protozoa, 72 _sqq._ - - Lime, effect on fungi in soil, 121, 126. - - _Lyngbya_, 112. - - - MAGNESIUM compounds, effect on fungi, 139. - - Manganese compounds, effect on bacteria, 61. - - Manure, farmyard, effect on algæ, 109, 110; effect on numbers of - bacteria, 60; effect on numbers of fungi, 126; effect on numbers of - insects, 154, 155. - - Manure, Artificial, effect on fungi, 127. - - Manure, town stable, occurrence of disease organisms in, 132. - - _Mastigophora_, classification of, 71; species of, 71. - - Media, containing nitrates, chemical analysis of, 111; for counting - soil bacteria, 54; for counting protozoa, 79; for counting fungi, - 119, 123. - - _Melanconium_, 134. - - _Melolontha_, 150. - - Methane, oxidation of, by bacteria, 26, 27. - - Millipedes, see _Diplopoda_. - - Mites, see _Acarina_. - - _Mollusca_, 149, 157. - - _Moniliaceæ_, 136. - - _Mucor_, 120, 121, 136, 138. - - _Mucorales_, 121, 134. - - _Mucorineæ_, 118. - - _Mycetophilidæ_, 155. - - Mycorrhiza, 132, 135, 139, 140. - - _Myriapoda_, 150, 156. - - _Myxophyceæ_, 100 (see also _Cyanophyceæ_ and blue-green algæ). - - - NAPHTHALENE, decomposition of, by bacteria, 31. - - _Naviculoideæ_, 100. - - _Nematoda_, 149, 151, 157. - - Nitrate, assimilation by algæ, 105, 108, 111; assimilation by - bacteria, 33, 40, 44, 51; assimilation by fungi, 136, 138; removal - from soil, 12, 112, 171; variations in amount in soil, 11. - - Nitre-beds, 1. - - Nitrification, and bacteria, 34; chemical changes in, 171; and fungi, - 136; energy supply in, 35; mechanism of, 1, 3; and soil fertility, 1, - 3. - - Nitrites and fungi, 136; formation by bacteria, 34. - - _Nitrobacter_, 35. - - Nitrogen, changes in amount in soil, 167; cycle in soil, 161; - fixation by bacteria, 6, 40 _sqq._; fixation by fungi, 135, 136, 141; - fixation of, in clover plant, 5; increase by protozoa of fixation - of, 94, 95 (fig.); fixation sources of energy for, 43, 49; gain of, - in soil, 174; in invertebrates, 162; loss of, by leaching, 112; - loss of, from cultivated soils, 173; relationships of fungi, 135; - relationships of algæ, 110-112; relationships of bacteria, 32 _sqq._, - 40 _sqq._; relationships of insects, 162. - - _Nitrosococcus_, 35. - - _Nitrosomonas_, 35. - - Nodule Organism of the Leguminosæ, 6, 46 _sqq._ - - _Nostocaceæ_, 100, 101, 102, 107. - - - _Oligochæta_, 149, 151, 153, 157. - - _Oospora_, 120. - - _Orcheomyces_, 132. - - Orchid cultivation and fungi, 132, 140. - - _Orthoptera_, 154. - - _Oscillatoriaceæ_, 100, 102. - - Osmotic pressure, influencing effect of salts on bacteria, 50. - - Oxalic acid, formation of, by fungi, 139. - - Oxidations effected by soil organisms; by bacteria, 26 _et seq._; by - fungi, 139. - - Oxygen, absorption by soils, 4. - - - PARTIAL sterilisation of soil, 8, 66 _sqq._, 96, 178; influence of - organic antiseptics, 177; limiting factor in, 67, 68. - - Pectin, effect of, on fungi, 134. - - _Pedras negras_, 112. - - _Penicillia_, 134. - - Pentosans, effect of, on fungi, 134. - - Peptones, decomposition of, by fungi, 136, 138; source of nitrogen - for algæ, 108. - - Periodicity, of protozoa in soil, 90 _sqq._ (fig.), 92 (fig.), 93. - - Phenol, decomposition of, by bacteria, 24, 25, 31. - - Phenylalanine, formation of, by algæ, 108. - - _Phoma_, 132. - - _Phormidium_, 106. - - Phosphates, availability of, influenced by bacteria, 52; by fungi, - 139; effect on bacteria, 46, 51, 60. - - Photosynthesis, 99, 100, 107, 110, 113. - - Phycocyanin, 100. - - Physical conditions in soil, 16. - - Physiological criteria, of bacteria, 22; of fungi, 128. - - _Phycomycetes_, 119. - - _Phytophthora_, 132. - - Plant disease, and fungi, 139. - - Plant residues, decomposition of, in soil, 168; influence of soil - reaction on, 165; relation to soil fertility, 1, 165. - - Plasticity of fungi, 119. - - _Plectonema_, 106. - - Potassium salts, effect on bacteria, 60; influence of bacteria on the - availability of, 52. - - Protein, decomposition of, in soil, 169; decomposition by bacteria, - 32; decomposition by fungi, 138, 140. - - _Protococcales_, 100. - - _Protoderma viride_, 105. - - Protonema of mosses, 100, 105, 106, 109. - - Protophyta, chlorophyll-bearing, 100. - - Protozoa, inoculation into soil of, 85 _sqq._; isolation from soil, - 69; classification of, 69 _sqq._; life histories of, 72 _sqq._; - species of, in soil, 70 _sqq._; distribution of, in soil, 74 _sqq._; - retention of, by soil, 78 (fig.); size of, 90; reproductive rates, - 93; inverse relation with bacteria, 79 _sqq._; presence of trophic - forms in soil, 9; numbers of, in soil, 90, 96, 97; fluctuations in - numbers of, 10, 81 (fig.), 82; external conditions, effect on, 82; - seasonal changes, effect on, 87 _sqq._; weight of, 90. - - _Pteridophyta_, 132. - - _Pythium_, 132. - - - REACTION of soil, 17. - - Reaction of soil, effect on bacteria, 36, 37, 46, 48, 61; effect on - protozoa, 93, 94 (see also hydrogen ion concentration). - - Relationships of Fungi, commensal, 132; mycorrhizal, 132; symbiotic, - 132. - - _Rhizopoda_; classification of, 70, 71; species of, 70, 71. - - Rhythm, supposed in ammonification by fungi, 137. - - _Rhizoctonia_, 132. - - _Rhizopus_, 119, 120. - - Rice plant, aeration of roots, 113; physiological disease of, 113. - - Rock Phosphate as base for nitrifying organisms, 36. - - Rothamsted, Broadbalk plot 2 (Farmyard Manure) algæ, 109; fungi, 125, - 127; Insects, 152. - - Rothamsted, Broadbalk plot 3 (Unmanured) algæ, 109; fungi, 120, 122, - 127; Insects, 152. - - Rothamsted, Broadbalk Plots 10, 11, and 13; 122, 127. - - Rothamsted, Barnfield Plot 1-0 (Farmyard Manure), Protozoa, 80. - - Rothamsted, unmanured grass plot, 120. - - _Russula_, 132. - - Rusts, 119. - - - _Saccharomyces_, 120. - - Saprophytes, facultative, 131. - - Saprophytism and algæ, 108, 110. - - _Scenedesmus_, 108. - - Seasonal fluctuations in numbers of soil organisms, 12, 87 _et seq._, - 125. - - Selective media, use of, in isolation of soil bacteria, 21. - - Serological tests, separation of varieties of _B. radicicola_ by, 48. - - Slugs, see _Mollusca_. - - Smuts, 119. - - Snails, see _Mollusca_. - - Soil; comparison of, by volume, 17; effect of depth below surface - on algæ, 101, 104, 109, 110, 113; effect of depth below surface on - insects, 151; effect of depth below surface on fungi, 121, 126, 127; - effect of various treatments on fungi, 126, 127, 132; environmental - factors in, 16; inoculation of, for leguminous plants, 50; moisture - (see Water supply); population, control of, 177 _sqq._; population, - methods of investigation, 10, 15; sterilisation and fungi, 137, 138, - 141 (see Partial Sterilisation); stored, survival of algæ in, 107; - type and fungi, 121, 126, 127. - - Soil conditions, effect on bacteria, 33, 36, 37, 40, 46, 48, 50, 59 - _sqq._; effect on protozoa, 82. - - Soil fertility, see Fertility of soil. - - _Spicaria_, 120. - - Spiders, see _Areinida_. - - _Spirochæta cytophaga_, 28, 43. - - Spore forming bacteria in soil, 23, 34. - - Spore, fungus, inhibition of formation, 123; presence in air of, 118. - - Standardisation of cultural methods for soil bacteria, 54 _sqq._ - - Starch, decomposition of, by fungi, 134. - - _Stichococcus_, 108. - - Straw; effect on nitrate production in soil, 33; manure, 29; rotting - of, 30. - - Sulphur oxidation, by bacteria, 37; by fungi, 139. - - Symbiosis, of Azotobacter with other organisms, 42, 43, see also - Mycorrhiza and Nodule organism. - - _Symphyla_, 150, 151, 157. - - _Symploca_, 112. - - - _Tachinidæ_, 150. - - Tannins, used by fungi, 134. - - Temperature of soil and fungi, 127, 140. - - Termites, 160. - - _Testacella_, 149. - - _Thiospirillum_, 37. - - _Thysanura_, 154. - - _Thysanoptera_, 154. - - _Tipula_, 150. - - Toluene, decomposition by soil bacteria, 31. - - _Tolypothrix_, 112. - - _Trichoderma_, 119, 120, 122, 134. - - _Trochiscia_, 105. - - Tropisms, 157. - - - _Ulothrix_, 105. - - _Ulotrichales_, 100. - - Urea, by fungi, 136, 138. - - Uric acid, utilisation of, by fungi, 138. - - - _Vaucheria_, 104, 106. - - Vitality, retention of, by algæ and moss protonema, 105, 107. - - - WATER; supply in soil, 17; and algæ, 112; bacteria, 50, 61, 82; - fungi, 127; protozoa, 82. - - Wireworms, 155. - - Wood, decay of, 134. - - Woodlice, 150; (see also _Isopoda_). - - - YEASTS, 138. - - - _Zygnema_, 104. - - _Zygorrhynchus mœlleri_, 119, 120, 121. - - -PRINTED IN GREAT BRITAIN BY THE UNIVERSITY PRESS, ABERDEEN - - - - - Transcriber’s Notes - - - Inconsistent and archaic or unusual spelling, capitalisation, - italicisation, hyphenation, etc. have been retained, unless mentioned - below. The names and classifications of the organisms as used in the - book do not always conform to modern names and classifications; these - have not been changed. - - Depending on the hard- and software used and their settings, not all - elements may display as intended. - - Page 14, table, lower right hand cell: the data given add up to 8, - not to 9. - - Page 47, ·9 × ·18 in size: the source document does not include the - units; presumably the sizes are in microns. - - Page 118, endnote 8c (2×): this note does not exist. - - Subject Index, entry Zygorrhynchus mœlleri: also refers to - Zygorrhynchus vuilleminii. - - - Changes made: - - Footnotes, tables and illustrations have been moved out of text - paragraphs; some tables have been split or re-arranged. - - Several minor obvious typographical, punctuation and spelling errors - (including accents) have been corrected silently. In several cases - spelling differences (mainly of proper names) between the text and - the index and endnotes have been standardised. In the indexes and in - tables some ditto marks have been replaced with the dittoed - text. Some page references have been corrected to indicate the - correct page number. - - Indented text under illustrations is not present as such in the - source document, but has been transcribed from the illustration for - legibility and ease of reference. Some tables have been re-arranged - or split to fit the available width. - - Page 28, 29: MacBeth changed to McBeth as elsewhere (in the Author - Index the entries MacBeth and McBeth have been merged). - - Page 32, formula: 30 changed to 3O. - - Page 58: From Barnfeild, ... changed to From Barnfield, .... - - Page 85: closing bracket deleted after ... Table VII. and Fig. 13. - - Page 90, Table VIII, column 5: 350·000 and 150·000 changed to 350,000 - and 150,000. - - Page 97: No creature lies or dies to itself, ... changed to No - creature lives or dies to itself, ... - - Page 104: Danske Aerofile Alghe changed to Danske Aërofile Alger. - - Page 114: Recherche sulla Malattia del Riso ... changed to Ricerche - sulla Malattia del Riso .... - - Page 115: ... sur de polymorphisme ... changed to ... sur le - polymorphisme .... - - Page 116: literature notes 38 (Robbins) and 48 (Schindler) changed to - 33 and 34 respectively. - - Page 120: Zygorrhynchus vuillemini changed to Zygorrhynchus - vuilleminii as elsewhere. - - Page 126: references to Waksman[24] and [24_e_] changed to [25] and - [25_e_]. - - Page 129: ... preparée de la pres de Russum ... changed to ... - préparée de la terre humeuse du Spanderswoud, près de Bussum .... - - Page 134: reference to Kohshi[24] changed to [34]. - - Page 143: Sämenbildung changed to Säurenbildung (entry 5); - Wurzelbranderregern im Baden changed to Wurzelbranderregern im Boden - (entry 11). - - Page 144: ... Umwandlung von Aminosamen in Oxysämen ... changed to - ... Umwandlung von Aminosäuren in Oxysäuren ....; ... Wirkungen der - Schimmelze ... changed to ... Wirkungen der Schimmelpilze ....; - Hydrogen-iron concentration changed to Hydrogen-ion concentration. - - Page 145: Ztschr. f. Garungs. Physiol. changed to Ztschr. f. - Gärungsphysiol. - - Page 146: einige Pilze gegen Hemizellulosen changed to einiger Pilze - gegen Hemicellulosen. - - Page 157: Such responses are known chemotropism ... changed to Such - responses are known as chemotropism .... - - Page 170: ... alphatic amino-acids ... changed to ... aliphatic - amino-acids .... - -*** END OF THE PROJECT GUTENBERG EBOOK THE MICRO-ORGANISMS OF THE -SOIL *** - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the -United States without permission and without paying copyright -royalties. 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width: .01em;} - td.dontwrap - {white-space: nowrap;} - td.highline25, - th.highline25 - {line-height: 2.5em;} - td.thinline - {line-height: .01em;} - th - {font-weight: normal; white-space: nowrap; text-align: center; vertical-align: bottom; padding-left: .25em; padding-right: .25em;} - th.w3 - {width: 3em;} - th.w4 - {width: 4em;} - th.w5 - {width: 5em;} - .tnbot - {border: dashed thin; margin: 1em 10%; padding: .5em 1.25em;} - .tnbot h2 - {font-size: 1em;} - .tnbot p - {text-indent: -1em; margin-left: 1em;} - .tnbox - {border: dashed thin; margin: 1em 20%; padding: 1em;} - ul.index - {list-style: none; margin: 1.5em 0;} - ul.index li - {text-align: justify; margin: 0; text-indent: -1em;} - .x-ebookmaker ul.index li {margin: 0; text-indent: 0;} - ul.index li.newletter - {margin-top: .75em;} - .underl - {text-decoration: underline;} - -</style> - -</head> -<body> -<p style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The micro-organisms of the soil, by Sir E. John Russell</p> -<div style='display:block; margin:1em 0'> -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online -at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The micro-organisms of the soil</p> -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Authors: Sir E. John Russell</p> -<p style='display:block; margin-top:0; margin-bottom:0; margin-left:2em;'>Members of the biological staff of The Rothamsted Experimental Station</p> -<p style='display:block; text-indent:0; margin:1em 0'>Release Date: August 2, 2022 [eBook #68670]</p> -<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: Charlene Taylor, Harry Lamé and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/Canadian Libraries)</p> -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE MICRO-ORGANISMS OF THE SOIL ***</div> - -<div class="tnbox"> - -<p class="noindent">Please see the <a href="#TN">Transcriber’s Notes</a> at the end of this text.</p> - -<p class="noindent blankbefore75">The cover image has been created for this text and is in the public domain.</p> - -</div><!--tnbox--> - -<div class="x-ebookmaker-drop"> - -<hr class="chap" /> - -<div class="container w30em"> -<img src="images/cover.jpg" alt="cover image" /> -</div> - -</div><!--scr only--> - -<hr class="chap x-ebookmaker-drop" /> - -<p class="center"><b><span class="fsize125"><i><span class="underl">THE ROTHAMSTED MONOGRAPHS ON<br /> -AGRICULTURAL SCIENCE</span></i></span><br /> -<span class="fsize70">EDITED BY</span><br /> -<span class="smcap">Sir</span> E. J. RUSSELL, D.Sc. (<span class="smcap">Lond.</span>), F.R.S.</b></p> - -<p class="center fsize175 highline8">THE MICRO-ORGANISMS OF THE SOIL</p> - -<hr class="chap x-ebookmaker-drop" /> - -<p class="center highline15"><span class="fsize125 gesp2">THE ROTHAMSTED MONOGRAPHS ON<br /> -AGRICULTURAL SCIENCE.</span><br /> -<span class="smcap">Edited by Sir</span> E. JOHN RUSSELL, D.Sc., F.R.S.</p> - -<hr class="sec" /> - -<p class="noindent blankbefore15">During the past ten years there have been marked developments -in knowledge of the relations between the soil and the -growing plant. The subject involves physical, biological, and -chemical considerations, and its ramifications are now so wide -that they cannot be satisfactorily dealt with in detail in any -one book. These monographs collectively cover the whole -ground. In “Soil Conditions and Plant Growth” the general -outlines are presented: in the monographs the various divisions -are fully and critically dealt with by the Heads of the Departments -concerned at Rothamsted. A homogeneous treatment -is thus secured that will, it is hoped, much facilitate the use of -the series.</p> - -<p class="hind02 blankbefore15 blankafter75">SOIL CONDITIONS AND PLANT GROWTH, Fourth Edition.<br /> -By <span class="smcap">Sir E. John Russell</span>, F.R.S. 16<i>s.</i> net.</p> - -<p class="hind02 highline15">The following volumes are in <span class="nowrap">preparation:—</span></p> - -<table class="monographs"> - -<tr> -<td class="book">MANURING OF GRASS-LANDS FOR HAY</td> -<td class="author">By <span class="smcap">Winifred E. Brenchley</span>, D.Sc., F.Z.S.</td> -</tr> - -<tr> -<td class="book">THE MICRO-ORGANISMS OF THE SOIL</td> -<td class="author">By Sir <span class="smcap">E. John Russell</span>, F.R.S., and Members of the Biological Staff of -the Rothamsted Experimental Station.</td> -</tr> - -<tr> -<td class="book">SOIL PHYSICS</td> -<td class="author">By <span class="smcap">B. A. Keen</span>, B.Sc.</td> -</tr> - -<tr> -<td class="book">SOIL PROTOZOA</td> -<td class="author">By <span class="smcap">D. W. Cutler</span>, M.A., and <span class="smcap">L. M. Crump</span>, M.Sc.</td> -</tr> - -<tr> -<td class="book">SOIL BACTERIA</td> -<td class="author">By <span class="smcap">H. G. Thornton</span>, M.A.</td> -</tr> - -<tr> -<td class="book">SOIL FUNGI AND ALGÆ</td> -<td class="author">By <span class="smcap">W. B. Brierley</span>, <span class="smcap">S. T. Jewson</span>, B.Sc., and -<span class="smcap">B. M. Roach</span> (Bristol), D.Sc.</td> -</tr> - -<tr> -<td class="book">CHEMICAL CHANGES IN THE SOIL</td> -<td class="author">By <span class="smcap">H. J. Page</span>, B.Sc.</td> -</tr> - -</table> - -<hr class="sec" /> - -<p class="center"><span class="fsize90">LONGMANS, GREEN AND CO.,</span><br /> -<span class="fsize70">LONDON, NEW YORK, TORONTO, BOMBAY, CALCUTTA, AND MADRAS.</span></p> - -<hr class="chap" /> - -<h1>THE MICRO-ORGANISMS<br /> -OF THE SOIL</h1> - -<p class="center"><span class="fsize60 highline15">BY</span><br /> -<span class="highline2"><span class="smcap">Sir</span> E. JOHN RUSSELL, F.R.S.</span><br /> -<span class="fsize60 highline15">AND</span><br /> -<span class="fsize90">MEMBERS OF THE BIOLOGICAL STAFF OF THE<br /> -ROTHAMSTED EXPERIMENTAL STATION</span></p> - -<p class="center highline8 fsize90"><i>WITH DIAGRAMS</i></p> - -<p class="center highline15"><span class="fsize125"><span class="gesp1">LONGMANS</span>, -<span class="gesp1">GREEN AND CO</span>.</span><br /> -39 PATERNOSTER ROW, LONDON, E.C. 4<br /> -<span class="fsize80">NEW YORK, TORONTO<br /> -BOMBAY, CALCUTTA and MADRAS</span><br /> -1923</p> - -<hr class="chap" /> - -<p class="center highline8 fsize80"><i>Made in Great Britain</i></p> - -<hr class="chap" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Pagev">[v]</span></p> - -<h2 class="nobreak">INTRODUCTION.</h2> - -</div><!--chapter--> - -<p>The purpose of this volume is to give the broad outlines -of our present knowledge of the relationships of the -population of living organisms in the soil to one another -and to the surface vegetation. It is shown that there is -a close relationship with vegetation, the soil population -being dependent almost entirely on the growing plant -for energy material, while the plant is equally dependent -on the activities of the soil population for removing the -residues of previous generations of plants and for the -continued production in the soil of simple materials, -such as nitrates, which are necessary to its growth. It -is also shown, however, that the soil population takes -toll of the plant nutrients and that some of its members -may directly injure the growing plant.</p> - -<p>The soil population is so complex that it manifestly -cannot be dealt with as a whole in any detail by any one -person, and at the same time it plays so important a part -in the soil economy that it must be seriously studied. -Team work therefore becomes indispensable, and fortunately -this has been rendered possible at Rothamsted.</p> - -<p>Each group of organisms is here dealt with by the -person primarily responsible for that particular section -of the work. The plan of the book has been carefully -discussed by all the authors, and the subject matter has -already been presented in a course of lectures given at -University College, London, under the auspices of the<span class="pagenum" id="Pagevi">[vi]</span> -Botanical Board of Studies of the London University. -The interest shown in these lectures leads us to hope -that the subject may appeal to a wider public, and above -all to some of the younger investigators in biological -science. They will find it bristling with big scientific -problems, and those who pursue it have the satisfaction, -which increases as the years pass by, of knowing that -their work is not only of interest to themselves, but of -great importance in ministering to the intellectual and -material needs of the whole community.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Pagevii">[vii]</span></p> - -<h2 class="nobreak">CONTENTS.</h2> - -</div><!--chapter--> - -<table class="toc"> - -<tr> -<th class="right fsize80">CHAP.</th> -<th> </th> -<th class="right fsize80">PAGE</th> -</tr> - -<tr> -<td rowspan="2" class="chapno">I.</td> -<td class="chaptit"><span class="smcap">Development of the Idea of a Soil Population</span></td> -<td class="pagno"><a href="#Page1">1</a></td> -</tr> - -<tr> -<td class="author">Sir <span class="smcap">E. John Russell</span>, F.R.S., Director.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">II.</td> -<td class="chaptit"><span class="smcap">Occurrence of Bacteria in Soil—Activities connected with the Acquirement of Energy</span></td> -<td class="pagno"><a href="#Page20">20</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">H. G. Thornton</span>, B.A., Head of the Department of Bacteriology.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">III.</td> -<td class="chaptit"><span class="smcap">Conditions affecting Bacterial Activities in the Soil—Activities connected with the -Intake of Protein Building Materials</span></td> -<td class="pagno"><a href="#Page39">39</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">H. G. Thornton</span>, B.A., Head of the Department of Bacteriology.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">IV.</td> -<td class="chaptit"><span class="smcap">Protozoa of the Soil, I.</span></td> -<td class="pagno"><a href="#Page66">66</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">D. W. Cutler</span>, M.A., Head of the Department of Protozoology.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">V.</td> -<td class="chaptit"><span class="smcap">Protozoa of the Soil, II.</span></td> -<td class="pagno"><a href="#Page77">77</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">D. W. Cutler</span>, M.A., Head of the Department of Protozoology.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">VI.</td> -<td class="chaptit"><span class="smcap">Soil Algæ</span></td> -<td class="pagno"><a href="#Page99">99</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">B. Muriel Bristol</span>, D.Sc., Algologist.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">VII.</td> -<td class="chaptit"><span class="smcap">Soil Fungi—The Occurrence of Fungi in the Soil</span></td> -<td class="pagno"><a href="#Page118">118</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">W. B. Brierley</span>, D.Sc., Head of the Department of Mycology.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">VIII.</td> -<td class="chaptit"><span class="smcap">Soil Fungi—The Life of Fungi in the Soil</span></td> -<td class="pagno"><a href="#Page131">131</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">W. B. Brierley</span>, D.Sc., Head of the Department of Mycology.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">IX.</td> -<td class="chaptit"><span class="smcap">The Invertebrate Fauna of the Soil (other than Protozoa)</span></td> -<td class="pagno"><a href="#Page147">147</a></td> -</tr> - -<tr> -<td class="author"><span class="smcap">A. D. Imms</span>, D.Sc., Head of the Department of Entomology.</td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="chapno">X.</td> -<td class="chaptit"><span class="smcap">The Chemical Activities of the Soil Population and their Relation to the Growing -Plant</span></td> -<td class="pagno"><a href="#Page164">164</a></td> -</tr> - -<tr> -<td class="author">Sir <span class="smcap">E. John Russell</span>, F.R.S., Director.</td> -<td> </td> -</tr> - -<tr> -<td> </td> -<td class="chaptit"><span class="smcap">Index</span></td> -<td class="pagno"><a href="#Page181">181</a></td> -</tr> - -</table> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page1">[1]</span></p> - -<h2 class="nobreak">CHAPTER I.<br /> -<span class="chaptitle">THE DEVELOPMENT OF THE IDEA OF A SOIL -POPULATION.</span></h2> - -</div><!--chapter--> - -<p>From the earliest times agriculturists have been familiar -with the idea that decomposition of vegetable and animal -matter takes place in the soil, and that the process is intimately -connected with soil fertility.</p> - -<p>By the middle of the nineteenth century three different -ways were known in which the decomposition occurred. -One had been since early times specially associated with soil -fertility, in that it gave rise to humus, the black sticky -substance in farmyard manure or in soil—which was supposed -up to 1840 to be the special food of plants. No good account -of the process or of the conditions in which it occurred is, -however, given by the older writers.</p> - -<p>A second resulted in the formation of nitrates. This -process became known as nitrification: it was described by -Georgius Agricola (1494-1555) in his book “De Re Metallica,” -and it was of great importance in the seventeenth and -eighteenth centuries, because it was used for the manufacture -of gunpowder in the great wars of that period. The conditions -for the making of successful nitre beds were so -thoroughly investigated that little fresh knowledge was -added to that of 1770<a id="FNanchor1" href="#Footnote1" class="fnanchor">[A]</a> until quite recently. This process, -however, was not usually associated with soil fertility, -although both Glauber (1656) and Mayow (1674) had insisted -on the connection.</p> - -<div class="footnote"> - -<p><a id="Footnote1" href="#FNanchor1" class="label">[A]</a> See the remarkable -collection of papers entitled “Instructions sur l’établissement -des nitrières,” publié par les Régisseurs-généraux des Poudres et -Salpêtre. Paris, 1777.</p> - -</div><!--footnote--> - -<p><span class="pagenum" id="Page2">[2]</span></p> - -<p>A third type of decomposition was brought into prominence -by Liebig in 1840.<a href="#Endnote1_7" class="fnanchor">[7]</a> -<a id="FNanchor2" href="#Footnote2" class="fnanchor">[B]</a> Reviewing the decomposition -of organic matter in the light of the newer chemistry, he -concluded that the process was a slow chemical oxidation, -to which he gave the name “Eremacausis.” He recognised -that humus was formed, but he regarded it only as an intermediate -product, and emphatically denied its importance in -soil fertility. The true fertility agents, in his view, were the -final products—CO<sub>2</sub>, potassium and other alkaline salts, -phosphates, silicates, etc. He went on to argue brilliantly -that instead of applying farmyard or similar manures to the -soil it was altogether quicker and better to apply these -mineral compounds obtained from other sources than to -wait for the slow process of liberation as the result of decomposition. -For some reason, difficult to understand, he overlooked -nitrification and the part that nitrates might play in -soil fertility. Lawes and Gilbert<a href="#Endnote1_6" class="fnanchor">[6]</a> were much attracted by -this new idea; they showed that it was incomplete because -it took no account of the necessity for supplying nitrogen -compounds to the crop. When ammonium salts were added -to Liebig’s ash constituents the resulting mixture had almost -as good a fertilising effect as farmyard manure. Lawes at -once saw the enormous practical importance of this discovery, -and set up a factory for the manufacture of artificial fertilisers. -He did not, however, follow it up more closely on -the scientific side.</p> - -<div class="footnote"> - -<p><a id="Footnote2" href="#FNanchor2" class="label">[B]</a> The numbers refer to the short bibliography on -<a href="#Page18">p. 18</a>.</p> - -</div><!--footnote--> - -<p>Both Lawes and Gilbert were in constant touch with the -idea of decomposition in the soil, and they attached so much -importance to nitrogen compounds in plant nutrition that it -is not easy to understand how they missed the connection -with nitrification. But they did so, and like other English -and German workers of the day, considered that plant roots -assimilated their nitrogen as ammonia. For the first ten -years of the history of Rothamsted only few experiments -with nitrates were made, and not till thirty-five years had -elapsed were they systematically studied.</p> - -<p><span class="pagenum" id="Page3">[3]</span></p> - -<p>It was by Boussingault<a href="#Endnote1_2" class="fnanchor">[2]</a> and in France that the connection -between nitrification and soil fertility was first recognised. -The news came to England, but it was not accepted, -although Way, one of the most brilliant agricultural chemists -of his time, showed that nitrates were formed in soils to -which nitrogenous fertilisers were added, and that they were -comparable in their fertiliser effects with ammonium salts.<a href="#Endnote1_12" class="fnanchor">[12]</a> -“The French chemists,” he wrote in 1856, “are going further, -several of them now advocating the view that it is in the -form of nitric acid that plants make use of compounds of -nitrogen. With this view I do not at present coincide, and -it is sufficient here to admit that nitric acid in the form of -nitrates has at least a very high value as manure.” Indeed, -Kuhlmann went so far as to argue that the nitrates found in -the soil were there reduced to ammonia before assimilation -by plants could take place. The water-culture work of the -plant physiologists of the ’sixties finally showed the correctness -of the French view.</p> - -<p>Even when the importance of nitrification was realised -its mechanism was not understood: some thought it was -chemical, some physical. Again the explanation came from -France. Pasteur in 1862 had expressed the view that nitrification -would probably be a biological action, since purely -chemical oxidation of organic matter was of very limited -occurrence. “Pénétrés de ces idées,” as Schloesing tells us, -he and Müntz in a memorable investigation cleared up the -whole problem, and in 1877 opened the way to a most -fruitful field of research.<a href="#Endnote1_10" class="fnanchor">[10]</a> The formal description is given -in his papers in the “Comptes Rendus,” but a more lively -account is given in his lectures before the <i>École d’application -des Manufacteurs de l’état</i>, which, though not printed, were -collected and issued in script by his distinguished son, and -a copy of this work is among the treasures of the Rothamsted -Library.</p> - -<p>He had been asked to study the purification of sewage, -and he and Müntz showed that it was bound up with nitrification. -The process was slow in starting, then it proceeded<span class="pagenum" id="Page4">[4]</span> -rapidly. Why, they asked, was the delay? There should -be none if the process were physical or chemical, and the -fact that it occurred strongly suggested biological action. -The process was stopped by chloroform vapour, but could be -restarted after the removal of the vapour by the addition -of a little fresh soil.</p> - -<p>The importance of this work in connection with soil -fertility was immediately realised by Warington, who had -recently come to Rothamsted.<a href="#Endnote1_11" class="fnanchor">[11]</a> He quickly confirmed the -result, and made the valuable discovery that two stages -were involved—the conversion of ammonia to a nitrite by -one organism, and of the nitrite to nitrate by another. He -made long and persistent attempts to isolate the organisms -from the soil, using the best technique of his time, but though -he found many bacteria none of them could nitrify ammonium -salts; yet the soil did it easily. For years he continued his -efforts to find the nitrifying organism, but always failed. -His health was not good, his life at Rothamsted was not -happy owing to disagreements with Gilbert, and although -his other research work was succeeding, this investigation on -which he had set his heart was not coming out; bacterial -technique was not yet sufficiently far advanced. Ten bitter, -disappointing years passed, and the crown of disappointment -came when Winogradsky, a young bacteriologist in Paris, -changed the technique and succeeded at once in isolating -both the nitrite and the nitrate-forming organisms.<a href="#Endnote1_13" class="fnanchor">[13]</a></p> - -<p>The numerous bacteria found by Warington in the soil -suggested the presence of a soil population, and this idea was -greatly strengthened by another line of investigation which -was being followed up in France. Boussingault had shown -that soils absorb oxygen and give out carbon dioxide; -Schloesing extended this discovery, as also did Wollny. It -was concluded that oxidation was the result of the activities -of the soil organisms in decomposing the organic matter of -the soil, and thus preparing the way for the nitrifying -organisms.</p> - -<p>A third important function of soil bacteria was revealed<span class="pagenum" id="Page5">[5]</span> -by Berthelot.<a href="#Endnote1_1" class="fnanchor">[1]</a> It was known that considerable loss of nitrogen -from the soil took place as the result of the conversion -of nitrogen compounds into nitrates, which were subsequently -washed out in the drainage water. It followed inevitably -that the stock of nitrogen compounds in the soil must long -ago have become exhausted had there been no addition of -nitrogen compounds to the soil. Berthelot argued that there -must be fixation of atmospheric nitrogen, and, following the -prevailing trend of thought in France, he attributed it to -bacteria. He confirmed the anticipation by exposing soil to -air in such conditions that dust, rain, etc., were excluded, -and he found an increase in the percentage of nitrogen.</p> - -<p>Looking back over the work, it is difficult to understand -the result. The fixation of nitrogen is a process that absorbs -energy, and should have necessitated some source of energy, -which apparently was not supplied. But in spite of this -drawback the investigation was immediately fruitful in that -it gave the key to another problem which had long puzzled -agriculturists.</p> - -<p>It had long been known that the growth of leguminous -crops, unlike that of others, enriched the ground,<a id="FNanchor3" href="#Footnote3" class="fnanchor">[C]</a> and -Lawes and Gilbert had shown that this was due to an increase -of soil nitrogen. But no explanation could be found till -Hellriegel and Wilfarth solved the problem.<a href="#Endnote1_4" class="fnanchor">[4]</a> In studying -the nitrogen nutrition of gramineous and leguminous crops, -they discovered that the gramineous plants died in absence -of nitrate, and in its presence made growth which increased -regularly with nitrate supply; while leguminous plants -sometimes died and sometimes flourished in absence of -nitrate, and behaved equally erratically with increasing -nitrate supply. When the plants flourished nodules were<span class="pagenum" id="Page6">[6]</span> -invariably present on the roots, but not otherwise. They -concluded, therefore, that the nitrogen nutrition of leguminous -plants differed from that of the gramineæ, and depended -on some factor which sometimes came into their -experiments and sometimes did not, and, in any case, was -associated with the nodule. Knowing that the nodules on -the roots of leguminous plants contained bacteria-like bodies, -and remembering Berthelot’s results, they explored the -possibility of bacterial fixation. They sterilised the sand -and found that peas invariably failed to develop nodules and -died, but after adding a little garden soil nodules were found -and vigorous growth was obtained.</p> - -<div class="footnote"> - -<p><a id="Footnote3" href="#FNanchor3" class="label">[C]</a> “Of the leguminous plants the bean best reinvigorates the ground ... -because the plant is of loose growth and rots easily, wherefore the people of -Macedonia and Thessaly turn over the ground when it is in flower” (i.e. dig it -into the ground if the soil is poor). Theophrastus, “Enquiry into Plants,” -bk. viii. 2, and bk. ix. <span class="smcapall">I</span>. This book is of profound interest to agriculturists -and botanists. An excellent translation by Sir Arthur Hort is now available. -(Loeb’s Classical Library.)</p> - -</div><!--footnote--> - -<p>Chemical analysis showed considerable fixation of gaseous -nitrogen, which Hellriegel associated with the nodule -organism. This has proved to be correct, and the fixation -of nitrogen by bacteria is now a well-recognised process, -the conditions of which are being thoroughly worked out. -Two types of organisms are known—those associated with -leguminous plants, and those living in a free and independent -state in the soil. Of the latter the Clostridium, isolated by -Winogradsky, is anaerobic, and the Azotobacter of Beijerinck -is aerobic. The essential conditions are that a source of -energy must be supplied—usually given as sugar—that the -medium must not be acid, and that sufficient phosphate -must be present.</p> - -<p>All this brilliant work had been accomplished in the -short space of the ten years 1880 to 1890. The inspiration -had in each instance come from France, and is traceable -direct to Pasteur, although coming long after his own work -on bacteriology. It is impossible for us now to realise the -thrill of wonder and astonishment with which students, -teachers, and writers of those days learned that the nutrition -of plants, and therefore the growth of crops and the feeding -of themselves, was largely the result of the activity of -bacteria in the dark recesses of the soil. It is not surprising -that the ideas were pushed somewhat too far, that the soil -population was regarded as solely bacterial, and that important<span class="pagenum" id="Page7">[7]</span> -chemical and physical changes were sometimes overlooked.</p> - -<p>Gradually there came the inevitable reaction and a somewhat -changed outlook. Continued examination showed the -presence in soil of almost every kind of bacteria for which -search was made. Some of them were almost certainly in -the resting condition as spores, and the new generation of -workers had an uneasy feeling that the case for the overwhelming -importance of bacteria in the economy of the soil -was not too well founded. It was shown that the decomposition -of nitrogen compounds to form ammonia would -take place without micro-organisms if, as presumably would -happen, the plant enzymes continued to act after they got -into the soil. Even the oxidation of ammonia to nitrate—the -great stronghold of the biological school—was accomplished -by chemical agents. The fixation of nitrogen in soil -conditions was beyond the power of chemists to achieve, -and here it was universally agreed that bacteria were the -active agents. And finally, chemists were themselves bringing -into prominence a set of bodies—the colloids—whose -remarkable properties seemed indefinitely expansible, and -were in addition sufficiently incomprehensible to the ordinary -student to attain much of the magnificence of the unknown.</p> - -<p>All the time, however, a faithful body of workers was -busy exploring the ground already won, improving the -technique, making counts of the numbers of bacteria in the -soil, and trying to measure the amount of bacterial activity. -Much of the value of this work was limited by the circumstance -that the bacteria were regarded as more or less constant -in numbers and activities, so that a single determination -was supposed to characterise the position in a given soil.</p> - -<p>This was the condition of the subject when it was seriously -taken up at Rothamsted. Before turning to agriculture, -the writer had been studying the mechanism of certain slow -chemical oxidations, and one of his first experiments in -agriculture was to examine the phenomena of oxidation in -soil. The results accorded with the biological explanation<span class="pagenum" id="Page8">[8]</span> -of Schloesing: when the soil was completely sterilised oxidation -almost ceased. But the striking discovery was made, -as the result of an accident to an autoclave, that partial -sterilisation increased the rate of oxidation, and therefore -presumably the bacterial activity. This remarkable phenomenon -had, however, already been observed, and it had been -shown that both bacterial numbers and soil fertility were -increased thereby. A full investigation was started in 1907 -by Dr. Hutchinson and the writer.<a href="#Endnote1_9" class="fnanchor">[9]</a> From the outset the -phenomena were recognised as dynamic and not static, and -the rates of change were always determined: thus the -bacterial numbers, the nitrate and ammonia present were -estimated after the several periods. Close study of the -curves showed that the chemical and bacterial changes were -sufficiently alike to justify the view that bacteria were in -the main the causes of the production of ammonia and of -nitrate; although non-biological chemical action was not -excluded, there was no evidence that it played any great -part. Thus the importance of micro-organisms in the soil -was demonstrated.</p> - -<p>The factor causing the increased bacterial numbers after -partial sterilisation was studied by finding out what agents -would, and what would not, allow the numbers to increase, -e.g. it was found that the bacterial increases became possible -when soil had been heated at 56° C., but not at 40° C. Again, -it was shown that the high numbers in partially sterilised -soils rose for a time even higher if a little fresh untreated -soil were incorporated into the partially sterilised soil, but -afterwards they fell considerably. Putting all the results -together, it appeared that some biological cause was at work -depressing the numbers of bacteria in normal soils, but not—or -not so much—in the partially sterilised soils. Studied in -detail, the data suggested protozoa as the agent keeping -down bacterial numbers, and they were found in the untreated, -but not in the treated, soils. The hypothesis was -therefore put forward that bacteria are not the only members -of the soil population, but that protozoa are also present<span class="pagenum" id="Page9">[9]</span> -keeping them in check, and therefore adversely affecting the -production of plant food.</p> - -<p>This conclusion aroused considerable controversy. It -was maintained that protozoa were not normal inhabitants -of the soil, but only occasional visitants, and, in any case, -they were only there as cysts; the soil conditions, it was -urged, were not suitable to large organisms like protozoa. -The objection was not to be treated lightly, but, on the other -hand, the experiments seemed quite sound. As neither -Dr. Hutchinson nor the writer were protozoologists, Dr. T. -Goodey and (after he left) Mr. Kenneth R. Lewin were -invited to try and find out, quite independently of the partial -sterilisation investigation, whether protozoa are normal inhabitants -of the soil, and if so, whether they are in a trophic -condition, and what is their mode of life and their relation to -soil bacteria. Had it turned out that protozoa had nothing -to do with the matter, search would have been made for -some other organism. Goodey showed that the ciliates were -not particularly important; Lewin soon demonstrated the -existence of trophic amœbæ and flagellates. Unfortunately -he was killed in the war before he had got far with the work. -After the Armistice, Mr. Cutler accepted charge of the work: -he will himself relate in <a href="#Page66">Chapters IV.</a> and <a href="#Page77">V.</a> what he has -done.</p> - -<p>At first sight it might be thought comparatively easy to -settle a question of this kind by examining soil under a -microscope or by sterilising it and introducing successively -bacteria and known types of protozoa. Unfortunately -neither method is simple in practice. It is impossible to -look into the soil with a microscope, and methods of teasing-out -small pieces of soil on a slide under the high, or even the -low power, give no information, because the particles of -soil have the remarkable power of attracting and firmly -retaining protozoa, and no doubt bacteria as well; indeed, -for protozoa (which have been the more fully investigated) -there seems to be something not unlike a saturation capacity -(see <a href="#Fig9">Fig. 9</a>, p. 78). Further, complete sterilisation of soil<span class="pagenum" id="Page10">[10]</span> -cannot be effected without at the same time altering its -chemical and physical properties, and changing it as a -habitat for micro-organisms. Cutler has, however, overcome -the difficulties and shown that the introduction of protozoa -into soils sterilised and then reinfected with bacteria -considerably reduces the numbers of these organisms.</p> - -<p>The method adopted, therefore, is to take a census of -population and of production. Counting methods are -elaborated, and estimates as accurate as possible are made -of the numbers of the various organisms in a natural field -soil at stated intervals. Simultaneously, wherever possible -some measure is taken of the work done. The details of the -census are finally arranged in consultation with the Statistical -Department, to ensure that the data shall possess -adequate statistical value. From the results it is possible -to adduce information of great value as to the life of the -population, the influence of external conditions, etc.</p> - -<p>The most important investigation of this kind carried out -at Rothamsted was organised by Mr. Cutler.<a href="#Endnote1_3" class="fnanchor">[3]</a> A team of -six workers was assembled, and for 365 days without a -break they counted every day the ciliates, the amœbæ, -the flagellates, and the bacteria in a plot of arable ground, -distinguishing no less than seventeen different kinds of -protozoa. The conclusions arrived at were carefully tested -by the Statistical Department.</p> - -<p>Of the protozoa the flagellates were found to be the most -numerous, the amœbæ came next, and the ciliates were by -far the fewest. The numbers of each organism varied from -day to day in a way that showed conclusively the essentially -trophic nature of the protozoan population. The numbers -of amœbæ—especially <i>Dimastigamœba</i> and of a species called -α—were sharply related to the numbers of bacteria: when the -amœbae were numerous the bacteria were few, and vice versa. -Detailed examination showed that the amœbæ were probably -the cause of the fluctuations in the bacterial numbers, but -Mr. Cutler has not yet been able to find why the amœbæ -fluctuated; it does not appear that temperature, moisture<span class="pagenum" id="Page11">[11]</span> -content, air supply or food supply were determining causes. -The flagellates and ciliates also showed large fluctuations, -amounting in one case—<i>Oicomonas</i>—to a definite periodicity, -apparently, however, not related to bacterial numbers, or, -so far as can be seen, to external conditions of moisture, -temperature and food supply, and showing no agreement -with the fluctuations of the amœbæ. However, one cannot -be certain that lack of agreement between curves expressing -protozoan numbers and physical factors implies absence of -causal relationships: the observations (though the best that -can yet be made) are admittedly not complete. If we saw -only the end of the bough of a tree, and could see no connection -with a trunk, we might have much difficulty in finding -relationships between its motion and the wind; whatever the -direction of the wind it would move backwards and forwards -in much the same way, and even when the wind was blowing -along the plane of its motion it would just as often move -against the wind as with it.</p> - -<p>Meanwhile evidence was obtained that the twenty-four -hour interval adopted by the protozoological staff was too -long for bacteria, and accordingly the Bacteriological Department, -under Mr. Thornton, refined the method still further. -Bacterial counts were made every two hours, day and night, -for several periods of sixty or eighty hours without a break. -The shape of the curve suggests that two hours is probably -close enough, and for the present counts at shorter intervals -are not contemplated. But there is at least one maximum -and one minimum in the day, although the bacterial day -does not apparently correspond with ours, nor can any relationship -be traced with the diurnal temperature curve.</p> - -<p>The nitrate content of the soil was simultaneously determined -by Mr. Page and found to vary from hour to hour, -but the variations did not sharply correspond with the bacterial -numbers; this, however, would not necessarily be -expected. The production of nitrate involves various -stages, and any lag would throw the nitrate and bacterial -curves out of agreement. There is a suggestion of a lag,<span class="pagenum" id="Page12">[12]</span> -but more counts are necessary before it can be regarded as -established.</p> - -<p>Examination of these and other nitrate curves obtained -at Rothamsted has brought out another remarkable phenomenon. -No crop is growing on these plots, and no rain fell -during the eighty hours, yet nitrate is disappearing for a -considerable part of the time. Where is it going to? At -present the simplest explanation seems to be that it is taken -up by micro-organisms. A similar conclusion had to be -drawn from a study of the nitrogen exhaustion of the soil. -The whole of the nitrate theoretically obtainable from the -organic matter of the soil is not obtained in the course of -hours or even days; in one of our experiments at Rothamsted -nitrification is still going on, and is far from complete, -even after a lapse of fifty-three years. The explanation at -present offered is that part of the nitrate is constantly being -absorbed by micro-organisms and regenerated later on.</p> - -<p>Now what organisms could be supposed to absorb nitrates -from the soil? Certain bacteria and fungi are known to -utilise nitrates, and one naturally thinks of algæ as possible -agents also. Dr. Muriel Bristol was therefore invited to -study the algæ of the soil. Her account is given in <a href="#Page99">Chapter -VI.</a> She has found them not only on the surface, but -scattered throughout the body of the soil, even in the darkness -of 4 inches, 5 inches, or 6 inches depth, where no light -can ever penetrate, and where photosynthesis as we understand -it could not possibly take place. Some modification -in their mode of life is clearly necessary, and it may well -happen that they are living saprophytically. Dr. Bristol -has not yet, however, been able to count the algæ in the soil -with any certainty, although she has made some estimates of -the numbers.</p> - -<p>The quantitative work on the soil population indicates -other possibilities which are being investigated. There is not -only a daily fluctuation in the numbers, but so far as measurements -have gone, a seasonal one also. There seems to be -some considerable uplift in numbers of bacteria, protozoa, and<span class="pagenum" id="Page13">[13]</span> -possibly algæ and fungi in the spring-time, followed by a fall -in summer, a rise in autumn, and a fall again in winter. At -present we are unable to account for the phenomenon, nor -can we be sure that it is general until many more data are -accumulated.</p> - -<p>In the cases of the protozoa and the algæ, there was a -definite reason for seeking them in the soil.</p> - -<p>Another section of the population, the fungi, was simply -found, and at present we have only limited views as to their -function. The older workers considered that they predominated -in acid soils, while bacteria predominated in -neutral soils. Present-day workers have shown that fungi, -including actinomycetes, are normal inhabitants of all soils. -The attempts at quantitative estimations are seriously complicated -by the fact that during the manipulations a single -piece of mycelium may break into fragments, each of which -would count as one, while a single cluster of spores might be -counted as thousands. Little progress has therefore been -made on the quantitative lines which have been so fruitful -with protozoa. Dr. Brierley gives, in <a href="#Page118">Chapters VII.</a> and -<a href="#Page131">VIII.</a>, a critical account of the work done on fungi.</p> - -<p>In addition to the organisms already considered there are -others of larger size. The nematodes are almost visible to the -unaided eye, most of them are free living and probably help -in the disintegration of plant residues, though a few are -parasitic on living plants and do much injury to clover, -oats, and less frequently to onions, bulbs, and potatoes. -Further, there are insects, myriapods and others, the effects -of which in the soil are not fully known. Special importance -attaches to the earthworms, not only because they are the -largest in size and in aggregate weight of the soil population, -but because of the great part they play in aerating the soil, -gradually turning it over and bringing about an intimate -admixture with dead plant residues, as first demonstrated -by Darwin. Earthworms are the great distributors of energy -material to the microscopic population. Systematic quantitative -work on these larger forms is only of recent date, -and Dr. Imms, in <a href="#Page147">Chapter IX.</a>, discusses our present knowledge.</p> - -<p><span class="pagenum" id="Page14">[14]</span></p> - -<p class="tabhead" id="TabI">TABLE I.<br /> -<span class="smcap">Soil Population, Rothamsted, 1922.</span></p> - -<p class="center fsize90">(The figures for algæ and fungi are first approximations only, and have considerably -less value than those for bacteria and protozoa.)</p> - -<table class="table1a"> - -<tr class="bt bb"> -<th rowspan="2" class="bl br"> </th> -<th rowspan="2" colspan="2" class="br">Numbers<br />per Gram<br />of Soil.</th> -<th colspan="6" class="br">Approximate Weight<br />per Acre of—</th> -</tr> - -<tr class="bb"> -<th colspan="2" class="br">Living<br />Organisms.</th> -<th colspan="2" class="br">Dry Matter<br />in<br />Organisms.</th> -<th colspan="2" class="br">Nitrogen<br />in<br />Organisms.</th> -</tr> - -<tr> -<td class="organism"><i>Bacteria</i>— </td> -<th colspan="2" class="br"> </th> -<th colspan="2" class="br">lb.</th> -<th colspan="2" class="br">lb.</th> -<th colspan="2" class="br">lb.</th> -</tr> - -<tr> -<td class="organism level1">High level</td> -<td class="data">45,000,000</td> -<td rowspan="12" class="brace br"> </td> -<td class="data">50</td> -<td rowspan="2" class="brace br"><span class="fsize150">}</span></td> -<td rowspan="2" class="data braced">2</td> -<td rowspan="18" class="brace br"> </td> -<td rowspan="2" class="data braced">0·2</td> -<td rowspan="18" class="brace br"> </td> -</tr> - -<tr> -<td class="organism level1">Low level</td> -<td class="data">22,500,000</td> -<td class="data">25</td> -</tr> - -<tr> -<td class="organism"><i>Protozoa</i>—</td> -<td class="data"> </td> -<td class="data"> </td> -<td rowspan="5" class="brace br"> </td> -<td class="data"> </td> -<td class="data"> </td> -</tr> - -<tr> -<td class="organism level1"><i>Ciliates</i>—</td> -<td class="data"> </td> -<td class="data"> </td> -<td class="data"> </td> -<td class="data"> </td> -</tr> - -<tr> -<td class="organism level2">High level</td> -<td class="data">1,000</td> -<td class="data">—</td> -<td class="data">—</td> -<td class="data">—</td> -</tr> - -<tr> -<td class="organism level2">Low level</td> -<td class="data">100</td> -<td class="data">—</td> -<td class="data">—</td> -<td class="data">—</td> -</tr> - -<tr> -<td class="organism level1"><i>Amœbæ</i>—</td> -<td class="data"> </td> -<td class="data"> </td> -<td class="data"> </td> -<td class="data"> </td> -</tr> - -<tr> -<td class="organism level2">High level</td> -<td class="data">280,000</td> -<td class="data">320</td> -<td rowspan="2" class="brace br"><span class="fsize150">}</span></td> -<td rowspan="2" class="data braced">12</td> -<td rowspan="2" class="data braced">1·2</td> -</tr> - -<tr> -<td class="organism level2">Low level</td> -<td class="data">150,000</td> -<td class="data">170</td> -</tr> - -<tr> -<td class="organism level1"><i>Flagellates</i>—</td> -<td class="data"> </td> -<td class="data"> </td> -<td class="brace br"> </td> -<td class="data"> </td> -<td class="data"> </td> -</tr> - -<tr> -<td class="organism level2">High level</td> -<td class="data">770,000</td> -<td class="data">190</td> -<td rowspan="2" class="brace br"><span class="fsize150">}</span></td> -<td rowspan="2" class="data braced">7</td> -<td rowspan="2" class="data braced">0·7</td> -</tr> - -<tr> -<td class="organism level2">Low level</td> -<td class="data">350,000</td> -<td class="data">85</td> -</tr> - -<tr> -<td class="organism level1"><i>Algæ</i> (not blue-green)</td> -<td class="data">[100,000</td> -<td class="brace br">] </td> -<td class="data">125</td> -<td rowspan="3" class="brace br"> </td> -<td class="data">6</td> -<td class="data">0·6</td> -</tr> - -<tr> -<td class="organism level2">Blue-green</td> -<td class="data">Not known.</td> -<td rowspan="2" class="brace br"> </td> -<td class="data">—</td> -<td class="data">Say 6</td> -<td class="data">Say 0·6</td> -</tr> - -<tr> -<td class="organism level1"><i>Fungi</i>—</td> -<td class="data"> </td> -<td class="data"> </td> -<td class="data"> </td> -<td class="data"> </td> -</tr> - -<tr> -<td class="organism level2">High level</td> -<td class="data">[1,500,000</td> -<td class="brace br">] </td> -<td class="data">1700</td> -<td rowspan="2" class="brace br"><span class="fsize150">}</span></td> -<td rowspan="2" class="data braced">60</td> -<td rowspan="2" class="data braced">6·0</td> -</tr> - -<tr> -<td class="organism level2">Low level</td> -<td class="data">[700,000</td> -<td class="brace br">] </td> -<td class="data">800</td> -</tr> - -<tr> -<td class="bl br"> </td> -<td class="data"> </td> -<td class="brace br"> </td> -<td class="data">93</td> -<td class="brace br"> </td> -<td class="brace"> </td> -<td class="data">9·3</td> -</tr> - -<tr class="bb"> -<td class="bl br"> </td> -<td colspan="2" class="br"> </td> -<td colspan="2" class="br"> </td> -<td colspan="4" class="data bt br dontwrap">= 4 parts nitrogen per<br />1,000,000 of soil.</td> -</tr> - -</table> - -<table class="table1b"> - -<tr class="bb"> -<th colspan="9" class="highline25 bl br"><span class="smcap">Larger Organisms.</span></th> -</tr> - -<tr class="bb"> -<th rowspan="3" class="bl br"> </th> -<th rowspan="2" colspan="2" class="br">Numbers<br />per Acre.<a id="FNanchor4" href="#Footnote4" class="fnanchor">[D]</a></th> -<th colspan="6" class="br">Approximate Weight<br />per Acre of—</th> -</tr> - -<tr class="bb"> -<th colspan="2" class="br">Living<br />Organisms.</th> -<th colspan="2" class="br">Dry Matter<br />in<br />Organisms.</th> -<th colspan="2" class="br">Nitrogen<br />in<br />Organisms.</th> -</tr> - -<tr class="bb"> -<th class="br">Ma-<br />nured.</th> -<th class="br">Un-<br />ma-<br />nured.</th> -<th class="br">Ma-<br />nured.</th> -<th class="br">Un-<br />ma-<br />nured.</th> -<th class="br">Ma-<br />nured.</th> -<th class="br">Un-<br />ma-<br />nured.</th> -<th class="br">Ma-<br />nured.</th> -<th class="br">Un-<br />ma-<br />nured.</th> -</tr> - -<tr> -<td class="organism bl"><i>Oligochaeta</i> (<i>Limicolae</i>)—</td> -<th class="br"> </th> -<th class="br"> </th> -<th class="br">lb.</th> -<th class="br">lb.</th> -<th class="br">lb.</th> -<th class="br">lb.</th> -<th class="br">lb.</th> -<th class="br">lb.</th> -</tr> - -<tr> -<td class="organism level1">Nematoda, etc.</td> -<td class="data br">3,609,000</td> -<td class="data br">794,000</td> -<td class="data br">9</td> -<td class="data br">2</td> -<td class="data br">3</td> -<td class="data br">1</td> -<td class="data br">—</td> -<td class="data br">—</td> -</tr> - -<tr> -<td class="organism level1">Myriapoda</td> -<td class="data br">1,781,000</td> -<td class="data br">879,000</td> -<td class="data br">203</td> -<td class="data br">99</td> -<td class="data br">85</td> -<td class="data br">42</td> -<td class="data br">4</td> -<td class="data br">2</td> -</tr> - -<tr> -<td class="organism level1">Insects</td> -<td class="data br">7,727,000</td> -<td class="data br">2,475,000</td> -<td class="data br">34</td> -<td class="data br">16</td> -<td class="data br">14</td> -<td class="data br">6</td> -<td class="data br">1</td> -<td class="data br">1</td> -</tr> - -<tr class="bb"> -<td class="organism level1">Earthworms</td> -<td class="data br">1,010,000</td> -<td class="data br">458,000</td> -<td class="data br">472</td> -<td class="data br">217</td> -<td class="data br">108</td> -<td class="data br">50</td> -<td class="data br">10</td> -<td class="data br">5</td> -</tr> - -<tr class="bb"> -<td colspan="5" class="right padr2 bl br">Total</td> -<td class="data br">210</td> -<td class="data br">99</td> -<td class="data br">15</td> -<td class="data br">9</td> -</tr> - -<tr> -<td colspan="9" class="text">Total organic matter (dry weight) in this soil = 126,000 lb. per acre.</td> -</tr> - -<tr> -<td colspan="9" class="text">Total nitrogen = 5700 lb. per acre. (1 lb. nitrogen per acre = 0·4 parts per -1,000,000 of soil.)</td> -</tr> - -<tr> -<td colspan="9" class="text"><a id="Footnote4" href="#FNanchor4" class="label">[D]</a> To a depth of 9 inches. The weight of -soil is approximately 1,000,000 kilos.</td> -</tr> - -</table> - -<p><span class="pagenum" id="Page15">[15]</span></p> - -<p>Are there any other members of the soil population that -are of importance? As already shown, the method of -investigating the soil population in use at Rothamsted is to -find by chemical methods the changes going on in the soil; -to find by biological methods what organisms are capable of -bringing about these changes; and then to complete the -chain of evidence by tracing the relationships between the -numbers or activities of these organisms and the amount of -change produced. The list as we know it to-day is given -in <a href="#TabI">Table I.</a></p> - -<p>The method, however, does not indicate whether the -account is fairly complete, or whether there are other -organisms to be found. We might, of course, trust to -empirical hunting for organisms, or to chance discoveries -such as led Goodey to find the mysterious Proteomyxan -Rhizopods, which cannot yet be cultured with certainty, so -that they are rarely found by soil workers. It is possible -that there are many such organisms, and it is even conceivable -that these unknown forms far outnumber the known. The -defect of the present method is that it always leaves us in -doubt as to the completeness of the list, and so we may -have to devise another.</p> - -<p>Reverting to <a href="#TabI">Table I.</a>, it obviously serves no purpose to -add the numbers of all the organisms together. We can -add up the weights of living organisms, of their dry matter -or nitrogen, so as to form some idea of the proportion of -living to non-living organic matter, and this helps us to -visualise the different groups and place them according to -their respective masses. But a much better basis for -comparing the activities of the different groups would be -afforded by the respective amounts of energy they transform, -if these could be determined. It is proposed to attempt -such measurements at Rothamsted. The results when -added would give the sum of the energy changes effected<span class="pagenum" id="Page16">[16]</span> -by the soil population as we know it: the figure could be -compared with the total energy change in the soil itself -as determined in a calorimeter. If the two figures are of -the same order of magnitude, we shall know that our -list is fairly well complete; if they are widely different, -search must be made for the missing energy transformers. -There are, of course, serious experimental difficulties -to be overcome, but we believe the energy relationships -will afford the best basis for further work on the soil -population.</p> - -<p>Finally, it is necessary to refer to the physical conditions -obtaining in the soil. These make it a much better habitat -for organisms than one might expect. At first sight one -thinks of the soil as a purely mineral mass. This view is -entirely incorrect. Soil contains a considerable amount of -plant residues, rich in energy, and of air and water. The -usual method of stating the composition of the soil is by -weight, but this is misleading to the biologist because the -mineral matter has a density some two and a half times that -of water and three times that of the organic matter. For -biological purposes composition by volume is much more -useful, and when stated in this way the figures are very -different from those ordinarily given. <a href="#TabII">Table II.</a> gives the -results for two Broadbalk arable plots, one unmanured and -the other dunged; it includes also a pasture soil.</p> - -<p>The first requirement of the soil population is a supply -of energy, without which it cannot live at all. All our -evidence shows that the magnitude of the population is -limited by the quantity of energy available. The percentage -by weight of the organic matter is about two to four or five, -and the percentage by volume runs about four to twelve. -Not all of this, however, is of equal value as source of -energy. About one-half is fairly easily soluble in alkalis, -and may or may not be of special value, but about -one-quarter is probably too stable to be of use to soil -organisms.</p> - -<p><span class="pagenum" id="Page17">[17]</span></p> - -<p>A second requirement is water with which in this country -the soil is usually tolerably well provided. Even in prolonged -dry weather the soil is moist at a depth of 3 inches -below the surface. It is not uncommon to find 10 per cent. -or 20 per cent. by volume of water present, spread in a thin -film over all the particles, and completely saturating the -soil atmosphere.</p> - -<p class="tabhead" id="TabII">TABLE II.<br /> -<span class="smcap">Volume of Air, Water and Organic Matter in 100 Volumes of -Rothamsted Soil.</span></p> - -<table class="standard"> - -<tr class="bt bb"> -<th rowspan="2" class="bl br"> </th> -<th colspan="2" class="br">Solid Matter.</th> -<th rowspan="2" class="br">Pore<br />Space.</th> -<th colspan="2" class="br">In Pore Space.<br />Values Commonly<br />Obtained.</th> -</tr> - -<tr class="bb"> -<th class="br">Mineral.</th> -<th class="br">Organic.</th> -<th class="br">Water.</th> -<th class="br"><span class="padl1 padr1">Air.</span></th> -</tr> - -<tr> -<td class="general bl br"><span class="padl1 padr1">(1)</span></td> -<td class="general br">62</td> -<td class="general br"> 4</td> -<td class="general br">34</td> -<td class="general br">23</td> -<td class="general br">11</td> -</tr> - -<tr> -<td class="general bl br"><span class="padl1 padr1">(2)</span></td> -<td class="general br">51</td> -<td class="general br">11</td> -<td class="general br">38</td> -<td class="general br">30</td> -<td class="general br"> 8</td> -</tr> - -<tr class="bb"> -<td class="general bl br"><span class="padl1 padr1">(3)</span></td> -<td class="general br">41</td> -<td class="general br">12</td> -<td class="general br">47</td> -<td class="general br">40</td> -<td class="general br"> 7</td> -</tr> - -</table> - -<p class="center fsize90 blankbefore75 blankafter75">(1) Arable, no manure applied to soil. -(2) Arable, dung applied to soil. -(3) Pasture.</p> - -<p>The air supply is usually adequate owing to the rapidity -with which diffusion takes place. Except when the soil is -water-logged, the atmosphere differs but little from that of -the one we breathe. There is more CO<sub>2</sub>, but only a little -less oxygen.<a href="#Endnote1_8" class="fnanchor">[8]</a> The mean temperature is higher than one -would expect, being distinctly above that of the air, while -the fluctuations in temperature are less.<a href="#Endnote1_5" class="fnanchor">[5]</a></p> - -<p>The reaction in normal soils is neutral to faintly alkaline; -<i>p</i>H values of nearly 8 are not uncommon. Results from -certain English soils are shown on <a href="#Page18">p. 18</a>.</p> - -<p>The soil reaction is not easily altered. A considerable -amount of acid must accumulate before any marked increase -in intensity of <i>p</i>H value occurs; in other words, the soil is -well buffered. The same can be said of temperature, of -water, and of energy supply. Like the reaction, they alter<span class="pagenum" id="Page18">[18]</span> -but slowly, so that organisms have considerable time in -which to adapt themselves to the change.</p> - -<p class="tabhead"><span class="smcap">Hydrogen Ion Concentration and Soil Fertility.</span></p> - -<table class="soilph"> - -<tr> -<th colspan="3"> </th> -<th colspan="2"><i>p</i>H</th> -<th> </th> -</tr> - -<tr> -<td colspan="2" class="acidity">Alkaline</td> -<td rowspan="2" class="ph">10</td> -<td class="scale bb"> </td> -<td class="scale bb"> </td> -<td rowspan="2" class="consequence">Sterile: Alkali soil.</td> -</tr> - -<tr> -<td rowspan="2" class="br"> </td> -<td rowspan="2"> </td> -<td class="scale br"> </td> -<td class="scale"> </td> -</tr> - -<tr> -<td rowspan="2" class="ph">9</td> -<td class="scale br bb"> </td> -<td class="scale bb"> </td> -<td rowspan="2" class="consequence"> </td> -</tr> - -<tr> -<td rowspan="2" class="br"> </td> -<td rowspan="2"> </td> -<td class="scale br"> </td> -<td class="scale"> </td> -</tr> - -<tr> -<td rowspan="2" class="ph">8</td> -<td class="scale br bb"> </td> -<td class="scale bb"> </td> -<td rowspan="2" class="consequence">Fertile: Arable.</td> -</tr> - -<tr> -<td class="br"> </td> -<td> </td> -<td class="scale br"> </td> -<td class="scale"> </td> -</tr> - -<tr> -<td rowspan="2" colspan="2" class="acidity">Neutral</td> -<td rowspan="2" class="ph">7</td> -<td class="scale br bb"> </td> -<td class="scale bb"> </td> -<td rowspan="2" class="consequence"> </td> -</tr> - -<tr> -<td class="br"> </td> -<td> </td> -</tr> - -<tr> -<td rowspan="2" class="br"> </td> -<td rowspan="2"> </td> -<td rowspan="2" class="ph">6</td> -<td class="scale br bb"> </td> -<td class="bb"> </td> -<td rowspan="2" class="consequence"> </td> -</tr> - -<tr> -<td class="scale br"> </td> -<td class="scale"> </td> -</tr> - -<tr> -<td rowspan="2" class="br"> </td> -<td rowspan="2"> </td> -<td rowspan="2" class="ph">5</td> -<td class="scale br bb"> </td> -<td class="scale bb"> </td> -<td rowspan="4" class="consequence">Potato Scab fails.<br /> -<span class="higher">Nitrification hindered.</span><br />Barley fails.</td> -</tr> - -<tr> -<td class="scale br"> </td> -<td class="scale"> </td> -</tr> - -<tr> -<td rowspan="2" class="br"> </td> -<td rowspan="2"> </td> -<td rowspan="2" class="ph">4</td> -<td class="scale br bb"> </td> -<td class="scale bb"> </td> -</tr> - -<tr> -<td class="scale br"> </td> -<td class="scale"> </td> -</tr> - -<tr> -<td rowspan="2" colspan="2" class="acidity">Acid</td> -<td rowspan="2" class="ph">3</td> -<td class="scale br bb"> </td> -<td class="scale bb"> </td> -<td rowspan="2" class="consequence">Sterile: Peat.</td> -</tr> - -<tr> -<td class="scale"> </td> -<td class="scale"> </td> -</tr> - -</table> - -<h3>A SELECTED BIBLIOGRAPHY.</h3> - -<div class="footnote"> - -<p><span id="Endnote1_1" class="label"> [1]</span> Berthelot, Marcellin, “Fixation directe de l’azote atmosphérique -libre par certains terrains argileux,” Compt. Rend., 1885, ci., -775-84.</p> - -<p><span id="Endnote1_2" class="label"> [2]</span> Boussingault, J. B., and Léwy, “Sur la composition de l’air -confiné dans la terre végétale,” Ann. Chim. Phys., 1853, -xxxvii., 5-50.</p> - -<p><span id="Endnote1_3" class="label"> [3]</span> Cutler, D. W., Crump, L. M., and Sandon, H., “A Quantitative -Investigation of the Bacterial and Protozoan Population of the -Soil, with an Account of the Protozoan Fauna,” Phil. Trans. -Roy. Soc., Series B, 1922, ccxi., 317-50.</p> - -<p><span id="Endnote1_4" class="label"> [4]</span> Hellriegel, H., and Wilfarth, H., “Untersuchungen über die -Stickstoffnahrung der Gramineen und Leguminosen,” Zeitsch. -des Vereins f. d. Rübenzucker-Industrie, 1888.</p> - -<p><span id="Endnote1_5" class="label"> [5]</span> Keen, B. A., and Russell, E. J., “The Factors determining Soil -Temperature,” Journ. Agric. Sci., 1921, xi., 211-37.</p> - -<p><span id="Endnote1_6" class="label"> [6]</span> Lawes, J. B., and Gilbert, J. H., “On Agricultural Chemistry, -Especially in Relation to the Mineral Theory of Baron Liebig,” -Journ. Roy. Agric. Soc., 1851, xii., 1-40.</p> - -<p><span id="Endnote1_7" class="label"> [7]</span> Liebig, Justus, “Chemistry in its Application to Agriculture and -Physiology,” 1st and 2nd editions (1840 and 1841), 3rd and -4th editions (1843 and 1847); “Natural Laws of Husbandry,” -1863.</p> - -<p><span id="Endnote1_8" class="label"> [8]</span> Russell, E. J., and Appleyard, A., “The Composition of the Soil -Atmosphere,” Journ. Agric. Sci., 1915, vii., 1-48; 1917, viii., -385-417.</p> - -<p><span class="pagenum" id="Page19">[19]</span></p> - -<p><span id="Endnote1_9" class="label"> [9]</span> Russell, E. J., and Hutchinson, H. B., “The Effect of Partial -Sterilisation of Soil on the Production of Plant Food,” Journ. -Agric. Sci., 1909, iii., 111-14; Part II., Journ. Agric. Sci., -1913, v., 152-221.</p> - -<p><span id="Endnote1_10" class="label">[10]</span> Schloesing, Th., and Müntz, A., “Sur la Nitrification par les ferments -organisés,” Compt. Rend., 1877, lxxxiv., 301-3; 1877, -lxxxv., 1018-20; and 1878, lxxxvi., 892-5. “Leçons de -chimie agricole,” 1883.</p> - -<p><span id="Endnote1_11" class="label">[11]</span> Warington, R., “On Nitrification,” Part I., Journ. Chem. Soc., -1878, xxxiii., 44-51; Part II, Journ. Chem. Soc., 1879, xxxv., -429-56; Part III., Journ. Chem. Soc., 1884, xlv., 637-72; -Part IV., Journ. Chem. Soc., 1891, lix., 484-529.</p> - -<p><span id="Endnote1_12" class="label">[12]</span> Way, J. T., “On the Composition of the Waters of Land Drainage -and of Rain,” Journ. Roy. Agric. Soc., 1856, xvii., 123-62.</p> - -<p><span id="Endnote1_13" class="label">[13]</span> Winogradsky, S., “Recherches sur les organismes de la nitrification,” -Ann. de l’Inst. Pasteur, 1890, iv., 1<sup>e</sup> Mémoire, 213-31; -2<sup>e</sup> Mémoire, 257-75; 3<sup>e</sup> Mémoire, 760-71.</p> - -<p>“Recherches sur l’assimilation de l’azote libre de l’atmosphère par -les microbes.” Arch. des Sci. Biolog. St. Petersburg, 1895, iii, -297-352.</p> - -</div><!--footnote--> - -<p class="fsize90">For further details and fuller bibliography, see E. J. Russell, -“Soil Conditions and Plant Growth,” Longmans, Green & Co.</p> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page20">[20]</span></p> - -<h2 class="nobreak">CHAPTER II.<br /> -<span class="chaptitle">SOIL BACTERIA.</span></h2> - -<h3 class="nobreak"><i>A.</i> <span class="smcap">Occurrence and Methods of Study.</span></h3> - -</div><!--chapter--> - -<p>To understand the development of our knowledge of soil -bacteria, it must be remembered that bacteriology is under -the disadvantage that it started as an applied science. -Although bacteria were first seen by Leeuwenhoeck about the -middle of the seventeenth century, and some of their forms -described by microscopists of the eighteenth and early nineteenth -centuries, it was only with the work of Pasteur on fermentation, -and of Duvaine, Pasteur, and their contemporaries -on disease bacteria, that bacteriology may be said to have -started. From the outset, therefore, attention has been -directed mainly to the bacteria in their specialised relationship -to disease or to fermentation and similar processes. As -a result, little research was done on the pure biology of the -bacteria, so that even now many of the most fundamental -and elementary problems concerning them are quite unsolved.</p> - -<p>In their work on fermentations and disease bacteria, the -earlier workers were assisted by the fact that under both -sets of conditions the causative bacteria exist, as a rule, -either in practically pure culture, or else in preponderating -numbers. The study and elucidation of such a mixed -micro-population as exists in the soil, became possible only -when methods had been devised for isolating the different -kinds of bacteria, and thus studying them apart from each -other. It was the development of the gelatine plate method -of isolating pure cultures by Koch<a href="#Endnote3_36" class="fnanchor">[36]</a> in 1881 that made the -study of the soil bacteria practicable. The plating method<span class="pagenum" id="Page21">[21]</span> -opened up two lines of research. In the first place, it provided -a simple means of isolating organisms from the mixed -population of the soil, and thus enabled a qualitative study -to be made of each organism in pure culture, and, in the -second place, from it was developed a counting technique -for estimating differences in bacterial numbers between -samples of soil, from which has sprung much of our knowledge -of the quantitative side.</p> - -<p>The earliest studies of the soil bacteria consisted of such -estimations of numbers, and showed that the soil contained -a very numerous population of bacteria. About 20,000,000 -bacteria per gram of soil is now considered a fair average -number. The number and variety of bacteria existing in the -soil is so enormous that the method of separating out all the -different forms, and of discovering their characters and -functions, has proved impracticable. In practice, therefore, -the problem has been approached from the biochemical -standpoint. That is to say, the special chemical changes -that the bacteria produce in the soil have first been investigated, -and this has been followed by the isolation and study -of the various groups of bacteria that bring about the -changes under investigation.</p> - -<p>The method commonly employed in isolating the organisms -that produce a given chemical change in the soil -is called the “elective” method. The soil is inoculated into -a culture medium that will especially favour the group of -bacteria to be isolated, to the exclusion of others. For -example, if it is desired to isolate the organisms that attack -cellulose, a medium is made up containing no other organic -carbon compounds except cellulose. Such a selective -medium encourages the growth of the group of organisms -to be investigated, so that after several transfers to fresh -medium a culture is obtained containing only two or three -different types of organisms. These are separated by plating -and pure cultures obtained.</p> - -<p>Another difficulty which has not yet been completely -overcome is that of adequately describing an organism when<span class="pagenum" id="Page22">[22]</span> -it is isolated. The morphology of bacteria is not the constant -thing that is seen in the more stable higher organisms. In -many cases the appearance of a single strain is entirely -different on different media, and may be quite altered by -such conditions as changes in acidity of the medium or -temperature of incubation. Even on a single medium -remarkable changes in morphology occur, at any rate, in -some bacteria. This is well seen in a cresol-decomposing -organism under investigation at Rothamsted. In cultures a -few days old this organism develops as bent and branching -rods; these rods then break up into chains of cocci and -short rods, which separate, and in old cultures all the organisms -may be in the coccoid form (<a href="#Fig1">Fig. 1</a>). It is claimed -by Löhnis<a href="#Endnote3_47" class="fnanchor">[47<i>b</i>]</a> that the possession of a complex life-cycle of -changing forms is a universal character in the bacteria. The -instability of shape in many bacteria makes it necessary -to standardise very carefully the cultural conditions under -which they are kept when their appearance is described.</p> - -<div class="container w40em" id="Fig1"> - -<img src="images/illo030.png" alt="" /> - -<p class="caption left padl6">Culture 15 hours old. -<span class="righttext padr6">Culture 3 days old.</span></p> - -<p class="caption"><span class="smcap">Fig. 1.</span>—Change in appearance, in culture, -of a cresol decomposing bacterium.</p> - -</div><!--container--> - -<p>The inadequacy of mere morphology as a basis for -describing bacteria led to the search for diagnostic characters, -based on the biochemical changes that they produced in -their culture media, and the appearance of their growth in -the mass on various media. These characters unfortunately -have also proved to be very much influenced by the exact -composition of the medium and other conditions of culture.<span class="pagenum" id="Page23">[23]</span> -Recently an attempt has been made by the American Society -of Bacteriologists to standardise the diagnostic characters -used in describing bacteria, and also the media and cultural -conditions under which they are grown for the purpose of -description. The need for such precautions, however, was -not sufficiently realised by the early workers, many of whose -descriptions cannot now be referred to any definite organism.</p> - -<p>The large number of organisms found in the soil, and the -difficulty and labour of adequately describing them, is such -that even now we have no comprehensive description of the -common soil bacteria that appear on gelatine platings. A -careful study based on modern methods of characterisation -has been made of certain selected groups of bacteria, and it -is hoped that the laborious systematic work of describing -the common forms will gradually be completed.</p> - -<p>Several attempts have been made to classify the bacteria -that appear commonly on gelatine platings. This -work was commenced by Hiltner and Stormer in Germany, -and continued by Chester, Harding, and Conn in America. -<span class="nowrap">Conn<a href="#Endnote3_10" class="fnanchor">[10]</a><sup>,</sup> -<a href="#Endnote3_14" class="fnanchor">[14]</a></span> -found that the common organisms fell into the -following main <span class="nowrap">groups:—</span></p> - -<p>(1) Large spore-forming bacteria, related to <i>Bacillus -subtilis</i>, which form about 5-10 per cent. of the numbers. -He adduced <span class="nowrap">evidence<a href="#Endnote3_12" class="fnanchor">[12]</a><sup>,</sup> -<a href="#Endnote3_13" class="fnanchor">[13]</a></span> -that these organisms exist in the -soil mainly as spores, so that they may not form an important -part of the active soil population.</p> - -<p>(2) Short non-sporing organisms, related to <i>Pseudomonas -fluorescens</i>, that are rapid gelatine liquefiers. These form -another 10 per cent. of the numbers.</p> - -<p>(3) Short rod forms that liquefy gelatine slowly or not at -all, and develop colonies very slowly. These form 40-75 per -cent. of the numbers, and may therefore be of considerable -importance in the soil.</p> - -<p>(4) A few micrococci also occur.</p> - -<p>These groups comprise the larger portion of the bacterial -flora of the soil, but, in addition to these organisms, that -develop on the media commonly used for plating, there are<span class="pagenum" id="Page24">[24]</span> -special and important groups that appear only on special -media, either owing to their being unable to grow on ordinary -media or because they get swamped by other forms. Examples -of such groups are the ammonia and nitrite oxidising -bacteria, the nitrogen fixing groups, the cellulose decomposing -organisms, and the sulphur bacteria.</p> - -<p>In order that we may apply the results of the study -of a definite organism to other localities, a knowledge of -the geographical distribution of the soil bacteria is clearly -needed. We have, unfortunately, very little knowledge of -the distribution of soil organisms. The common spore-forming -groups appear to be universally distributed. Thus -Barthel, in a study of the bacterial flora of soils from Greenland -and the island of Disko, obtained soil organisms belonging -to the groups of <i>Bacillus subtilis</i>, <i>B. amylobacter</i>, <i>B. -fluorescens</i>, <i>B. caudatus</i>, and <i>B. Zopfii</i>, which are common -groups in European soil, indicating that the general constitution -of the bacterial flora of the soil in arctic regions is -not widely different from that of Western Europe. Bredemann, -who made an extensive study of the <i>Bacillus amylobacter</i> -group, obtained soil samples from widely scattered -localities, and found these organisms in soil from Germany, -Holstein, Norway, Italy, Morocco, Teneriffe, Russia, Japan, -China, the East Indies, Samoa, Illinois, Arizona, German -East Africa, and the Cameroons. Some soil organisms, on -the other hand, are apparently absent from certain districts. -This may be due to the conditions, such as climatic environment, -being unfavourable to them. A study has recently -been made at Rothamsted of the distribution over Great -Britain of a group of bacteria that are capable of decomposing -phenol and cresol. One of these organisms, apparently -related to the acid-fast <i>B. phlœi</i>, has an interesting -distribution. It has been found in 50 per cent. of the soils -samples examined from the drier region, where the annual -rainfall is less than 30 inches, but in only 20 per cent. of the -samples in the wetter parts of Britain. Another example of -limited distribution is found in the case of <i>Bacillus radicicola</i>,<span class="pagenum" id="Page25">[25]</span> -the organism that produces tubercles on the roots of leguminous -plants. The distribution of the varieties of this -organism follows that of the host plants with which they are -associated, so that when a new leguminous crop is introduced -into a country, nodules may not appear on the roots unless -the soil be specially inoculated with the right variety of -organism. In cases where a group of soil organisms is -widely distributed over the globe, it may yet be absent -from many soils owing to the soil conditions not suiting it. -Thus, phenol decomposing bacteria, though abundant in the -neighbourhood of Rothamsted, are yet absent from field -plots that have been unmanured for a considerable period. -The occurrence of the nitrifying organisms and the nitrogen -fixing <i>Azotobacter</i> is also very dependent on the soil conditions.</p> - -<p>Owing to the method by which our knowledge of soil -bacteria has been acquired, by studying first the chemical -changes in the soil and then the bacteria that produce them, -it is natural for us to divide them into physiological groups -according to the chemical changes that they bring about. -This grouping is the more reasonable since so little is known -as to the true relationships of the different groups of bacteria -that a classification based on morphology is well-nigh -impossible. In considering the activities of bacteria in the -soil, it is convenient to group the changes which they bring -about into the two divisions into which they naturally fall -in the economy of the organisms.</p> - -<p>In the first place, there are the changes that result in a -release of energy, which the bacteria utilise for their vital -processes.</p> - -<p>In the second place, there are the processes by which the -bacteria build up the material of their bodies. These building -up processes involve an intake of energy for their accomplishment.</p> - -<p>It will be convenient to deal first with the release of -energy for their own use by bacteria, and its consequences.</p> - -<div class="chapter"> - -<p><span class="pagenum" id="Page26">[26]</span></p> - -<h3 class="nobreak"><i>B.</i> <span class="smcap">Activities Connected with the Acquirement of -Energy.</span></h3> - -</div> - -<p>Unlike the green plants, most bacteria are unable to -obtain the energy that is required for their metabolism from -sunlight. They must, therefore, make use of such chemical -changes as will involve the release of energy.</p> - -<p>As an example of the acquirement of energy in this way -may be taken the oxidation of methane by <i>B. methanicus</i>. -This organism, described by Söhngen, obtains its energy -supply by the conversion of methane into CO<sub>2</sub> and H<sub>2</sub>O.</p> - -<p class="chemform">CH<sub>4</sub> + 2O<sub>2</sub> = CO<sub>2</sub> + 2H<sub>2</sub>O 220 Cal.</p> - -<p>A further example is the acetic organism that obtains its -energy through the oxidation of alcohol to acetic acid.</p> - -<p class="chemform">C<sub>2</sub>H<sub>6</sub>O + O<sub>2</sub> = C<sub>2</sub>H<sub>4</sub>O<sub>2</sub> + H<sub>2</sub>O 115 Cal.</p> - -<p>The decomposition processes brought about by micro-organisms -in obtaining energy are usually oxidations, but -this is not necessarily so, as can be seen in case of the fermentation -of sugar into alcohol.<a id="FNanchor5" href="#Footnote5" class="fnanchor">[E]</a></p> - -<p class="chemform">C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> = 2C<sub>2</sub>H<sub>6</sub>O + 2CO<sub>2</sub> 50 Cal.</p> - -<div class="footnote"> - -<p><a id="Footnote5" href="#FNanchor5" class="label">[E]</a> These examples are from Orla-Jensen (Centralblatt f. Bakt., II., Bd. 22, -p. 305).</p> - -</div><!--footnote--> - -<p>By far the greater part of the decomposition of organic -matter is brought about by bacteria in the process of acquiring -energy. In the soil, nearly the whole of the material utilised -by bacteria as a source of energy is derived ultimately from -green plants. The energy materials left in the soil by the -plant fall into two groups, the non-nitrogenous compounds, -which are mainly carbohydrates and their derivatives, and -the nitrogenous compounds, principally derived from proteins.</p> - -<h4>(1) <i>Decomposition of Non-nitrogenous Compounds.</i></h4> - -<p>The simpler carbohydrates and starches are attacked and -decomposed by a large variety of bacteria. The addition<span class="pagenum" id="Page27">[27]</span> -of such substances to soil causes a rapid increase in bacterial -numbers. In nature the sugars are in all probability among -the first plant constituents to be destroyed during the decay -processes.</p> - -<p>A large proportion of plant tissues consist of cellulose -and its derivatives. These compounds are consequently of -great importance in the soil. Unfortunately our knowledge -of the processes by which cellulose is broken down in the -soil is very inadequate. The early experimental study of -cellulose decomposition, such as that of Tappeiner<a href="#Endnote3_60" class="fnanchor">[60]</a> and -Hoppe-Seyler,<a href="#Endnote3_33" class="fnanchor">[33]</a> was mostly carried out under conditions of -inadequate aeration, and the products of decomposition were -found to include methane and CO<sub>2</sub>, and sometimes fatty -acids and hydrogen. The bacteriology of this anaerobic -decomposition was studied by Omelianski,<a href="#Endnote3_54" class="fnanchor">[54]</a> who described -two spore-bearing organisms, one of which attacked cellulose -with the production of hydrogen, and the other with the -production of methane. Both species also produce fatty -acids and CO<sub>2</sub>. It is probable that these organisms operate -in the soil under conditions of inadequate aeration. In -swamp soils, in which rice is grown, it has been shown that -methane, hydrogen, and CO<sub>2</sub> are evolved in the lower layers. -In these soils, however, the methane and hydrogen are -oxidised when they reach the surface layers. This oxidation -is also effected by micro-organisms. Bacteria capable of -deriving energy by the oxidation of hydrogen gas have been -isolated and studied by Kaserer,<a href="#Endnote3_37" class="fnanchor">[37]</a> and by Nabokich and -Lebedeff,<a href="#Endnote3_52" class="fnanchor">[52]</a> while -Söhngen<a href="#Endnote3_57" class="fnanchor">[57]</a> has isolated an organism which -he named <i>Bacillus methanicus</i>, that was capable of oxidising -methane.</p> - -<p>Under normal conditions in cultivated soils, however, the -decomposition of cellulose takes place in the presence of an -adequate air supply, and so follows a different course from -that studied by Omelianski. Our knowledge of this aerobic -decomposition is very scanty. A number of bacteria, -capable of decomposing cellulose aerobically, are known. -A remarkable organism was investigated by Hutchinson and<span class="pagenum" id="Page28">[28]</span> -Clayton,<a href="#Endnote3_30" class="fnanchor">[30]</a> who named it <i>Spirochæta cytophaga</i>. This -organism, which they isolated from Rothamsted soil, though -placed among the <i>Spirochætoidea</i>, is of doubtful affinities. -During the active condition it exists for the most part as -thin flexible rods tapered at the extremities. This form -passes into a spherical cyst-like stage, at first thought to be -a distinct organism (<a href="#Fig2">Fig. 2</a>). <i>Spirochæta cytophaga</i> is very -aerobic, working actively, only at the surface of the culture -medium. It is very selective in its action. It appears -unable to derive energy from any carbohydrate other than -cellulose. Indeed, many of the simple carbohydrates, especially -the reducing sugars, are toxic to the organism in -pure culture. An extensive study of aerobic cellulose decomposition -by bacteria was made by McBeth and Scales,<a href="#Endnote3_50" class="fnanchor">[50]</a> -who isolated fifteen bacteria having this power. Five of -these were spore-forming organisms. Unlike <i>Spirochæta cytophaga</i>, -they are all able to develop on ordinary media such -as beef agar or gelatine, and are thus not nearly so selective -in their food requirements.</p> - -<div class="container w40em" id="Fig2"> - -<img src="images/illo036.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 2.</span>—<i>Spirochæta cytophaga.</i> Changes occurring in culture. (After -<span class="smcap">Hutchinson</span> and <span class="smcap">Clayton</span>.)</p> - -</div><!--container--> - -<p>We are at present ignorant as to which organisms are -most effective in decomposing cellulose in the soil under -field conditions, or what are the conditions best suited to -their activity. It is possible that fungi also help in the -decomposition of cellulose to a great extent. This subject -of the decomposition of cellulose offers one of the most -promising fields of research in soil bacteriology. The difficulty -of the subject is further increased by our present ignorance -of the chemical aspect of cellulose decomposition. It -has been supposed that the early decomposition products -are simpler sugars, but these are not found under conditions<span class="pagenum" id="Page29">[29]</span> -in which cellulose is being decomposed by pure cultures of -the bacteria mentioned above. Hutchinson and Clayton -found that their organism produced volatile acids, mucilage, -and a carotin-like pigment. The organisms isolated by -McBeth and Scales also produce acids, and in some cases -yellow pigments. It is known, however, that the decomposition -products of cellulose can be utilised as energy supply -for other organisms, such as nitrogen fixing bacteria.</p> - -<p>When plant remains decompose in the soil there are -ultimately produced brown colloidal bodies collectively -known as humus. The processes by which this humus is -produced are not yet properly understood. Humus is of -great importance in the soil, in rendering the soil suitable -for the growth of crops. It affects the physical properties -of the soil to a great extent. In the first place, it improves -the texture of the soil, making heavy clay soils more friable, -and loose sandy soils more coherent. Secondly, it has great -water-retaining powers, so that soils rich in organic matter -suffer comparatively little during periods of drought. And -lastly, it exerts a strong buffering effect against soil acids. -Now, it is one of the problems of present-day farming that -soil is becoming depleted of its humus. This is due to the -increasing scarcity of farmyard manure in many districts, -and the consequent use of mineral fertilisers to supply -nitrogen, potash, and phosphate to the crop. A need has -therefore arisen for a substitute for farmyard manure, by -means of which the humus content of soils may be kept up -in districts where natural manure is scarce.</p> - -<div class="container w40em" id="Fig3"> - -<img src="images/illo038.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 3.</span>—Cellulose decomposed by <i>S. cytophaga</i> in media with increasing -amounts of nitrogen. (After <span class="smcap">Hutchinson</span> and <span class="smcap">Clayton</span>.)</p> - -<div class="illotext w30em"> - -<p>X-axis: Milligrams of nitrogen supplied as sodium-ammonium phosphate.</p> - -<p>Y-axis: Milligrams of cellulose decomposed in 21 days.</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>It is well known that if fresh unrotted manure or straw -be added to the soil, it often produces harmful effects on the -succeeding crop. The problem, therefore, was to develop a -method by which fresh straw, before application to the soil, -could be made to rot down to a mixture of humus compounds -such as occur in well-rotted farmyard manure. The -solution of this problem came as a result of an investigation -by Hutchinson and Richards,<a href="#Endnote3_30b" class="fnanchor">[30<i>b</i>]</a> at Rothamsted, into food -requirements of the cellulose decomposing bacteria. They<span class="pagenum" id="Page30">[30]</span> -realised that since more than 10 per cent. of the dry weight -of bacteria consists of nitrogen, it would be necessary to supply -the cellulose decomposing bacteria with a supply of nitrogen, -in order that they should attain their greatest activity. -Experiments with cultures of <i>Spirochæta cytophaga</i> showed -that the amount of cellulose decomposed depended upon -an adequate supply of nitrogen for the organism (<a href="#Fig3">Fig. 3</a>). -Similarly, materials such as straw will scarcely decompose -at all if wetted with pure water. An adequate supply of -nitrogen compounds is needed to enable decomposition to -take place. Hutchinson and Richards tested the effect of -ammonium sulphate, and discovered experimentally the proportion -of ammonia to straw that produced the most rapid -decomposition. They found that if a straw heap was treated -with the correct proportion of ammonia, it decomposed into -a brown substance having the appearance of well-rotted -manure. This has resulted in the development of a commercial -process for making synthetic farmyard manure from -straw. The method of manufacture is as follows: A straw -stack is made and thoroughly wetted with water. The -correct amount of ammonium sulphate is then sprinkled on -the top and wetted, so that the solution percolates through<span class="pagenum" id="Page31">[31]</span> -the straw. The cellulose bacteria attack the straw, breaking -it down and assimilating the ammonia. This ammonia is not -wasted, as it is converted into bacterial protoplasm that -eventually decays in the soil. Field trials of this synthetic -manure show that it produces an effect closely similar to -that of natural farmyard manure.</p> - -<p>While cellulose and related carbohydrates are by far the -most important non-nitrogenous compounds left in the soil -by plants, there are other compounds whose destruction by -bacteria is of special interest. Such, for example, is the case -of phenol. This compound is produced by bacterial action -as a decomposition product of certain amino-acids. It -occurs in appreciable amounts in cow urine. It is probable -that it forms a common decomposition product in soil -and also in farmyard manure. If this phenol were to -persist in the soil, it would eventually reach a concentration -harmful to plant growth. It does not, however, -accumulate in the soil; indeed, if pure phenol or cresol -be added to ordinary arable soil, a rapid disappearance -occurs. This disappearance is of some practical importance, -since it limits the commercial use of these compounds as -soil sterilising agents. The cause of the disappearance has -been to some extent elucidated at Rothamsted,<a href="#Endnote3_58" class="fnanchor">[58]</a> where it -was found to be in part a purely chemical reaction with -certain soil constituents, and partly due to the activity of -bacteria capable of decomposing it. A large number of soil -bacteria have now been isolated that can decompose phenol, -meta-, para-, and ortho-cresol, and are able to use these -substances as the sole sources of energy for their life -processes. These organisms have a wide distribution, -having been found in soil samples taken from all over Great -Britain, from Norway, the Tyrol, Gough Island, Tristan da -Cunha and South Georgia. Soil bacteria have also been isolated -that are able to decompose and derive their energy from -naphthalene and from toluene. The ability of the bacteria -to break up the naphthalene is very remarkable, and all the -more so since they can hardly have come across this compound<span class="pagenum" id="Page32">[32]</span> -in the state of nature. The naphthalene organisms -have a distribution as world-wide as the phenol group.</p> - -<h4>(2) <i>Ammonia Production.</i></h4> - -<p>The second main group of products left in the soil by -higher plants are the nitrogen-containing compounds, such as -the proteins and amino-acids. Plant remains are not the -only source of organic nitrogen compounds available to soil -bacteria. There are, in addition, the dead bodies of other -soil organisms, such as protozoa and algæ. The relative -importance of these sources of nitrogen is not known, but -almost certainly varies greatly with the state of activity of -the various groups of the soil population. Bacteria are able -to utilise organic nitrogen compounds as energy sources, as -can be exemplified in the oxidation of a simple <span class="nowrap">amino-acid:—</span></p> - -<div class="container"> -<img src="images/illo040.png" alt="CH₂NH₂-COOH + 3O = 2CO₂ + H₂O +NH₃ + 152 Cal." class="high5em" /> -</div> - -<p>It will be seen that, in the acquirement of energy from -such a compound, ammonia is released as a by-product. It -is not certainly known what is the exact course of the -reactions brought about by bacteria in soil during the breaking-down -of organic nitrogen compounds, but they result in -the splitting off of most of the nitrogen as ammonia. Herein -lies the great importance of the process, for the production -of ammonia is an essential stage in the formation of nitrate -in the soil, and on the supply of nitrate the growth of -most crops largely depends.</p> - -<div class="container w40em" id="Fig4"> - -<img src="images/illo041.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 4.</span>—Quantities of ammonia produced by pure cultures from 5 grams of -casein in the presence of varying quantities of dextrose. (After <span class="smcap">Doryland</span>.)</p> - -<div class="illotext w20em"> - -<p>X-axis: Percentage of dextrose added.</p> - -<p>Y-axis: Milligrams of NH<sub>3</sub> produced.</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>It is very important to note that the production of this -ammonia is only a by-product in the economy of the bacteria, -the benefit that they derive from the reactions being due to -the release of energy involved in the decomposition. The -common ammonia-producing bacteria in the soil have been -found equally capable of deriving their energy by the oxidation<span class="pagenum" id="Page33">[33]</span> -of sugars and similar non-nitrogenous compounds. -<a href="#Fig4">Fig. 4</a> shows an experiment by Doryland,<a href="#Endnote3_17" class="fnanchor">[17]</a> in which -cultures of common soil bacteria were grown in peptone -solution, to which increasing quantities of sugar were -added. One can see that, as the amount of sugar is increased, -the production of ammonia is lowered, since the -bacteria are obtaining energy from the sugar instead of -from the nitrogen compound, peptone. Consequently, if -soil contains a quantity of easily decomposible carbohydrate -material, bacteria will derive their energy from -this source, and the production of ammonia and nitrate will -be lowered. Thus the addition of sugar or unrotted straw -to the soil often lowers the nitrate production, and consequently -reduces the crop yield. If the soil is sufficiently -rich in carbohydrate material, the bacteria may multiply -until the supply of organic nitrogen is used up, and then will -actually assimilate some of the ammonia and nitrate already -existing. There is thus a balance of conditions in the soil -due to varying proportions of nitrogenous and non-nitrogenous -energy material. When nitrogen compounds are the -predominant energy source, the bacteria utilise them, and<span class="pagenum" id="Page34">[34]</span> -ammonia is released. When a non-nitrogenous energy source -predominates, this is utilised and little or no ammonia is -released, and in extreme cases ammonia may be assimilated.</p> - -<p>Although a large number of the common organisms in -the soil produce ammonia in culture media containing peptone, -the relative importance of these in the soil has yet to -be decided. It was supposed that the spore-forming organisms -related to <i>Bacillus mycoides</i> were of chief importance. -This supposition dates from the work of Marchal,<a href="#Endnote3_49" class="fnanchor">[49]</a> who -studied the production of ammonia by an organism of this -group in culture solution, and found it to be a very active -ammonifier. As already mentioned, however, there is some -doubt as to whether the large spore-forming organisms are -very active under soil <span class="nowrap">conditions.<a href="#Endnote3_12" class="fnanchor">[12]</a><sup>,</sup> -<a href="#Endnote3_13" class="fnanchor">[13]</a></span> The existence of -rapid fluctuations in nitrate content, found to exist in soil, -may in the future indicate which are the most active of the -common bacteria in the soil itself by enabling us to observe -which types increase during periods of rapid ammonia and -nitrate formation.</p> - -<h4>(3) <i>Nitrate Production.</i></h4> - -<p>The ammonia produced in the soil under normal field -conditions is rapidly oxidised successively to nitrite and to -nitrate, a process known as nitrification. The process of -nitrification is more rapid than that of ammonia production, -with the consequence that no more than traces of ammonia -are able to accumulate. The rate at which nitrate is formed -in the soil is consequently set by the slower process of -ammonia production.</p> - -<p>The work of Schloesing and of Warington showed that the -oxidation of ammonia was the work of living organisms. It -is, however, to Winogradsky’s isolation and study of the -causative organisms that we owe our present knowledge of -the biology of the process. By a new and ingenious technique, -he isolated from soil two remarkable groups of bacteria -that bring about nitrification. The first group oxidises -ammonium carbonate to nitrite, and was divided by<span class="pagenum" id="Page35">[35]</span> -Winogradsky into the two genera, <i>Nitrosomonas</i>, a very -short rod-like organism bearing a single flagellum, and -<i>Nitrosococcus</i>, a non-motile form found in South America. -The second group oxidises nitrites to nitrates. They are -minute pear-shaped rods to which he gave the name <i>Nitrobacter</i>.</p> - -<p>Winogradsky found that the first, or nitrite-producing -group, would live in a culture solution <span class="nowrap">containing:—</span></p> - -<table class="standard"> - -<tr> -<td class="number">2·25</td> -<td class="general">grams</td> -<td class="text">ammonium sulphate,</td> -</tr> - -<tr> -<td class="number">2·0 </td> -<td class="general">„</td> -<td class="text">sodium chloride,</td> -</tr> - -<tr> -<td class="number">1·0 </td> -<td class="general">„</td> -<td class="text">magnesium carbonate,</td> -</tr> - -<tr> -<td> </td> -<td colspan="2" class="text">to the litre of well water.</td> -</tr> - -</table> - -<p>Nitrobacter would grow in a similar medium containing -sodium nitrite instead of ammonium sulphate. There being -no organic carbon in these media, the organisms had no -source of carbon for their nutrition, except the CO<sub>2</sub> of the -air, or possibly that of bicarbonate in solution. It therefore -followed that the organisms must obtain their carbon supply -from one of these sources. Unlike green plants, the nitrous -and nitric organisms are able to carry on this carbon assimilation -in the dark, and must therefore obtain the energy -needed for the process from some chemical reaction. The -only sources of energy in Winogradsky’s solutions were the -nitrogen compounds, and it consequently followed that the -organisms must derive their energy supply by the oxidation -of ammonia and nitrite respectively. The release of energy -obtained by these two reactions has been calculated by Orla-Jensen -to be as <span class="nowrap">follows:—</span></p> - -<p class="chemform">(NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> + 3O<sub>2</sub> = 2HNO<sub>2</sub> + CO<sub>2</sub> + 3H<sub>2</sub>O + 148 Cals.</p> - -<p class="chemform">KNO<sub>2</sub> + O = KNO<sub>3</sub> + 22 Cals.</p> - -<p>The exact process by which ammonium carbonate is -converted into nitrite is not at present known. The two -groups of organisms are extremely selective in their source -of energy. The nitrous organisms can derive their energy -only by the oxidation of ammonia to nitrite, and the nitric<span class="pagenum" id="Page36">[36]</span> -organisms only by the oxidation of nitrite to nitrate. In -culture media they are, indeed, inhibited by soluble organic -compounds such as sugars. Under natural conditions, however, -they appear to be less sensitive, since ammonium carbonate -is readily nitrified in substrata rich in organic matter. -The rapid nitrification that takes place during the purification -of sewage is an example of this. The conditions in -culture, with regard to aeration and the removal of metabolic -products from the neighbourhood of the organisms, are -very different from those in the soil, and perhaps account -for the discrepancies found.</p> - -<p>The oxidation of ammonium carbonate by nitrosomonas -results in the formation of nitrous acid. The organisms are -very sensitive to acidity, and can only operate if the nitrous -acid produced is neutralised by an available base. In normal -soils calcium carbonate supplies this base, and in acid soils -the formation of nitrite is, as a rule, increased by the addition -of lime, or of calcium or magnesium carbonate. There is -evidence that in the absence of calcium carbonate, other -compounds can be used as a base. It was found by Hopkins -and Whiting<a href="#Endnote3_32" class="fnanchor">[32]</a> that in culture solution the nitrifying organisms -could use insoluble rock phosphate as a base, producing -therefrom the soluble acid phosphate. There is evidence, -however, that in ordinary soil containing calcium carbonate -very little solution of phosphate takes place in this way. -The further oxidation of nitrite to nitrate by <i>Nitrobacter</i> -does not produce acid, and requires no further neutralising -base.</p> - -<p>The nitrate produced in this way is the main source of -nitrogen supply to plants under normal conditions. Experiments -have shown that a number of plants are capable of -utilising ammonia as a source of nitrogen, and Hesselmann<a href="#Endnote3_34" class="fnanchor">[34]</a> -has found forest soils in Sweden where no nitrification was -proceeding, and where, therefore, plants would presumably -obtain their nitrogen in this way, but such cases must be -regarded as exceptional.</p> - -<p>Another group of bacteria capable of deriving their energy<span class="pagenum" id="Page37">[37]</span> -from an inorganic source exists in the soil. This comprises -the sulphur bacteria, which are able to derive energy by the -oxidation of sulphur, sulphides, or thiosulphates to sulphuric -<span class="nowrap">acid:—</span></p> - -<p class="chemform">S + 3O + H<sub>2</sub>O = H<sub>2</sub>SO<sub>4</sub> + 141 Cals.</p> - -<p>One organism studied by Waksman and Joffe<a href="#Endnote3_63" class="fnanchor">[63]</a> is able -to live in inorganic solution, deriving its carbon from carbon -dioxide. The sulphur bacteria have recently come into -prominence in America owing to their faculty for producing -acid. Thus Thiospirillum will increase the acidity of its -medium to a reaction of P<sub>H</sub> 1·0 before growth ceases. The -potato scab disease in America is now treated by composting -with sulphur. This treatment depends on the production of -sulphuric acid by the sulphur oxidising bacteria, which -renders the soil too acid for the parasite. There is some -evidence also that acid thus produced can be used to render -insoluble phosphatic manures more available in the soil.</p> - -<p>Analogous to the sulphur organisms are certain bacteria -isolated from sheep dig tanks in South Africa by Green,<a href="#Endnote3_28b" class="fnanchor">[28<i>b</i>]</a> -which can derive energy by the oxidation of sodium arsenite -to arsenate.</p> - -<h4>(4) <i>Anaerobic Respiration.</i></h4> - -<p>As is seen in the examples mentioned, energy is commonly -obtained by bacteria through an oxidation process -in which free oxygen is utilised. In water-logged soil, however, -or in soil overloaded with organic matter, anaerobic -bacteria may develop, which obtain their oxygen from oxidised -compounds. Thus there are soil organisms described -by Beijerinck<a href="#Endnote3_2" class="fnanchor">[2]</a> and others which can obtain oxygen by -reducing sulphates to sulphides.</p> - -<p>A more important source of oxygen under these conditions -is nitrate, which can supply oxygen to a larger -number of bacteria. The stage to which the reduction can -be carried varies according to the organism. A very large -number of bacteria are capable of reducing nitrates to -nitrites. Many can reduce nitrate to ammonia, and some<span class="pagenum" id="Page38">[38]</span> -can produce an evolution of nitrogen gas from nitrate. The -effects of nitrate reduction, therefore, appear under water-logged -conditions in soils. For example, in swamp soils in -which rice is grown, it has been found by Nagaoka,<a href="#Endnote3_53" class="fnanchor">[53]</a> in -Japan, that treatment with nitrate of soda depresses the -yield, probably owing to the formation of poisonous nitrites -by reduction.</p> - -<p>Under normal conditions of well aerated soil, however, it -is unlikely that the reduction of nitrate is of great importance. -In such soils the activities through which bacteria acquire -their energy are, as we have seen, of vital importance to the -plant, resulting in the disintegration of plant tissues, with -the ultimate formation of humus, and in the production of -nitrate.</p> - -<p>In their activities connected with the building up of their -protoplasm, bacteria may, on the other hand, compete with -the plant. These activities and their consequences will be -reviewed in the following chapter.</p> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page39">[39]</span></p> - -<h2 class="nobreak">CHAPTER III.<br /> -<span class="chaptitle">SOIL BACTERIA.</span></h2> - -<h3 class="nobreak"><i>C.</i> <span class="smcap">Activities Connected with the Building-up of -Bacterial Protoplasm.</span></h3> - -<h4>(1) <i>Composition of Bacteria.</i></h4> - -</div><!--chapter--> - -<p>The activities of the soil bacteria that we have yet to -consider are those connected with the building-up from -simpler materials of the protoplasm of the bacterial cell. -It is important to bear in mind that this process is one -requiring an expenditure of energy on the part of the -organism. The sources of energy we have already considered.</p> - -<p>The bodies of bacteria contain the same elements common -to other living matter. Analyses of various bacteria have -been made by a number of workers. About 85 per cent. of -their weight is made up of water. This analysis of Pfeiffer’s -Bacillus by Cramer<a href="#Endnote3_15" class="fnanchor">[15]</a> shows the typical percentages of -carbon, nitrogen, hydrogen, and ash in the dry <span class="nowrap">matter:—</span></p> - -<p class="tabhead"><i>Composition of Pfeiffer’s Bacillus (Cramer).</i></p> - -<table class="standard"> - -<tr> -<td class="text">C</td> -<td class="numbers">50  </td> -<td class="text">per cent.</td> -</tr> - -<tr> -<td class="text">N</td> -<td class="numbers">12·3</td> -<td class="general">„</td> -</tr> - -<tr> -<td class="text">H</td> -<td class="numbers">6·6</td> -<td class="general">„</td> -</tr> - -<tr> -<td class="text"><span class="padr2">Ash</span></td> -<td class="numbers">9·1</td> -<td class="general">„</td> -</tr> - -</table> - -<p class="noindent">About 65-70 per cent. of the dry matter of bacteria consists -of protein.</p> - -<h4>(2) <i>Sources of Carbon.</i></h4> - -<p>The biggest constituent of the dry matter of bacteria is -therefore carbon. In the soil, bacteria find an abundance of<span class="pagenum" id="Page40">[40]</span> -organic matter from which they may derive their carbon -supply. A special case, however, is furnished by the nitrifying -organisms, certain sulphur oxidising bacteria, and others -that derive their carbon from the CO<sub>2</sub> of the soil atmosphere. -The sources from which these special groups obtain the -necessary energy to accomplish this, we have already -considered.</p> - -<h4>(3) <i>Assimilation of Nitrogen Compounds.</i></h4> - -<p>Of chief importance in its consequences are the means -adopted by bacteria to obtain their nitrogen supply.</p> - -<p>There is some reason to believe that soil bacteria do not -take up protein and peptones as such, but must first break -down these bodies into simpler compounds. When a sufficient -amount of easily decomposable organic nitrogen is -present in the soil, the ammonifying bacteria use such -compounds as sources of energy, and in this case have a -nitrogen supply exceeding their requirements.</p> - -<p>But where there is an excess of carbohydrate or other -non-nitrogenous source of energy available in the soil, the -case is different. Here the organisms have a supply of -energy which enables them to multiply rapidly until the -organic nitrogen is insufficient for their needs. Hence they -turn to the ammonia and nitrate present in the soil, and -build up their proteins from this source. Doryland<a href="#Endnote3_17" class="fnanchor">[17]</a> has -shown that many common soil ammonifiers assimilate -ammonia and nitrate when supplied with carbohydrate. -There may thus be a temporary loss of nitrate from soil -when sugar, starch, straw, or such materials are added to it.</p> - -<h4>(4) <i>Fixation of Free Nitrogen.</i></h4> - -<p>The bacteria that we have so far considered take up their -nitrogen directly from compounds containing this element. -There remain, however, a comparatively small but very -important group of bacteria possessing the power of causing -elemental nitrogen to combine, and of building it up into<span class="pagenum" id="Page41">[41]</span> -their proteins. This fixation of nitrogen by micro-organisms -is a vital step in the economy of nature. Losses of nitrogen -from the land are continually occurring through the washing-out -of nitrates by rain, and through the evolution of gaseous -nitrogen during the processes of decay. To maintain the -supply of combined nitrogen which is essential to living -organisms, there must therefore be a compensating process -by which the supply of nitrogen compounds in the soil is -kept up.</p> - -<p>It was discovered in the middle of the nineteenth century -that if soil were kept moist and exposed to the air, there was -an increase in the amount of nitrogen compounds present. -Berthelot, in 1893, studied the nitrogen relationships of soil, -and recognised that this fixation of nitrogen in soil was the -work of micro-organisms.</p> - -<p>Winogradsky followed up his work and isolated from soil -a large anaerobic spore-forming organism, capable of fixing -nitrogen, to which he gave the name <i>Clostridium pasteurianum</i>. -In 1901 the investigations of Beyerinck, in Holland, led to -the important discovery of a group of large aerobic organisms, -which he named <i>Azotobacter</i>. These were found to be very -active in fixing free nitrogen. More recently, a number of -other nitrogen-fixing bacteria have been described, and the -property has been found to exist to a small extent in several -previously well-known organisms.</p> - -<p>It becomes important to determine which are the groups -of bacteria whose nitrogen-fixing powers are of chief importance -in the soil.</p> - -<p>On account of its energetic fixation of nitrogen in culture -media, <i>Azotobacter</i> has attracted the greatest attention of -workers. The evidence seems to be consistent with the view -that <i>Azotobacter</i> is of importance in the soil. Thus the -distribution of <i>Azotobacter</i> would appear to be world-wide. -It is found all over Western Europe and the United States. -Lipman and Burgess<a href="#Endnote3_45" class="fnanchor">[45]</a> found it in soils collected from Italy -and Spain, Smyrna, Cairo, the Fayum, the Deccan in India, -Tahiti, Hawaii, Mexico, Guatemala, and Canada. C. M.<span class="pagenum" id="Page42">[42]</span> -Hutchinson<a href="#Endnote3_29" class="fnanchor">[29]</a> found it to be distributed throughout India. -It was found by Omelianski<a href="#Endnote3_55" class="fnanchor">[55]</a> to be widely distributed in -European and Asiatic Russia, and by Groenewege<a href="#Endnote3_28" class="fnanchor">[28]</a> in Java. -Ashby<a href="#Endnote3_1" class="fnanchor">[1]</a> at Rothamsted, isolated it from soils from the -Transvaal, East Africa, and Egypt. Also, an association -has sometimes been found between the ability of a soil to -fix nitrogen and the occurrence and vigour of its <i>Azotobacter</i> -flora. Thus Lipman and Waynick<a href="#Endnote3_46" class="fnanchor">[46]</a> found that if soil from -Kansas were removed to California, its power to produce -a growth of <i>Azotobacter</i>, when inoculated into a suitable -medium, was lost, and, at the same time, its nitrogen-fixing -power was greatly reduced. Moreover, it is known that -conditions favourable to the fixation of nitrogen by <i>Azotobacter</i> -in cultures on the whole favour nitrogen fixation in -soils. The conditions that favour other aerobic nitrogen-fixing -bacteria are, however, not sufficiently distinct to make such -evidence of great value.</p> - -<p>It is usually found that nitrogen fixation is most active -in well-aerated soil. Thus Ashby,<a href="#Endnote3_1" class="fnanchor">[1]</a> at Rothamsted, found -the nitrogen-fixing power of a soil to decrease rapidly with -depth. Similar results were obtained in Utah by Greaves. -This suggests, at first sight, that anaerobic nitrogen fixers -are unimportant under normal soil conditions. It is, however, -quite possible that they may assume an importance -when acting in conjunction with aerobic organisms. Thus -Omelianski and Salunskov<a href="#Endnote3_55" class="fnanchor">[55]</a> found that beneficial association, -or symbiosis, could occur between <i>Azotobacter</i> and -<i>Clostridium pasteurianum</i>, the former absorbing oxygen from -the surroundings, and thus creating a suitable anaerobic -environment for the <i>Clostridium</i>.</p> - -<p>The question of symbiosis of nitrogen-fixing bacteria with -each other and with other organisms offers an inviting field -for research. There is evidence that this factor may have -considerable importance. Beijerinck and Van Delden<a href="#Endnote3_3" class="fnanchor">[3]</a> early -recognised that <i>Azotobacter</i> in mixed cultures fixed more -nitrogen than in pure cultures. <i>Granulobacter</i>, an organism -which they found to be commonly associated with <i>Azotobacter</i><span class="pagenum" id="Page43">[43]</span> -in crude cultures, appears to increase its nitrogen-fixing -powers (Krzeminiewski).<a href="#Endnote3_41" class="fnanchor">[41]</a> It -was also found by Hanzawa<a href="#Endnote3_31" class="fnanchor">[31]</a> -that a greater fixation of nitrogen was obtained when two -strains of <i>Azotobacter</i> were grown together. A symbiosis -between <i>Azotobacter</i> and green algæ has been described, and -will be further <a href="#Page99">discussed</a> by Dr. Bristol. It is likely that this -association may be of importance under suitable conditions -on the soil surface where the algæ are exposed to light.</p> - -<p>The combination of elemental nitrogen is an endothermic -process which requires a very considerable amount of energy -for its accomplishment. This fact is well illustrated by the -various commercial processes in use for fixation of atmospheric -nitrogen. The nitrogen-fixing bacteria obtain this -energy from the carbon compounds in the soil. A number -of compounds were compared as sources of energy by Löhnis -and Pillai,<a href="#Endnote3_47" class="fnanchor">[47]</a> who tested their effect on the amounts of nitrogen -fixed by <i>Azotobacter</i> in culture. It was found that mannitol -and the simpler sugars give the best results as sources of -energy, but that other organic compounds can also be used. -Mockeridge<a href="#Endnote3_51" class="fnanchor">[51]</a> has adduced evidence that ethylene glycol, -methyl-, ethyl-, and propyl-alcohol, lactic, malic, succinic, -and glycocollic acids could also be utilised. Since so large -a part of the organic matter added to soil is in the form of -celluloses, it is of great importance to ascertain how far these -compounds and their decomposition products can be utilised -in nitrogen fixation. Stubble, corn-stalks and roots, oak -leaves, lupine and lucerne tops, maple leaves, and pine -needles may all serve as useful sources of energy to nitrogen-fixing -organisms in the soil. Pure cellulose cannot apparently -be used as a source of energy, but when acted upon -by cellulose decomposing organisms, it becomes available as -a source of energy. Hutchinson and Clayton, at Rothamsted, -found that a fixation of nitrogen could be brought about by -mixed cultures of <i>Azotobacter</i>, and of the cellulose attacking -<i>Spirochæta cytophaga</i>, when grown in cultures containing -pure cellulose. It is not known how far cellulose decomposition -must proceed to produce an effective source of energy,<span class="pagenum" id="Page44">[44]</span> -nor what are the substances thus produced that are utilised. -This point will not be decided until something more is known -of the course of changes in the breaking-down of cellulose in -the soil.</p> - -<p>The amount of nitrogen fixed per unit of energy material -decomposed varies greatly, according to the organism and -the conditions. Winogradsky found that his <i>Clostridium</i> -assimilated 2-3 mgs. of nitrogen per gram of sugar consumed. -Lipman found that <i>Azotobacter</i> fixed 15-20 mgs. of nitrogen -per gram of mannite consumed.</p> - -<div class="container w40em" id="Fig5"> - -<img src="images/illo052.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 5.</span></p> - -<div class="illotext w40em"> - -<p>Caption: Azotobacter. Decrease in efficiency in N fixation with age of culture. (Koch & Seydel.)</p> - -<p>X-axis: Days.</p> - -<p>Y-axis: Milligrams of Nitrogen fixed per gram of dextrose consumed.</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>It is found, however, that in liquid culture, the ratio of -nitrogen fixed to carbohydrates oxidised varies according to -the age of the culture, falling off rapidly as the age increases<a href="#Endnote3_42" class="fnanchor">[42]</a> -(<a href="#Fig5">Fig. 5</a>). This decreasing efficiency in cultures may be due -to the accumulation of metabolic products such as would -not occur under soil conditions. Indeed, the efficiency of -<i>Azotobacter</i> in a sand culture has been found by Krainskii<a href="#Endnote3_39" class="fnanchor">[39]</a> -to be considerably greater than in solution. It is thus probable -that in soil the nitrogen-fixing organisms are less<span class="pagenum" id="Page45">[45]</span> -wasteful of energy material than under the usual laboratory -conditions. It is to be hoped that future research will indicate -what are the conditions that produce the greatest -economy of energy material in nitrogen fixation.</p> - -<p>The fixation of nitrogen in soil is depressed by the presence -of considerable amounts of nitrates. This is, in all probability, -due to the fact that nitrogen-fixing organisms are able -to utilise compounds of nitrogen where these are available. -The energy needed to build up amino-acids and proteins -from nitrate or ammonia is, of course, far less than that -required to build up these substances from elemental nitrogen. -It is, therefore, not surprising that where nitrate is -available, <i>Azotobacter</i> will use it in preference to fixing -atmospheric nitrogen.<a href="#Endnote3_5" class="fnanchor">[5]</a></p> - -<p class="tabhead" id="TabIII">TABLE III.—ASSIMILATION OF NITRATES.<br /> -<span class="smcap">By Azotobacter in Pure Culture</span>—(<i>Bonazzi</i>).</p> - -<table class="standard"> - -<tr class="bt bb"> -<th class="bl br"> </th> -<th class="br">Nitrate<br />and<br />Nitrite<br />Present.</th> -<th class="br">Organic<br />Nitrogen<br />and<br />Ammonia<br />Present.</th> -<th class="br">Total<br />Fixed<br />or Lost.</th> -</tr> - -<tr> -<th class="bl br"> </th> -<th class="w4 br">mgs.</th> -<th class="w4 br">mgs.</th> -<th class="w4 br">mgs.</th> -</tr> - -<tr> -<td class="text bl br"><span class="padr1"><i>Culture with nitrate</i>—</span></td> -<td class="br"> </td> -<td class="br"> </td> -<td class="br"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2">At beginning</span></td> -<td class="general br">8·55</td> -<td class="general br">0·76</td> -<td class="general br">—</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2">After growth</span></td> -<td class="general br">0·2 </td> -<td class="general br">8·71</td> -<td class="general br">- 0·4 </td> -</tr> - -<tr> -<td class="text blankbefore75 bl br"><span class="padr1"><i>Culture without nitrate</i>—</span></td> -<td class="br"> </td> -<td class="br"> </td> -<td class="br"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2">At beginning</span></td> -<td class="general br">—</td> -<td class="general br">0·76</td> -<td class="general br">—</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padl2">After growth</span></td> -<td class="general br">—</td> -<td class="general br">4·50</td> -<td class="general br">+ 3.74</td> -</tr> - -<tr> -<td colspan="4" class="general">(Growth period—24 days at 25° C.)</td> -</tr> - -</table> - -<p>The chemical process by which nitrogen is fixed is quite -unknown, although a number of speculative suggestions -have been made. The appearance of considerable amounts -of amino acids in young cultures of <i>Azotobacter</i> suggests that -these may be a step in the process, but at present the data -are too inconclusive to form a basis for theorising.</p> - -<p><i>Azotobacter</i> is very rich in phosphorus, an analysis of the -surface growth in <i>Azotobacter</i> cultures, made by Stoklasa, -giving about 60 per cent. of phosphoric acid in the ash. In -cultures it has been found that a considerable amount of<span class="pagenum" id="Page46">[46]</span> -phosphate is needed to produce full development. As would -be expected, therefore, nitrogen fixation in soil is often -greatly stimulated by the addition of phosphates. Christensen -has, indeed, found soils where lack of phosphate was the -limiting factor for <i>Azotobacter</i> growth.</p> - -<p><i>Azotobacter</i> is very intolerant of an acid medium, and is -very dependent on the presence of an available base. In -cultures this is usually provided in the form of calcium or -magnesium carbonate. Gainey<a href="#Endnote3_21" class="fnanchor">[21]</a> found that <i>Azotobacter</i> -occurred in soils having an acidity not greater than P<sub>H</sub> 6·0, -and <span class="nowrap">Christensen,<a href="#Endnote3_7" class="fnanchor">[7]</a><sup>,</sup> -<a href="#Endnote3_9" class="fnanchor">[9]</a></span> -in Denmark, has found a close association -between the occurrence of <i>Azotobacter</i> in soils and the presence -of an adequate supply of calcium carbonate. So close was -this association that he devised a technique based on this -fact for detecting a deficiency of lime in a soil sample.</p> - -<p>In addition to the groups already discussed, there is a -remarkable and important group of nitrogen-fixing bacteria -that inhabit and can carry on their functions within the root -tissues of higher plants. It has been known at least from -classical times that certain leguminous plants would, under -suitable conditions, render the soil more productive. On -the roots of leguminosæ small tubercles are commonly found. -These were noted and figured by Malpighi in the seventeenth -century, and for a long time were regarded as root-galls. As -was described in <a href="#Page1">Chapter I.</a>, the true nature of these tubercles -was finally elucidated by Hellriegel and Wilfarth in 1886. -As the result of a series of pot experiments, they made the -very brilliant deduction that the ability to fix nitrogen, -possessed by the legumes, was due to bacteria associated -with them in the tubercles.</p> - -<p>These bacteria were finally isolated and studied in pure -culture by Beijerinck. Since then a very great deal of -literature has accumulated on the subject of the nodule-producing -bacteria, which it is impossible to deal with in a -small space. The nodule organism, <i>Bacillus radicicola</i>, when -grown on suitable media, passes through a number of different -changes in morphology. The most connected account of<span class="pagenum" id="Page47">[47]</span> -these changes is given in a paper by Bewley and Hutchinson.<a href="#Endnote3_4" class="fnanchor">[4]</a> -In a vigorous culture the commonest type is a rod-shaped -bacillus which may or may not be motile. As these get -older they often become branched, or irregular in shape, the -formation of these branched forms being perhaps due to -conditions in the medium. These irregular forms, known -as “bacteroids,” are a characteristic type in the nodules. -Their production in culture media has been found to be -stimulated by sugars and organic acids such as would occur -in their environment within the host plant. In the older -rods and bacteroids the staining material becomes condensed -into granules, and finally the rods disintegrate or break up -into coccoid forms. By suitable culture conditions, Bewley -and Hutchinson obtained cultures consisting almost entirely -of this stage. If such a culture be inoculated into a fresh -medium rich in sugar, the swarmer stage appears in great -numbers. These swarmers are very minute coccoid rods, -·9 × ·18 in size, that are actively motile. They apparently -develop later into the rod stage.</p> - -<div class="container w40em" id="Fig6"> - -<img src="images/illo055.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 6.</span>—<i>Bacillus radicicola.</i> -Stages in the life cycle. (After <span class="smcap">Hutchinson</span> and -<span class="smcap">Bewley</span>.)</p> - -<div class="illotext w20em"> - -<p>Motile Rods</p> - -<p class="right">Vacuolated Stage</p> - -<p>“Swarmers”</p> - -<p class="right">“Bacteroids”</p> - -<p>“Pre-swarmers”</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>Very little is known as to the life of the organism in the -soil. It is able, however, to fix nitrogen in cultures, and it<span class="pagenum" id="Page48">[48]</span> -has been <span class="nowrap">claimed<a href="#Endnote3_35" class="fnanchor">[35]</a><sup>,</sup> -<a href="#Endnote3_48" class="fnanchor">[48]</a></span> -that it can do so in the soil outside -the plant, so that it is possible that we must take it into -consideration in this connection. More knowledge is needed -as to the optimum conditions for the growth of the organism -in the soil. It seems to be more tolerant of acid soil conditions -than <i>Azotobacter</i>. The limiting degree of acidity has -been found to vary among different varieties of the organism -from P<sub>H</sub> 3·15 to P<sub>H</sub> 4·9.</p> - -<p>A long controversy has been held as to whether the nodule -organisms found in different host-plants all belong to one -species, or whether there are a number of separate species, -each capable of infecting a small group of host-plants. As -the term “species” has at present no exact meaning when -applied to bacteria, the discussion in this form is unlikely -to reach a conclusion. The evidence seems to show that -the nodule organisms form a group that is in a state of -divergent specialisation to life in different host-plants, and -that this specialisation has reached different degrees with -different hosts. Thus the organisms from the nodule of the -pea (<i>Pisum sativum</i>) will also produce nodules on vicia, -Lathyrus, and Lens, but seem to have lost the ability normally -to infect other legumes. On the other hand, the bacteria -from the nodules of the Soy Bean (<i>Glycine hispida</i>) have -become so specialised that they do not infect any other -genus of host-plant, and soy beans are resistant to infection -by other varieties of the nodule organism. Burrill and -Hansen,<a href="#Endnote3_6" class="fnanchor">[6]</a> after an extensive study, divided the nodule -bacteria into eleven groups, within each of which the host-plants -are interchangeable. The existence of different -groups of nodule organisms has been confirmed by the -separate evidence of serological tests (Zipfel, Klimmer, and -Kruger).<a href="#Endnote3_40" class="fnanchor">[40]</a> The results of cross-inoculation tests have sometimes -been conflicting. It seems, indeed, that the host-plant -has a variable power of resisting infection, so that when its -resistance is lowered it may be capable of infection by a -strange variety of the nodule organism. The question that -has thus arisen of the ability of the legume to resist infection<span class="pagenum" id="Page49">[49]</span> -is of fundamental importance, and its elucidation should -throw light on the relation of plants to bacterial infection -as a whole.</p> - -<p>The stage of the organism that infects the plant is not at -present known. It may be supposed that it is the motile -“swarmer.” The entry is normally effected through the -root-hairs. The hair is attacked close to the tip, and an -enzyme is apparently produced which causes the tip to bend -over in a characteristic manner. The organisms multiply -within the root hair and pass down it, producing a characteristic -gelatinous thread filled with bacteria, in the rod form. -This “infection thread” passes down into the cells of the -root tissue, where it branches profusely. In young stages of -nodule formation the branches can be seen penetrating cells -in the pericycle layer. Rapid cell division of these root -cells is induced. In the course of this cell division abnormal -mitotic figures are sometimes found, such as occur in pathological -growths. The cells push outward the root cortical -layer, and so form a nodule.</p> - -<p>Certain of the cells in the centre of the nodule become -greatly enlarged, and in the fully grown nodule are seen to -be filled with bacteria. Differences have been described in -the morphology of the organisms in different parts of the -nodule.<a href="#Endnote3_62" class="fnanchor">[62]</a> Whether the different stages of the organism are -equally capable of fixing nitrogen, or what is the significance -of these stages within the nodule, is not certainly known. -It has been held that it is the irregular bacteroid forms -that are chiefly concerned with nitrogen fixation. In older -nodules the organisms become irregular and stain faintly, -and the bacteroidal tissue breaks down, the nodule finally -decaying. In the fixation of nitrogen that occurs in the -nodules, the bacteria without doubt derive the necessary -energy from the carbohydrates of the host-plant. There is -evidence that the plant assists the process of fixation by -removing soluble metabolic products from the neighbourhood -of the bacteria. Golding<a href="#Endnote3_22" class="fnanchor">[22]</a> was able to obtain a -greatly increased fixation of nitrogen in artificial cultures<span class="pagenum" id="Page50">[50]</span> -by arranging a filtering device so as to remove the products -of metabolism.</p> - -<p>The great practical importance of leguminous crops in -agriculture has led to numerous attempts being made to -increase their growth, and the fixation of nitrogen in them, -by inoculating the seed or the soil with suitable nodule-bacteria. -This inoculation can be effected either with soil in -which the host-plant has been successfully grown, and which -should consequently contain the organism in fair numbers, -or else pure cultures of the organisms isolated from nodules -may be used. Very varying results have been obtained with -inoculation trials.</p> - -<p>In farm practice a leguminous crop has often been introduced -into a new area where it has never previously grown. -In such soil it is very probable that varieties of the nodule -organism capable of infecting the roots may not exist. In -such cases inoculation with the right organism or with -infected soil often produces good results.</p> - -<p>The more difficult case, however, is that in which the -legume crop has been grown for a long time in the locality, -and where the soil is already infected with right organisms. -This, the more fundamental problem, applies especially to -this country. Here it would seem that inoculation with a -culture of the organism will benefit the plant only (1) if the -naturally occurring organisms are present in very small -numbers; or (2) if the organisms in the culture added are -more virulent than those already in the soil. The problem -of successful inoculation would therefore seem to be bound -up with that of grading up the infective virulence of the -organism to a higher level.</p> - -<p>Successful nodule development in a legume crop is also -dependent to a large degree on the soil conditions. The -effects of soil conditions on nodule development have been -studied by numerous workers. Moisture has been found -very greatly to affect the nodule development. Certain salts -have a very definite effect on nodule formation.<a href="#Endnote3_64" class="fnanchor">[64]</a> Their -effect on the number of nodules developing has been studied,<span class="pagenum" id="Page51">[51]</span> -but the reason for this effect is unusually difficult to decide. -The action is usually a complex one. Thus phosphates are -known to stimulate nodule formation. They probably act -in several ways. In the first place, they may cause the -nodule organisms to multiply in the soil; in the second -place, they produce a greater root development in the plant, -thus increasing the chances of infection; and in the third -place, Bewley and Hutchinson<a href="#Endnote3_4" class="fnanchor">[4]</a> have found that phosphates -cause the appearance of the motile stage of the organism in -cultures. A real understanding of the influence of environment -on nodule production will produce great improvements -in our methods of legume cropping.</p> - -<h3><i>D.</i> <span class="smcap">The Relation of Bacterial Activities to Soil -Fertility.</span></h3> - -<p>The various activities of the soil bacteria have a vital -importance to the growth of higher plants, which are dependent -for their existence on certain of these processes. In -the first place, as we have seen, bacteria decompose the -tissues of higher plants and produce humus materials, which -are essential to the maintenance of good physical properties -in the soil. Then the nitrate supply on which most higher -plants depend is produced by the decomposition of organic -nitrogen compounds by bacteria in their search for energy. -The depletion of the total nitrogen content of the soil through -rain and through the removal of nitrogen in the crops, is to -some extent compensated by the fixation of atmospheric -nitrogen by certain bacteria. On the other hand, in the -assimilation of nitrogen compounds to build up protein, the -bacteria are competing with higher plants for one of their -essential food constituents, and their action may, under -certain conditions, cause a temporary nitrogen starvation. -One must remember, however, that large quantities of -nitrate are lost from field soils by washing-out through rain -action, especially in winter. The assimilation of nitrate and -ammonia by micro-organisms keeps some of this nitrogen in -the soil, and at certain periods may thus be beneficial.</p> - -<p><span class="pagenum" id="Page52">[52]</span></p> - -<p>There is another important respect in which soil bacteria -influence plant growth. Their activities result in the release -of inorganic salts, such as potash and phosphates, in a form -available for the use of plants. The release of phosphorus -and potassium compounds takes place in two ways. In the -first place, organic matter containing phosphorus and potassium, -in an insoluble form, is attacked by bacteria, resulting -in these elements being set free as inorganic salts available -to the higher plant. Secondly, much of the phosphorus -supplied to the soil from rock minerals is present as insoluble -phosphates, such as apatite and iron phosphate. Much of -the potassium, too, is derived from insoluble silicate minerals. -In both cases the conversion of the insoluble minerals into -soluble phosphates and potassium compounds is brought -about to a large extent by solution in water containing carbonic -and other acids. These acids are largely produced by -micro-organisms, which, in addition to carbonic acid, produce -organic acids, and in specialised cases, sulphuric and nitrous -acids. It has been found, for example, that in a compost of -soil with sulphur and insoluble phosphate, sufficient sulphuric -acid may be produced by the oxidation of the sulphur by -bacteria to convert an appreciable amount of phosphate into -a soluble form. When we consider the functions performed -by soil bacteria, therefore, it is not surprising to find that -high bacterial activity in the soil is associated, as a rule, with -fertility.</p> - -<h3><i>E.</i> <span class="smcap">Changes in Bacterial Numbers and Activities, -and their Relation to External Factors.</span></h3> - -<p>The object of soil bacteriologists is to discover means -of favouring the activity of soil bacteria, especially those -activities that are useful to the higher plant. Knowledge is -therefore needed of the changes in numbers and activities of -the soil bacteria, and of the influence of soil conditions on -them. The necessity of studying these changes has required -the development of a quantitative technique by which the<span class="pagenum" id="Page53">[53]</span> -numbers of bacteria in the soil and their activities can be -estimated.</p> - -<p>The method commonly used in counting bacteria in soil -is a modification of the plating method of Koch. In counting -bacteria two difficulties have to be overcome—their immense -numbers and their small size. The numbers of bacteria in -soil are so large that the bacterial population of a gram of -soil could not, of course, be counted directly. The method -adopted, therefore, is to make a suspension of soil in sterile -salt solution, and to dilute this suspension to a convenient -and known extent, which will depend on the numbers of -bacteria expected. In ordinary field soils it is found convenient, -for example, to dilute the soil suspension so that -one cubic centimeter of the diluted suspension will contain -<sup>1</sup>⁄<sub>250,000</sub>th of a gram of soil. Such a volume will commonly -contain a number of bacteria sufficiently small to count. The -second difficulty is that the organisms are microscopic, and -yet cannot be readily counted under the microscope owing -to the presence of soil particles in the suspension. Hence -recourse is had to plating. One cubic centimeter of diluted -suspension is placed in a petri dish and mixed with -a suitable nutrient agar medium, melted, and cooled to -about 40° C. The medium sets, and after a few days’ incubation -the organisms multiply and produce colonies visible to -the naked eye. By counting these colonies we obtain an -estimate of the number of bacteria in the one cubic centimeter -of suspension, it being assumed that every organism has -developed into one colony, and by multiplying this number -by the degree of dilution we obtain the numbers per gram -of soil. In practice a number of parallel platings are made -from one cubic centimeter portions of the diluted suspension -and the mean number of colonies per plate is taken. -By this means the error due to the random distribution of -bacteria in the suspension is reduced, because of the greater -number of organisms counted.</p> - -<p>In drawing conclusions from bacterial count data, it is -necessary to distinguish between the indication which the<span class="pagenum" id="Page54">[54]</span> -method gives of the absolute numbers of bacteria in the soil -and the accuracy with which it enables the numbers of -two soil samples to be compared. The method cannot be -used for the former purpose at present. We do not know -how far the figures obtained by this counting method fall -short of the actual number of bacteria in the soil. One -reason for this is the difficulty of effecting a complete separation -of the clumps of bacteria into discrete individuals in -the suspension. Then again, there is no known medium -upon which all the physiological groups of bacteria will -develop and produce colonies. And even on a suitable -medium some of the individuals may fail to multiply.</p> - -<p>In comparing the bacterial numbers in two soil samples, -however, the case is different. Within each bacterial -group investigated the plate method should give counts -proportional to the bacterial numbers in the soil. Thus, -by the method one should be able to tell whether the -bacterial numbers are increasing or decreasing over a period -of time, or whether a certain soil treatment produces an -increase or a decrease. With this end in view the technique -of the method has been improved by recent workers. It -was found that, when carefully standardised, the process of -dilution of the soil could be carried out without significant -variation in result (<a href="#TabIV">Table IV.</a>), and that the accuracy of the -method is limited mainly by the variation in colony numbers -on parallel platings, due in part to random distribution of -bacteria throughout the final suspension, and partly to the -uneven development of colonies on the medium. The question -of the medium was therefore taken up with a view to -improving the uniformity of results obtained with it. Lipman, -Conn, and others effected an improvement by using -chemical compounds as nutrient ingredients, thus making -their media more closely reproducible. On most agar media, -an important disturbing factor is the growth of spreading -colonies, which prevent the development of some of the other -colonies. A medium has been devised at Rothamsted on -which these spreading organisms are largely restricted.<a href="#Endnote3_61" class="fnanchor">[61]</a><span class="pagenum" id="Page55">[55]</span> -A statistical examination<a href="#Endnote3_19" class="fnanchor">[19]</a> has shown that on this medium -errors due to the uneven development of colonies, except in -special cases, are prevented, so that in fact the variation in -colony numbers between parallel plates is found to be that -produced merely by random distribution of bacteria in the -diluted suspension (see <a href="#TabIV">Table IV.</a>). In this case the accuracy -of the counts of the bacteria in the diluted suspension depend -directly on the number of colonies counted, and can be -known with precision.</p> - -<p class="tabhead" id="TabIV">TABLE IV.—BACTERIAL COUNTS OF A SOIL SAMPLE.<br /> -<span class="smcap">Parallel Plate Counts from Four Sets of Dilutions made by -Different Workers.</span></p> - -<table class="standard"> - -<tr class="bt bb"> -<th colspan="5" class="bl br">Counts of Colonies on each Plate.</th> -</tr> - -<tr class="bb"> -<th class="w4 bl br">Plate.</th> -<th class="w4 br">Set I.</th> -<th class="w4 br">Set II.</th> -<th class="w4 br">Set III.</th> -<th class="w4 br">Set IV.</th> -</tr> - -<tr> -<td class="general bl br">1</td> -<td class="general br">72   </td> -<td class="general br">74   </td> -<td class="general br">78   </td> -<td class="general br">69   </td> -</tr> - -<tr> -<td class="general bl br">2</td> -<td class="general br">69   </td> -<td class="general br">72   </td> -<td class="general br">74   </td> -<td class="general br">67   </td> -</tr> - -<tr> -<td class="general bl br">3</td> -<td class="general br">63   </td> -<td class="general br">70   </td> -<td class="general br">70   </td> -<td class="general br">66   </td> -</tr> - -<tr> -<td class="general bl br">4</td> -<td class="general br">59   </td> -<td class="general br">69   </td> -<td class="general br">58   </td> -<td class="general br">64   </td> -</tr> - -<tr> -<td class="general bl br">5</td> -<td class="general br">59   </td> -<td class="general br">66   </td> -<td class="general br">58   </td> -<td class="general br">62   </td> -</tr> - -<tr> -<td class="general bl br">6</td> -<td class="general br">53   </td> -<td class="general br">58   </td> -<td class="general br">56   </td> -<td class="general br">58   </td> -</tr> - -<tr class="bb"> -<td class="general bl br">7</td> -<td class="general br">51   </td> -<td class="general br">52   </td> -<td class="general br">56   </td> -<td class="general br">54   </td> -</tr> - -<tr class="bb"> -<td class="general bl br">Mean</td> -<td class="general br">60·86</td> -<td class="general br">65·86</td> -<td class="general br">64·28</td> -<td class="general br">62·86</td> -</tr> - -<tr> -<td colspan="5" class="text">Standard deviation between the four sets = 5·62.</td> -</tr> - -<tr> -<td colspan="5" class="text">Standard deviation between plates within the sets = 7·76.</td> -</tr> - -</table> - -<p>The knowledge obtained from counts of soil bacteria is -subject to another serious limitation. We do not know -which of the bacteria counted are the most effective in -bringing about the various changes that take place in the -soil. It is not even known which of them are active in the -soil and which are in a resting condition. It is thus possible -to have two soils containing equal numbers of bacteria but -showing widely different biochemical activity, if one soil -contains organisms of a higher efficiency. Moreover, as has -been pointed out, many important groups of soil bacteria -do not develop on the plating media, and so are not counted. -These considerations led to the development of supplementary<span class="pagenum" id="Page56">[56]</span> -methods by which it was hoped to estimate the actual biochemical -activity of the soil microflora. The first of these -methods was developed by Remy, who attempted to study -the biochemical activity of a soil by placing weighed amounts -into sterile solutions of suitable and known composition, -keeping them under standard conditions for a definite time -and then estimating the amount of the chemical change that -was being studied. Thus, to test the activity of the organisms -that produce ammonia from organic nitrogen compounds, he -inoculated soil into 1 per cent. peptone solution and measured -the amount of ammonia produced in a given time. By -similar methods the power of a soil to oxidise ammonia to -nitrate, to reduce nitrate, or to fix atmospheric nitrogen, is -tested. This method has been extensively used and developed -by more recent workers. It suffers, however, from the -same serious disadvantage that it was designed to avoid, for -we cannot be certain that those bacteria that develop in the -nutrient solution are the types that are active in the soil, and, -moreover, even where the same types do function in the two -conditions, we do not know that the degree of their activity -is the same in soil and in solution cultures. For instance, -<i>Nitrosomonas</i> appears to show very different degrees of -activity in soil and in culture.</p> - -<p>Another method, therefore, of studying the activity of -soil micro-organisms is the obvious one of estimating the -chemical changes that they produce in the soil itself. This -method has obvious advantages over the unnatural methods -developed from Remy’s, but it has a number of limitations -that make its actual application difficult. In the first place, -we cannot always tell whether changes found to occur in -soil are due to the activity of micro-organisms, or are purely -chemical reactions unassisted by biological agencies. Then, -if we succeed in showing that the changes are due to micro-organisms, -it is very difficult to determine which organisms -are effecting them. This cannot be definitely tested by -isolating suspected organisms and testing their activity in -sterile soil, because in sterilising soil its nature and composition<span class="pagenum" id="Page57">[57]</span> -is altered. In spite of these difficulties, however, the -study of the chemical changes that take place in the soil has -produced valuable knowledge, when it has been combined -with a study of the changes in the number and variety of -the micro-organisms that accompany these reactions. This -method of investigation is well illustrated by the work of -Russell and Hutchinson on the effects of heat and volatile -antiseptics on soil, where a study of the chemical changes -such as ammonia production, that occurred in these treated -soils, combined with a study of the changes in bacterial -numbers, led to the realisation that the soil micro-population -was a complex one, containing active protozoa.</p> - -<p>A great difficulty in applying quantitative methods to -bacteria in the field is the great variation in the density of -the bacterial population over a plot of field soil, which may -be so great that a bacterial count from a single sample is -quite valueless. For example, the distribution of bacterial -numbers over a plot of arable soil near Northampton was -studied by taking sixteen samples distributed over an area -about 12 feet square. The result showed that in some cases -the bacterial numbers in samples taken 6 inches apart -differed by nearly 100 per cent. Fortunately, under favourable -conditions, a remarkably uniform distribution of bacterial -numbers over a plot of soil can be found.</p> - -<p>On such a plot it is possible to investigate the rapidity -with which the numbers of the soil micro-organisms alter in -point of time. For example, on the dunged plot of Barnfield, -Rothamsted, which has been cropped with mangolds -for forty-seven successive years, the area distribution of -bacteria has been found to be so uniform that if a number -of samples of soil are taken from the plot at the same time, -the difference in bacterial numbers between the samples -cannot be detected by means of the counting technique (see -<a href="#TabV">Table V.</a>). The work of Cutler, Crump, and Sandon<a href="#Endnote3_16" class="fnanchor">[16]</a> on this -plot showed that the bacterial numbers vary very greatly -from one day to the next, and that these fluctuations took -place over the whole plot, since two series of samples, taken<span class="pagenum" id="Page58">[58]</span> -in two rows 6 feet apart, showed similar fluctuations (see -<a href="#Fig7">Fig. 7</a>). The discovery of these big daily fluctuations in -numbers led to an inquiry as to how quickly bacterial -numbers change, and samples from Barnfield, taken at two-hourly -intervals, showed that significant changes in numbers -took place even at such short intervals.</p> - -<p class="tabhead" id="TabV">TABLE V.—BACTERIAL COUNTS OF FOUR SOIL SAMPLES.<br /> -<span class="smcap">From Barnfield, Taken Simultaneously.</span></p> - -<table class="standard"> - -<tr class="bt bb"> -<th colspan="5" class="bl br">Counts of Colonies on each Plate.</th> -</tr> - -<tr class="bb"> -<th class="w4 bl br">Plate.</th> -<th class="w4 br">Sample<br />I.</th> -<th class="w4 br">Sample<br />II.</th> -<th class="w4 br">Sample<br />III.</th> -<th class="w4 br">Sample<br />IV.</th> -</tr> - -<tr> -<td class="general bl br">1</td> -<td class="general br">38  </td> -<td class="general br">45  </td> -<td class="general br">43  </td> -<td class="general br">27  </td> -</tr> - -<tr> -<td class="general bl br">2</td> -<td class="general br">32  </td> -<td class="general br">40  </td> -<td class="general br">34  </td> -<td class="general br">41  </td> -</tr> - -<tr> -<td class="general bl br">3</td> -<td class="general br">52  </td> -<td class="general br">45  </td> -<td class="general br">52  </td> -<td class="general br">35  </td> -</tr> - -<tr> -<td class="general bl br">4</td> -<td class="general br">32  </td> -<td class="general br">31  </td> -<td class="general br">55  </td> -<td class="general br">36  </td> -</tr> - -<tr class="bb"> -<td class="general bl br">5</td> -<td class="general br">40  </td> -<td class="general br">43  </td> -<td class="general br">38  </td> -<td class="general br">45  </td> -</tr> - -<tr class="bb"> -<td class="general bl br">Mean</td> -<td class="general br">38·8</td> -<td class="general br">40·8</td> -<td class="general br">44·4</td> -<td class="general br">36·8</td> -</tr> - -<tr> -<td colspan="5" class="text">Standard deviation between the four samples = 7·25.</td> -</tr> - -<tr> -<td colspan="5" class="text">Standard deviation between parallel plates within the sets = 7·55.</td> -</tr> - -</table> - -<div class="container" id="Fig7"> - -<img src="images/illo066.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 7.</span></p> - -<div class="illotext w30em"> - -<p>X-axis (top): Days.</p> - -<p>Y-axis (left): (Series A) Bacteria—millions per gramme of soil.</p> - -<p>Y-axis (right): (Series B) Bacteria—millions per gramme.</p> - -<p>Caption: Daily changes in bacterial numbers in field soil.<br /> -Counts from two series of soil samples taken 6 feet apart.<br /> -(After Cutler.)</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>Since the bacteria involved in this fluctuation are of -great importance to the crops, being for the most part<span class="pagenum" id="Page59">[59]</span> -ammonia producing types, further knowledge as to the cause -of this fluctuation and of its effect on the ammonia and -nitrate in the soil is of fundamental importance. There is -evidence, which will be discussed later, that the cause is -connected with the changing activities of certain soil protozoa, -since the daily changes in the numbers of active -amœbæ in the soil have been found to be in the reverse -direction to those of the bacterial numbers. It appears, -therefore, that we are dealing with an equilibrium between -the various members of the soil population, the point of -equilibrium changing at frequent intervals.</p> - -<p>In addition to daily changes, it is possible to detect -changes in the numbers and activity of the soil population -related to the season. There is a well-marked increase in the -spring and autumn (see <a href="#Fig15">Figs. 15</a>, <a href="#Fig16">16</a>, pp. 89, 90). This is well -seen when the fortnightly averages of the daily bacterial and -protozoal counts from Barnfield soil are plotted. These spring -and autumn increases comprise both the bacterial and the -protozoal population, and therefore differ from the short time -fluctuations in being due, not to a disturbance of the bacteria-protozoa -equilibrium, but to a general rise in activity of both -groups of organisms.</p> - -<p>When we consider the action of external conditions on the -soil bacteria, the existence of a complex soil population and -the interdependence of its members must be borne in mind. -Changes in external conditions may affect the different components -of the population in different ways or to different -degrees, thus upsetting the equilibrium between the various -groups. For example, the addition of a mild aromatic -antiseptic to the soil apparently affects the protozoa in such -a way as to disturb the bacteria-protozoa equilibrium in -favour of the bacteria, while in some cases the aromatic -compound affords a food supply to special bacteria, causing -these to increase, upsetting the equilibrium between the -different bacterial groups. When our knowledge of the -effect of external factors on the soil population becomes -sufficient, it will probably be found that in nearly all cases<span class="pagenum" id="Page60">[60]</span> -a change in the soil conditions produces some disturbance in -the equilibrium between the components of the soil population, -though at present there are only certain examples where -this disturbance is a probable explanation of the facts.</p> - -<p>Since bacteria are dependent on adequate supplies of -energy and food, it is to be expected that additions of organic -matter or of inorganic food materials will greatly benefit -their activities. The effect of added farmyard manure in -increasing bacterial activities has been much studied.<a href="#Endnote3_27" class="fnanchor">[27]</a> -Some of the increased bacterial numbers and activities in -this case may be due to the addition of bacteria with the -manure, but it is thought that this factor is of less importance -than the added energy and food supply which the -general soil flora obtain from it. Nutritive salts such as -phosphates and salts of potassium usually increase the -bacterial activities.</p> - -<p>The effect of alkali salts on soil bacteria has been especially -studied in the Western United States, where the existence of -alkali in the soil is a serious problem.<a href="#Endnote3_23" class="fnanchor">[23]</a> Soil bacteria are -usually stimulated by small doses of alkali salts that are toxic -in higher concentration. As a rule, chlorides are the most -toxic salts, the electronegative ion playing an important part -in the effect of the salt. Salts affect bacteria both owing to -the changes in osmotic pressure which they produce, and -through their specific action on the bacterial protoplasm.<a href="#Endnote3_26" class="fnanchor">[26]</a> -When equal weights of various salts are added to soil, their -toxic action on bacteria shows so little association with their -respective osmotic pressures that we must conclude that this -factor is the less important. There is reason to suppose -that the toxic action of salts on bacteria is often connected -with an effect of the specific ions on the permeability of the -bacterial cell-wall. This conclusion is based on the changes -in electrical conductivity of bacterial suspensions in the -presence of various salts.<a href="#Endnote3_59" class="fnanchor">[59]</a></p> - -<p>A definite antagonism between various salts has been -found to exist. It is possible that future work in this line -may indicate what are the proportions of common electrolytes<span class="pagenum" id="Page61">[61]</span> -which will produce a properly “balanced” soil solution -so that the harmful excess of one salt may be antagonised.</p> - -<p>Certain salts, such as those of arsenic<a href="#Endnote3_24" class="fnanchor">[24]</a> and manganese, -seem to exercise a stimulating action on bacterial activities; -the causes of this action are not at present understood.</p> - -<p>The acidity of the soil has an important effect on the -bacterial processes. The acidity of soils may increase to -such a point that the decomposition of plant tissues by -bacteria is hindered, a peat layer being thus produced. The -degree of acidity that is toxic varies very greatly with different -soil bacteria, some of them, like Azotobacter and Nitrosomonas -being very intolerant of acidity.</p> - -<p>The conditions of aeration, water content, and temperature -are inter-related in field soil. Ammonifying organisms -are not greatly dependent on aeration, but this factor is -sometimes a limiting one in the case of the very aerobic -nitrifying bacteria. Hence efficient soil cultivation is beneficial -to nitrification.</p> - -<p>Many attempts have been made to correlate the temperature -and moisture of field soils with the bacterial numbers -and activities. These attempts have given very discordant -results. It is generally agreed that a plentiful moisture -supply is beneficial. Thus Greaves, in Utah, found the optimum -water content for ammonia and nitrate production to -be about 60 per cent. of the water-holding capacity. On the -other hand, Prescott<a href="#Endnote3_56" class="fnanchor">[56]</a> found that the summer desiccation of -soil in Egypt was followed by increased bacterial activities. -Fabricius and Feilitzen,<a href="#Endnote3_18" class="fnanchor">[18]</a> using moor soil, found a direct -relationship between soil temperature and bacterial numbers, -showing that temperature can be a limiting factor under -certain conditions. With normal arable soils, however, no -such direct effect of temperature or moisture can be found<a href="#Endnote3_16" class="fnanchor">[16]</a> -(see <a href="#Fig8">Fig. 8</a>). It has even been found by Conn<a href="#Endnote3_11" class="fnanchor">[11]</a> that -freezing of the soil may cause a marked increase in bacterial -numbers. The erratic effects of temperature and moisture -on the soil bacteria probably afford instances of a disturbance -of the equilibrium between the bacteria and other<span class="pagenum" id="Page62">[62]</span> -components of the soil micro-population. Thus desiccation -and freezing, though they harmfully affect the bacteria, may -inhibit other micro-organisms to a greater degree, thus freeing -the bacteria from competition. It is in the investigation of<span class="pagenum" id="Page63">[63]</span> -this equilibrium, and of the factors that can control it to -our benefit, that the great advances in soil biology in the -future are to be expected.</p> - -<div class="container w30em" id="Fig8"> - -<img src="images/illo070.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 8.</span>—Effect of frost on the bacterial numbers in the soil. (After -<span class="smcap">Conn</span>.)</p> - -<div class="illotext"> - -<p>X-axis: Nov.-May</p> - -<p>Y-axis (bottom): Temperature—Degrees C.</p> - -<p>Y-axis (top): Bacteria—Millions per Gramme of Soil.</p> - -</div><!--illotext--> - -</div><!--container--> - -<h3>REFERENCES TO CHAPTERS II. AND III.</h3> - -<div class="footnote"> - -<p><span id="Endnote3_1" class="label"> [1]</span> Ashby, S. F., Journ. Agric. Sci., 1907, vol. ii., p. 35.</p> - -<p><span id="Endnote3_2" class="label"> [2]</span> Beijerinck, M. W., Centr. f. Bakt., 1900, Abt. II., Bd. 6, p. 1.</p> - -<p><span id="Endnote3_3" class="label"> [3]</span> Beijerinck, M. W., and Van Delden, A., Centr. f. Bakt., 1902, -Abt. II., Bd. 9, p. 3.</p> - -<p><span id="Endnote3_4" class="label"> [4]</span> Bewley, W. F., and Hutchinson, H. B., Journ. Agric. Sci., 1920, -vol. x., p. 144.</p> - -<p><span id="Endnote3_5" class="label"> [5]</span> Bonazzi, E., Journ. Bact., 1921, vol. vi., p. 331.</p> - -<p><span id="Endnote3_6" class="label"> [6]</span> Burrill, T. J., and Hansen, R., Illin. Exp. Sta., 1917, Bulletin 202.</p> - -<p><span id="Endnote3_7" class="label"> [7]</span> Christensen, H. R., Centralblatt. f. Bakt., 1915, Abt. II., Bd. 43, -p. 1.</p> - -<p><span id="Endnote3_8" class="label"> [8]</span> Christensen, H. R., Centralblatt. f. Bakt., 1907, Abt. II., Bd. 17, -pp. 109, 161.</p> - -<p><span id="Endnote3_9" class="label"> [9]</span> Christensen, H. R., and Larsen, O. H., Centralblatt. f. Bakt., 1911, -Abt. II., Bd. 29, p. 347.</p> - -<p><span id="Endnote3_10" class="label">[10]</span> Conn, H. J., Centralblatt. f. Bakt., 1910, Abt. II., Bd. 28, p. 422.</p> - -<p><span id="Endnote3_11" class="label">[11]</span> Conn, H. J., Centralblatt. f. Bakt., 1914, Abt. II., Bd. 42, p. 510.</p> - -<p><span id="Endnote3_12" class="label">[12]</span> Conn, H. J., Journ. Bact., 1916, vol. i., p. 187.</p> - -<p><span id="Endnote3_13" class="label">[13]</span> Conn, H. J., Journ. Bact., 1917, vol. ii., p. 137.</p> - -<p><span id="Endnote3_14" class="label">[14]</span> Conn, H. J., Journ. Bact., 1917, vol. ii., p. 35.</p> - -<p><span id="Endnote3_15" class="label">[15]</span> Cramer, E., Arch. f. Hyg., 1893, Bd. 16, p. 151.</p> - -<p><span id="Endnote3_16" class="label">[16]</span> Cutler, W., Crump, L. M., and Sandon, H., Phil. Trans. Roy. Soc., -1923, Series B, vol. ccxi., p. 317.</p> - -<p><span id="Endnote3_17" class="label">[17]</span> Doryland, C. J. T., N. Dakota Agr. Exp. Sta., 1916, Bulletin 116.</p> - -<p><span id="Endnote3_18" class="label">[18]</span> Fabricius, O., and Feilitzen, H., Centr. f. Bakt., 1905, Abt. II., -Bd. 14, p. 161.</p> - -<p><span id="Endnote3_19" class="label">[19]</span> Fisher, R. A., Thornton, H. G., and Mackenzie, W. A., Ann. Appl. -Biol., 1922, vol. ix., p. 325.</p> - -<p><span id="Endnote3_20" class="label">[20]</span> Fred, E. B., and Hart, E. B., Wisconsin Agr. Exp. Sta. Research, -1915, Bulletin 35.</p> - -<p><span id="Endnote3_21" class="label">[21]</span> Gainey, P. L., Journ. Agric. Research, 1918, vol. xiv., p. 265.</p> - -<p><span id="Endnote3_22" class="label">[22]</span> Golding, J., Journ. Agric. Sci., 1905, vol. i., p. 59.</p> - -<p><span id="Endnote3_23" class="label">[23]</span> Greaves, J. E., Soil Sci., 1916, vol. ii., p. 443.</p> - -<p><span id="Endnote3_24" class="label">[24]</span> Greaves, J. E., Journ. Agric. Res., 1916, vol. vi, p. 389.</p> - -<p><span id="Endnote3_25" class="label">[25]</span> Greaves, J. E., Soil Sci., 1920, vol. x., p. 77.</p> - -<p><span id="Endnote3_26" class="label">[26]</span> Greaves, J. E., and Lund, Y., Soil Sci., 1921, vol. xii., p. 163.</p> - -<p><span id="Endnote3_27" class="label">[27]</span> Greaves, J. E., and Carter, E. G., Journ. Agric. Research, 1916, -vol. vi., p. 889.</p> - -<p><span id="Endnote3_28" class="label">[28]</span> Groenewege, J., Arch. Suikerindust., 1913, Bd. 21, p. 790.</p> - -<p><span class="pagenum" id="Page64">[64]</span></p> - -<p><span id="Endnote3_28b" class="label">[28<i>b</i>]</span> Green, H. H., Union of S. Africa Dept. Agr., Rept. of Director -Vet. Res., 1918, p. 592.</p> - -<p><span id="Endnote3_29" class="label">[29]</span> Hutchinson, C. M., Rept. Agr. Res. Inst. and Col. of Pusa, 1912, -p. 85.</p> - -<p><span id="Endnote3_30" class="label">[30]</span> Hutchinson, H. B., and Clayton, J., Journ. Agric. Sci., 1919, -vol. ix., p. 143.</p> - -<p><span id="Endnote3_30b" class="label">[30<i>b</i>]</span> Hutchinson, H. B., and Richards, H. H., Journ. Min. Agric., -1921, vol. xxviii., p. 398.</p> - -<p><span id="Endnote3_31" class="label">[31]</span> Hanzawa, J., Centr. f. Bakt., 1914, Abt. II., Bd. 41, p. 573.</p> - -<p><span id="Endnote3_32" class="label">[32]</span> Hopkins, C. G., and Whiting, A. L., Ill. Agr. Exp. Sta., 1916, -Bulletin 190, p. 395.</p> - -<p><span id="Endnote3_33" class="label">[33]</span> Hoppe-Seyler, G., Ztschr. Phys. Chem., 1886, vol. x, pp. 201, 401; -1887, vol. xi., p. 561.</p> - -<p><span id="Endnote3_34" class="label">[34]</span> Hesselmann, H., Skogsvårdsför. Tidskr., 1917, No. 4, p. 321.</p> - -<p><span id="Endnote3_35" class="label">[35]</span> Joshi, N. V., Mem. Dept. Agr. in India, Bact. Ser., 1920, vol. i., -No. 9.</p> - -<p><span id="Endnote3_36" class="label">[36]</span> Koch, R., Mitt. Kais. Gesundh., 1881, vol. i., p. 1.</p> - -<p><span id="Endnote3_37" class="label">[37]</span> Kaserer, H., Centr. f. Bakt., 1906, Abt. II., Bd. 16, p. 681.</p> - -<p><span id="Endnote3_38" class="label">[38]</span> Kaserer, H., Centr. f. Bakt., 1905, Abt. II., Bd. 15, p. 573.</p> - -<p><span id="Endnote3_39" class="label">[39]</span> Krainskii, A. V., Centr. f. Bakt., 1910, Abt. II., Bd. 26, p. 231.</p> - -<p><span id="Endnote3_40" class="label">[40]</span> Klimmer, M., and Kruger, R., Centr. f. Bakt., 1914, Abt. II., Bd. -40, p. 257.</p> - -<p><span id="Endnote3_41" class="label">[41]</span> Krzeminiewski, S., Centr. f. Bakt., 1909, Abt. II., Bd. 23, p. 161.</p> - -<p><span id="Endnote3_42" class="label">[42]</span> Koch, A., and Seydel, S., Centr. f. Bakt., 1912, Abt. II., Bd. 31, -P. 570.</p> - -<p><span id="Endnote3_43" class="label">[43]</span> Lipman, C. B., Bot. Gaz., 1909, vol. xlviii., p. 106.</p> - -<p><span id="Endnote3_44" class="label">[44]</span> Lipman, C. B., and Burgess, P. S., Centr. f. Bakt., 1914, Abt. II., -Bd. 41, p. 430.</p> - -<p><span id="Endnote3_45" class="label">[45]</span> Lipman, C. B., and Burgess, P. S., Centr. f. Bakt., 1915, Abt. II., -Bd. 44, p. 481.</p> - -<p><span id="Endnote3_46" class="label">[46]</span> Lipman, C. B., and Waynick, D. O., Soil Sci., 1916, vol. i., p. 5.</p> - -<p><span id="Endnote3_47" class="label">[47]</span> Löhnis, F., and Pillai, N. K., Centr. f. Bakt., 1908, Abt. II., Bd. 20, -p. 781.</p> - -<p><span id="Endnote3_47b" class="label">[47<i>b</i>]</span> Löhnis, F., and Smith, T., Journ. Agric. Res., 1914, vol. vi., p. 675.</p> - -<p><span id="Endnote3_48" class="label">[48]</span> Mackenna, J., Rept. Prog. Agric., India, 1917, p. 101.</p> - -<p><span id="Endnote3_49" class="label">[49]</span> Marchal, E., Bull. Acad. Roy. Belgique, 1893, vol. xxv., p. 727.</p> - -<p><span id="Endnote3_50" class="label">[50]</span> McBeth, I. G., and Scales, F. M., U.S. Dept. Ag., Bureau Plant -Indus., 1913, Bulletin 266.</p> - -<p><span id="Endnote3_51" class="label">[51]</span> Mockeridge, J., Biochem. Journ., 1915, vol. ix., p. 272.</p> - -<p><span id="Endnote3_52" class="label">[52]</span> Nabokich, A. J., and Lebedeff, A. F., Centr. f. Bakt., 1906, Abt. II., -Bd. 17, p. 350.</p> - -<p><span id="Endnote3_53" class="label">[53]</span> Nagaoka, M., Bull. Coll. Agr., Tokyo, 1900, vol. vi., No. 3.</p> - -<p><span id="Endnote3_54" class="label">[54]</span> Omelianski, W. L., Comptes Rendus Acad. Sci., 1895, vol. cxxi., -p. 653; 1897, vol. cxxv., pp. 907, 1131; Arch. Sci. Bio., (St. -Petersburg), 1899, vol. vii., p. 411.</p> - -<p><span class="pagenum" id="Page65">[65]</span></p> - -<p><span id="Endnote3_55" class="label">[55]</span> Omelianski, W. L., and Sohmskov, M., Arch. Sci. Biol., Publ. -Inst. Imp. Med. Exp. (Petrograd), 1916, vol. xviii., pp. 327, -338, 459; vol. xix., p. 162.</p> - -<p><span id="Endnote3_56" class="label">[56]</span> Prescott, J. A., Journ. Agr. Sci., 1920, vol. x., p. 177.</p> - -<p><span id="Endnote3_57" class="label">[57]</span> Söhngen, N. L., Centr. f. Bakt., 1905, Abt. II., Bd. 15, p. 513.</p> - -<p><span id="Endnote3_58" class="label">[58]</span> Sen Gupta, N., Journ. Agr. Sci., 1921, vol. xi., p. 136.</p> - -<p><span id="Endnote3_59" class="label">[59]</span> Shearer, C., Journ. Hyg., 1919, vol. xviii., p. 337.</p> - -<p><span id="Endnote3_60" class="label">[60]</span> Tappeiner, Ber. Deut. Chem. Gesell., 1883, vol. xvi., p. 1734; -Zeitsch. Biol., 1884, vol. xx., p. 52.</p> - -<p><span id="Endnote3_61" class="label">[61]</span> Thornton, H. G., Ann. Appl. Biol., 1922, vol. ix., p. 241.</p> - -<p><span id="Endnote3_62" class="label">[62]</span> Wallin, I. E., Journ. Bact., 1922, vol. vii., p. 471.</p> - -<p><span id="Endnote3_63" class="label">[63]</span> Waksman, S. A., and Joffe, J. S., Journ. Bact., 1922, vol. vii., -p. 239.</p> - -<p><span id="Endnote3_64" class="label">[64]</span> Wilson, J. K., Cornell Agric. Exp. Sta., 1917, Bulletin 386.</p> - -</div><!--footnote--> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page66">[66]</span></p> - -<h2 class="nobreak">CHAPTER IV.<br /> -<span class="chaptitle">PROTOZOA OF THE SOIL, I.</span></h2> - -</div><!--chapter--> - -<p>That protozoa could be isolated from the soil was a matter -of common knowledge to the biologists of the nineteenth -century, but not until the early part of the present century -was it suggested that these organisms might be playing -some part in the general economy of the soil micro-population. -Of recent years a great deal of our knowledge of the cytology -of the different groups of protozoa, especially the Amœbæ, -has been obtained from the study of representatives normally -living in the soil; but unfortunately little or no knowledge -has been gained of the biology of these animals in their -natural habitat.</p> - -<p>The view that the presence of these organisms in excessive -numbers may lead to “soil sickness” was first put forward -by Russell and Hutchinson in 1909, and elaborated in their -further papers dealing with “Partial Sterilisation of the -Soil.”</p> - -<p>It is unnecessary to discuss in detail this important branch -of agriculture, but to obtain a clear idea of the development -of the study of soil protozoa it is necessary to give as -briefly as possible the conclusions deduced by Russell and -Hutchinson from their extensive experiments on soils treated -with steam and various volatile <span class="nowrap">antiseptics<a href="#Endnote5_21" -class="fnanchor">[21]</a><sup>,</sup> <a href="#Endnote5_22" class="fnanchor">[22]</a>:—</span></p> - -<p>“(1) Partial sterilisation of the soil causes first a fall, then -a rise, in bacterial numbers, which goes on till the numbers -considerably exceed those present in the original soil.</p> - -<p>“(2) Simultaneously there is a marked increase in the rate -of accumulation of ammonia which is formed from organic -nitrogen compounds.</p> - -<p><span class="pagenum" id="Page67">[67]</span></p> - -<p>“(3) The increase in bacterial numbers is the result of -improvement in the soil as a medium for bacterial growth, -and not an improvement in the bacterial flora.</p> - -<p>“(4) The improvement in the soil brought about by partial -sterilisation is permanent, the high bacterial numbers being -kept up even for 200 days or more. It is evident from -(3) and (4) that the factor limiting bacterial numbers in -ordinary soil is not bacterial, nor is it any product of bacterial -activity, nor does it arise spontaneously in soils.</p> - -<p>“(5) But if some of the untreated soil is introduced into -partially sterilised soil, the bacterial numbers, after the -initial rise, begin to fall. Thus the limiting factor can be -reintroduced from untreated soils.</p> - -<p>“(6) Evidence of the limiting factor in untreated soils is -obtained by studying the effect of temperature on bacterial -numbers. Untreated soils were maintained at 10°, 20°, 30° C. -in a well-moistened aerated condition, and periodical counts -were made of the numbers of bacteria per gram. Rise in -temperature rarely caused any increase in bacterial numbers. -But after the soil was partially sterilised the bacterial numbers -showed the normal increase with increasing temperatures.</p> - -<p class="tabhead" id="TabVI">TABLE VI.</p> - -<table class="standard"> - -<tr class="bt bb"> -<th rowspan="2" class="bl br">Temperature<br />of Storage.<br />°C.</th> -<th colspan="4" class="br">Untreated Soil.</th> -<th colspan="4" class="br">Soil Treated with Toluene.</th> -</tr> - -<tr class="bb"> -<th class="w3 bl br">At<br />Start.</th> -<th class="w3 br">After<br />13<br />Days.</th> -<th class="w3 br">After<br />25<br />Days.</th> -<th class="w3 br">After<br />70<br />Days.</th> -<th class="w3 br">At<br />Start.</th> -<th class="w3 br">After<br />13<br />Days.</th> -<th class="w3 br">After<br />25<br />Days.</th> -<th class="w3 br">After<br />70<br />Days.</th> -</tr> - -<tr> -<td class="general bl br">5°-12°</td> -<td class="general br">65</td> -<td class="general br">63</td> -<td class="general br">41</td> -<td class="general br">32</td> -<td class="general br">8·5</td> -<td class="general br"> 73</td> -<td class="general br">101</td> -<td class="general br">137</td> -</tr> - -<tr> -<td class="general bl br">20°</td> -<td class="general br">65</td> -<td class="general br">41</td> -<td class="general br">22</td> -<td class="general br">23</td> -<td class="general br">8·5</td> -<td class="general br">187</td> -<td class="general br">128</td> -<td class="general br">182</td> -</tr> - -<tr> -<td class="general bl br">30°</td> -<td class="general br">65</td> -<td class="general br">27</td> -<td class="general br">50</td> -<td class="general br">16</td> -<td class="general br">8·5</td> -<td class="general br">197</td> -<td class="general br">145</td> -<td class="general br"> 51</td> -</tr> - -<tr class="bb"> -<td class="general bl br">40°</td> -<td class="general br">65</td> -<td class="general br">14</td> -<td class="general br"> 9</td> -<td class="general br">33</td> -<td class="general br">8·5</td> -<td class="general br">148</td> -<td class="general br"> 52</td> -<td class="general br">100</td> -</tr> - -</table> - -<p>“(7) It is evident, therefore, that the limiting factor in -the untreated soils is not the lack of anything, but the presence -of something active. The properties of the limiting -factor <span class="nowrap">are:—</span></p> - -<div class="factors"> - -<p>“(<i>a</i>) It is active and not a lack of something.</p> - -<p>“(<i>b</i>) It is not bacterial.</p> - -<p><span class="pagenum" id="Page68">[68]</span></p> - -<p>“(<i>c</i>) It is extinguished by heat or poisons.</p> - -<p>“(<i>d</i>) It can be re-introduced into soils from which it has -been extinguished by the addition of a little untreated -soil.</p> - -<p>“(<i>e</i>) It develops more slowly than bacteria.</p> - -<p>“(<i>f</i>) It is favoured by conditions favourable to trophic -life in the soil, and finally becomes so active that -the bacteria become unduly depressed.</p> - -</div><!--factors--> - -<p>“It is difficult to see what agent other than a living -organism can fulfil these conditions. Search was therefore -made for a larger organism capable of destroying bacteria, -and considerable numbers of protozoa were found. The -ciliates and amœbæ are killed by partial sterilisation. Whenever -they are killed the detrimental factor is found to be put -out of action; the bacterial numbers rise and maintain a -high level. Whenever the detrimental factor is not put out -of action, the protozoa are not killed. To these rules we -have found no exception.”</p> - -<p>From such premises as the above Russell and Hutchinson -founded the “protozoa theory of partial sterilisation,” and -at Rothamsted there was commenced the serious study of -these soil organisms.</p> - -<p>Goodey was one of the early workers on this new subject, -and added considerably to our knowledge of the species -living in normal soils, and of the chemical constitution of -the cyst wall of ciliates. He also made investigations on -the effects of various chemicals on the micro-population of -soils, but was unable to draw very definite conclusions.<a href="#Endnote5_11" class="fnanchor">[11]</a></p> - -<p>One of the first criticisms raised against the protozoa -theory of partial sterilisation was that the protozoa were not -normal inhabitants of the soil, and were present only in -small numbers, all of them in the cystic, quiescent condition. -It was further held that these cysts were carried by the wind -from dried-up ponds and streams. It is difficult to trace the -origin of this view, since early observers, viz., Ehrenberg -and Dujardin, in 1841, were of the opinion that the protozoa<span class="pagenum" id="Page69">[69]</span> -were living in the trophic active condition in the soil, and it -was not until 1878 that Stein showed that free living protozoa -can encyst. To Martin and Lewin, however, must be ascribed -the distinction of first proving that the soil possesses an -active protozoan population, for by a series of ingenious -experiments these observers isolated several flagellates and -amœbæ in a trophic condition from certain of the Rothamsted -soils.<a href="#Endnote5_18" class="fnanchor">[18]</a> The more recent work in this country has been in the -direction of devising new quantitative methods of research, -since by this means alone is it possible to elucidate many -fundamental questions.</p> - -<p>In America and elsewhere experiments have been devised -for testing the conclusions of Russell and Hutchinson. Cunningham -and Löhnis,<a href="#Endnote5_2" class="fnanchor">[2]</a> in America, Truffaut and Bezssonoff,<a href="#Endnote5_24" class="fnanchor">[24]</a> -in France, supply evidence in favour of the theory, but -most of the American work is in opposition to it.</p> - -<p>Sherman<a href="#Endnote5_23" class="fnanchor">[23]</a> is perhaps the most prominent in opposing -the phagocytic action of protozoa on soil bacteria in spite -of the fact that certain of his experimental results apparently -show enormous decreases in bacterial numbers in the -presence of protozoa. In many of his soil inoculation -experiments, however, it was not demonstrated that his -active cultures remained alive after entering the soil.</p> - -<p>The experimental difficulties of dealing with soil protozoa -are considerable, and without a thoroughly sound technique -investigators may easily go astray.</p> - -<h3><span class="smcap">Classification.</span></h3> - -<p>The animal kingdom is divided into two main groups or -sub-kingdoms—the Protozoa and the Metozoa. In the latter -the characteristic feature is that the body is composed of -several units, called cells, and consequently such animals are -often spoken of as multicellular. The Protozoa, on the other -hand, are usually designated as uni-cellular, since their bodies -are regarded as being homologous to a single unit or cell of -the metozoan body. For various reasons exception has been -taken by Dobell<a href="#Endnote5_9" class="fnanchor">[9]</a> and others to the use of the term uni-cellular,<span class="pagenum" id="Page70">[70]</span> -for, as Dobell says, “If we regard the whole organism as an -individual unit, then the whole protozoan is strictly comparable -with a whole metozoon, and not with a part of it. -But the body of a protozoan, though it shows great complexity -of structure, is not differentiated internally into cells, -like the body of a metozoon. Consequently it differs from -the latter not in the number of its cellular constituents, but -in lacking these altogether. We therefore define the sub-kingdom -of the protozoa as the group which contains <i>all -non-cellular animals</i>.”</p> - -<p>It should be pointed out that this view does not find -favour with many zoologists, but it is useful in bringing into -prominence the fact that each protozoan is comparable as -regards its functions with the multi-cellular animals.</p> - -<p>The protozoa are again further divided into four main -<span class="nowrap">classes:—</span></p> - -<table class="standard"> - -<tr> -<td class="numbers">I.</td> -<td class="text">Rhizopoda.</td> -</tr> - -<tr> -<td class="numbers">II.</td> -<td class="text">Mastigophora.</td> -</tr> - -<tr> -<td class="numbers">III.</td> -<td class="text">Ciliophora.</td> -</tr> - -<tr> -<td class="numbers">IV.</td> -<td class="text">Sporozoa.</td> -</tr> - -</table> - -<p>Of the above classes, representatives of each of the first -three are found living in the soil, but up to the present there -is no evidence that any sporozoon is capable of living an -active life in the soil, though the cysts of such organisms -may be present.</p> - -<p>The class <i>RHIZOPODA</i> consists of those protozoa whose -organs of locomotion and food capture are <i>pseudopodia</i>, that -is, temporary extensions of the living protoplasm. The body -is typically naked, that is to say, without any cuticular -membrane, though in some forms, ex. <i>Amœbæ terricola</i>, the -external layer of protoplasm is thickened to form a pellicle. -A skeleton or shell may be present.</p> - -<p>The class is further sub-divided into various sub-classes, -only two of which concern the soil protozoologist, viz., the -<i>Amœbæ</i> and the <i>Mycetozoa</i>, of which the most important -representative is <i>Plasmodiophora brassicæ</i>, which attacks the<span class="pagenum" id="Page71">[71]</span> -roots of many cruciferous plants, causing the disease familiarly -known as “Fingers and Toes.”</p> - -<p>The <i>Amœbæ</i> are again divided into two <span class="nowrap">orders:—</span></p> - -<p class="blankbefore75">(<i>a</i>) <i>Nuda</i>, without shell or skeleton;</p> - -<p>(<i>b</i>) <i>Testacea</i>, with shells often termed <i>Thecamœbæ</i>.</p> - -<p class="blankbefore75">Representatives of the “naked” amœbæ commonly -found in soils are <i>Nægleria (Dimastigamœba) gruberi</i>, <i>Amœba -diploidea</i> (possessing two nuclei) and <i>A. terricola</i>, the last -two forms possessing a comparatively thick skin or pellicles. -<i>Trinema enchelys</i>, <i>Difflugia constricta</i> and <i>Chlamydophrys -stercorea</i> are examples of soil Thecamœbæ.</p> - -<p>The class <i>MASTIGOPHORA</i> consists of those protozoa -whose typical modes of progression are by means of flagella, -whip-like filaments which, by their continual lashing motion, -cause movement of the animal.</p> - -<p>The body may be naked or corticate. The only organisms -which concern the soil biologist belong to the <i>Flagellata</i> order.</p> - -<p>The Flagellates differ considerably among themselves, -both as regards their mode of feeding, and the number of -flagella, thus making their classification difficult and outside -the scope of this book. Suffice it to say that in the soil such -organisms occur possessing one, two, three or four flagella, -ex. <i>Oicomonas termo</i>, <i>Heteromita globosus</i>, <i>Dallengeria</i> and -<i>Tetramitus spiralis</i>. Further, their mode of feeding may be -<i>saprophytic</i> in which nourishment is absorbed by diffusion -through the body surface in the form of soluble organic -substances, <i>holozoic</i> where solid food particles are taken in, -or <i>holophytic</i> in which food is synthesised by the energy of -sunlight. This last group is commonly spoken of as the -<i>Phyto flagellates</i>, which are to all intents and purposes unicellular -algæ, and as such will be dealt with in <a href="#Page99">Chapter VI.</a></p> - -<p>The class <i>CILIOPHORA</i> consists of those protozoa whose -typical organs of locomotion are threads or cilia. These -organisms can in one sense be regarded as the highest of -the protozoa, since in no other division does the body attain -so great a complexity of structure. Moreover, they are<span class="pagenum" id="Page72">[72]</span> -typically characterised by a complicated nuclear apparatus -with the vegetative and generative portions separated into -distinct bodies, the macro-nucleus and the micro-nucleus. -Their mode of nutrition is <i>holozoic</i>, though recently Peters has -brought forward evidence that certain species can obtain their -nourishment saprophytically.</p> - -<p>The sub-class Ciliata comprises four orders, all of which -are represented in the soil.</p> - -<p>I. <i>Holotricha.</i> The cilia are equal in length and uniformly -distributed over the whole body in the primitive -forms, though restricted to special regions in the specialised -forms. Typical soil forms are <i>Colpoda cucullus</i>, <i>Colpidium -colpoda</i>.</p> - -<p>II. <i>Heterotricha.</i> There is a uniform covering of cilia, -and a conspicuous spiral zone of larger cilia forming a -vibratile membrane and leading to the mouth.</p> - -<p>III. <i>Hypotricha.</i> The body is flattened dorso-ventrally -and the cilia are often fused to form larger appendages or -cirri confined to the ventral surface. Movement is typically -a creeping one. Typical soil forms are <i>Pleurotricha</i>, <i>Gastrostylis</i>, -<i>Oxytricha</i>.</p> - -<p>IV. <i>Peritricha.</i> Typically of a sedentary habit and the -cilia are reduced to a zone round the adoral region of the -body. A typical soil form is <i>Vorticella microstomum</i>.</p> - -<p>The above classification is far from complete, but should -be sufficient to give an idea of the general grouping of the -organisms. For a more detailed account reference must -be made to the numerous text books on protozoa.</p> - -<h3><span class="smcap">Life Histories.</span></h3> - -<p>The life history of each species has its own characteristic -features as regards nuclear division, etc., and in many forms, -notably the amœbæ, it is impossible to identify them with -certainty unless the chief stages of the life history are known. -In general, however, the soil protozoa pass through very -similar phases and develop in a perfectly straightforward -way. Broadly speaking, there are two main phases of the<span class="pagenum" id="Page73">[73]</span> -life history—a period of activity often mistermed vegetative, -and a period of rest. In the former the animal moves, -feeds and reproduces, while in the latter there is secreted -round the body a thick wall, capable of resisting adverse -external influences. This condition is termed the cystic -stage, and by means of it the animals are distributed from -place to place by air, water, etc. Indeed, so resistant are -the cysts that many of them are capable of withstanding -the action of the digestive juices of the intestines of animals, -through which they pass to be deposited by the fæces on -fresh ground.</p> - -<p>This cystic stage of the life history is found in practically -all free-living protozoa, though it is not formed in exactly -the same manner in every case. In the majority of instances -the cyst is the product of a single organism, round which is -formed a delicate gelatinous substance which soon hardens -and gradually acquires the peculiar characters of the wall. -Concerning the chemical nature of this wall there is little -known, but Goodey,<a href="#Endnote5_11" class="fnanchor">[11]</a> working on the cysts of <i>Colpoda -cucullus</i>, found it to be formed of a carbohydrate, different -from all carbohydrates previously described, to which the -name “Cytose” was given. When in this state the animals are -able to remain dormant for considerable periods until favourable -conditions once more obtain when the wall is ruptured -and the animal again resumes the active phase of its life -history. This simple process is characteristic of such species -as <i>Heteromita globosus</i>, <i>Cercomonas spp.</i>, and many others. -It will be noted that no increase of numbers, i.e. reproduction, -occurs. A more complex condition is, however, -sometimes found, as, for example, in the ciliate <i>Colpoda -steinii</i>, where actual reproduction into small animals takes -place within the cyst.</p> - -<p>Finally there is the less common type of cyst formation, -such as is found in the flagellate <i>Oicomonas termo</i> described -by Martin.<a href="#Endnote5_19" class="fnanchor">[19]</a> This flagellate, in common with all other forms, -reproduces by dividing into two; the division of the nucleus -initiating the process. At certain undetermined periods of<span class="pagenum" id="Page74">[74]</span> -the life history, however, conjugation occurs between two -similar animals forming a large biflagellate body known as -the zygote. After swimming about for varying periods of -time, during which the size increases and a large vacuole -appears, the zygote secretes a thick wall, loses its flagella, -and becomes a cyst. While in this condition the two gamete -nuclei fuse to form one, and eventually a single <i>Oicomonas</i> -emerges from its cyst.</p> - -<p>Similarly in <i>A. diploidea</i> the cysts are formed after two -individuals have come together. In the young cysts two -amœbæ are found in close association, and according to -Hartmann and Nägler<a href="#Endnote5_12" class="fnanchor">[12]</a> a sexual process occurs inside the -cyst involving a “reductive” division of the nuclei. This -requires confirmation, but it is certain that only one individual -comes out of the cysts, which originally contained two -amœbæ.</p> - -<p>Such cysts have been termed by some writers “reproductive,” -evidently a misleading term, since no increase in -numbers, but rather a decrease, results from the process. -A better term is, perhaps, conjugation cyst.</p> - -<p>In soil protozoa, then, three different modes of cyst -formation obtain, and failure to make the distinction -inevitably leads to confusion.</p> - -<p>Before leaving the question of life histories, reference -must be made to a peculiar and characteristic feature of -<i>Nægleria gruberi</i>. This amœba under certain circumstances -assumes a free-swimming biflagellate stage. After variable -periods of time the flagella are lost and the ordinary amœboid -condition resumed. What are the factors concerned -in the production of flagellates is unknown, but flooding the -coverslips with distilled water is an effective method for -causing their appearance.</p> - -<h3><span class="smcap">Distribution of Soil Protozoa.</span></h3> - -<p>For both the bacteria and algæ observations have been -made regarding their distribution through successive depths<span class="pagenum" id="Page75">[75]</span> -of the soil; little can be said, however, about the protozoa -in this connection. It is certain that they occur throughout -the first six inches of the Rothamsted soils, though their -relative frequencies in the successive inches has not been -determined, but probably they are most abundant in the -2nd to the 4th inch.</p> - -<p>In this country experiments have not been made to determine -whether sub-soil normally contains protozoa; but -from some South African soil, taken under sterile conditions -4 ft. down and examined in this laboratory, large numbers -of protozoa were cultivated.</p> - -<p>This soil, however, could not, for various reasons, be -regarded as a typical sub-soil.</p> - -<p>Kofoid records the presence of <i>Nægleria gruberi</i> in clay -and rock talus taken from the sides of excavations of over -20 ft. depth, but the possibility of external infection does not -appear to have been excluded.</p> - -<p>The presence of protozoa is not peculiar to British soil -since they have been found by various workers in Germany, -France, the United States, and elsewhere. In view of their -probable importance in the soil economy there has been instituted -a survey of the protozoan species of soil from all parts -of the world.</p> - -<p>This work is in charge of Mr. Sandon, to whom I am indebted -for the following summary of his as yet unpublished -research.</p> - -<p>“The majority of soil protozoa (like the fresh-water -forms) appear to be quite cosmopolitan, for the species -found in such widely separated localities as England, Spitsbergen, -Africa, West Indies, Gough Island (in the South -Atlantic) and Nauru (in the Pacific) are, with few exceptions, -identical. This distribution indicates an ability to withstand -an extremely wide range of conditions, for the same -species occurring in Arctic soils, which are frozen for the -greater part of the year, are found also in soils exposed to -the direct rays of the tropical sun. Even sand from the -Egyptian desert contains protozoa, though it seems probable<span class="pagenum" id="Page76">[76]</span> -that in such cases they must be present only in the encysted -condition for the greater part of the time.</p> - -<p>“Not every sample of soil, however, contains all the -species capable of living in soil, but the local conditions -determining the presence or absence of any species are at -present unknown. In general the numbers, both of species -and of individuals present, follow the number of bacteria. -They are consequently most numerous in rich moist soils. -The statement sometimes made that protozoa are most -numerous in peaty soils is based solely on the number of -Rhizopod shells found in such localities; but as most of -these shells are empty, their abundance is probably due -simply to the slowness with which they disintegrate in these -soils where bacterial activity is low, they do not indicate a -great protozoal activity. Active protozoa do occur even in -extremely acid soils, but their numbers in such cases are -low. The common soil protozoa, in fact, appear to be as -tolerant of differences in soil acidity as they are of differences -in climate, for many of the same forms which occur in acid -soils are found also in soils containing high percentages of -chalk. It is possible that some of the less common species -may be confined within closer limits of external conditions -but the information available on this point is inadequate. -All the species, however, which in Rothamsted soils occur -in the highest numbers (e.g. <i>Oicomonas termo</i>, <i>Heteromita -spp.</i>, <i>Cercomonas crassicauda</i>, <i>Nægleria gruberi</i>, <i>Colpoda -cucullus</i>, <i>C. steinii</i>) occur in practically every soil which is -capable of supporting vegetation, though, of course, in very -varying numbers.”</p> - -<p>It is evident, therefore, that the protozoa must be regarded -as constituting part of the normal micro-organic population -of soils, and as such are probably playing an important -rôle. Unfortunately our knowledge of the physiology -of these organisms is extremely scant, and much of future -research must be directed towards elucidating their functions -and their responses to varying environmental conditions.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page77">[77]</span></p> - -<h2 class="nobreak">CHAPTER V.<br /> -<span class="chaptitle">PROTOZOA OF THE SOIL, II.</span></h2> - -</div><!--chapter--> - -<p>In the preceding <a href="#Page66">chapter</a> an outline has been given of the -development of the study of soil protozoa, with especial -reference to its qualitative aspects.</p> - -<p>Here it is proposed to deal with the quantitative methods -which have been devised for studying these organisms and -the results obtained.</p> - -<p>From the beginning great difficulty has been encountered -in finding means for counting protozoa; and most of -the early results have been obtained by the use of one of the -following methods: (1) direct counts in a known volume of -soil suspension by means of a microscope; (2) dilution method -as used for counting bacteria, and suggested by Rahn, who -made dilutions of the soil and determined, by examination -at periodic intervals, the one above which protozoa did not -grow; (3) Agar plating as used by Killer; (4) counting per -standard loop of suspension as devised by Müller. Of these -the two last have been little used, and for various reasons are -now discarded by most workers. Direct methods have been -used extensively in the United States by Koch<a href="#Endnote5_13" class="fnanchor">[13]</a> -and others,<a href="#Endnote5_16" class="fnanchor">[16]</a> -who claim to have got satisfactory results; they are, however, -highly inaccurate and should be discontinued. The present -writer<a href="#Endnote5_3" class="fnanchor">[3]</a> has shown that there exists a surface energy relationship -between the soil particles and the protozoa, so that the -two are always in intimate contact; thus rendering it impossible -to count under the microscope the number of organisms -in a given weight of soil suspension (<a href="#Fig9">Fig. 9</a>). Further, -in a clay soil, such as is found at Rothamsted, the clay -particles alone make it very difficult to use such methods.</p> - -<p><span class="pagenum" id="Page78">[78]</span></p> - -<p>The demonstration of this surface energy relationship -affords an effective rejoinder to the criticism made against -Russell and Hutchinson’s hypothesis, viz., that soil protozoa -must be very few in numbers, since it was impossible to see -them on examining soil under the microscope.</p> - -<div class="container w45em" id="Fig9"> - -<img src="images/illo086.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 9.</span>—Showing the number of amœbæ and flagellates withdrawn from -suspensions of varying strengths by different types of solid matter. A = -clay: B = partially sterilized soil: C = ignited soil: D = fine sand: E = -waste sand. Since complete withdrawal occurs when the numbers of organisms -added are less than the capacity of the solid matter, the first part of -each of the above curves is coincident with the ordinate. The numbers of -organisms are given in thousands. (From Journ. Agric. Soc., vol. ix.)</p> - -<div class="illotext w30em"> - -<p>X-axis: Number of Organisms per c.c. left in Solution.</p> - -<p>Y-axis: Number of Organisms per c.c. taken up by Solid Matter.</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>The second or dilution method is the one, therefore, that -has been most extensively developed.</p> - -<p><span class="pagenum" id="Page79">[79]</span></p> - -<p>Cunningham obtained concordant results in this way, -and his method, modified by L. M. Crump, was as follows: -10 grams of soil were added to 125 c.c. of sterile tap-water -and shaken for three minutes. This gives a 1 in 12·5 dilution. -From it further dilutions were made until a sufficiently -high one was obtained. Petri dishes, containing -nutrient agar, were inoculated with 1 c.c. of each of the -dilutions and incubated. At intervals covering 28 days -the plates were examined and the presence or absence of -protozoa on each recorded. In this way the approximate -number of organisms per gram of soil could be found.</p> - -<p>By methods essentially similar to this numerous counts -have been made of the bacteria and protozoa in field soil -and in partially sterilized soils. They were, however, inconclusive; -thus, on the one hand, Goodey and several American -observers, found no correlation between the numbers of -protozoa and bacteria, while Miss Crump and Cunningham -obtained evidence pointing to the reverse conclusion.</p> - -<p>Such divergence of opinion was probably mainly due to -two causes: firstly, that the time elapsing between the -successive counts was too long, for it has been shown recently -that the number of bacteria and protozoa fluctuate very -rapidly; and secondly, the method was not completely -satisfactory since only the total numbers of protozoa were -considered, no means having been found of differentiating -between the cystic and active forms. This was a particularly -serious source of error for it is possible for soil to contain -large numbers of bacteria and protozoa, of which a -high percentage of the latter are in the form of cysts. A -count made on such a soil would give results apparently -opposed to the theory that protozoa act as depressors of -bacteria.</p> - -<p>This difficulty has, however, been overcome by a further -modification of the dilution method, and it is now possible -in any soil sample to count both the numbers of cysts and -active forms. Also a further advance in technique has made -it possible to recognise and enumerate the common species<span class="pagenum" id="Page80">[80]</span> -of protozoa, instead of simply grouping them as Ciliates, -Flagellates, and Amœbæ, as was done in the past.<a href="#Endnote5_7" class="fnanchor">[7]</a></p> - -<p>Briefly the method consists in dividing the soil sample -into equal portions (usually 10 grams each) one of which -is counted, thus giving the total numbers of protozoa -(active + cystic) present. The second portion is -treated over-night with 2 per cent. hydrochloric acid, the -HCl used being B.P. pure 31·8 per cent. Previous experiments -have shown that such acid kills all the active protozoa, -leaving viable the cysts. The number of cysts is therefore -found by counting this treated sample, and the number -obtained subtracted from the total gives the active number.<a id="FNanchor6" href="#Footnote6" class="fnanchor">[F]</a></p> - -<div class="footnote"> - -<p><a id="Footnote6" href="#FNanchor6" class="label">[F]</a> The proof of the accuracy of this method will be found in the following -<span class="nowrap">papers:—</span></p> - -<p>(1) Cutler, D. W. (1920), Journ. Agric. Sci., vol. x., 136-143.</p> - -<p>(2) Cutler, D. W., and Crump, L. M. (1920), Ann. App. Biol., vol. vii., 11-24.</p> - -</div><!--footnote--> - -<p>The discovery of this method at once puts into the -hands of the investigator a much more efficient instrument -for studying the activities of the soil micro-population, -especially since at a slightly later date Thornton’s method -for counting bacteria was devised.</p> - -<p>Early in 1920 Cutler and Crump<a href="#Endnote5_6" class="fnanchor">[6]</a> decided to make a preliminary -survey of the protozoon and bacterial populations of -one of the Rothamsted field soils (Broadbalk dunged plot). -The investigation was continued for 28 days, daily soil samples -being taken. The results so obtained showed that an extended -investigation of the micro-population of field soil -would yield interesting and important results, especially as -it was evident that certain views held by soil biologists -required modification.</p> - -<p>In July of the same year, therefore, it was decided to start -an extended investigation of the soil protozoa and bacteria. -The method adopted was to make counts of the numbers of -bacteria and of six<a id="FNanchor7" href="#Footnote7" class="fnanchor">[G]</a> species of protozoa in soil samples taken -daily direct from the field (Barnfield dunged plot) and by -statistical methods to correlate these counts one with another -and with the data for external conditions. Observations at -shorter periods than 24 hours could not be made, but it -was found possible to continue the research for 365 days.<a href="#Endnote5_7" class="fnanchor">[7]</a></p> - -<div class="footnote"> - -<p><a id="Footnote7" href="#FNanchor7" class="label">[G]</a> Actual counts were made of six species, -though, as stated on <a href="#Page10">p. 10</a>, -observations were made on seventeen.</p> - -</div><!--footnote--> - -<p><span class="pagenum" id="Page81">[81]</span></p> - -<div class="container" id="Fig10"> - -<img src="images/illo089.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 10.</span>—Daily numbers of active amœbæ (Dimastigamœba and -Species α) and bacteria in 1 gram of field soil, -from August 29 to October 8, 1920. (From Phil. Trans. Roy. Soc., vol. ccxi.)</p> - -<div class="illotext w25em"> - -<p>X-axis: August September October</p> - -<p>Y-axis (left): Amoebae Active numbers per gramme of soil</p> - -<p>Y-axis (right): Bacteria in millions per gramme of soil</p> - -<p>Legend: Dimastigamoeba<br />Species α<br />Bacteria</p> - -</div><!--illotext--> - -</div><!--container--> - -<p><span class="pagenum" id="Page82">[82]</span></p> - -<p>The number of all the organisms showed large fluctuations -of two kinds, daily and seasonal. The size of the -changes that took place within so short a period as 24 -hours was, perhaps, the most surprising fact that the experiment -revealed. Thus three consecutive samples gave 58·0, -14·25 and 26·25 millions of bacteria per gram respectively; -and the changes exhibited by any of the species of protozoa -were at times even larger. This fact is of extreme importance, -since in the past it has always been assumed that the -number of bacteria remained fairly constant from day to -day, and investigators have not hesitated to separate the -taking of soil samples by long periods. It is now obvious -that such a procedure is of little use for comparative -purposes (<a href="#Fig10">Fig. 10</a>).</p> - -<p>It has usually been assumed that the changes in the -external conditions markedly affect the density of the soil -population. To test this the environmental conditions—temperature, -moisture content and rainfall were examined; -but contrary to all expectation no connection could be -traced between any of these and the daily changes in numbers -of any of the organisms investigated, and moreover -the species of protozoa appeared in the main to be living -independently of one another.</p> - -<p>It is difficult to believe that external conditions are -as inoperative as appears from the above; and in view of the -known complexity of the soil it is possible that further -research will show that certain combinations of external -conditions are important agents in effecting the changes.</p> - -<p><span class="pagenum" id="Page83">[83]</span></p> - -<div class="container w40em" id="Fig11"> - -<img src="images/illo091.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 11.</span>—Numbers of active amœbæ (Dimastigamœba and Species α) and -bacteria to 1 gram of field soil for typical periods in February and April, -1921. (From Phil. Trans. Roy. Soc., vol. ccxi.)</p> - -<div class="illotext w25em"> - -<p>X-axis: Feby. Feby. April</p> - -<p>Y-axis (left): Amoebae Active numbers per gramme of soil</p> - -<p>Y-axis (right): Bacteria millions</p> - -<p>Legend: Dimastigamoeba<br />Species α<br />Bacteria</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>In the case of the bacteria, however, the agent causing -the fluctuations is mainly the active amœbæ. This was -well shown during the year’s count, for with only 14 per -cent. of exceptions, 10 per cent. of which can be explained -as due to rapid excystation or encystation, a definite inverse -relationship was established between the active numbers of -amœbæ and the number of bacteria (<a href="#Fig11">Figs. 11</a> and <a href="#Fig12">12</a>). Thus -a rise from one day to the next in the amœbic population -was correlated with a fall in the numbers of bacteria and -vice versa. It must not be supposed that the flagellates -are of no account in this process; some species, known to -eat bacteria, undoubtedly induce slight depressions, but,<span class="pagenum" id="Page84">[84]</span> -owing to their small size, any effect is masked by the greater -one of the amœbæ.</p> - -<div class="container w35em" id="Fig12"> - -<img src="images/illo092.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 12.</span>—Numbers of active amœbæ (Dimastigamœba and Species α) and -bacteria in 1 gram of field soil for typical periods in September, October, -and November, 1920.</p> - -<div class="illotext w25em"> - -<p>X-axis: August September October</p> - -<p>Y-axis (left): Amoebae, thousands</p> - -<p>Y-axis (right): Millions, Bacteria</p> - -<p>Legend: Dimastigamoeba<br />Species α<br />Bacteria</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>These experiments seem to admit of no doubt that in -field soil the active protozoa are instrumental in keeping -down, below the level they might otherwise have attained,<span class="pagenum" id="Page85">[85]</span> -the numbers of bacteria; but a further proof of this contention -ought to be obtained by inoculation experiments. -It should be possible, by inoculating sterile soil with bacteria -alone and with bacteria plus protozoa, to demonstrate -fluctuations in bacterial numbers in the latter, while those -of the former remained constant. This admittedly crucial -test has often been tried, but owing to difficulties in technique, -etc., has always failed. Recently, however, by using new -methods confirmatory results have been obtained.<a href="#Endnote5_5" class="fnanchor">[5]</a></p> - -<p>Ordinary field soil was sterilised by heat at 100° C. -for 1 hour on four successive days; it was then divided into -equal portions, one of which was inoculated with three -known species of bacteria, and the other inoculated with -the same number of bacteria plus the cysts of the common -soil amœba <i>Nægleria gruberi</i>. The numbers of bacteria in -each soil were counted daily for the first eight days and then -daily from the 15th to the 21st day after the experiment -started. The results are given in <a href="#TabVII">Table VII.</a> and <a href="#Fig13">Fig. 13</a>.</p> - -<p class="tabhead" id="TabVII">TABLE VII.</p> - -<table class="standard"> - -<tr class="bt bb"> -<th class="w5 bl br">Numbers<br />of Days<br />after<br />Inoculation.</th> -<th class="w5 br">Control<br />(Bacteria<br />alone).</th> -<th class="w5 br">Control<br />Bacteria<br />+ Amœbæ.</th> -</tr> - -<tr> -<td class="general bl br"> 0</td> -<td class="general br"> 13·0</td> -<td class="general br"> 12·2</td> -</tr> - -<tr> -<td class="general bl br"> 1</td> -<td class="general br"> 48·6</td> -<td class="general br"> 35·4</td> -</tr> - -<tr> -<td class="general bl br"> 2</td> -<td class="general br"> 97·6</td> -<td class="general br">117·2</td> -</tr> - -<tr> -<td class="general bl br"> 3</td> -<td class="general br">127·0</td> -<td class="general br">178·4</td> -</tr> - -<tr> -<td class="general bl br"> 4</td> -<td class="general br">154·8</td> -<td class="general br">154·4</td> -</tr> - -<tr> -<td class="general bl br"> 5</td> -<td class="general br">196·8</td> -<td class="general br">177·0</td> -</tr> - -<tr> -<td class="general bl br"> 6</td> -<td class="general br">214·4</td> -<td class="general br">151·8</td> -</tr> - -<tr> -<td class="general bl br"> 7</td> -<td class="general br">193·4</td> -<td class="general br"> 75·6</td> -</tr> - -<tr> -<td class="general bl br"> 8</td> -<td class="general br">165·2</td> -<td class="general br"> 65·8</td> -</tr> - -<tr> -<td class="general bl br">15</td> -<td class="general br">169·2</td> -<td class="general br"> 72·8</td> -</tr> - -<tr> -<td class="general bl br">16</td> -<td class="general br">174·8</td> -<td class="general br"> 30·2</td> -</tr> - -<tr> -<td class="general bl br">17</td> -<td class="general br">175·6</td> -<td class="general br"> 53·2</td> -</tr> - -<tr> -<td class="general bl br">18</td> -<td class="general br">168·4</td> -<td class="general br"> 82·8</td> -</tr> - -<tr> -<td class="general bl br">19</td> -<td class="general br">160·4</td> -<td class="general br"> 43·8</td> -</tr> - -<tr> -<td class="general bl br">20</td> -<td class="general br">171·2</td> -<td class="general br"> 70·8</td> -</tr> - -<tr class="bb"> -<td class="general bl br">21</td> -<td class="general br">176·2</td> -<td class="general br"> 28·2</td> -</tr> - -<tr> -<td colspan="3" class="text">The numbers of bacteria are given in<br />millions per gram of soil.</td> -</tr> - -</table> - -<p><span class="pagenum" id="Page86">[86]</span></p> - -<div class="container w35em" id="Fig13"> - -<img src="images/illo094.png" alt="" /> - -<p class="caption nobotmargin"><span class="smcap">Fig. 13.</span>—Numbers of bacteria counted daily in soils containing</p> - -<div class="centerblock fsize90"> - -<p>A. Bacteria alone.<br /> -B. Same Bacteria as in A + Amœbæ.<br /> -C. Same Bacteria as in A + Flagellates.</p> - -</div><!--centerblock--> - -<p class="center fsize90">(From Ann. Appl. Biol., vol. x.)</p> - -</div><!--container--> - -<p><span class="pagenum" id="Page87">[87]</span></p> - -<p class="blankbefore75">It will be noted that the numbers of bacteria in each -soil rose steadily until a maximum was reached 6-8 days -after inoculation. This is in accordance with expectation, -since the reproductive rate of bacteria is much greater than -that of the amœbæ, which, until their active forms are -numerous, will not exert any appreciable influence on the -bacterial population. Further, since the protozoa were -inoculated as cysts an appreciable time would elapse before -excystation took place. The last seven days of the experiment -are of particular interest. During this period the -amœbæ were known to be active in the soil, and were depressing -the bacterial numbers, for in the control (protozoa-free) -soil the variation in numbers was within experimental error, -while in the other soil the variations were considerable and -well outside experimental error. In fact the variations -were comparable with those found from day to day in untreated -field soils. Finally, the experiment shows that the -bacteria in protozoa-free soil are able to maintain high -numbers for a longer period than those living in association -with protozoa.</p> - -<h3><span class="smcap">Seasonal Changes.</span></h3> - -<p>Superimposed on the daily variations in numbers there -are seasonal changes, as is clearly shown when fourteen day -averages are made of the numbers for each species. Bacteria -have long been known to show autumn and spring -rises, but recent research has demonstrated that the protozoan -population also rises to a maximum at the end of -November, with a less marked spring rise at the end of -March and beginning of April (<a href="#Fig14">Figs. 14</a> and <a href="#Fig15">15</a>).</p> - -<p>It has sometimes been claimed that the numbers of soil -organisms are closely linked with the soil moisture, but no -support for this view was found during the course of the -experiment. Similarly, as in the case of the daily variations, -no connection could be traced between the seasonal changes -and any of the external conditions considered.</p> - -<p>It is interesting to note, however, that the seasonal -variations in the numbers of soil organisms is very similar to -those recorded for many aquatic organisms. Miss Delf,<a href="#Endnote5_8" class="fnanchor">[8]</a> -for<span class="pagenum" id="Page88">[88]</span> -instance, found that in ponds at Hampstead the algæ are -most numerous in spring and again in the autumn, and like -changes are recorded in British lakes by West and West<a href="#Endnote5_25" class="fnanchor">[25]</a> -and in the Illinois river by Kofoid.<a href="#Endnote5_14" class="fnanchor">[14]</a></p> - -<div class="container w40em" id="Fig14"> - -<img src="images/illo096.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 14.</span>—Fortnightly averages of total numbers of Oicomonas, Species γ, and -Species α, and of bacteria, moisture, and temperature. (From Phil. Trans. -Roy. Soc., vol. ccxi.)</p> - -<div class="illotext"> - -<p>X-axis: Fortnight beginning 1920. July. Aug. Sep<sup>t</sup>. Oct. Nov. Dec. Jan 1921. Feb<sup>y</sup>. Mch. April. May. June.</p> - -<p>Y-axis (bottom left): Percentage of moisture</p> - -<p>Y-axis (top left): Logarithms of numbers of active protozoa per gramme of soil</p> - -<p>Y-axis (bottom right): Temperature F</p> - -<p>Y-axis (top right): Bacteria in millions per gramme</p> - -<p>Legend: Oicomonas<br />Species γ<br />Species α<br />Bacteria<br />Temperature<br />Moisture</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>It is difficult to resist the conclusion that these annual -variations are produced by similar causes, from which it -follows that the increase in the numbers of protozoa in the -soil is not wholly conditioned by an increased food supply—the<span class="pagenum" id="Page89">[89]</span> -bacteria—for the algæ are not dependent on such a form -of nourishment. This is substantiated by the fact that the -numbers of protozoa, except those of <i>Oicomonas</i>, rose during -March, whereas the corresponding increase in the bacteria -was delayed till the early part of April.</p> - -<div class="container w40em" id="Fig15"> - -<img src="images/illo097.png" alt="" /> - -<p class="caption long" ><span class="smcap">Fig. 15.</span>—Fortnightly averages of total numbers of Heteromita, Cercomonas, -and Dimastigamœba and of bacteria, moisture, and temperature. (From -Phil. Trans. Roy. Soc., vol. ccxi.)</p> - -<div class="illotext"> - -<p>X-axis: Fortnight beginning July 1920. Aug. Sep<sup>t</sup>. Oct. Nov. Dec. Jan. 1921. Feb. Mar. April May June</p> - -<p>Y-axis (bottom left): Percentage of moisture.</p> - -<p>Y-axis (top left): Logarithms of numbers of active protozoa per gramme of soil.</p> - -<p>Y-axis (bottom right): Temperature F</p> - -<p>Y-axis (top right): Bacteria in millions per gramme</p> - -<p>Legend: Heteromita<br />Cercomonas<br />Dimastigamoeba<br />Bacteria<br />Temperature<br />Moisture</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>Owing to the variations in the numbers of both protozoa -and bacteria, little reliance can be placed on figures obtained -from an isolated count, since on one day the total numbers of<span class="pagenum" id="Page90">[90]</span> -flagellates may be nearly 2,000,000 per gram and drop by -more than half this figure in 24 days. It is certain, however, -that the numbers recorded in the past are much too low, -since the total flagellate and amœbæ species were lumped -together in two groups. Some idea of the size of the soil -population can be obtained, nevertheless, by using the fourteen-day -averages mentioned above. In <a href="#TabVIII">Table VIII.</a> are -tabulated the average total numbers of flagellates, and -amœbæ for the two periods of the year when the population -was at its maximum and minimum respectively. An -endeavour has also been made to strike a rough balance -sheet as to the amount of protoplasm represented by protozoa -and bacteria in a ton of soil. For this purpose it has -been assumed that the organisms have a specific gravity of -1·0 and are spheres of diameters, 6μ for the flagellates, 10μ -for the amœbæ, and 1μ for the bacteria; and that they are -uniformly distributed through the top nine inches of soil. -The top nine inches of soil is taken as weighing 1000 tons.</p> - -<p class="tabhead" id="TabVIII">TABLE VIII.</p> - -<table class="standard"> - -<tr class="bt bb"> -<th rowspan="2" class="bl br"> </th> -<th colspan="3" class="br">Maximum Period.</th> -<th colspan="3" class="br">Minimum Period.</th> -</tr> - -<tr class="bb"> -<th class="w5 br">No.<br />per<br />Gram.</th> -<th class="w5 br">Weight<br />in Gram<br />per Gram.</th> -<th class="w5 br">Weight<br />in Tons<br />per Acre.</th> -<th class="w5 br">No.<br />per<br />Gram.</th> -<th class="w5 br">Weight<br />in Gram<br />per Gram.</th> -<th class="w5 br">Weight<br />in Tons<br />per Acre.</th> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Flagellates</span></td> -<td class="general br">   770,000</td> -<td class="general br">0·000087</td> -<td class="general br">0·087</td> -<td class="general br">   350,000</td> -<td class="general br">0·000039</td> -<td class="general br">0·039</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Amœbæ</span></td> -<td class="general br">   280,000</td> -<td class="general br">0·000147</td> -<td class="general br">0·147</td> -<td class="general br">   150,000</td> -<td class="general br">0·000078</td> -<td class="general br">0·078</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Bacteria</span></td> -<td class="general br">40,000,000</td> -<td class="general br">0·000020</td> -<td class="general br">0·02 </td> -<td class="general br">22,500,000</td> -<td class="general br">0·000012</td> -<td class="general br">0·012</td> -</tr> - -</table> - -<p>It must be remembered that the above figures are minimum -ones, as many species of bacteria and protozoa, known -to occur in the soil, are not included in the statement owing -to their not appearing on the media used for counting -purposes.</p> - -<div class="container w35em" id="Fig16"> - -<img src="images/illo099.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 16.</span>—Daily variations in the numbers of active individuals of a species of -flagellate, <i>Oicomonas termo</i> (Ehrenb.) during March, 1921. (From Phil. -Trans. Roy. Soc., vol. ccxi.)</p> - -<div class="illotext w20em"> - -<p>X-axis: March</p> - -<p>Y-axis: Active numbers per gramme of soil</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>Before leaving the discussion of daily variations in -numbers of protozoa, reference must be made to the flagellate -species. As already mentioned, their active numbers -fluctuate rapidly, and for the most part entirely irregularly. -One species, however, <i>Oicomonas termo</i>, is characterised by<span class="pagenum" id="Page91">[91]</span> -possessing a periodic change; high active numbers on one -day being succeeded by low, which are again followed by -high on the third day. This rhythm was maintained, with -few exceptions, for 365 days (<a href="#Fig16">Fig. 16</a>), and has been shown<span class="pagenum" id="Page92">[92]</span> -to take place in artificial culture kept under controlled -laboratory conditions (<a href="#Fig17">Fig. 17</a>).</p> - -<div class="container w35em" id="Fig17"> - -<img src="images/illo100.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 17.</span>—Daily variations in the numbers of active individuals of <i>Oicomonas -termo</i> (Ehrenb.) in artificial culture media kept at a constant temperature -of 20° C. A, in hay infusion; B, in egg albumen.</p> - -<div class="illotext w20em"> - -<p>X-axis: Days</p> - -<p>Y-axis: Thousands</p> - -</div><!--illotext--> - -</div><!--container--> - -<p><span class="pagenum" id="Page93">[93]</span></p> - -<p>It was thought that an explanation of this phenomenon -might be found in alternate excystation and encystation, -since the latter is a constituent part of the animals’ life -history (see <a href="#Page73">p. 73</a>). This, however, does not hold, for the -cyst curve is not the inverse of that of the active; and, -moreover, statistical treatment demonstrated that cyst formation -is wholly unperiodic in character.</p> - -<p>An explanation must therefore be sought in the changes -in the organisms during the active period of their life, and the -deduction can be drawn that, increased active numbers tend -to be followed by death, conjugation, or both, while decreases -in the active numbers are followed by rises in total numbers, -i.e., reproduction, and this rhythmically.</p> - -<p>This somewhat surprising conclusion appears to hold, in -a lesser degree, for other soil protozoa, and is of sufficient -importance to warrant further research. The direction in -which this is being pursued is by a study of the reproductive -rates of pure cultures of certain ciliates and flagellates under -varying external conditions. Space does not admit of adequate -discussion of this problem, but the results already -obtained justify the view that such lines of work will elucidate -some of the baffling problems of soil micro-biology.</p> - -<h3><span class="smcap">Soil Reaction.</span></h3> - -<p>The development of the artificial fertiliser industry -has in many ways revolutionised farm practice, with the -inevitable result that new problems have arisen, not the least -of which are biological in character.</p> - -<p>If, as seems to be indubitable, the micro-organisms of -the soil are of importance to soil fertility, it is necessary for -us to know in what way this population is affected by the -application of fertilisers, and a start has been made by -investigating the effects of hydrogen ion concentration on -soil protozoa. Much has already been written concerning -this question, but almost entirely on results obtained in -artificial cultures. It is always dangerous to argue from -the artificial to the natural environment of organisms and<span class="pagenum" id="Page94">[94]</span> -particularly so in respect to the soil. Also, as Collett has -shown, the toxic effects of acids are probably not entirely -a function of the hydrogen ion concentration, but that the -molecules of certain acids are in themselves toxic, an -action which can, however, be diminished by the antagonistic -powers of many substances such as NaCl.</p> - -<p>In this laboratory S. M. Nasir, by unpublished work, -has shown that the limiting value on the acid side for <i>Colpoda -cucullus</i> was P<sub>H</sub> 3·3; for a flagellate (<i>Heteromita globosus</i>), -3·5; and for an amœba (<i>Nægleria gruberi</i>), 3·9.</p> - -<p>Also Mlle. Perey, investigating the numbers of protozoa -in one of the Rothamsted grass plots of P<sub>H</sub> 3·65, found a -total of 13,600 protozoa, of which 90 per cent. were active.</p> - -<p>The tolerance, therefore, of these organisms to varying -external conditions is greater than has formerly been supposed, -a conclusion which is becoming more evident from the -researches mentioned in <a href="#Page66">Chapter IV.</a> on soils from different -parts of the world.</p> - -<h3><span class="smcap">Protozoa and the Nitrogen Cycle.</span></h3> - -<p>In partially-sterilised soil from which protozoa were absent -Russell and Hutchinson obtained an increased ammonia -production, a result also obtained by Cunningham. Hill, on -the other hand, concluded that protozoa have no effect on -ammonification, but his technique is open to criticism.</p> - -<p>Lipman, Blair, Owen and McLean’s work<a href="#Endnote5_17" class="fnanchor">[17]</a> contains many -figures obtained by adding dried blood, tankage, soluble -blood flour, cottonseed meal, soy-bean meal, wheat flour, -corn meal, etc., to soil. It is difficult to understand how -accurate results could be expected when, to an already little -understood complex substance, such as soil, is added a series -of substances whose effects are practically unknown.</p> - -<p>Free nitrogen-fixation in soils is an important process, -more especially in soils of a light sandy nature, from which -crops are taken year after year without any application of -manure. The effect of protozoa on the organisms causing -this process has in the past received little attention. Recently,<span class="pagenum" id="Page95">[95]</span> -however, Nasir<a href="#Endnote5_20" class="fnanchor">[20]</a> has studied the influence of protozoa -on Azotobacter, both in artificial culture and in sand. From -a total of 36 experiments done in duplicate or triplicate, -31 showed a decided gain in nitrogen fixation over the control, -while only 5 gave negative results.</p> - -<div class="container w40em" id="Fig18"> - -<img src="images/illo103.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 18.</span>—Showing the highest fixations of nitrogen above the control recorded -for Azotobacter in the presence of different species of Protozoa. -(From Ann. Appl. Biol., vol. ii.)</p> - -<div class="illotext w20em"> - -<p>X-axis (left): Artificial Media C A F AF AC ACF</p> - -<p>X-axis (right): Sand Cultures C A AF AC</p> - -<p>Legend: C represents CILIATES.<br />A -do.- AMOEBAE.<br />F -do.- FLAGELLATES.</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>As might be expected, the fixation figures varied from -culture to culture, the highest recorded being 36·04 per cent. -above the control and this in a sand culture (<a href="#Fig18">Fig. 18</a>).<span class="pagenum" id="Page96">[96]</span> -Reference to the details of the experiments shows that the -criticisms made against similar work done in the past do -not hold here, and one must conclude that Azotobacter is -capable of fixing more atmospheric nitrogen in the presence -of protozoa than in their absence.</p> - -<p>At present it is impossible to say how this occurs, but -it is highly improbable that the protozoa are themselves -capable of fixing nitrogen. A more likely explanation is -that the protozoa, by consuming the Azotobacter, kept down -the numbers, and transfer the nitrogen to their own bodies. -This will tend to prevent the bacteria from reaching a maximum -density, and reproduction, involving high metabolism, -will be maintained for a longer period than would have -otherwise occurred. This and other possible explanations, -are being tested.</p> - -<p>Little has been said regarding the application of protozoology -to the question of soil partial sterilisation. As -already pointed out, in the past much work has been done, -but the results were conflicting. In view, however, of our -recently acquired knowledge of the life of protozoa in ordinary -field soil, most of the early experiments require repeating. -A beginning has already been made, but the work is not -sufficiently advanced to warrant discussion.</p> - -<p>What is urgently needed, however, is to increase our -knowledge of the general physiology of these unicellular -animals. Until we know what are the inter-relationships -between the members of the micro-organic population of -normal soil it is almost impossible to hope that means will -be devised by which they can be controlled.</p> - -<p>At present we are almost entirely ignorant of the simplest -of physiological reactions, such as the exact effect of various -inorganic salts found in the soil.</p> - -<p>Also some experiments in Germany and the States indicate -that amœbæ are selective as regards the bacteria they -ingest. If this is substantiated it may prove of importance -to economic biology.</p> - -<p>It has been shown that the flagellates occur in the soil<span class="pagenum" id="Page97">[97]</span> -in large numbers, and many of them feed on bacteria. It is -probable, however, that certain of them feed saprophytically -and must therefore exert some influence on the soil -solution, though what this may be is entirely unknown.</p> - -<p>Finally, as Nasir has shown, the protozoa play a part in -the complicated nitrogen cycle, and work of this type needs -extending.</p> - -<p>Such, then, are a few of the outstanding problems that -confront the soil protozoologist; but he must always remember -that the organisms he studies are but a small -fraction of the total, and that any influence affecting one -part of the complex will be reflected in another. As Prof. -Arthur Thomson said in his Gifford Lectures, “No creature -lives or dies to itself, there is no insulation. Long nutritive -chains often bind a series of organisms together in the very -fundamental relation that one kind eats the others.” Such -nutritive chains obtain in the soil as markedly as in other -haunts of living creatures.</p> - -<h3>SELECTED BIBLIOGRAPHY.</h3> - -<p class="center fsize90">* <i>Papers giving extensive bibliographies.</i></p> - -<div class="footnote"> - -<p><span id="Endnote5_1" class="label"> [1]</span> Cunningham, A., Journ. Agric. Sci., 1915, vol. xvii., p. 49.</p> - -<p><span id="Endnote5_2" class="label"> [2]</span> Cunningham, A., and Löhnis, F., Centrlb. f. Bakt. Abt. II., 1914, -vol. xxxix., p. 596.</p> - -<p><span id="Endnote5_3" class="label"> [3]</span> Cutler, D. W., Journ. Agric. Sci., 1919, vol. ix., p. 430.</p> - -<p><span id="Endnote5_4" class="label"> [4]</span> Cutler, D. W., Journ. Agric. Sci., 1920, vol. x., p. 136.</p> - -<p><span id="Endnote5_5" class="label"> [5]</span> Cutler, D. W., Ann. App. Biol., 1923, vol. x., p. 137.</p> - -<p><span id="Endnote5_6" class="label"> [6]</span> Cutler, D. W., and Crump, Ann. App. Biol., 1920, vol. vii., p. 11.</p> - -<p><span id="Endnote5_7" class="label"> [7]</span> * Cutler, D. W., Crump and Sandon, Phil. Trans. Roy. Soc. B., -1922, vol. ccxi., p. 317.</p> - -<p><span id="Endnote5_8" class="label"> [8]</span> Delf, E. M., New Phytologist, 1915, vol. xiv., p. 63.</p> - -<p><span id="Endnote5_9" class="label"> [9]</span> Dobell, C. C., Arch. f. Protisenk., 1911, vol. xxiii., p. 269.</p> - -<p><span id="Endnote5_10" class="label">[10]</span> * Goodey, T., Roy. Soc. Proc. B., 1916, vol. lxxxix., p. 279.</p> - -<p><span id="Endnote5_11" class="label">[11]</span> Goodey, T., Roy. Soc. Proc. B., 1913, vol. lxxxvi, p. 427.</p> - -<p><span id="Endnote5_12" class="label">[12]</span> Hartmann, M., and Nägler, K., Sitz-Ber. Gesellsch. Naturf. -Freunde, 1908, Berlin, No. 4.</p> - -<p><span id="Endnote5_13" class="label">[13]</span> Koch, G. P., Journ. Agric. Res., 1916, vol. li., p. 477.</p> - -<p><span id="Endnote5_14" class="label">[14]</span> Kofoid, C. A., Bull. Illinois State Lab. Nat. Hist., 1903 and 1908.</p> - -<p><span id="Endnote5_15" class="label">[15]</span> * Kopeloff, N., Lint, H. C., and Coleman, D. A., Centrlb. f. Bakt. -Abt. II., 1916, vol. xlvi., p. 28.</p> - -<p><span class="pagenum" id="Page98">[98]</span></p> - -<p><span id="Endnote5_16" class="label">[16]</span> Kopeloff, N., Lint, H. C., and Coleman, D. A., Centrlb. f. Bakt. -Abt. II., 1916, vol. xlv., p. 230.</p> - -<p><span id="Endnote5_17" class="label">[17]</span> Lipman, J. G., Blair, A. W., Owen, L. L., and McLean, H. C., -N.J. Agric. Exp. Sta. 1912, Bull., No. 248.</p> - -<p><span id="Endnote5_18" class="label">[18]</span> Martin, C. H. and Lewin, K. R., Journ. Agric. Soc., 1915, vol. vii., -p. 106.</p> - -<p><span id="Endnote5_19" class="label">[19]</span> Martin, C. H., Roy. Soc. Proc. B., 1912, vol. lxxxv., p. 393.</p> - -<p><span id="Endnote5_20" class="label">[20]</span> * Nasir, S. M., Ann. App. Biol., 1923, vol. x., p. 122.</p> - -<p><span id="Endnote5_21" class="label">[21]</span> Russell, E. J., Roy. Soc. Proc. B., 1915, vol. lxxxix., p. 76.</p> - -<p><span id="Endnote5_22" class="label">[22]</span> * Russell, E. J., “Soil Conditions and Plant Growth,” 1921, 4th ed.</p> - -<p><span id="Endnote5_23" class="label">[23]</span> * Sherman, J. M., Journ. Bact., 1916, vol. i., p. 35, and vol. ii., -p. 165.</p> - -<p><span id="Endnote5_24" class="label">[24]</span> Truffaut, G., and Bezssonoff, H., Compt. Rend. Acad. Sci., 1920, -vol. clxx., p. 1278.</p> - -<p><span id="Endnote5_25" class="label">[25]</span> West, W., and West, G. S., Journ. Linn. Soc. Bot., 1912, vol. xl., -p. 395.</p> - -</div><!--footnote--> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page99">[99]</span></p> - -<h2 class="nobreak">CHAPTER VI.<br /> -<span class="chaptitle">ALGÆ.</span></h2> - -</div><!--chapter--> - -<h3>I. <span class="smcap">General and Historical Introduction.</span></h3> - -<p>Speaking broadly, the organisms of the soil may be classified -into several distinct groups differing conspicuously in their -general characters and physiological functions and therefore -in their economic significance; among such groups may -be mentioned the bacteria, protozoa, algæ and fungi. It is -found, however, that though typical members of these groups -are conspicuously different from one another, yet there -exist a number of unicellular forms which have characters -in common with more than one of these big groups, and the -lines of demarcation between them become difficult to define. -It becomes advisable, therefore, to depart a little from the -systematist’s rigid definitions and to adopt a somewhat -more logical grouping of the soil organisms based on their -mode of life.</p> - -<p>To give but a single example: <i>Euglena viridis</i> occurs -quite commonly in soil. Through its single flagellum, its -lack of a definite cellulose wall, its changeable shape and its -ability to multiply by simple fission in the motile state it -definitely belongs systematically to the group of protozoa. -But its possession of chlorophyll, in enabling it to synthesise -complex organic substances from CO<sub>2</sub> and water in a manner -entirely typical of plants, connects it physiologically so closely -with the lower green algæ that in studying the biology of -the soil it seems best to include it and other nearly related -forms with the algæ.</p> - -<p>On this physiological basis “soil-algæ” may be defined as -those micro-organisms of the soil which have the power,<span class="pagenum" id="Page100">[100]</span> -under suitable conditions, to produce chlorophyll. Such a -definition has the advantage that it is wide enough to include -the filamentous protonema of mosses, which, though alga-like -in form and in physiological action, is nevertheless -separated from the true algæ by a wide gulf. A more -accurate name for such a group of organisms would be the -“chlorophyll-bearing protophyta” of the soil; they may -be classified briefly as follows <span class="nowrap">(<a href="#TabIX">Table IX.</a>):—</span></p> - -<p class="tabhead" id="TabIX">TABLE IX.</p> - -<table class="classification"> - -<tr class="bt bb"> -<th class="bl br"> </th> -<th colspan="4" class="bl br">Group.</th> -<th class="bl br">Colour.</th> -<th class="bl br">Pigments.</th> -</tr> - -<tr> -<td class="class">I.</td> -<td colspan="2" class="text"><i>Flagellatæ.</i></td> -<td colspan="2" class="text">Euglenaceæ.<br />Cryptomonadineæ.</td> -<td class="text wrappable">Green.</td> -<td class="text"><i>Chlorophyll.</i></td> -</tr> - -<tr> -<td rowspan="5" class="class">II.</td> -<td colspan="2" class="text"><i>Algæ</i>—</td> -<td colspan="2" class="text"> </td> -<td class="text"> </td> -<td class="text"> </td> -</tr> - -<tr> -<td class="number unhigh">1.</td> -<td class="text unhigh">Myxophyceæ.</td> -<td colspan="2" class="text wrappable unhigh">Mostly filamentous, chiefly Oscillatoriaceæ and Nostocaceæ.</td> -<td class="text wrappable unhigh">Blue-green to violet or brown.</td> -<td class="text unhigh">Phycocyanin.<br /><i>Chlorophyll.</i><br />Carotin.</td> -</tr> - -<tr> -<td class="number">2.</td> -<td class="text">Bacillariaceæ.</td> -<td colspan="2" class="text wrappable">Mostly pennate, chiefly Naviculoideæ.</td> -<td class="text wrappable">Golden-brown.</td> -<td class="text">Carotin.<br />Xanthophyll.<br /><i>Chlorophyll.</i></td> -</tr> - -<tr> -<td rowspan="2" class="number">3.</td> -<td rowspan="2" class="text">Chlorophyceæ.</td> -<td class="number">(i)</td> -<td class="text wrappable">Protococcales, Ulotrichales, Conjugatæ, etc.</td> -<td class="text wrappable">Green.</td> -<td class="text"><i>Chlorophyll.</i></td> -</tr> - -<tr> -<td class="number unhigh">(ii)</td> -<td class="text unhigh">Heterokontæ.</td> -<td class="text wrappable unhigh">Yellow-green.</td> -<td class="text unhigh"><i>Chlorophyll.</i><br />Xanthophyll.</td> -</tr> - -<tr class="bb"> -<td class="class">III.</td> -<td colspan="2" class="text"><i>Bryophyta.</i></td> -<td colspan="2" class="text">Filamentous moss protonema.</td> -<td class="text">Green.</td> -<td class="text"><i>Chlorophyll.</i></td> -</tr> - -</table> - -<p>The importance of the lower algæ from a biological standpoint -has long been recognised, since their extremely primitive -organisation, coupled with their ability to synthesise organic -compounds from simple inorganic substances, singles them -out as being not very distantly removed from the group of -organisms in which life originated upon the earth. But the -possibility of their having a very much wider economic -significance was completely overlooked until about a quarter -of a century ago, when Hensen demonstrated their importance -in marine plankton as the producers of the organic substance<span class="pagenum" id="Page101">[101]</span> -upon which the whole of the animal life of the ocean is -ultimately dependent. In consequence, it has been generally -assumed that the growth of algæ, since they contain chlorophyll, -is entirely dependent on the action of light. Hence -the recent idea of the existence of algæ which actually -inhabit the soil has been received with a certain amount of -scepticism, though the results of modern physiological -research on a number of the lower algæ show that there -is very good reason to believe that such a soil flora is entirely -possible.</p> - -<p>In considering the alga-flora of a soil it is necessary -to distinguish between two very different sets of conditions -under which the organisms may be growing. In the first -place, they may grow on the surface of the soil, being -subjected directly to insolation, rain, the deposition of -dew, the drying action of wind, relatively quick changes of -temperature and other effects of climate. Certain combinations -of these conditions present so favourable an environment -for the growth of algæ that at times there appears on -the surface of the soil a conspicuous green stratum, sometimes -so dark in colour as to appear almost black. Strata -of this nature are well known, and in systematic works -there are constant references to species growing “on damp -soil”; for instance, of the 51 well-defined species of <i>Nostoc</i> -recognised by Forte, no less than 31 are characterised as -terrestrial. Such appearances, however, seem to have been -regarded as sporadic and more or less accidental, rather than -as the unusually luxuriant development of an endemic -population, and have been frequently attributed to an -excessively moist condition of the soil due to defective -drainage.</p> - -<p>In the second place, the algæ may be living within the -soil itself, away from the action of sunlight and under somewhat -more uniform conditions of moisture and temperature.</p> - -<p>Up to the present time the greater number of the investigations -carried out in this subject have been of a systematic -nature, and extremely little direct evidence has<span class="pagenum" id="Page102">[102]</span> -been obtained which can throw any light on the subject -of the economic significance of the soil algæ.</p> - -<p>The earliest systematic work was carried out by Esmarch, -in 1910-11, who investigated by means of cultures the blue-green -algæ of a number of soils from the German African -Colonies, the samples being taken from the surface and also -from the lower layers of the soil. He obtained a considerable -number of species and observed that in cultivated soils -they were not confined to the surface but occurred regularly -to a depth of 10-25 cms. and occasionally as low as 40-50 -cms. He attributed their existence in the lower layers to -the presence of resting spores carried down in the processes -of cultivation, since his samples from uncultivated -soils were unproductive.</p> - -<p>Later, Esmarch extended his investigations to a far larger -number of samples, 395 in all, of soils of different types from -Schleswig-Holstein. He found that blue-green algæ were -very widely distributed in soils of certain types, though -they occurred rarely in uncultivated soils of low water-content, -and he described no less than 45 species of which -34 belonged to the <i>Oscillatoriaceæ</i> and <i>Nostocaceæ</i>. Certain -of the commoner species were obtained from soils of widely -different types, as shown in <a href="#TabX">Table X.</a>, while other forms -occurred only rarely and with a much more limited distribution.</p> - -<p class="tabhead" id="TabX">TABLE X.—FREQUENCY OF OCCURRENCE OF CERTAIN -COMMON SPECIES IN ESMARCH’S SOIL SAMPLES.</p> - -<table class="standard"> - -<tr class="bt bb"> -<th rowspan="3" class="bl br">Species.</th> -<th colspan="6" class="br">Percentage of Samples containing<br />given Alga.</th> -</tr> - -<tr class="bb"> -<th colspan="3" class="br">Uncultivated<br />Damp Sandy Soil.</th> -<th colspan="3" class="br">Cultivated Soils.</th> -</tr> - -<tr class="bb"> -<th class="w3 br">Shores<br />of<br />Elbe.</th> -<th class="w3 br">Shores<br />of<br />Lakes.</th> -<th class="w3 br">Sea-<br />shore.</th> -<th class="w3 br">Sandy.</th> -<th class="w3 br">Clay.</th> -<th class="w3 br">Marsh-<br />land.</th> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Anabæna variabilis</span></td> -<td class="general br">46  </td> -<td class="general br">43  </td> -<td class="general br"> 9  </td> -<td class="general br">10·3</td> -<td class="general br">60  </td> -<td class="general br">46  </td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Anabæna torulosa</span></td> -<td class="general br">31  </td> -<td class="general br">14·3</td> -<td class="general br">63·6</td> -<td class="general br">27·6</td> -<td class="general br">34·3</td> -<td class="general br">56·4</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Cylindrospermum muscicola</span></td> -<td class="general br">23  </td> -<td class="general br">28·6</td> -<td class="general br"> 0  </td> -<td class="general br">24  </td> -<td class="general br">48·6</td> -<td class="general br">59  </td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Cylindrospermum majus</span></td> -<td class="general br"> 0  </td> -<td class="general br">14·3</td> -<td class="general br"> 0  </td> -<td class="general br">38  </td> -<td class="general br">40  </td> -<td class="general br">33·3</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Nostoc Sp. III.</span></td> -<td class="general br"> 7·7</td> -<td class="general br"> 0  </td> -<td class="general br"> 0  </td> -<td class="general br">38  </td> -<td class="general br">37  </td> -<td class="general br">48·7</td> -</tr> - -</table> - -<p><span class="pagenum" id="Page103">[103]</span></p> - -<p>Taking the number of samples containing blue-green -algæ as a rough measure of their relative abundance, Esmarch -obtained the following interesting figures <span class="nowrap">(<a href="#TabXI">Table XI.</a>):—</span></p> - -<p class="tabhead" id="TabXI">TABLE XI.</p> - -<table class="standard"> - -<tr class="bt bb"> -<th colspan="4" class="text bl br">Kind of Soil.</th> -<th class="br">Percentage<br />of Samples<br />Containing<br />Blue-green<br />Algæ.</th> -<th class="br">Number<br />of Samples<br />Examined.</th> -</tr> - -<tr> -<td colspan="4" class="text bl br"><span class="padr2">Cultivated marshland</span></td> -<td class="general br">95  </td> -<td class="general br">40</td> -</tr> - -<tr> -<td colspan="4" class="text bl br"><span class="padr2">Cultivated clay soil</span></td> -<td class="general br">94·6</td> -<td class="general br">37</td> -</tr> - -<tr> -<td colspan="4" class="text bl br"><span class="padr2">Uncultivated moist sandy soils</span></td> -<td class="general br">88·6</td> -<td class="general br">35</td> -</tr> - -<tr> -<td colspan="4" class="text bl br"><span class="padr2">Cultivated sandy soil</span></td> -<td class="general br">64·4</td> -<td class="general br">45</td> -</tr> - -<tr> -<td colspan="4" class="thinline bl br"> </td> -<td class="thinline bl br"> </td> -<td class="thinline bl br"> </td> -</tr> - -<tr> -<td rowspan="3" class="text mid bl">Uncultivated</td> -<td rowspan="3" class="brace">-</td> -<td rowspan="3" class="brace bt bb bl"> </td> -<td class="text br"><span class="padr2">Woodland</span></td> -<td class="general br">12·5</td> -<td class="general br">40</td> -</tr> - -<tr> -<td class="text br"><span class="padr2">Sandy heathland</span></td> -<td class="general br"> 9  </td> -<td class="general br">34</td> -</tr> - -<tr> -<td class="text br"><span class="padr2">Moorland</span></td> -<td class="general br"> 0  </td> -<td class="general br">35</td> -</tr> - -<tr class="bb"> -<td colspan="4" class="thinline bl br"> </td> -<td class="thinline bl br"> </td> -<td class="thinline bl br"> </td> -</tr> - -</table> - -<p>In noting that the soils fell into two groups, those relatively -rich and those poor in blue-green algæ, Esmarch -concluded that the two chief factors governing the distribution -of the <i>Cyanophyceæ</i> on the surface of soils are, (1) the -moisture content of the soil, (2) the availability of mineral -salts, cultivated soils being especially favoured in both of -these respects. He further distinguished between cultivated -land of two kinds, viz. arable land and grass land, and -found that on all types of soil grassland was richer in species -than was arable land.</p> - -<p>Esmarch examined, in addition, 129 samples taken from -the lower layers of the soil immediately beneath certain of -his surface samples, 107 at 10-25 cms. and the rest at 30-50 -cms. depth.</p> - -<p>In cultivated soils, whether grassland or arable land, he -found that blue-green algæ occurred almost invariably in -the lower layers in those places bearing algæ on the surface -and that, with rare exceptions, the algæ found in the lower -layers corresponded exactly to those on the surface, except -that with increasing depth there was a progressive reduction -in the number of species.</p> - -<p><span class="pagenum" id="Page104">[104]</span></p> - -<p>In uncultivated, moist, sandy soils the agreement was -far less complete, for though algæ were rarely absent from -the lower layers their vertical distribution was frequently -disturbed by the action of wind and rain. Other uncultivated -soils not subject to periodic disturbance were found -to be uniformly lacking in algæ in the lower layers, but as -the limited number of samples examined came completely -from places where there were no algæ on the surface this -means very little.</p> - -<p>By direct microscopic examination of soil Esmarch -claims to have found living filaments of blue-green algæ at -various depths below the surface. He realised, however, -that there was no indication of the length of time that such -filaments had been buried, and therefore conducted a series -of experiments from which he concluded that the period -during which the algæ investigated could continue vegetatively -in the soil after burial varied with different species -from 5-12 weeks, but that during the later part of the period -the algæ gradually assumed a yellowish-green colour.</p> - -<p>It is unfortunate that Esmarch’s investigations were -directed only towards the blue-green algæ since observations -made in this country indicate that such a series of records -gives but a very incomplete picture of the soil flora as a -whole.</p> - -<p>Petersen, in his “Danske Aërofile Alger” (1915) added -considerably to our knowledge of soil algæ, especially of -diatoms. Unfortunately he confined his investigations of -the green algæ to forms growing visibly on the surface of the -ground. He observed, however, that acid soils possessed -a different flora from that commonly found on alkaline or -neutral soils, the former being dominated by <i>Mesotænium -violascens</i>, <i>Zygnema ericetorum</i>, and 2 spp. of <i>Coccomyxa</i>, -while the latter were characterised by <i>Mesotænium macrococcum -var.</i>, <i>Hormidium</i>, 2 spp., and <i>Vaucheria</i>, 3 spp.</p> - -<p>Of diatoms he obtained no less than 24 species and varieties -from arable and garden soils, and five characteristic of -marshy soils, while from forest soils and dry heathland they<span class="pagenum" id="Page105">[105]</span> -appeared to be often absent. He omitted all reference to -blue-green algæ.</p> - -<p>Meanwhile Robbins, examining a number of Colorado -soils that contained unprecedented quantities of nitrate, -obtained from them 18 species of blue-green algæ, 2 species -of green algæ, and one diatom. Moore and Karrer have -demonstrated the existence of a subterranean alga-flora of -which <i>Protoderma viride</i>, the most constantly occurring -species, was shown to multiply when buried to a depth of -one metre.</p> - -<p>In this country attention was first called to the subject -by Goodey and Hutchinson of Rothamsted who, in examining -certain old stored soils for protozoa, obtained also a -number of blue-green forms which were submitted to Professor -West for identification. This ability of certain algal -spores to retain their vitality for a long resting period -was so very striking that an investigation was begun -at Birmingham in 1915 to ascertain whether other forms -were equally resistant. The investigation was carried out -on a large number of freshly collected samples of arable -and garden soils which were first aseptically air-dried for -at least a month and then grown in culture. No less than -20 species or varieties of diatoms, 24 species of blue-green -and 20 species of green algæ were obtained from these -cultures (<a href="#TabXII">Table XII.</a>). In the majority of the samples there -was found a central group of algæ, including <i>Hantzschia -amphioxys</i>, <i>Trochiscia aspera</i>, <i>Chlorococcum humicola</i>, <i>Bumilleria -exilis</i> and rather less frequently <i>Ulothrix subtilis var. -variabilis</i>, while moss protonema was universally present. -These species were thought to form the basis of an extensive -ecological plant formation in which, by the inclusion of other -typically terrestrial but less widely distributed species -smaller plant-associations were recognised.</p> - -<p>In certain of the soils, associations consisting very largely -of diatoms were present, and it is to be noted that the majority -of the forms that have been described are of exceedingly -small size. It is doubtless this characteristic which enables<span class="pagenum" id="Page106">[106]</span> -them to withstand the conditions of drought to which the -organisms of the soil are liable to be subjected, small organisms -having been shown to be better able to resist desiccation -than are larger ones. Since the soil diatoms belong to -the pennate type, they are further adapted to their mode of -life by their power of locomotion, which enables them in -times of drought to retire to the moister layers of the soil.</p> - -<p>In the soils examined in this work blue-green algæ were -less universally present than were diatoms or green algæ, -and the species found appeared to be more local in occurrence. -There seemed to be, however, an association between the -three species, <i>Phormidium tenue</i>, <i>Ph. autumnale</i>, and <i>Plectonema -Battersii</i>, at least two of the three species having -been found together in no less than 16 of the samples, -while all three occurred in 7 of them.</p> - -<p class="tabhead" id="TabXII">TABLE XII.—ALGÆ IN DESICCATED ENGLISH SOILS. -(BRISTOL.)</p> - -<table class="standard"> - -<tr class="bt bb"> -<th rowspan="2" class="bl br">Group.</th> -<th rowspan="2" class="br">Number<br />of Samples<br />Productive.</th> -<th colspan="3" class="br">Number of Species.</th> -</tr> - -<tr class="bb"> -<th class="br">Maximum<br />per<br />Sample.</th> -<th class="br">Average<br />per<br />Sample.</th> -<th class="br">Total.</th> -</tr> - -<tr> -<th class="bl br"> </th> -<th class="w4 br">per cent.</th> -<th class="w4 br"> </th> -<th class="w4 br"> </th> -<th class="w4 br"> </th> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Diatoms</span></td> -<td class="general br"> 95·5</td> -<td class="general br"> 9</td> -<td class="general br"> 3·7</td> -<td class="general br">20</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Blue-green algæ</span></td> -<td class="general br"> 77·3</td> -<td class="general br"> 7</td> -<td class="general br"> 2·5</td> -<td class="general br">24</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Green algæ</span></td> -<td class="general br">100  </td> -<td class="general br"> 7</td> -<td class="general br"> 4·3</td> -<td class="general br">20</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Moss protonema</span></td> -<td class="general br">100  </td> -<td class="general br">—</td> -<td class="general br">—</td> -<td class="general br">—</td> -</tr> - -<tr class="bb"> -<td class="general bl br">Total</td> -<td class="general br">—</td> -<td class="general br">20</td> -<td class="general br">10·5</td> -<td class="general br">—</td> -</tr> - -</table> - -<p>It was generally noticeable that those soils found to be rich -in blue-green algæ contained only a few species of diatoms, -and vice versa. Diatoms appeared most frequently in soils -from old gardens, whereas blue-green algæ were more characteristic -of arable soils. The green algæ and moss protonema, -on the other hand, were distributed universally.</p> - -<p>The majority of green algæ typically found in soils are -unicellular, but a few filamentous forms occur. With the -exception of <i>Vaucheria</i> spp. these are characterised, however,<span class="pagenum" id="Page107">[107]</span> -by an ability to break down in certain circumstances -into unicellular or few-celled fragments, in which condition -identification is often very difficult.</p> - -<p>It was also found by cultural examination of a number of -old stored soils from Rothamsted that germination of the -resting forms of a number of algæ could take place after an -exceedingly long period of quiescence. No less than nine -species of blue-green algæ, four species of green algæ, and one -species of diatom were obtained from soils that had been -stored for periods of about forty years, the species with -the greatest power to retain their vitality being <i>Nostoc muscorum</i> -and <i>Nodularia Harveyana</i>.</p> - -<h3>II. <span class="smcap">The Soil as a Suitable Medium for Algal Growth.</span></h3> - -<p>Were it not for the recent advances that have been made -in our knowledge of the mode of nutrition of many of the -lower algæ, it would be very difficult to account for the widespread -occurrence of algæ in the soil, for it is undoubtedly -true of some of the more highly evolved algæ that their -mode of nutrition is entirely typical of that of green plants -in general. The application of bacteriological technique to -the algæ, however, by Beijerinck, by Artari, and by Chodat -and his pupils, and the introduction of pure-culture methods -have led to a study of the physiology of some of the lower -algæ, in the hope of getting to understand some of the -fundamental problems underlying the nutrition of organisms -containing chlorophyll. It is impossible here to do more -than mention the names of a few of the more important of -those who have worked along these lines, such as Chodat, -Artari, Grintzesco, Pringsheim, Kufferath, Nakano, Boresch, -Magnus and Schindler, and to condense into a few sentences -some of their more important conclusions.</p> - -<p>It is now established that although in the light the algæ -are able to build up their substance from CO<sub>2</sub> and water -containing dilute mineral salts, yet in such conditions -growth is sometimes very slow, and with some species at<span class="pagenum" id="Page108">[108]</span> -any rate it is greatly accelerated by the addition of a small -quantity of certain organic compounds. The ability of the -lower algæ to use organic food materials varies specifically, -quite closely related forms often reacting very differently -to the same substance, but there have been shown to be a -considerable number of forms which can make use of organic -compounds to such an extent that they can grow entirely -independently of light. In such cases the nutrition of the -organism becomes wholly saprophytic, and the chlorophyll -may be completely lost; it has frequently been observed, -however, that on suitable nutrient media, even in complete -darkness, certain algæ continue to grow and retain their -green colour, provided that a sufficient supply of a suitable -nitrogenous compound is present.</p> - -<p><i>Chlorella vulgaris</i>, an alga frequently found in soil, has -been shown to be extremely plastic in its relations to food -substances. Given only a dilute mineral-salts solution as -food source, it absorbs CO<sub>2</sub> from the air, and grows in sunlight -with moderate rapidity. The addition of glucose to -the medium in the light greatly increases the rate and -amount of growth and the size of the cells, while in the -dark the colonies not only remain green but have been shown -to develop more vigorously than in full daylight. The -organism is also able to use peptone as a source of nitrogen -in place of nitrates.</p> - -<p><i>Stichococcus bacillaris</i> and <i>Scenedesmus spp.</i>, also occurring -in soils, have been shown to be almost equally adaptable, -though in these cases the organisms grow more slowly in -the dark than on the corresponding medium in the light. -Liquefaction of gelatine by the secretion of proteolytic -enzymes has been shown to be a further property of certain -species, resulting in the formation of amino acids such as -glycocoll, phenylalanine, dipeptides, etc. This property is, -however, possessed by only a limited number of species and in -varying degree.</p> - -<p>Up to the present very little work of this kind has been -done upon algæ actually taken from the soil, and our knowledge<span class="pagenum" id="Page109">[109]</span> -is therefore very scanty. Of the species so far examined -all show considerable increase in growth on the -addition to the medium of glucose and other sugars, and -tend to be partially saprophytic; a few have been shown -to liquefy gelatine to some extent.</p> - -<p>Servettaz, Von Ubisch, and Robbins have also demonstrated -that the protonema of some mosses can make use -of certain organic substances, especially the sugars, and grow -vigorously in the dark. It has been shown, however, that -light is essential for the development of the moss plant.</p> - -<p>It was thought at Rothamsted that some light might be -thrown upon the activities of the soil-algæ by making -counts of the numbers present in samples of soil taken -periodically within a circumscribed area. A dilution method -similar to that in use in the protozoological laboratory -was adopted and applied to samples of arable soil taken -from the surface, and at depths of 2, 4, 6 and 12 inches -vertically beneath. A considerable number of samples -were examined in this way from two plots on Broadbalk -wheat-field, viz.: the unmanured plot and that receiving -a heavy annual dressing of farmyard manure. The numbers -in the unmanured soil were observed to fall far short of -those in that containing a large amount of organic matter, -while in both plots the numbers varied considerably at -different times of the year. The chief species in both plots -were identical, and their vertical distribution was fairly -uniform, but it was observed that the numbers of individuals -varied according to the depth of the sample. The 6th and -12th inch samples contained very few individuals of comparatively -few species, but the 4th inch samples yielded -numbers that were not significantly less than those in the -top inch. The 2nd inch sample was usually much poorer -in individuals than either the top or the 4th inch.</p> - -<p>It is unfortunate that this method of counting is not -really satisfactory for the algæ, chiefly because it takes no -account of the blue-green forms. The gelatinous envelope -which encloses the filaments of these algæ prevents their<span class="pagenum" id="Page110">[110]</span> -breaking up into measurable units. Assuming, as appears -to be the case for the two plots investigated, that the blue-green -algæ are at least as numerous as the green forms, the -total numbers should probably be at least twice as great -as those calculated. Taking 100,000 as a rough estimate -of the number of algæ per gram of manured soil in a given -sample, and assuming the cells to be spherical and of average -diameter 10µ, it has been calculated that the volume of algal -protoplasm present was at least 3 times that of the bacteria -though only one-third of that of the protozoa. This is -probably only a minimum figure for this sample.</p> - -<p>A soil population of this magnitude can not be without -effect on the fertility of the soil. When growing on the -surface of the ground exposed to sunlight the algæ must, -by photosynthesis, add considerably to the organic matter -of the soil, but when they live within the soil itself their -nutrition must be wholly saprophytic, and they can be -adding nothing either to the energy or to the food-content -of the soil. How these organisms fit into the general scheme -of life in the soil is at present undetermined, and there is a -wide field for research in this direction.</p> - -<h3>III. <span class="smcap">Relation of Algæ to the Nitrogen Cycle</span></h3> - -<p>Probably the most important limiting factor in British -agriculture is the supply of nitrogen available for the growing -crop, and it seems likely that the soil-algæ are intimately -connected with this question in several ways.</p> - -<p>Periodic efforts have been made during the last half -century to establish the fact that a number of the lower -organisms, including the green algæ, have the power of fixing -atmospheric nitrogen and converting it into compounds -which are then available for higher plants. This property -has been definitely established for certain bacteria, and rather -doubtfully for some of the fungi, but until recently no -authentic proof had been produced that algæ by themselves -could fix nitrogen. The subject is too wide to be discussed -in much detail here.</p> - -<p><span class="pagenum" id="Page111">[111]</span></p> - -<p>Schramm in America, working with pure cultures of -algæ, tried for ten years to establish the fact of nitrogen -fixation, and failed completely; more recently Wann has -extended Schramm’s work, and claims to have proved -indisputably that, given media containing nitrates as a -source of nitrogen and a small amount of glucose, the seven -species of algæ tested by him fixed atmospheric nitrogen to -the extent of 4-54 per cent. of the original nitrogen content -of the medium. So important a result needed corroboration, -and Wann’s experiment, with some slight improvements, -was therefore repeated at Rothamsted last summer.</p> - -<p>This work has not yet been published, but in the whole -series of ninety-six cultures, with four different species, each -growing on six different media, there is no evidence that -nitrogen fixation has taken place; but there has been a -total recovery at the end of the experiment of 98·93 per -cent. of the original nitrogen supplied. On the other hand, -a flaw has been detected in Wann’s method of analysing -those media containing nitrates, sufficiently great to account -for the differences he obtained between the initial and -final nitrogen content of his cultures. Hence, though one -hesitates to say that the algæ are unable, given suitable -conditions, to fix atmospheric nitrogen, one must admit -that no one has yet proved that they can do so.</p> - -<p>It is far more likely, however, that the experiments of -Kossowitsch and others throw more light on the relation -of soil algæ to nitrogen fixation. They affirm that greater -fixation of nitrogen is effected by mixtures of bacteria and -certain gelatinous algæ than by nitrogen-fixing bacteria -alone, and that the addition of algæ to cultures of bacteria -produces a stimulating effect only slightly less than that of -sugar. It is probable, therefore, that the algæ, in their -gelatinous sheaths, provide easily available carbohydrates -from which the bacteria derive the energy essential to their -work, and that nitrogen fixation in nature is due to the -combined working of a number of different organisms rather -than to the individual action of single species.</p> - -<p><span class="pagenum" id="Page112">[112]</span></p> - -<p>Russell and Richards have shown that the rate of loss of -nitrogen by leaching from uncropped soils is far less than -would be expected from a purely chemical standpoint, and -suggest that certain organisms are present in the soil which, -by absorbing nitrates and ammonium salts as they are formed, -remove them from the soil solution and so help to conserve -the nitrogen of the soil. It is probable that the soil algæ -act in this manner, though to what extent has not yet been -determined.</p> - -<h3>IV. <span class="smcap">Relation of Algæ to Soil Moisture and to the -Formation of Humus Substances.</span></h3> - -<p>In warmer countries than our own, especially those with -an adequate rainfall, the significance of soil algæ is perhaps -more obvious to a casual observer. Treub states that after -the complete destruction of the island of Krakatoa by -volcanic eruption in 1883, the first colonists to take possession -of the island were six species of blue-green algæ, viz., -<i>Tolypothrix</i> sp., <i>Anabæna</i> sp., <i>Symploca</i> sp., <i>Lyngbya</i> 3 spp. -Three years after the eruption these organisms were observed -to form an almost continuous gelatinous and hygroscopic -layer over the surface of the cinders and stones -constituting the soil, and by their death and decay they -rapidly prepared it for the growth of seeds brought to the -island by visiting birds. Hence the new flora which soon -established itself upon the island can be said to have had its -origin in the alga-flora which preceded it. Fritsch has also -emphasised the importance of algæ in the colonisation of -new ground in Ceylon.</p> - -<p>Welwitsch ascribes the characteristic colour from which -the “pedras negras” in Angola derive their name to the -growth of a thick stratum of <i>Scytonema myochrous</i>, a blue-green -alga, which gradually becomes black and completely -covers the soil. At the close of the rainy season this gelatinous -stratum dries up very slowly, enabling the underlying -soil to retain its moisture for a longer period than would -otherwise be the case.</p> - -<p><span class="pagenum" id="Page113">[113]</span></p> - -<p>The gelatinous soil algæ are probably very important in -this respect, for their slow rate of loss of water is coupled -with a capacity for rapid absorption, and they are therefore -able to take full advantage of the dew that may be -deposited upon them and increase the power of the soil to -retain moisture.</p> - -<h3>V. <span class="smcap">Relation of Algæ to Gaseous Interchanges in the -Soil.</span></h3> - -<p>In the cultivation of rice the algæ of the paddy field have -been found to be of extreme importance. Brizi in Italy -has shown that although rice is grown under swamp conditions -yet the roots of the rice plant are typical of those -of ordinary terrestrial plants and have none of the structural -adaptations to aquatic life so characteristic of ordinary marsh -plants. Hence the plants are entirely dependent for healthy -growth upon an adequate supply of oxygen to their roots -from the medium in which they are growing. A serious -disease of the rice plant, characterised by the browning and -dying off of the leaves, which was thought at first to be due -to the attacks of fungi, was found to be the effect of the -inadequate aeration of the roots, while the entry of the -fungi was shown to be subsequent to the appearance of the -physiological disease. The presence of algæ in the swamp -water was found to prevent the appearance of this disease, -in that they unite with other organisms to form a more or -less continuous stratum over the surface of the ground, -and add to the gases which accumulate there large quantities -of oxygen evolved during photosynthesis. The concentration -of dissolved oxygen in the water percolating through -the soil is thereby raised to a maximum, and the healthy -growth of the crop ensured.</p> - -<p>This work has been corroborated by Harrison and Aiyer -in India, and a sufficient supply of algæ in the swamp water -is now regarded as one of the essentials for the production -of a good rice crop.</p> - -<p>From what has been said, it appears that, although our<span class="pagenum" id="Page114">[114]</span> -knowledge of the soil algæ is extremely limited, and our -conception of the part they play is largely based on speculation, -yet the subject is one of enormous interest and -worthy of investigation in many directions. In its present -undeveloped state, it is a little difficult to foresee which lines -of study are likely to prove most profitable, but there is -little doubt that eventually the soil algæ will be shown -to play a significant part in the economy of the soil.</p> - -<h3>SELECTED BIBLIOGRAPHY.</h3> - -<p class="center fsize90">* <i>Papers giving extensive bibliographies.</i></p> - -<h4>I. <span class="smcap">General.</span></h4> - -<div class="footnote"> - -<p><span id="Endnote6_1" class="label"> [1]</span> Bristol, B. M., “On the Retention of Vitality by Algæ from Old -Stored Soils,” New Phyt., 1919, xviii., Nos. 3 and 4.</p> - -<p><span id="Endnote6_2" class="label"> [2]</span> Bristol, B. M., “On the Alga-Flora of some Desiccated English -Soils: an Important Factor in Soil Biology,” Annals of Botany, -1920, vol. xxxiv., No. 133.</p> - -<p><span id="Endnote6_3" class="label"> [3]</span> Brizi, U., “Ricerche sulla Malattia del Riso detta ‘Brusone,’ -Sect. IV. Influenza che le alghe verdi esercitano in risaia,” -Annuario dell Instituzione Agraria Dott. A. Ponti, Milan, 1905, -vol. vi., pp. 84-89.</p> - -<p><span id="Endnote6_4" class="label"> [4]</span> Esmarch, F., “Beitrag zur Cyanophyceen-Flora unserer Kolonien,” -Jahrb. der Hamburgischen wissensch. Anstalten, 1910, xxviii., -3. Beiheft, S. 62-82.</p> - -<p><span id="Endnote6_5" class="label"> [5]</span> Esmarch, F., “Untersuchungen über die Verbreitung der Cyanophyceen -auf und in verschiedenen Boden,” Hedwigia, 1914, Band -lv., Heft 4-5.</p> - -<p><span id="Endnote6_6" class="label"> [6]</span> Fritsch, F. E., “The Rôle of Algal Growth in the Colonisation of -New Ground and in the Determination of Scenery,” Geog. -Journal, 1907.</p> - -<p><span id="Endnote6_7" class="label"> [7]</span> Harrison, W. H., and Aiyer, P. A. Subramania, “The Gases of -Swamp Rice Soils,” Mem. Dept. Agr. in India, Chem. Ser. (I.) -“Their Composition and Relationship to the Crop,” 1913, vol. -iii., No. 3; (II.) “Their Utilisation for the Aeration of the -Roots of the Crop,” 1914, vol. iv., No. 1; (IV.) “The Source of -the Gaseous Soil Nitrogen,” 1916, vol. v., No. 1.</p> - -<p><span id="Endnote6_8a" class="label"> [8<i>a</i>]</span> Hensen, V., “Ueber die Bestimmung des Planktons oder des im -Meere treibenden Materials am Pflanzen und Thieren.” Fünfter -Ber. Komm. wiss. Unters. deutschen Meere, 1887.</p> - -<p><span id="Endnote6_8" class="label"> [8]</span> Moore, G. T., and Karrer, J. L., “A Subterranean Alga Flora,” Ann. -Miss. Bot. Gard., 1919, vi., pp. 281-307.</p> - -<p><span class="pagenum" id="Page115">[115]</span></p> - -<p><span id="Endnote6_9" class="label"> [9]</span> Nadson, G., “Die perforierenden (kalkbohrende) Algen und ihre -Bedeutung in der Natur,” Scripta bot. hort. Univ. Imp. Petrop., -1901, Bd. 17.</p> - -<p><span id="Endnote6_10" class="label">[10]</span> Petersen, J. B., “Danske Aërofile Alger,” D. Kgl. Danske Vidensk. -Selsk. Skrifter, 7 Raekke, Naturv. og mathem., 1915, Bd. xii., -7, Copenhagen.</p> - -<p><span id="Endnote6_11" class="label">[11]</span> Robbins, W. W., “Algæ in some Colorado Soils,” Agric. Exp. -Sta., Colorado, 1912, Bulletin 184.</p> - -<p><span id="Endnote6_12" class="label">[12]</span> Treub, “Notice sur la nouvelle Flora de Krakatau,” Ann. Jard. -Bot. Buitenzorg, 1888, vol. vii., pp. 221-223.</p> - -</div><!--footnote--> - -<h4>II. <span class="smcap">Relation of Algæ to Light and Carbon.</span></h4> - -<div class="footnote"> - -<p><span id="Endnote6_13" class="label">[13]</span> Artari, A., “Zur Ernährungsphysiologie der grünen Algen,” Ber. -der D. bot. Ges., 1901, Bd. xix., S. 7.</p> - -<p><span id="Endnote6_14" class="label">[14]</span> Artari, A., “Zur Physiologie der Chlamydomonaden (Chlam. -Ehrenbergii);” (I.) Jahrb. f. Wiss. Bot., 1913, Bd. lii., S. -410; (II.) <i>Ibid.</i>, 1914, Bd. liii., S. 527.</p> - -<p><span id="Endnote6_15" class="label">[15]</span> Adjarof, M., “Recherches expérimentales sur la Physiologie de -quelques Algues vertes,” Université de Genève—Institut Botanique, -Prof. R. Chodat—1905, 6 serie, vii. fascicule, Genève.</p> - -<p><span id="Endnote6_16" class="label">[16]</span> Beijerinck, M. W., “Berichte über meine Kulturen niederer Algen -auf Nährgelatine,” Centr. f. Bakt. u. Paras., 1893, Abt. I., -Bd. xiii., S. 368, Jena.</p> - -<p><span id="Endnote6_17" class="label">[17]</span> Boresch, K., “Die Färbung von Cyanophyceen und Chlorophyceen -in ihrer Abhängigkeit vom Stickstoffgehalt des Substrates,” -Jahrbücher für Wiss. Botanik., 1913, lii., pp. 145-85.</p> - -<p><span id="Endnote6_18" class="label">[18]</span> Chodat, R., “Étude critique et expérimentale sur le polymorphisme -des Algues,” Genève, 1909.</p> - -<p><span id="Endnote6_19" class="label">[19]</span> Chodat, R., “La crésol-tyrosinase, réactif des peptides et des -polypeptides, des protéides et de la protéolyse,” Archiv. des -Sciences physiques et naturelles, 1912.</p> - -<p><span id="Endnote6_20" class="label">[20]</span> Chodat, R., “Monographie d’Algues en Culture pure: Matériaux -pour la Flore Cryptogamique Suisse,” 1913, vol. iv., fasc. 2, -Berne.</p> - -<p><span id="Endnote6_21" class="label">[21]</span> Dangeard, P. A., “Observations sur une Algue cultivée à -l’obscurité depuis huit ans,” Compt. Rend. Acad. Sci. (Paris), -1921, vol. clxxii., No. 5, pp. 254-60.</p> - -<p><span id="Endnote6_22" class="label">[22]</span> Étard et Bouilhac, “Sur la présence de la chlorophyll dans un Nostoc -cultivé à l’abri de la lumière,” Compt. Rend., t. cxxvii, 1898.</p> - -<p><span id="Endnote6_23" class="label">[23]</span> Grintzesco, J., “Recherches expérimentales sur la morphologie -et la physiologie expérimentale de <i>Scenedesmus acutus</i>,” Meyen. -Bull. herb. Boiss., 1902, Bd. ii., pp. 219-64 and 406-29.</p> - -<p><span id="Endnote6_24" class="label">[24]</span> Grintzesco, J., “Contribution à l’étude des Protococcoidées: -<i>Chlorella vulgaris</i> Beyerinck,” Revue générale de Botanique, -1903, xv., pp. 5-19, 67-82.</p> - -<p><span class="pagenum" id="Page116">[116]</span></p> - -<p><span id="Endnote6_25" class="label">[25]</span> * Kufferath, H., “Contribution à la physiologie d’une protococcacée -nouvelle, <i>Chlorella luteo-viridis</i> Chod. n. sp. var., <i>lutescens</i> Chod. -n. var.,” Recueil de l’institut bot. Léo Errera, 1913, t. ix, p. 113.</p> - -<p><span id="Endnote6_26" class="label">[26]</span> Kufferath, H., “Recherches physiologiques sur les algues vertes -cultivées en culture pure,” Bull. Soc. Roy. Bot. Belgique, 1921, -liv., pp. 49-77.</p> - -<p><span id="Endnote6_27" class="label">[27]</span> Magnus, W., and Schindler, B., “Ueber den Einflusz der Nährsalze -auf die Färbung der Oscillarien,” Ber. der D. Bot. Gesellschaft, -1912-13, xxx., p. 314.</p> - -<p><span id="Endnote6_28" class="label">[28]</span> * Nakano, H., “Untersuchungen über die Entwicklungs- und -Ernährungsphysiologie einiger Chlorophyceen,” Journ. College -of Sci. Imp. Univ. Tokyo, 1917, vol. xl., Art. 2.</p> - -<p><span id="Endnote6_29" class="label">[29]</span> Pringsheim, E., “Kulturversuche mit chlorophyll-führenden -Mikroorganismen,” Cohns Beiträge Z. Biol. d. Pflanzen. (I.) Die -Kultur von Algen in Agar, 1912, Bd. xi., S. 249; (II.) Zur Physiologie -der <i>Euglena gracilis</i>, 1913, Bd. xii., S. 1.; (III.) Zur Physiologie -der Schizophyceen, 1913, Bd. xii., S. 99.</p> - -<p><span id="Endnote6_30" class="label">[30]</span> Radais, “Sur la culture pure d’une algue verte; formation de -chlorophylle à l’obscurité,” Comptes Rendus, 1900, cxxx., p. -793.</p> - -<p><span id="Endnote6_31" class="label">[31]</span> Richter, O., “Zur Physiologie der Diatomeen.” (I.) Sitzber. -d. kais. Akad. d. W. in Wien, math, naturw. Kl., 1906, Bd. -cxv., Abt. I., S. 27; (II.) Denkschrift d. math. naturw. Kl. d. -kais. Akad. d. W. in Wien, 1909, Bd. lxxxiv., S. 666; (III.) -Sitzber. d. Kais. Akad., etc., 1909, Bd. cxviii., Abt. I., S. 1337.</p> - -<p><span id="Endnote6_32" class="label">[32]</span> Richter, O., “Ernährung der Algen,” 1911.</p> - -<p><span id="Endnote6_33" class="label">[33]</span> Robbins, W. J., “Direct Assimilation of Organic Carbon by -<i>Ceratodon purpureus</i>,” Bot. Gaz., 1918, lxv., pp. 543-51.</p> - -<p><span id="Endnote6_34" class="label">[34]</span> Schindler, B., “Ueber den Farbenwechsel der Oscillarien,” Zeitsch. -f. Bot., 1913, v., pp. 497-575.</p> - -<p><span id="Endnote6_35" class="label">[35]</span> Ternetz, Charlotte, “Beiträge zur Morphologie und Physiologie -der <i>Euglena gracilis</i>,” Jahrb. f. Wiss. Bot., 1912, Bd. 51, S. 435.</p> - -</div><!--footnote--> - -<h4>III. <span class="smcap">Relation of Algæ to Nitrogen.</span></h4> - -<div class="footnote"> - -<p><span id="Endnote6_36" class="label">[36]</span> Berthelot, “Recherches nouvelles sur les microorganismes fixateurs -de l’azote,” Comptes Rend., 1893, cxvi., pp. 842-49.</p> - -<p><span id="Endnote6_37" class="label">[37]</span> Bouilhac, R., “Sur la fixation de l’azote atmosphérique par -l’association des algues et des bactéries,” Comptes Rend., -1896, cxxiii., pp. 828-30.</p> - -<p><span id="Endnote6_38" class="label">[38]</span> Bouilhac and Giustiniani, “Sur une culture de sarrasin en -présence d’un mélange d’algues et de bactéries,” Comptes -Rendus, 1903, cxxxvii., pp. 1274-76.</p> - -<p><span id="Endnote6_39" class="label">[39]</span> Charpentier, P. G., “Alimentation azotée d’une algue: Le Cystococcus -humicola,” Ann. Inst. Pasteur, 1903, 17, pp. 321-34.</p> - -<p><span class="pagenum" id="Page117">[117]</span></p> - -<p><span id="Endnote6_40" class="label">[40]</span> Fischer, Hugo, “Über Symbiose von Azotobacter mit Oscillarien,” -Centr. f. Bakt., 1904, xii.</p> - -<p><span id="Endnote6_41" class="label">[41]</span> Frank, B., “Uber den experimentellen Nachweis der Assimilation -freien Stickstoffs durch Erdbewohnende Algen,” Ber. der D. Bot. -Gesellsch., 1889, vol. vii., pp. 34-42.</p> - -<p><span id="Endnote6_42" class="label">[42]</span> Frank, B., “Ueber den gegenwärtigen Stand unserer Kenntnisse -der Assimilation elementaren Stickstoffs durch die Pflanze,” -Ber. der. D. Bot. Ges., 1889, vii., 234-47.</p> - -<p><span id="Endnote6_43" class="label">[43]</span> Frank, B., and Otto, R., “Untersuchungen über Stickstoff Assimilation -in der Pflanze,” Ber. der D. Bot. Ges., 1890, -viii., 331-342.</p> - -<p><span id="Endnote6_44" class="label">[44]</span> Gautier and Drouin, “Recherches sur la fixation de l’azote par le -sol et les végétaux,” Compt. Rend., 1888, cvi., pp. 1174-76; -General Conclusions, p. 1232.</p> - -<p><span id="Endnote6_45" class="label">[45]</span> Kossowitsch, P., “Untersuchungen über die Frage, ob die Algen -freien Stickstoff fixiren,” Bot. Zeit., 1894, Heft 5, S. 98-116.</p> - -<p><span id="Endnote6_46" class="label">[46]</span> Krüger, W., und Schneidewind, “Sind niedere chlorophyllgrüne -Algen imstande, den freien Stickstoff der Atmosphäre zu assimilieren -und Boden an Stickstoff zu bereichern?” Landwirtschaftliche -Jahrb., 1900, Bd. 29, S. 771-804.</p> - -<p><span id="Endnote6_47" class="label">[47]</span> Moore, Benjamin, and T. Arthur Webster, “Studies of the photosynthesis -in f.w.a.” (I.) “The fixation of both C and N from -atmosphere to form organic tissue by green plant cell”; -(II.) “Nutrition and growth produced by high gaseous dilutions -of simple organic compounds, such as formaldehyde and methylic -alcohol”; (III.) “Nutrition and growth by means of high dilution -of CO<sub>2</sub> and oxides of N without access to atmosphere,” -Proc. Roy. Soc., London, 1920, B. xci., pp. 201-15.</p> - -<p><span id="Endnote6_47a" class="label">[47<i>a</i>]</span> Moore, B., Whiteley, Webster, T. A., Proc. Roy. Soc., London, B., -1921; xcii., pp. 51-60.</p> - -<p><span id="Endnote6_48" class="label">[48]</span> Reinke, J., “Symbiose von Volvox und Azotobacter,” Ber. der -d. Bot. Ges., 1903, Bd. xxi., S. 481.</p> - -<p><span id="Endnote6_49" class="label">[49]</span> Russell, E. J., and Richards, E. H., “The washing out of Nitrates -by Drainage Water from Uncropped and Unmanured Land,” -Journ. Agric. Sci., 1920, vol. x., Part I.</p> - -<p><span id="Endnote6_50" class="label">[50]</span> Schloesing, fils, and Laurent, E., “Recherches sur la fixation de -l’azote libre par les plantes,” Ann. de l’Institut Pasteur, 1892, -vi., pp. 65-115.</p> - -<p><span id="Endnote6_51" class="label">[51]</span> Schramm, J. R., “The Relation of Certain Grass Green Algæ to -Elementary Nitrogen,” Ann. Mo. Bot. Gard., 1914, i., No. 2.</p> - -<p><span id="Endnote6_52" class="label">[52]</span> Wann, F. B., “The Fixation of Nitrogen by Green Plants,” -Amer. Journ. Bot., 1921, viii., pp. 1-29.</p> - -</div><!--footnote--> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page118">[118]</span></p> - -<h2 class="nobreak">CHAPTER VII.<br /> -<span class="chaptitle">THE OCCURRENCE OF FUNGI IN THE SOIL.</span></h2> - -</div><!--chapter--> - -<div class="footnote"> - -<p><span class="smcap">Note.</span>—I am indebted to my late colleague Miss Sibyl S. Jewson, M.Sc., -for permission to include unpublished data from our investigations on the soil -fungi.</p> - -</div><!--footnote--> - -<p>In 1886 Adametz,<a href="#Endnote7_1" class="fnanchor">[1]</a> investigating the biochemical changes -occurring in soils, isolated several species of fungi. It was, -however, only with the work of Oudemans and Koning,<a href="#Endnote7_17" class="fnanchor">[17]</a> in -1902 when forty-five species were isolated and described, the -majority as new to science, that the real study of the fungus -flora of the soil commenced. There is now no doubt that -fungi form a large and very important section of the permanent -soil population, and certain forms are found only -in the soil. Indeed, Takahashi<a href="#Endnote7_22" class="fnanchor">[22]</a> has reversed the earlier -ideas by suggesting that fungus spores in the air are derived -from soil forms. The majority of investigations on this -subject fall, perhaps, into one or more of three classes: (<i>a</i>) -purely systematic studies such as those of Oudemans and -Koning,<a href="#Endnote7_17" class="fnanchor">[17]</a> Dale,<a href="#Endnote7_5" class="fnanchor">[5]</a> Jensen,<a href="#Endnote7_9" class="fnanchor">[9]</a> Waksman,<a href="#Endnote7_25" class="fnanchor">[25a]</a> Hagem,<a href="#Endnote7_8" class="fnanchor">[8c]</a> Lendner,<a href="#Endnote7_12" class="fnanchor">[12]</a> -and others, which consist in the isolation and identification -of species from various soils: (<i>b</i>) physiological researches, -such as those of Hagem<a href="#Endnote7_8" class="fnanchor">[8c]</a> on the Mucorineæ of Norway, or the -many investigations on the biochemical changes in soils -produced by fungi, such as those of Muntz and Coudon,<a href="#Endnote7_15" class="fnanchor">[15]</a> -McLean and Wilson,<a href="#Endnote7_15" class="fnanchor">[15]</a> Kopeloff,<a href="#Endnote7_11" class="fnanchor">[11]</a> Goddard,<a href="#Endnote7_7" class="fnanchor">[7]</a> McBeth and -Scales,<a href="#Endnote7_14" class="fnanchor">[14]</a> and others: (<i>c</i>) quantitative studies, such as those -of Remy,<a href="#Endnote7_20" class="fnanchor">[20]</a> Fischer,<a href="#Endnote7_6" class="fnanchor">[6]</a> Ramann,<a href="#Endnote7_18" class="fnanchor">[18]</a> Waksman,<a href="#Endnote7_25" class="fnanchor">[25c]</a> and Takahashi,<a href="#Endnote7_22" class="fnanchor">[22]</a> -which involve numerical estimates of the fungus -flora in soils.</p> - -<h3><span class="smcap">Qualitative Study.</span></h3> - -<p>With very rare exceptions soil fungi cannot be examined -in situ, and the necessary basis of any qualitative research is<span class="pagenum" id="Page119">[119]</span> -the isolation of the organisms in pure culture. Most soil -forms belong to the <i>Fungi imperfecti</i>, and often show considerable -plasticity on artificial media. This makes it very -difficult to determine them by comparison with type herbarium -specimens or published morphological diagnoses. -In consequence many soil fungi have not infrequently been -given new specific names, as <i>humicola</i>, <i>terricola</i>, and so forth, -which is very unsatisfactory, and means that the determinations -have little significance.</p> - -<p>Furthermore, most artificial media are slight variations -on a few common and simple themes, and are very selective, -permitting the growth of a moiety only of the fungi present. -In addition, many fungi grow so slowly that they are overwhelmed -by the more rapidly germinating or spreading -forms, or on the other hand, they may be eliminated by the -metabolic products of different adjacent colonies. The extremely -selective nature of the technique commonly used -is shown if one tabulates systematically all the fungi which -have been recorded or described in soil investigations. -Of <i>Phycomycetes</i> there are fifty-six species of eleven genera; -of <i>Ascomycetes</i> twelve species of eight genera; and of <i>Fungi -imperfecti</i>, including <i>Actinomycetes</i> but not sterile <i>Mycelia</i>, -197 species of sixty-two genera. Rusts and Smuts one -might not expect, but that of the multitudes of <i>Basidiomycetes</i> -growing in wood and meadow not one should have -been recorded is indeed startling. It was at first thought -that many imperfect fungi might be conidial stages of <i>Basidiomycetes</i>, -but much search among forms isolated at Rothamsted -has, up to the present, failed to reveal clamp connections -in the hyphæ.</p> - -<p>Since various species of soil fungi have different optimum -temperature, humidity and other conditions<a href="#Endnote7_3" class="fnanchor">[3]</a> one would not -expect to find an even geographic distribution. Very little -is yet known of this aspect, but <i>Rhizopus nigricans</i>, <i>Mucor -racemosus</i>, <i>Zygorrhynchus vuilleminii</i>, <i>Aspergillus niger</i>, -<i>Trichoderma koningi</i>, <i>Cladosporium herbarum</i>, and many -species of <i>Aspergillus</i>, <i>Penicillium</i>, <i>Fusarium</i>, <i>Alternaria</i>, and<span class="pagenum" id="Page120">[120]</span> -<i>Cephalosporium</i> have been commonly found throughout -North America and Europe wherever soils have been examined. -Species of <i>Aspergillus</i>, however, would appear to -be more common in the soils of south temperate regions -and species of <i>Penicillium</i>, <i>Mucor</i>, <i>Trichoderma</i>, and <i>Fusarium</i> -more abundant in northern soils.</p> - -<p>It is well known that in many plant and animal communities -there occurs a definite rhythm, various species -following each other in a regular sequence as dominants in -the population. Although it is not yet possible to make -any definite statement there would seem indications that -this may also be true of the soil fungi.</p> - -<p>Much work has been done on the distribution of species -at different depths in the soil, but the results are still confusing. -Thus, examining eighteen species, Goddard<a href="#Endnote7_7" class="fnanchor">[7]</a> found no -difference in relative distribution down to 5<sup>1</sup>⁄<sub>2</sub> -inches. Werkenthin<a href="#Endnote7_26" class="fnanchor">[26]</a> -found identical species from 1-4 inches, and then -an absence of fungi from 5-7 inches, which latter was the -greatest depth he examined. Waksman<a href="#Endnote7_25" class="fnanchor">[25]</a> found little difference -in the first six inches, but very few species below -8 inches except <i>Zygorrhynchus vuilleminii</i>, which extended -down to 30 inches and was often the only species occurring -below 12 inches. Taylor<a href="#Endnote7_23" class="fnanchor">[23]</a> has reported species of -<i>Fusarium</i> at practically every depth to 24 inches. Rathbun<a href="#Endnote7_19" class="fnanchor">[19]</a> -found <i>Aspergillus niger</i>, <i>Rhizopus nigricans</i>, and -species of Fusarium and Mucor down to 34 inches, and -<i>Oospora lactis</i>, <i>Trichoderma koningi</i>, <i>Zygorrhynchus vuilleminii</i> -and species of <i>Penicillium</i>, <i>Spicaria</i> and <i>Saccharomyces</i> -as deep as 44 inches. Eleven species were isolated from -the alimentary canal of grubs and worms, and Rathbun concluded -that soil fungi may be spread by these organisms.</p> - -<p>On an unmanured grass plot at Rothamsted twenty -species were isolated from a depth of 1 inch, nineteen -from 6 inches, and eleven from 12 inches, whereas on the -unmanured plot of Broadbalk wheat field twenty-six species -were obtained from 1 inch, seven from 6 inches, and -five from 12 inches. There appeared to be no conspicuous<span class="pagenum" id="Page121">[121]</span> -differences between the floras of the two plots save -that in the Broadbalk plot there were fewer Mucorales, -and <i>Zygorrhynchus mœlleri</i> and <i>Absidia cylindrospora</i> were -absent. In the grass plot samples about one-half the forms -occurring at the lower levels were isolated also from the -upper levels, but in the Broadbalk sample the five forms -isolated from 12 inches, and five out of seven of those at -6 inches occurred only at those levels, i.e. each of the -three levels appeared to have a specific flora. The difference -in depth distribution in these two cases may be due to the -fact that in the Broadbalk plot the stiff clay subsoil occurs -at 5-7 inches, whereas in the grass plot the depth of soil -is greater than 12 inches. Much further work needs to -be done on this aspect before any definite conclusion can be -reached.</p> - -<p>Much scattered information is available concerning the -effect of soil type, manuring, treatment, cropping, and so -forth upon the fungus content, but no clear issue as yet -emerges from the results. Hagem<a href="#Endnote7_8" class="fnanchor">[8]</a> found that cultivated soils -vary greatly from forest soils in the species of <i>Mucor</i> present, -and that certain species seem to be associated in similar -environments. Thus in pinewoods <i>Mucor ramannianus</i> is -usually found, together with <i>M. strictus</i>, <i>M. flavus</i>, and -<i>M. sylvaticus</i>, and with this “<i>M. Ramannianus Society</i>,” -<i>M. racemosus</i>, <i>M. hiemalis</i>, and <i>Absidia orchidis</i>, are frequently -associated. The differences found by Hagem between -the species of <i>Mucor</i> from forest and cultivated land -could not, however, be confirmed by Werkenthin.<a href="#Endnote7_26" class="fnanchor">[26]</a></p> - -<p>Dale,<a href="#Endnote7_5" class="fnanchor">[5]</a> examining sandy, chalky, peaty and black earth -soils, found specific differences, although many of the species -were common to all. A soil which had been manured continuously -for thirty-eight years with ammonium sulphate -alone, contained twenty-two species, whereas the same -soil with the addition of lime only had thirteen species. -Both Goddard<a href="#Endnote7_7" class="fnanchor">[7]</a> and -Werkenthin,<a href="#Endnote7_26" class="fnanchor">[26]</a> in their investigations, -found a constant and characteristic fungus flora regardless -of soil type, tillage, or manuring. Waksman’s<a href="#Endnote7_25" class="fnanchor">[25]</a> -studies of<span class="pagenum" id="Page122">[122]</span> -forest soils showed few species of <i>Mucor</i> but many of <i>Penicillium</i> -and <i>Trichoderma</i><a href="#Endnote7_2" class="fnanchor">[2]</a>; orchard soil contained no species of -<i>Trichoderma</i>, very few of <i>Penicillium</i>, but a large number -of species of <i>Mucor</i>; species of <i>Trichoderma</i> were common in -acid soils, whilst cultivated garden soil contained all forms. -The examination of very differently manured plots on the -Broadbalk wheat field at Rothamsted has not shown any -striking differences in the fungus flora, all the more important -groups of species being represented in every plot, -but significant minor differences are present. Thus, plot -13, manured with double ammonium salts, superphosphate -and sulphate of potash, is especially rich in “species” of -Trichoderma, whereas the unmanured plot contains large -numbers of species of green <i>Penicillium</i>, <i>Trichoderma</i>, and -a species of <i>Botrytis</i> (pyramidalis?).</p> - -<p>The effect of the crop upon the fungus flora is seen in -cases where the same crop is grown year after year as in -certain flax areas, where species of <i>Fusarium</i> accumulate -in the soil and tend to produce “flax sickness.”<a href="#Endnote7_13" class="fnanchor">[13]</a></p> - -<h3><span class="smcap">Quantitative Study.</span></h3> - -<p>As it is not possible to count the soil fungi <i>in situ</i>, any -estimation of the numbers present in a soil must be arrived -at by indirect means. The method adopted is to make -as fine a suspension as possible of a known quantity of soil -sample in a known amount of water, dilute this to <sup>1</sup>⁄<sub>5000</sub>, -<sup>1</sup>⁄<sub>10000</sub>, and so forth by regular gradations, incubate cubic -centimetres of the final dilution on artificial media in petri -dishes, and count the colonies of fungi developing in each -plate. Using the average figures from a series of duplicate -plates, the number of “individual” fungi in a gram of -the original soil sample may then be calculated. The very -few students who have made quantitative estimations have -obtained very unsatisfactory results. In bacterial or protozoal -estimations, the shaking of the soil suspension separates -the unicellular individuals, so that in the final platings -each individual from the soil theoretically gives rise to one<span class="pagenum" id="Page123">[123]</span> -colony on the medium. In the case of fungi, the organisms -may be in the form of unicellular or multicellular spores -or larger or smaller masses of unicellular or multicellular -mycelium differing for each particular species or phase of -development within the single species. The organisms may -be sterile in the soil or form fruiting bodies, consisting -of few or myriads of locally or widely distributed spores. -In the process of shaking the soil-suspension fungi of different -organisation or of differing developmental stages may be -broken up and moieties fragmented in totally different ways -or to very different degrees. With protozoa and bacteria -the relation of soil individual to plate colony is direct; -with fungi we do not know what is the soil “individual” -nor whether it is the same for different fungi; nor can we -yet profitably discuss any significant numerical relationship -of plate colonies to soil organisms. Thus Conn<a href="#Endnote7_4" class="fnanchor">[4]</a> has pointed -out that the plate count of a fungus indicates only the -ability to produce reproductive bodies and found that the -spores of one colony of <i>Aspergillus</i>, if distributed evenly -through a kilogram of soil, could produce the average plate -counts obtained by Waksman. Abundant vegetative growth -may, in some species, reduce or inhibit spore formation, -so that of two species the one giving a lower count might -really be much the more important and plentiful in the -soil. Further, the colonies developing in the final plates -represent only a selected few of the fungi present in the soil -sample, the <i>Basidiomycetes</i>, and no doubt many other forms, -being absent. In addition, different media differ among -themselves in the average number of colonies developing -on the plates, each medium giving, as it were, its own point -of view. Thus, in one experiment carried out at Rothamsted -by Miss Jewson, using the same soil suspension, twenty -plates of Coon’s Agar gave 357 colonies, of Cook’s Agar 246, -of Czapek’s Agar 215, and of Prune Agar 366. Thus if one -only used Coon’s Agar and Prune Agar one would obtain -a total of 723 colonies, whereas the same suspension on -Cook’s Agar and Czapek’s Agar would give only 461, and the<span class="pagenum" id="Page124">[124]</span> -calculated numbers of fungi per gram of soil would be -totally different. Further, if a single medium be taken, it -is found that slight alterations in the degree of acidity may -make very considerable differences in the final numbers. -Thus Coon’s Agar acidified to a hydrogen ion concentration -of 5·0 gave as the results of four series the following average -numbers of colonies per plate, 17, 23·75, 18, 23. When, -however, the medium was acidified to a PH of 4·0 to 4·3, -corresponding averages from three series were 38, 46·3, and -44·8; i.e. the final estimations of numbers of fungi in the -soil was about twice as great. Again, the degree of dilution -of soil suspension used in plating may also be a very serious -factor. Thus, if a series of dilutions be made of <sup>1</sup>⁄<sub>80,000</sub>, -<sup>1</sup>⁄<sub>40,000</sub>, <sup>1</sup>⁄<sub>20,000</sub>, <sup>1</sup>⁄<sub>10,000</sub>, <sup>1</sup>⁄<sub>5,000</sub> and <sup>1</sup>⁄<sub>2,500</sub>, the average plate -numbers should be in the proportions of 1, 2, 4, 8, 16, and 32 -respectively. In an actual experiment, the following average -plate numbers were obtained, 15·4, 32·8, 59·1, 104·0, -150, 224·5, which show a very decided reduction in the -higher numbers. If, however, dilutions of a suspension of -spores of a single species be made, this reduction does not -occur.</p> - -<p>These are but three of the very numerous factors involved -in the technique of quantitative estimation, and every -single factor may be the source of errors of similar magnitude, -minute fluctuations in the operations leading to the -final platings having very considerable effect upon the -numbers of colonies that develop.</p> - -<p>By critically evaluating each particular factor in the -method, and making statistical correction, it has, however, -been found possible to obtain series of duplicate plates -comparing very favourably and thus to extract certain -figures which, whilst not possessing any final value, have yet -a certain general and comparative worth. Thus, 20·0, 18·2, -and 16·8 were obtained as the averages of six plates each, -of a soil suspension divided into three parts, and the individual -plate numbers in all three series were within the range -of normal distribution. The meaning of these numerical<span class="pagenum" id="Page125">[125]</span> -estimates in relation to fungi per gram of soil sample is, -however, entirely hypothetical, and to have value quantitative -comparison should only be made between single -species or groups of species closely related physiologically, -and where the technique is standardised.</p> - -<div class="container w40em" id="Fig19"> - -<img src="images/illo133.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 19.</span>—Monthly Counts of Numbers of Fungi per gramme of Dry Soil. -Broadbalk Plot 2 (Farmyard Manure), Rothamsted.</p> - -<div class="illotext"> - -<p>X-axis: <span class="underl">Apr.</span> 1921 May Jun. <span class="underl">Jul.</span> Aug. Sep. -<span class="underl">Oct.</span> Nov. <span class="underl">Dec.</span> Jan. 1922 <span class="underl">Feb.</span> -Mar. Apr. May <span class="underl">Jun.</span> Jul. Aug. <span class="underl">Sep.</span> <span class="underl">Oct.</span></p> - -<p>Y-axis: 10.000 per Gramme of Soil</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>No comparative estimations have been made of the -number of fungi in the soils of different regions. There -are, however, certain figures which show that decided -seasonal differences exist. Thus, correcting and averaging -certain of Waksman’s results<a href="#Endnote7_25" class="fnanchor">[25]</a> the following numbers of -fungi per gram of soil at 4 inches deep are obtained; -September, 768,000; October, 522,000; November, 310,000; -January, 182,000. At Rothamsted results have been obtained -which would appear to mark a clear seasonal rhythm,<span class="pagenum" id="Page126">[126]</span> -corresponding in the time of its maxima in Autumn and -Spring with the periodicities known for many other ecological -communities (<a href="#Fig19">Fig. 19</a>).</p> - -<p>The numbers of fungi at various depths in the soil show -very clearly marked differences. The distribution in the -top 4-6 inches depending probably upon the depth of soil, -is more or less equal, but there is a very rapid falling off in -numbers, especially between 5-9 inches, until at 20-30 inches -fungi are either very few in number or absent. Thus Takahashi<a href="#Endnote7_22" class="fnanchor">[22]</a> -found 590,000 fungi per gram at a depth of 2 cms. -and only 160,000 at 8 cms.</p> - -<p class="tabhead" id="TabXIII">TABLE XIII.—INFLUENCE OF SOIL TREATMENT UPON THE -NUMBERS OF FUNGI AS DETERMINED BY THE PLATE -METHOD—(AFTER WAKSMAN).</p> - -<table class="standard"> - -<tr class="bt bb"> -<th class="bl br">Soil Fertilisation.</th> -<th class="br">Reaction.</th> -<th class="br">Numbers<br />of Fungi<br />per Gram<br />of Soil.</th> -</tr> - -<tr> -<th class="bl br"> </th> -<th class="br">P.H.</th> -<th class="br"> </th> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Minerals only</span></td> -<td class="general br">5·6</td> -<td class="general br"> 37,300</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Heavily manured</span></td> -<td class="general br">5·8</td> -<td class="general br"> 73,000</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Sodium nitrate</span></td> -<td class="general br">5·8</td> -<td class="general br"> 46,000</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Ammonium sulphate</span></td> -<td class="general br">4·0</td> -<td class="general br">110,000</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Minerals and lime</span></td> -<td class="general br">6·6</td> -<td class="general br"> 26,200</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Ammonium sulphate and lime</span></td> -<td class="general br">6·2</td> -<td class="general br"> 39,100</td> -</tr> - -</table> - -<p>The type of soil and its treatment exercise a great influence -over the number of fungi present. Fischer<a href="#Endnote7_6" class="fnanchor">[6]</a> found -that farmyard manure increased the number of fungi in -uncultivated “Hochmoor,” cultivated “Grunlandmoor,” -and a clay soil by two, three, and five times respectively. -Waksman’s results<a href="#Endnote7_25" class="fnanchor">[25]</a> indicate that the more fertile soils -contain more fungi, both in number and species, than the -less fertile ones, and if one averages his results, the following -figures are obtained: garden soil, 525,000 per gram; orchard -soil, 250,000; meadow soil, 750,000; and forest soil, 151,000. -Recently Waksman<a href="#Endnote7_25" class="fnanchor">[25<i>e</i>]</a> has found that manure and acid -fertilisers increase the numbers of fungi in the soil, whereas -the addition of lime decreases them (<a href="#TabXIII">Table XIII.</a>).</p> - -<p><span class="pagenum" id="Page127">[127]</span></p> - -<p>Jones and Murdock<a href="#Endnote7_10" class="fnanchor">[10]</a> examined surface and sub-surface -samples of forty-six soils representing seventeen soil types -in eastern Ontario. Molds were fairly uniform in numbers -in all soils except a sandy clay loam and sandy clay shale, -in which they were absent.</p> - -<p>It has also frequently been pointed out that acid and -water-logged soils are richer in fungus content than normal -agricultural soils. On the other hand, Brown and Halversen<a href="#Endnote7_2" class="fnanchor">[2]</a> -found, examining six plots receiving different -treatment and studied through a complete year, that the -numbers of fungi were unaffected by moisture, temperature, -or soil treatment. Against this, however, must be -set the work of Coleman<a href="#Endnote7_3" class="fnanchor">[3]</a> who studied the activities of -fungi in sterile soils and found such factors as temperature, -aeration and food supply to exercise a deciding -control.</p> - -<p>Investigations at Rothamsted show that Broadbalk plot -13, receiving double ammonium salts, superphosphate and -sulphate of potash and yielding 31 bushels per acre, and -plot 2, receiving farmyard manure and yielding 35·2 bushels, -contain approximately equal numbers of fungi. This -figure is about half as high again as that for plot 3, which is -unmanured and yields 12·6 bushels, plot 10, with double -ammonium salts alone and yielding 20 bushels, and plot -11, with double ammonium salts and superphosphate and -yielding 22·9 bushels per acre. A primary factor, however, -in all considerations such as these is the equality of distribution -of fungi laterally in any particular soil. There are -probably few soils so homogeneous as the Broadbalk plots -at Rothamsted, and on plot 2 (farmyard manure since -1852) samples taken from the lower and upper ends and the -middle region gave average numbers of colonies per plate -of 24, 23, and 25 respectively. On the other hand, soil -samples taken only a few yards apart in the middle region -of the plot gave average plate counts of 33·7 and 56·8.</p> - -<p><span class="pagenum" id="Page128">[128]</span></p> - -<h3><span class="smcap">Conclusion.</span></h3> - -<p>Surveying generally the field covered in this chapter, -one can only be impressed with the fragmentary character -of our knowledge and with the fact that, owing to the selective -nature of the technique, the data we possess, if assumed to -be representative, give an entirely partial and erroneous -picture of the soil fungi. From the qualitative aspect, -the chief impediment is the impossibility of obtaining -reliable specific determinations of very many of the soil -fungi. Lists of doubtfully-named forms from particular -soils or geographic regions are useless or a positive evil, and -there is imperative need for the systematising of selected -genera by physiological criteria, such as has been partially -done for <i>Penicillium</i>, <i>Fusarium</i>, and <i>Aspergillus</i>. Furthermore, -until a standardised and non-selective technique has -been devised, or a number of standardised selective methods -for particular groups, comparative investigations into -specific distribution can give little of value. This latter -criticism is also very applicable if regard be paid to the -quantitative aspect of soil work, for progress here largely -depends upon the elaboration of a standardised fractionation -technique. Every single factor in these methods needs -exact analysis, for each gives opportunity for great error, -and each error is magnified many thousand times in the -final results. Much has been done in this direction at -Rothamsted, but more remains to do. Finally, working -with single species in sterilised soil under standardised -conditions, there is fundamental work to be done on the -relation of plate colony to soil “individual.”</p> - -<div class="footnote"> - -<p><span id="Endnote7_1" class="label"> [1]</span> Adametz, I., “Untersuchungen über die niederen Pilze der Ackerkrume,” -Inaug. Diss., Leipzig, 1886.</p> - -<p><span id="Endnote7_2" class="label"> [2]</span> Brown, P. E., and Halversen, W. V., “Effect of Seasonal Conditions -and Soil Treatment on Bacteria and Molds in Soil,” -Iowa Agric. Expt. Sta. 1921, Res. Bull., 56.</p> - -<p><span id="Endnote7_3" class="label"> [3]</span> Coleman, D. A., “Environmental Factors Influencing the Activity -of Soil Fungi,” Soil Sci., 1916, v., 2.</p> - -<p><span class="pagenum" id="Page129">[129]</span></p> - -<p><span id="Endnote7_4" class="label"> [4]</span> Conn, H. J., “The Microscopic Study of Bacteria and Fungi in -Soil,” N.Y. Agric. Expt. Sta., 1918, Bull. 64.</p> - -<p><span id="Endnote7_5" class="label"> [5]</span> Dale, E., (<i>a</i>) “On the Fungi of the Soil,” Ann. Mycol., 1912, 10; -(<i>b</i>) “On the Fungi of the Soil,” Ann. Mycol., 1914, 12.</p> - -<p><span id="Endnote7_6" class="label"> [6]</span> Fischer, H., “Bakteriologisch-chemische Untersuchungen; Bakteriologischen -Teil,” Landw. Jahrb., 1909, 38.</p> - -<p><span id="Endnote7_7" class="label"> [7]</span> Goddard, H. M., “Can Fungi living in Agricultural Soil Assimilate -Free Nitrogen?” Bot. Gaz., 1913, 56.</p> - -<p><span id="Endnote7_8" class="label"> [8]</span> Hagem, O., (<i>a</i>) “Untersuchungen über Norwegische Mucorineen I., -Vidensk. Selsk, I.,” Math. Naturw. Klasse, 1907, 7; (<i>b</i>) “Untersuchungen -über Norwegische Mucorineen II., Vidensk. Selsk. -I.,” Math. Naturw. Klasse, 1910, 10.</p> - -<p><span id="Endnote7_9" class="label"> [9]</span> Jensen, C. N., “Fungus Flora of the Soil,” N.Y. (Cornell) Agric. -Expt. Sta., 1912, Bull. 315.</p> - -<p><span id="Endnote7_10" class="label">[10]</span> Jones, D. H., and Murdock, F. G., “Quantitative and Qualitative -Bacterial Analysis of Soil Samples taken in Fall of 1918,” -Soil Sci., 1919, 8.</p> - -<p><span id="Endnote7_11" class="label">[11]</span> Kopeloff, N., “The Effect of Soil Reaction on Ammonification by -Certain Soil Fungi,” Soil Sci., 1916, 1.</p> - -<p><span id="Endnote7_12" class="label">[12]</span> Lendner, A., “Les Mucorinées de la Suisse,” 1908.</p> - -<p><span id="Endnote7_13" class="label">[13]</span> Manns, S. F., “Fungi of Flax-sick Soil and Flax Seed,” Thesis, -N. Dak. Agric. Expt. Sta., 1903.</p> - -<p><span id="Endnote7_14" class="label">[14]</span> McBeth, I. G., and Scales, F. M., “The Destruction of Cellulose by -Bacteria and Filamentous Fungi,” U.S. Dept. Agric. Bur. Plant -Indust., 1913, Bull. 266.</p> - -<p><span id="Endnote7_15" class="label">[15]</span> McLean, H. C., and Wilson, G. W., “Ammonification Studies with -Soil Fungi,” N.J. Agric. Expt. Sta., 1914, Bull. 270.</p> - -<p><span id="Endnote7_16" class="label">[16]</span> Muntz, A., and Coudon, H., “La fermentation ammoniaque de -la terre,” Compt. Rend. Acad. Sci. (Paris), 1893, 116.</p> - -<p><span id="Endnote7_17" class="label">[17]</span> Oudemans, A. C., and Koning, C.J., “Prodrome d’une flore mycologique, -obtenue par la culture sur gelatin préparée de la terre humeuse du Spanderswoud, -près de Bussum,” Arch. Néerland. Sci. Exact et Nat., 1902, s. ii., 7.</p> - -<p><span id="Endnote7_18" class="label">[18]</span> Ramann, E., “Bodenkunde,” Berlin, 1905.</p> - -<p><span id="Endnote7_19" class="label">[19]</span> Rathbun, A. E., “The Fungus Flora of Pine Seed Beds,” Phytopath., -1918, 8.</p> - -<p><span id="Endnote7_20" class="label">[20]</span> Remy, T., “Bodenbakteriologischen Studien,” Centr. f. Bakt., -1902, ii., 8.</p> - -<p><span id="Endnote7_21" class="label">[21]</span> Sherbakoff, C. D., “Fusaria of Potatoes,” N.Y. (Cornell) Agric. -Expt. Sta., 1915, Mem. 6.</p> - -<p><span id="Endnote7_22" class="label">[22]</span> Takahashi, T., “On the Fungus Flora of the Soil,” Anns. Phytopath. -Soc., Japan, 1919, 1.</p> - -<p><span id="Endnote7_23" class="label">[23]</span> Taylor, M. W., “The Vertical Distribution of <i>Fusarium</i>,” Phytopath., -1917, 7.</p> - -<p><span class="pagenum" id="Page130">[130]</span></p> - -<p><span id="Endnote7_24" class="label">[24]</span> Thom, Ch., “Cultural Studies of Species of Penicillium,” U.S. -Dept. Agric. Bur. Animal Indus., 1910, Bull. 118.</p> - -<p><span id="Endnote7_25" class="label">[25]</span> Waksman, S. A., (<i>a</i>) “Soil Fungi and their Activities,” Soil Sci., -1916, 2; (<i>b</i>) “Do Fungi Actually Live in the Soil and Produce -Mycelium?” Science, 1916, 44; (<i>c</i>) “Is there any Fungus -Flora of the Soil?” Soil Sci., 1917, 3; (<i>d</i>) “The Importance of -Mold Action in the Soil,” Soil Sci., 1918, 6; (<i>e</i>) “The Growth -of Fungi in the Soil,” Soil Sci., 1922, xiv.</p> - -<p><span id="Endnote7_26" class="label">[26]</span> Werkenthin, F. C., “Fungus Flora of Texas Soils,” Phytopath., -1916, 6.</p> - -</div><!--footnote--> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page131">[131]</span></p> - -<h2 class="nobreak">CHAPTER VIII.<br /> -<span class="chaptitle">THE LIFE OF FUNGI IN THE SOIL.</span></h2> - -</div><!--chapter--> - -<p>In the last chapter fungi were considered as so many specific -but functionless units in the soil. Unless, however, they -are regarded merely as inert spore contaminations from the -air, a view which is now no longer tenable, their very presence -implies the existence of innumerable vital relationships -between the organisms and their environment. From this -point of view the studies treated in the previous chapter -are but the necessary first steps to an understanding of the -relation of soil fungi to living plants and of the part played -by them in the soil economy.</p> - -<h3><span class="smcap">Relation of Soil Fungi to Living Plants.</span></h3> - -<p>Older classifications of fungi frequently divided these -organisms into four categories—parasites, saprophytes, -facultative parasites, and facultative saprophytes, but the -further mycological studies are carried the more clearly it -is seen that these groups are entirely artificial. There are -probably few fungi that cannot, under particular conditions, -invade living tissues, and it only seems a question of time -before at all events the vast majority of fungi will be grown -on synthetic media in the laboratory. From our present -point of view the importance of this lies in the fact that -fungi living saprophytically in the soil may, given the right -conditions or the presence of some particular host plant, -become parasites or symbionts, and conversely well-known -pathogens may live a saprophytic existence. Thus Cucumber -Leaf Spot is caused by <i>Colletotrichum oligochætum</i>, and<span class="pagenum" id="Page132">[132]</span> -Bewley<a href="#Endnote8_3" class="fnanchor">[3]</a> has repeatedly isolated this fungus from glasshouse -manure and refuse of various kinds. In his early -studies, Butler<a href="#Endnote8_13" class="fnanchor">[13]</a> isolated many parasitic species of <i>Pythium</i> -from Indian soils, and the presence of <i>P. de Baryanum</i> as -a soil saprophyte has been confirmed by Bussey, Peters, and -Ulrich.<a href="#Endnote8_11" class="fnanchor">[11]</a> -De Bruyn<a href="#Endnote8_17" class="fnanchor">[17]</a> has recently found that most species -of <i>Phytophthora</i>, including <i>P. erythroseptica</i> and <i>P. infestans</i> -may live as saprophytes in the soil, whilst Pratt<a href="#Endnote8_53" class="fnanchor">[53]</a> has -isolated from virgin lands and desert soils various fungi, -which cause disease in potatoes. In 1912 Jensen<a href="#Endnote8_29" class="fnanchor">[29]</a> gave a -list of twenty-three “facultative parasites” isolated from -soil, and these are but a moiety of those which could be listed -to-day.</p> - -<p>Furthermore, it was shown by Frank<a href="#Endnote8_24" class="fnanchor">[24]</a> many decades -ago that forest humus is not merely a mass of the remains -of animals and plants, but that a considerable part of its -organic substance is made up of fungus hyphæ, which ramify -and penetrate in all directions. Evidence is rapidly accumulating -that this is also true of most other soils containing -organic matter. It is well known that many of the higher -plants live in symbiotic or commensal relationship with -these humus fungi, which are present in the host tissues -as mycorrhiza, and further studies only serve to show the -widespread and fundamental nature of this relationship. -Thus many <i>Basidiomycetes</i><a href="#Endnote8_50" class="fnanchor">[50]</a> (species of <i>Tricholoma</i>, <i>Russula</i>, -<i>Cortinarius</i>, <i>Boletus</i>, <i>Elaphomyces</i>, etc.) possess a mycorrhizal -relationship with various broad leaved trees, such as -beech, hazel, and birch<a href="#Endnote8_57" class="fnanchor">[57]</a> and with various conifers and -certain Ericales. Other Ericales show this relationship with -species of the genus <i>Phoma</i>,<a href="#Endnote8_62" class="fnanchor">[62]</a> many orchids, with species -of <i>Rhizoctonia</i><a href="#Endnote8_2" class="fnanchor">[2]</a> -(or <i>Orcheomyces</i><a href="#Endnote8_10" class="fnanchor">[10]</a>), whilst <i>Gastrodia elata</i> -contains <i>Armillaria mellea</i>.<a href="#Endnote8_36" class="fnanchor">[36]</a> Certain species of <i>Pteridophyta</i> -and <i>Bryophyta</i> are also known to certain mycorrhizal -fungi. Of the numerous fungi taking part in these mycorrhizal -relationships, only a small number have yet been -identified, but there is little doubt that perhaps the majority -of these organisms must be regarded as true soil <span class="nowrap">forms.<a href="#Endnote8_14" class="fnanchor">[14]</a><sup>,</sup> -<a href="#Endnote8_45" class="fnanchor">[45]</a></span><span class="pagenum" id="Page133">[133]</span> -The mycological flora of the soil thus plays an important -part in the life of many higher forms of vegetation, and -this relationship is a very fruitful field for study.</p> - -<h3><span class="smcap">Relation of Fungi to Soil Processes.</span></h3> - -<p>The great cycle of changes occurring in the soil whereby -organic matter is gradually transformed and again made -available as plant food is entirely dependent upon micro-organisms. -Until a decade ago it was thought that bacteria -were by far the most important group concerned in the -bringing about of these changes, but recent studies have -shown that, in at all events certain arcs of this great organic -cycle, the fungi have, perhaps, an equal part to play. -The life of fungi in the soil may, for our purposes, be considered -from three points of view—their part in the decomposition -of carbon compounds, their nitrogen relationships, -and their work in the mineral transformations of the -soil.</p> - -<h3><span class="smcap">Carbon Relationships.</span></h3> - -<p>Of primary importance in the carbon relationships of -soil fungi is the part played in the decomposition of the -celluloses, which compose almost all the structural remains -of plant tissues. Our first real knowledge of this subject -was given by Van Iterson<a href="#Endnote8_28" class="fnanchor">[28]</a> in 1904 when he showed the wide -extent of cellulose destruction by fungi, and devised methods -whereby fifteen cellulose-decomposing forms, many of which -have since proved to be common soil fungi, were isolated. -Three years later Appel<a href="#Endnote8_1" class="fnanchor">[1]</a> published his account of the genus -<i>Fusarium</i>, and showed that many of the species could -destroy filter paper. A difficulty was introduced in 1908 -by Schellenberg,<a href="#Endnote8_60" class="fnanchor">[60]</a> who, working with common soil forms, -found that only hemicelluloses and not pure cellulose were -destroyed. This has recently been supported by Otto,<a href="#Endnote8_48" class="fnanchor">[48]</a> -but from the practical point of view the discussion is academic -for the amount of pure cellulose in plants is insignificant.</p> - -<p><span class="pagenum" id="Page134">[134]</span></p> - -<p>In 1913 McBeth and Scales<a href="#Endnote8_43" class="fnanchor">[43]</a> showed that a considerable -number of common soil fungi were most active cellulose -destroyers, pure precipitated cellulose and cotton being -readily attacked. This was supported by McBeth in 1916,<a href="#Endnote8_42" class="fnanchor">[42]</a> -whilst Scales<a href="#Endnote8_59" class="fnanchor">[59]</a> has found that most species of <i>Penicillium</i> -and <i>Aspergillus</i> decompose cellulose, especially where ammonium -sulphate is the source of nitrogen. Waksman<a href="#Endnote8_65" class="fnanchor">[65]</a> -tested twenty-two soil fungi and found that eleven decomposed -cellulose rapidly and four slowly, whilst Dascewska,<a href="#Endnote8_16" class="fnanchor">[16]</a> -<span class="nowrap">Waksman,<a href="#Endnote8_66" class="fnanchor">[66]</a><sup>,</sup> -<a href="#Endnote8_67" class="fnanchor">[67]</a></span> and others have concluded that soil fungi -play a more important part in the decomposition of cellulose -and in “humification” than soil bacteria. Schmitz<a href="#Endnote8_61" class="fnanchor">[61]</a> -has recently shown that cellulose-destroying bacteria play -no important part in the decay of wood under natural -conditions.</p> - -<p>In addition to the celluloses, practically all simple and -complex organic carbon compounds are attacked by soil -fungi, and in many cases the decomposition is very rapid.<a href="#Endnote8_26" class="fnanchor">[26]</a> -Many <i>Actinomycetes</i>, <i>Aspergilli</i> and <i>Penicillia</i> are active -starch splitters, and it is of interest to note that some of the -strongest cellulose decomposers (<i>Melanconium sp.</i>, <i>Trichoderma -sp.</i>, and <i>Fusaria</i>) secrete little diastase.<a href="#Endnote8_66" class="fnanchor">[66]</a> The -<i>Mucorales</i> apparently do not attack cellulose, but can only -utilise pectin bodies, monosaccharides, and partly disaccharides.<a href="#Endnote8_26" class="fnanchor">[26]</a> -Dox and Neidig<a href="#Endnote8_19" class="fnanchor">[19]</a> have shown that various species -of <i>Aspergillus</i> and <i>Penicillium</i> are able to attack the soil -pentosans. Roussy,<a href="#Endnote8_58" class="fnanchor">[58]</a> -Kohshi,<a href="#Endnote8_24" class="fnanchor">[24]</a> Verkade and Söhngen,<a href="#Endnote8_64" class="fnanchor">[64]</a> -and many other workers have found that fats and fatty -acids are readily used as food by soil fungi, and Koch and -Oelsner<a href="#Endnote8_33" class="fnanchor">[33]</a> have recently shown that tannins are readily -assimilated. Klöcker,<a href="#Endnote8_32" class="fnanchor">[32]</a> -Ritter,<a href="#Endnote8_56" class="fnanchor">[56]</a> and others have shown -that the utilisation of many carbon compounds is to a -large extent determined by the source of nitrogen and its -concentration in the pabulum.</p> - -<p>There would seem, therefore, no doubt that the decomposition -of celluloses and other carbon compounds is -of primary importance in the life-activities of soil fungi.</p> - -<p><span class="pagenum" id="Page135">[135]</span></p> - -<h3><span class="smcap">Nitrogen Relationships.</span></h3> - -<p>In this section we shall consider the problems of nitrogen -fixation and nitrification, of ammonification, and of the -utilisation of nitrogenous compounds by soil fungi.</p> - -<p>As soil fungi form so large a part of the soil population, -the question of whether they can make use of the free -nitrogen of the air is of primary importance. During the -last two decades many investigators have attempted to -solve the problem, often studying allied or identical species; -but if one consults some thirty researches published during -this period, opinion is found to be about equally divided. -Even, however, in those studies where nitrogen fixation has -been recorded the amounts are very slight, usually being -below 5 mgrms. per 50 c.c. of solution, and often being -obviously within the limits of experimental error. Latham,<a href="#Endnote8_37" class="fnanchor">[37]</a> -however, working on <i>Aspergillus niger</i>, recorded variations -ranging from a nitrogen loss of 42·5 mgrms. to a nitrogen -fixation of 205·1 mgrms. per 50 c.c. of medium. Ternetz<a href="#Endnote8_63" class="fnanchor">[63]</a> -found that different strains of <i>Phoma radicis</i> may fix from -2·5 mgrms. of nitrogen in the lowest case, to 15·7 mgrms. -in the highest per 50 c.c. of nutrient solution. Duggar -and Davis<a href="#Endnote8_20" class="fnanchor">[20]</a> report that <i>Phoma betæ</i> may fix nitrogen in -quantities of 7·75 mgrms. per 50 c.c. of medium. The latter -authors, in a very able critique of the problem, indicate -certain possible sources of error in previous work, and if -one examines the studies in which nitrogen fixation has -been recorded in the light of these criticisms, it is difficult -not to think that, with the exception of the genus <i>Phoma</i>, -good evidence for nitrogen fixation by fungi is lacking. -<i>Phoma betæ</i> is a common pathogen attacking beets, whilst -<i>P. radicis</i> is a mycorrhizal form inhabiting various Ericales. -Apart from these exact quantitative studies, which have -given a negative verdict, there is a considerable amount of -positive but indirect evidence for nitrogen fixation by mycorrhizal -fungi,<a href="#Endnote8_55" class="fnanchor">[55]</a> and it is very unfortunate that more of these -forms have not been investigated quantitatively. As the<span class="pagenum" id="Page136">[136]</span> -evidence stands to-day, one must conclude that the fungus -flora does not play any part in the direct nitrogen enrichment -of the soil.</p> - -<p>Equally obscure is the question of nitrification and -denitrification by soil fungi, but this is the result of a lack -of study rather than of a plethora of indeterminate researches. -Direct nitrification or denitrification has not been established, -but the work of Laurent<a href="#Endnote8_38" class="fnanchor">[38]</a> and a few other workers -appears to show that soil fungi can reduce nitrates to nitrites.</p> - -<p>The second primary nitrogen relationship that we have -to consider is the process of ammonification. The ammonifying -power of soil fungi was first demonstrated by Muntz -and Coudon,<a href="#Endnote8_46" class="fnanchor">[46]</a> and by -Marchal<a href="#Endnote8_40" class="fnanchor">[40]</a> in 1893, the former showing -that <i>Mucor racemosus</i> and <i>Fusarium Muntzii</i> gave a larger -accumulation of ammonia in soil than any of the bacteria -tested; and the latter that <i>Aspergillus terricola</i>, <i>Cephalothecium -roseum</i> and other soil fungi were active ammonifiers, -especially in acid soils. Shibata,<a href="#Endnote8_62" class="fnanchor">[62]</a> -Perotti,<a href="#Endnote8_49" class="fnanchor">[49]</a> Hagem,<a href="#Endnote8_26" class="fnanchor">[26]</a> -Kappen,<a href="#Endnote8_31" class="fnanchor">[31]</a> Löhnis,<a href="#Endnote8_39" class="fnanchor">[39]</a> -and others, have observed that urea, -dicyanamide and cyanamide are decomposed with the -liberation of ammonia; and Hagem<a href="#Endnote8_26" class="fnanchor">[26]</a> has recorded the -same process for peptones, amino acids, and other organic -nitrogen compounds in plant and animal remains in the -soil. The latter author considers soil fungi more important -ammonifying agents in the soil than bacteria, a conclusion -in which McLean and Wilson,<a href="#Endnote8_44" class="fnanchor">[44]</a> and perhaps most later -workers concur. McLean and Wilson<a href="#Endnote8_44" class="fnanchor">[44]</a> found large differences -in the ammonifying powers of various soil fungi, -the <i>Moniliaceæ</i> being the strongest ammonifiers, the <i>Aspergillaceæ</i> -the weakest. Generic and specific differences have -been confirmed by Coleman,<a href="#Endnote8_15" class="fnanchor">[15]</a> -Waksman,<a href="#Endnote8_67" class="fnanchor">[67]</a> and other -authors. Waksman and Cook<a href="#Endnote8_70" class="fnanchor">[70]</a> suggested that such variations -may be due, not to innate differences in the metabolic -activities of the several organisms, but to differences in -reproductive times, and that there might be some relationship -between sporogeny and the ability to accumulate -nitrogen. Kopeloff<a href="#Endnote8_35" class="fnanchor">[35]</a> has -carried out experiments on the<span class="pagenum" id="Page137">[137]</span> -inoculation of sterilised soil with known quantities of spores -and found that, although the amount of ammonia accumulated -increased with the number of spores the proportion -was not direct but modified by the food supply. After the -first five days’ growth, the rate of ammonia production -varied markedly in a two-day rhythm which seemed to be -due to the metabolism of the fungus rather than to recurrent -stages of spore formation and germination in the -life history. The amount of ammonia liberated has been -shown by recent work<a href="#Endnote8_66" class="fnanchor">[66]</a> to depend upon the available -sources of carbon and nitrogen. In the absence of a carbohydrate -supply the protein is attacked both for carbon -and nitrogen, and since more of the former is required much -ammonia is liberated. In addition, however, to the carbon -and nitrogen control, the process of ammonification by soil -fungi is intimately related to physical conditions. Working -with pure cultures, McLean and Wilson,<a href="#Endnote8_44" class="fnanchor">[44]</a> -Coleman,<a href="#Endnote8_15" class="fnanchor">[15]</a> Kopeloff,<a href="#Endnote8_35" class="fnanchor">[35]</a> -Waksman and Cook,<a href="#Endnote8_70" class="fnanchor">[70]</a> and other students, have shown -that the amount of ammonia accumulated depends upon -such factors as the presence of phosphates, the period of -incubation of the fungi, aeration, the moisture in the soil, -the temperature, the degree of soil acidity, the type of soil, -and so forth.</p> - -<p>That fungi take a very important place as ammonifying -agents in the soil can no longer be doubted, but the question -yet remains to be considered of the balance of profit or loss -resulting from their activities. It has usually been considered -that a part of the ammonia freed is used by the fungi -themselves, but that the greater part is liberated, and so -rendered available to nitrifying organisms. Both Neller<a href="#Endnote8_47" class="fnanchor">[47]</a> -and Potter and Snyder<a href="#Endnote8_51" class="fnanchor">[51]</a> found that typical soil fungi -inoculated into sterile soil grew with a vigour approximately -equal to the growth induced by an inoculation of the entire -soil flora. This is largely to be accounted for by the fact -that when soils are sterilised by heat or by certain chemicals, -breaking-down changes occur, and substances are liberated -which are peculiarly favourable to fungus growth. This<span class="pagenum" id="Page138">[138]</span> -fact must be borne in mind when interpreting ammonification -and other studies where the method is that of inoculation -of fungi into sterilised soil. In many cases it tends to -nullify any application of the results to normal soils, whilst -in others the conclusions must be accepted with some reserve. -In all cases Potter and Snyder<a href="#Endnote8_51" class="fnanchor">[51]</a> found that fungi caused -a diminution in the amount of nitrates, that the ammonia was -not much changed in amount, and that there was a decrease -in the quantities of soluble non-protein nitrogen. The range -of organic and inorganic nitrogenous compounds utilisable -by soil fungi is very great. Ritter<a href="#Endnote8_56" class="fnanchor">[56]</a> has shown that certain -forms can use the nitrogen of “free” nitric acid in the medium; -Ritter,<a href="#Endnote8_56" class="fnanchor">[56]</a> -Hagem,<a href="#Endnote8_26" class="fnanchor">[26]</a> and others, that soil fungi can use ammonia -nitrogen equally with nitrate nitrogen, and Ehrenberg<a href="#Endnote8_21" class="fnanchor">[21]</a> -concluded that soil fungi play a more important part in the -building of albuminoids from ammonia than bacteria do. -Ehrlich<a href="#Endnote8_22" class="fnanchor">[22]</a> has shown that various heterocyclic nitrogen -compounds and alkaloids can serve as sources of nitrogen -to soil fungi, whilst Ehrlich and Jacobsen<a href="#Endnote8_23" class="fnanchor">[23]</a> have found that -soil fungi can form oxy-acids from amino-acids. Hagem,<a href="#Endnote8_26" class="fnanchor">[26]</a> -Povah,<a href="#Endnote8_52" class="fnanchor">[52]</a> -<span class="nowrap">Bokorny,<a href="#Endnote8_6" class="fnanchor">[6]</a><sup>,</sup> -<a href="#Endnote8_8" class="fnanchor">[8]</a></span> and others, state that for many soil -forms organic nitrogen sources are better than inorganic -sources, and that peptones, amino-acids, urea, and uric -acids, etc., are very quickly utilised by species of <i>Mucor</i>, -yeasts, and so forth. Butkevitch,<a href="#Endnote8_12" class="fnanchor">[12]</a> -and Dox<a href="#Endnote8_18" class="fnanchor">[18]</a> have recently -found that it depends on circumstances which compounds -of protein molecule can be utilised by particular -fungi, and that soil fungi can utilise both amino and amido -complexes for the formation of ammonia. In 1919 Boas<a href="#Endnote8_4" class="fnanchor">[4]</a> -showed for <i>Aspergillus niger</i> that if a number of nitrogenous -compounds are available the fungus absorbs the most -highly dissociated.</p> - -<p>In the welter of scattered observations on the utilisation -of nitrogenous compounds, it is difficult to trace any -clear issue. That proteins, amino-acids, and other complex -organic compounds are readily broken down to ammonia by -soil fungi is clear, and, on the other hand, it is also clear that<span class="pagenum" id="Page139">[139]</span> -soil fungi utilise extensively ammonia and nitrates as sources -of nitrogen. On which side the balance lies it is yet impossible -to say.</p> - -<h3><span class="smcap">Mineral Relationships.</span></h3> - -<p>Heinze<a href="#Endnote8_27" class="fnanchor">[27]</a> and -Hagem<a href="#Endnote8_26" class="fnanchor">[26]</a> have stated that soil fungi -make the insoluble calcium, phosphorus, and magnesium -compounds in soil soluble and available for plant food; -and Butkevitch<a href="#Endnote8_12" class="fnanchor">[12]</a> has used <i>Aspergillus niger</i> in determining -the availability of the mineral constituents, but practically -no work has yet been carried out on these problems. A -further matter on which sound evidence is greatly to be -desired is the part played by soil fungi in the oxidation -processes of iron and sulphur.</p> - -<p>A point which may be mentioned here, as it is of some -considerable practical importance, is the large quantity of -oxalic, citric, and other acids formed by certain common -soil fungi. Acid formation is partly dependent upon the -species of fungus—even more the physiological race within -the species—and partly upon the substratum, particularly -the source of <span class="nowrap">carbon.<a href="#Endnote8_5" class="fnanchor">[5]</a><sup>,</sup> -<a href="#Endnote8_54" class="fnanchor">[54]</a></span> It is interesting that as a group -<i>Actinomycetes</i> do not form acids from the carbon source but -alkaline substances from the nitrogen sources.<a href="#Endnote8_69" class="fnanchor">[69]</a></p> - -<h3><span class="smcap">Control of Soil Fungi.</span></h3> - -<p>In the preceding sections an attempt has been made to -sketch rapidly the chief outlines of the widespread relationships -of soil fungi and of the fundamental part that they -play in the biochemical changes occurring in the soil. It -will be evident, even from this survey, that their occurrence -is of the utmost agricultural importance, both when helpful -as in mycorrhizal relationships or as agents in making -complex organic materials available as plant food, or when -harmful as when causal agents of disease in plants. It is -clear that could the soil fungi be controlled to human ends -by the encouragement of the useful forms and the elimination -of the harmful, a valuable power would be placed in -the hands of the grower of plants. Certain aspects of this<span class="pagenum" id="Page140">[140]</span> -control, the cruder and more destructive perhaps, are already -practicable, whilst the finer and more constructive -aspects remain possibilities of to-morrow.</p> - -<p>Theoretically, the technique of control is selective in -that it aims to determine one or more particular fungi, -leaving the remaining flora untouched. Its highest expression -is seen, perhaps, in the utilisation of pure cultures of -mycorrhizal fungi for horticultural purposes, such as orchid -cultivation, but there is no reason why this should not be -done for other purposes on a field scale similar to the way -in which cultures of special strains of the root nodule organisms -of legumes are employed. A second aspect is the -direct encouragement of special components of the fungus -flora for particular purposes by selective feeding. Thus, -in a laboratory experiment, McBeth and Scales<a href="#Endnote8_43" class="fnanchor">[43]</a> record -an increase of 2000 times in cellulose-destroying and other -soil fungi by this method. It has been pointed out that -soil fungus activities such as ammonification, proteolysis -and carbohydrate decomposition are controlled by factorial -equilibria, and for special purposes it would seem feasible -to weight the balance so that particular activities may be -favoured. A further step in this direction is the controlling -of particular physical conditions so that the activities -of certain fungi may be restricted. Professor L. R. Jones<a href="#Endnote8_30" class="fnanchor">[30]</a> -and his colleagues at Madison have shown the primary -importance of the control of the soil temperature in certain -parasitic relationships; the work of Gillespie and Hurst<a href="#Endnote8_25" class="fnanchor">[25]</a> -and later workers has demonstrated that the parasitism of -certain species and strains of <i>Actinomyces</i> upon the potato -is conditioned by definite ranges of soil acidity; and many -other relationships of similar nature are known. Data -along such lines are rapidly accumulating, and in certain -cases are already susceptible of practical application. In -other cases, particular soil fungi are less open to persuasive -influences, and more drastic treatment needs to be adopted. -Certain chemicals mixed intimately with the soil increase -or diminish the numbers of particular fungi or groups of<span class="pagenum" id="Page141">[141]</span> -fungi; whilst these organisms may be totally eliminated -from the soil by wet or dry heat for definite periods or by -treatment with potent fungicides such as formaldehyde. -Although soil sterilisation and crude treatment in other ways -has been practised for decades, the possibility of a more -delicate control of soil fungi is only now being realised. -Its concrete expression will depend upon the progress that -is made in exact knowledge of the activities of soil fungi -under natural and controlled conditions, of the balance of -factors in the environment which controls any particular -function and of the genetic nature of the soil fungi which -occur. Each of these aspects is a fruitful field of study.</p> - -<h3><span class="smcap">Relation to Soil Fertility.</span></h3> - -<p>From a general survey of the researches that have been -carried out on soil fungi during the past two decades certain -issues emerge. It would seem clear that fungi occupy, -perhaps, a primary place as factors in the decomposition -of celluloses, and thus may be the chief agents in the transformation -of plant remains to humus and to soluble compounds -which can be used as food by the nitrogen-fixing -bacteria. Furthermore, soil fungi are very important -ammonifiers, but whether the balance of ammonia freed is -utilised by the fungi themselves, or whether it is made -available to nitrifying bacteria is not yet clear. If the latter -is the case, soil fungi play a valuable indirect rôle in the -accumulation of available plant food in the soil. On the other -hand, by utilising nitrates as sources of nitrogen, fungi -may play an important part in the depletion of the nitrogenous -food in the soil available to crop plants. Thirdly, -soil fungi apparently take no part in the direct nitrogen -enrichment of the soil. Thus, soil fungi would seem to be -the most important factor in the first half of that great -cycle whereby organic remains become again available as -organic food.</p> - -<p class="blankafter75">The impression left on one’s mind by the study of the life -of fungi in the soil is of an infinitely complex series of moving<span class="pagenum" id="Page142">[142]</span> -equilibria, the living activities being determined by both -biological and physico-chemical conditions. All these factors -play an integral part in the life of the soil fungi and must -be considered if a true picture is to be drawn. The principal -factors may be classified into the following groups: Most -evident, perhaps, are the natures and specificities of the fungi -and the relative composition of the fungus flora. Equally -important, however, are the quantity and quality of the foods -available and the non-biological environment which results -from the complex series of physical and chemical changes -occurring in the soil causally independent of the organisms -present, which interacts with the equally vast series of -changes resulting from fungus activities. Finally, one must -consider the interacting biological environment of surface -animals and plants and the microscopic fauna and flora. -The complexities are such that only the application of -Baconian principles can unravel them. A beginning has -been made in the study of pure cultures of soil fungi on -synthetic media, and much valuable data have accrued, but -it is obviously not possible to apply directly to soil the results -obtained in such work. They remain possibilities; in -certain cases probabilities, but nothing more. A further -step, one already taken and of great promise, is the investigation -of the changes occurring in sterilised soils inoculated -with known quantities of one or more pure cultures of -particular soil fungi. Such intensive study of single factors -in a standardised natural or artificial soil, to which has been -added a pedigreed fungus, is, perhaps, the most fruitful -avenue of progress. In all such work, however, one must -bear acutely in mind the fact that a sterilised soil and, -still more, an artificial soil, is a very different complex from -a normal soil, and that results obtained from the inoculation -of such soils are not applicable directly in the elucidation -of ordinary soil processes. At present there is no method -known of completely sterilising a soil which does not destroy -the original physico-chemical balance. It is evident that -the complexities are such that chemist, physicist, and<span class="pagenum" id="Page143">[143]</span> -biologist must all co-operate if the significance of the processes -is to be understood, and a solid foundation laid for -future progress and for practical application.</p> - -<div class="footnote"> - -<p><span id="Endnote8_1" class="label"> [1]</span> Appel, O., “Untersuchungen über die Gattung <i>Fusarium</i>,” Mitt. -Biol. Reichanst. Land- u. Forstw., 1907, 4.</p> - -<p><span id="Endnote8_2" class="label"> [2]</span> Bernard, N., “L’évolution dans la symbiose. Les Orchidées et -leurs Champignons commensaux,” Ann. Sci. Nat. (Bot.), Ser. -9, 1909, 9.</p> - -<p><span id="Endnote8_3" class="label"> [3]</span> Bewley, W. F., “Anthracnose of the cucumber under glass,” Journ. -Min. Agric., 1922, xxix.</p> - -<p><span id="Endnote8_4" class="label"> [4]</span> Boas, F., “Die Bildung löslicher Stärke im elektiven Stickstoff-Stoffwechsel,” -Ber. deut. bot. Ges., 1919, 37.</p> - -<p><span id="Endnote8_5" class="label"> [5]</span> Boas, F., und Leberle, H., “Untersuchungen über Säurenbildung -bei Pilzen und Hefen II.,” Biochem. Ztschr., 1918, 92.</p> - -<p><span id="Endnote8_6" class="label"> [6]</span> Bokorny, T., “Benzene derivatives as sources of nourishment,” -Zentr. Physiol., 1917, 32.</p> - -<p><span id="Endnote8_7" class="label"> [7]</span> Bokorny, T., “Sugar fermentation and assimilation,” Allg. Brau. -Hopfen Zeit., 1917, 57.</p> - -<p><span id="Endnote8_8" class="label"> [8]</span> Bokorny, T., “Verhaltung einiger organischer Verbindungen in der -lebenden Zelle,” Pflügers Archiv., 1917, 168.</p> - -<p><span id="Endnote8_9" class="label"> [9]</span> Brown, P. E., “Mould action in soils,” Science, 1917, 46.</p> - -<p><span id="Endnote8_10" class="label">[10]</span> Burgeff, H., “Die Wurzelpilze der Orchideen,” Jena. 1909.</p> - -<p><span id="Endnote8_11" class="label">[11]</span> Bussey, W., Peters, L., and Ulrich, P., “Ueber das Vorkommen -von Wurzelbranderregern im Boden,” Arb. Kais. Biol. Anst. -Land- u. Forstw., 1911, 8.</p> - -<p><span id="Endnote8_12" class="label">[12]</span> Butkevitch, V. S., “Ammonia as a product of protein transformations -caused by mould fungi, and the conditions of its formation,” -Recueil d’articles dedié au Prof. C. Timiriazeff, 1916.</p> - -<p><span id="Endnote8_13" class="label">[13]</span> Butler, E. J., “An account of the genus <i>Pythium</i> and some -<i>Chytridiaceæ</i>,” Mem. Dept. Agr. India, 1907, Bot. Ser. 5, 1.</p> - -<p><span id="Endnote8_14" class="label">[14]</span> Christoph, H., “Untersuchungen über die mykotrophen Verhältnisse -der Ericales und die Keimung von Pirolaceen,” Beihefte -Bot. Centr., 1921, 28.</p> - -<p><span id="Endnote8_15" class="label">[15]</span> Coleman, D. A., “Environmental factors influencing the activity -of soil fungi,” Soil Sci., 1916, 2.</p> - -<p><span id="Endnote8_16" class="label">[16]</span> Dascewska, W., “Étude sur la désagrégation de la cellulose dans -la terre de bruyère et la tourbe,” Univ. Genève, Inst. Bot., 1913, -S. 8.</p> - -<p><span id="Endnote8_17" class="label">[17]</span> De Bruyn, H. L. G., “The saprophytic life of <i>Phytophthora</i> -in the soil,” Meded. v. d. Landbouwhoogeschool Wageningen, -1922, xxiv.</p> - -<p><span id="Endnote8_18" class="label">[18]</span> Dox, A. W., “Amino acids and micro-organisms,” Proc. Iowa -Acad. Sci., 1917, 24.</p> - -<p><span class="pagenum" id="Page144">[144]</span></p> - -<p><span id="Endnote8_19" class="label">[19]</span> Dox, A. W., and Neidig, R. E., “Pentosans in lower fungi,” Journ. -Biol. Chem., 1911, 9.</p> - -<p><span id="Endnote8_20" class="label">[20]</span> Duggar, B. M., and Davis, A. R., “Studies in the physiology of the -fungi. (I.) Nitrogen fixation,” Ann. Mo. Bot. Gard., 1916, 3.</p> - -<p><span id="Endnote8_21" class="label">[21]</span> Ehrenberg, P., “Die Bewegung des Ammoniakstickstoffs in der -Natur,” Mitt. Landw. Inst., Breslau, 1907, 4.</p> - -<p><span id="Endnote8_22" class="label">[22]</span> Ehrlich, F., “Yeasts, moulds, and heterocyclic nitrogen compounds -and alkaloids,” Biochem. Ztschr., 1917, 79.</p> - -<p><span id="Endnote8_23" class="label">[23]</span> Ehrlich, F., and Jacobsen, K. A., “Über die Umwandlung von -Aminosäuren in Oxysäuren durch Schimmelpilze,” Ber. Deut. -Chem. Gesell., 1911, 44.</p> - -<p><span id="Endnote8_24" class="label">[24]</span> Frank, B., “Ueber die auf Wurzelsymbiose beruhende Ernährung -gewisser Bäume durch unterirdische Pilze,” Ber. d. Deut. -Bot. Gesell., 1885, 3.</p> - -<p><span id="Endnote8_25" class="label">[25]</span> Gillespie, L. J., and Hurst, L. A., “Hydrogen-ion concentration—soil -type—common potato scab,” Soil Sci., 1918, 6.</p> - -<p><span id="Endnote8_26" class="label">[26]</span> Hagem, O., “Untersuchungen über Norwegische Mucorineen,” -Vidensk. Selsk. I., Math. Naturw. Klasse, 1910, 7.</p> - -<p><span id="Endnote8_27" class="label">[27]</span> Heinze, B. H., “Sind Pilze imstande den elementaren Stickstoff -der Luft zu verarbeiten und den Boden an Gesamtstickstoff -anzureichen,” Ann. Mycol., 1906, 4.</p> - -<p><span id="Endnote8_28" class="label">[28]</span> Van Iterson, C., “Die Zersetzung von Cellulose durch Aërobe -Mikroorganismen,” Centr. f. Bakt., 1904, ii, 11.</p> - -<p><span id="Endnote8_29" class="label">[29]</span> Jensen, C. N., “Fungous flora of the soil,” Agric. Expt. Sta. Cornell, -Bull. 1912, 315.</p> - -<p><span id="Endnote8_30" class="label">[30]</span> Jones, L. R., “Experimental work on the relation of soil temperature -to disease in plants,” Trans. Wisc. Acad. Sci., 1922, -20.</p> - -<p><span id="Endnote8_31" class="label">[31]</span> Kappen, H., “Ueber die Zersetzung des Cyanamids durch Pilze,” -Centr. f. Bakt., 1910, ii, 26.</p> - -<p><span id="Endnote8_32" class="label">[32]</span> Klöcker, A., “Contribution à la connaissance de la faculté assimilatrice -de douze espèces de levure vis-à-vis de quatre Sucres,” -Compt. Rend. Trav. Lab., Carlsberg, 1919, 14.</p> - -<p><span id="Endnote8_33" class="label">[33]</span> Koch, A., und Oelsner, A., “Einfluss von Fichtenharz und Tannin -auf den Stickstoffhaushalt des Bodens und seiner physikalischen -Eigenschaften,” Centr. f. Bakt., 1916, ii, 45.</p> - -<p><span id="Endnote8_34" class="label">[34]</span> Kohshi, O., “Ueber die fettzehrenden Wirkungen der Schimmelpilze -nebst dem Verhalten des Organfettes gegen Fäulnis,” Biochem. -Ztschr., 1911, 31.</p> - -<p><span id="Endnote8_35" class="label">[35]</span> Kopeloff, N., “The inoculation and incubation of soil fungi,” Soil -Sci., 1916, 1.</p> - -<p><span id="Endnote8_36" class="label">[36]</span> Kusano, S., “<i>Gastrodia elata</i> and its symbiotic association with -<i>Armillaria mellea</i>,” Journ. Coll. Agric., Imp. Univ., Tokyo, 1911, -iv.</p> - -<p><span class="pagenum" id="Page145">[145]</span></p> - -<p><span id="Endnote8_37" class="label">[37]</span> Latham, M. E., “Nitrogen assimilation of <i>Sterigmatocystis niger</i> -and the effect of chemical stimulation,” Torrey Bot. Club, -Bull. 1909, 36.</p> - -<p><span id="Endnote8_38" class="label">[38]</span> Laurent, “Les reduction des nitrates en nitrites par les graines -et les tubercles,” Bull. Acad. Roy. Sci. Belg., 1890, 20.</p> - -<p><span id="Endnote8_39" class="label">[39]</span> Löhnis, F., “Ammonification of cyanamid,” Ztschr. f. Gärungsphysiol., 1914, v.</p> - -<p><span id="Endnote8_40" class="label">[40]</span> Marchal, E., “Sur la production de l’ammoniaque dans le sol -par les microbes,” Bull. Acad. Roy. Sci. Belg., 1893, 25.</p> - -<p><span id="Endnote8_41" class="label">[41]</span> Mazé, P., Vila et Lemoigne, “Transformation de la cyanamide -en urée par les microbes du sol,” Compt. Rend. Acad. Sci., -Paris, 1919, 169.</p> - -<p><span id="Endnote8_42" class="label">[42]</span> McBeth, I. G., “Studies on the decomposition of cellulose in soils,” -Soil Sci., 1916, I.</p> - -<p><span id="Endnote8_43" class="label">[43]</span> McBeth, I. G., and Scales, F. M., “The destruction of cellulose by -bacteria and filamentous fungi,” U.S. Dept. Agric, Bur. Pl. -Ind., 1913, Bull. 266.</p> - -<p><span id="Endnote8_44" class="label">[44]</span> McLean, H. C, and Wilson, G. W., “Ammonification studies with -soil fungi,” New Jersey Agric. Expt. Sta., 1914, Bull. 270.</p> - -<p><span id="Endnote8_45" class="label">[45]</span> Melin, E., “Ueber die mykorrhizenpilze von <i>Pinus silvestris</i> (L.) -und <i>Picea abies</i> (L.), Karst.” Svensk. Botan. Tidskr., 1921, xv.</p> - -<p><span id="Endnote8_46" class="label">[46]</span> Muntz, A., and Coudon, H., “La fermentation ammoniaque de la -terre,” Compt. Rend. Acad. Sci., Paris, 1893, 116.</p> - -<p><span id="Endnote8_47" class="label">[47]</span> Neller, J. R., “Studies on the Correlation between the production -of carbon dioxide and the accumulation of ammonia by soil -organisms,” Soil Sci., 1918, 5.</p> - -<p><span id="Endnote8_48" class="label">[48]</span> Otto, H, “Untersuchungen über die Auflösung von Zellulosen -und Zellwänden durch Pilze,” Dissert., Berlin, 1916.</p> - -<p><span id="Endnote8_49" class="label">[49]</span> Perotti, B., “Uber das physiologische Verhalten des Dicyanamides -mit Rücksicht auf seinen Wert als Düngemittel,” Centr. -f. Bakt., 1907, ii, 18.</p> - -<p><span id="Endnote8_50" class="label">[50]</span> Peyronel, B., “Nuovi casa di rapporti micorizici tra Basidiomiceti -e Fanerogame arboree,” Bull. Soc. Bot. Ital., 1922.</p> - -<p><span id="Endnote8_51" class="label">[51]</span> Potter, R. S., and Snyder, R. S., “The production of carbon dioxide -by moulds inoculated into sterile soil,” Soil Sci., 1918, 5.</p> - -<p><span id="Endnote8_52" class="label">[52]</span> Povah, A. H. W., “A critical study of certain species of <i>Mucor</i>,” -Bull. Torrey Bot. Club, 1917, 44.</p> - -<p><span id="Endnote8_53" class="label">[53]</span> Pratt, O. A., “Soil fungi in relation to diseases of the Irish potato -in Southern Idaho,” Journ. Agric. Res., 1918, 13.</p> - -<p><span id="Endnote8_54" class="label">[54]</span> Raistrick, H., and Clark, A. B., “On the mechanism of oxalic -acid formation by <i>Aspergillus niger</i>,” Biochem. Journ., 1919, -13.</p> - -<p><span id="Endnote8_55" class="label">[55]</span> Rayner, M. C., “Nitrogen fixation in Ericaceae,” Bot. Gaz., 1922, -73.</p> - -<p><span class="pagenum" id="Page146">[146]</span></p> - -<p><span id="Endnote8_56" class="label">[56]</span> Ritter, G. E., “Contributions to the physiology of mould fungi,” -Voronege, 1916.</p> - -<p><span id="Endnote8_57" class="label">[57]</span> Rosseels, E., “L’influence des microorganismes sur la croissance -des végétaux supérieurs,” Bull. Soc. Centrale Forest. Belg., -1916, 23.</p> - -<p><span id="Endnote8_58" class="label">[58]</span> Roussy, A., “Sur la vie des champignons en milieux Gras,” -Compt. Rend. Acad. Sci., Paris, 1909, 149.</p> - -<p><span id="Endnote8_59" class="label">[59]</span> Scales, F. M., “The Enzymes of <i>Aspergillus terricola</i>,” Journ. Biol. -Chem., 1914, 19.</p> - -<p><span id="Endnote8_60" class="label">[60]</span> Schellenberg, H. C., “Untersuchungen über das Verhalten einiger -Pilze gegen Hemizellulosen,” Flora, 1908, 98.</p> - -<p><span id="Endnote8_61" class="label">[61]</span> Schmitz, H., “The relation of bacteria to cellulose fermentation -induced by fungi with special reference to the decay of wood,” -Ann. Mo. Bot. Gard., 1919, vi.</p> - -<p><span id="Endnote8_62" class="label">[62]</span> Shibata, K., “Uber das Vorkommen vom Amide spaltenden Enzymen -bei Pilzen,” Beitr. Chem. Physiol. u. Path., 1904, 5.</p> - -<p><span id="Endnote8_63" class="label">[63]</span> Ternetz, C., “Über die Assimilation des atmosphärischen Stickstoffs -durch Pilze,” Jahrb. f. wiss. Bot., 1907, 44.</p> - -<p><span id="Endnote8_64" class="label">[64]</span> Verkade, P. E., and Söhngen, N. L., “Attackability of cis- and -trans-isomeric unsaturated acids by moulds,” Centr. f. Bakt., -1920, ii, 50.</p> - -<p><span id="Endnote8_65" class="label">[65]</span> Waksman, S. A., “Soil fungi and their activities,” Soil Sci., 1916, 2.</p> - -<p><span id="Endnote8_66" class="label">[66]</span> Waksman, S. A., “The influence of available carbohydrate upon -ammonia accumulation by micro-organisms,” Journ. Amer. -Chem. Soc., 1917, 39.</p> - -<p><span id="Endnote8_67" class="label">[67]</span> Waksman, S. A., “Proteolytic enzymes of soil fungi and <i>Actinomycetes</i>,” -Journ. Bact., 1918, 3.</p> - -<p><span id="Endnote8_68" class="label">[68]</span> Waksman, S. A., “On the metabolism of <i>Actinomycetes</i>,” Proc. -Soc. Amer. Bact. Abstract Bact., 1919, 3.</p> - -<p><span id="Endnote8_69" class="label">[69]</span> Waksman, S. A., “The influence of soil reaction upon the growth -of <i>Actinomycetes</i> causing potato scab,” Soil Sci., 1922, xiv.</p> - -<p><span id="Endnote8_70" class="label">[70]</span> Waksman, S. A., and Cook, R. C., “Incubation studies with soil -fungi,” Soil Sci., 1916, 1.</p> - -</div><!--footnote--> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page147">[147]</span></p> - -<h2 class="nobreak">CHAPTER IX.<br /> -<span class="chaptitle">THE INVERTEBRATE FAUNA OF THE SOIL -(OTHER THAN PROTOZOA).</span></h2> - -</div><!--chapter--> - -<p>The micro-organisms of the soil have been fully discussed -in the preceding chapters of this volume. There now -remains to be considered the fauna of invertebrate animals, -other than protozoa, which inhabit that same medium. -In the first place, it is necessary to define what groups of -invertebrate animals are to be regarded as coming under -the category of soil organisms. The latter expression has -rather a wide application and, for the present purpose, is -held to mean any organism of its kind which, in some stage -or stages of its life-cycle, lives on or below the surface of -the soil. It will be obvious that, with so comprehensive a -definition, the intimacy of the association of these animals -with the soil will vary within very wide limits. Some -animals pass their whole life-cycle in the soil; others are -only present during a limited phase, and not necessarily -in a trophic condition, but since their occurrence is constant, -they cannot be entirely omitted from consideration.</p> - -<p>Unlike the groups of organisms which have been dealt with -in the foregoing pages, the invertebrates of the soil do not -admit, as a rule, of investigation in culture media. It is, -in consequence, much more difficult to achieve in the laboratory -the same control over their environmental conditions. -This fact in itself largely explains why the interpretations -of field observations in animal ecology have not usually -been subjected to the test of laboratory experimentation. -The study of animal ecology, in so far as the denizens of -the soil are concerned, is of very recent birth. It has not,<span class="pagenum" id="Page148">[148]</span> -as yet, passed the preliminary stage of cataloguing empirical -data, and much spade work will be necessary before the -various factors controlling the phenomena actually observed -are understood.</p> - -<p>Owing to the paucity of information available, this -chapter is essentially based upon observations conducted -at Rothamsted. Its object is not so much to attempt to -evaluate the invertebrate fauna of the soil, as to suggest -a line of ecological work demanding investigation on land -of many different types.</p> - -<h3><span class="smcap">Method of Investigating the Soil Fauna.</span></h3> - -<p>The method adopted at Rothamsted consists in taking -weekly soil samples from a given area for a period of twelve -months. Each sample is a cube of soil, with a side dimension -of nine inches, and a total content of 729 cubic inches. -The samples are taken by means of an apparatus consisting -of four iron plates, which are driven into the ground down -to the required depth so as to form a kind of box, which -encloses a cube of soil (<i>vide</i> Morris, 1922 <span class="smcapall">A</span>). The latter is -then removed in layers, each layer being transferred to a -separate bag for the purpose. When the complete sample -has been extracted, there are five bags containing layers of -soil taken from the surface to a depth of 1″, from 1″ to 3″, -from 3″ to 5″, from 5″ to 7″, and 7″ to 9″ respectively. -Below a depth of 9″ no samples have been taken.</p> - -<p>The sample obtained in this manner may be gradually -worked into small fragments by hand, and examined whenever -necessary under a binocular microscope for the smaller -organisms present. This procedure, however, is very -tedious and has been replaced by the use of an apparatus -consisting of a series of three sieves, with meshes of decreasing -size (<i>vide</i> Morris, 1922). The soil is washed -through these sieves by means of a stream of water, and the -meshes of the final strainer are small enough to retain all -except the most minute organisms present, while at the same -time they allow the finest soil particles to be carried away.<span class="pagenum" id="Page149">[149]</span> -When desirable, the effluent can be passed through a bag -or sieve of bolting silk, in order to collect such organisms -that may have passed through the third sieve.</p> - -<p>In addition to the actual taking and examination of -the samples, a botanical survey of the area under investigation -is made; chemical and mechanical analyses of the soil -are also required. It is further necessary to take soil temperature -readings, to determine the moisture content of -the samples taken, and the amount of organic matter which -they contain.</p> - -<h3><span class="smcap">Groups of Invertebrata Represented in the Soil.</span></h3> - -<p>The various groups of invertebrates represented in the -soil may be briefly referred to in zoological order.</p> - -<p><i>Nematoda.</i>—The Nematoda or thread-worms are chiefly -animal parasites, nevertheless they usually lead an independent -existence in the soil in certain stages of their development. -The numerous small species belonging to the family -<i>Anguillididæ</i>, or eel-worms, form a definite constituent of -the soil fauna; they are generally free-living and non-parasitic. -Certain members of this family, however, are -enemies of cultivated plants.</p> - -<p><i>Annelida.</i>—Terrestrial Annelida are almost entirely -confined to the order <i>Oligochæta</i>, the majority of which are -earthworms (<i>Terricolæ</i>), whose whole life-cycle is passed -within the confines of the soil. The small white worms of -the family <i>Enchytræidæ</i> belong to the aquatic section -(<i>Limicolæ</i>) of the order, but they have various representatives -which are abundant in damp soil containing organic matter.</p> - -<p><i>Mollusca.</i>—The terrestrial Mollusca are included in the -sub-order <i>Pulmonata</i> of the <i>Gastropoda</i>. These organisms, -which include the snails (<i>Helicidæ</i>) and slugs (<i>Limacidæ</i>), -regularly deposit their eggs in moist earth. Slugs adopt the -soil as a frequent habitat, only leaving it for feeding purposes -in the presence of sufficient moisture. They are frequent -consumers of vegetation, with the exception of <i>Testacella</i>, -which is carnivorous.</p> - -<p><span class="pagenum" id="Page150">[150]</span></p> - -<p><i>Crustacea.</i>—The few species of Crustacea inhabiting the -soil belong to the order <i>Isopoda</i>, family <i>Oniscidæ</i>, which are -popularly referred to as “woodlice,” “slaters,” etc.</p> - -<p><i>Myriapoda.</i>—The <i>Diplopoda</i> or millipedes include enemies -of various crops and are common denizens of the soil. -The <i>Chilopoda</i> or centipedes are usually less abundant and -are carnivorous. The minute <i>Symphyla</i> are often evident -but are of minor importance.</p> - -<p><i>Insecta.</i>—Insects form the dominant element in the -invertebrate fauna. Phytophagous species devour the subterranean -parts of plants, and notable examples are afforded -by the larvæ of <i>Melolontha</i>, <i>Agriotes</i> and <i>Tipula</i>. -Saprophagous forms are abundantly represented by the -<i>Collembola</i>, and by numerous larval <i>Diptera</i> and <i>Coleoptera</i>. -Predaceous species preying upon other members of the -soil fauna are exemplified by the <i>Carabidæ</i> and many larval -<i>Diptera</i>. Parasitic species pass their larval stages on or -within the bodies of other organisms. The groups of -<i>Hymenoptera</i>, and the dipterous family <i>Tachinidæ</i>, which -exhibit this habit, constitute, along with predaceous forms, -one of the most important natural agencies controlling -the multiplication of insect life. There are also insects -(ants, and other of the aculeate <i>Hymenoptera</i>) which utilize -the soil as a suitable medium wherein to construct their -habitations or brood chambers, without necessarily deriving -their food from the soil. Lastly, there are many insects, -notably <i>Lepidoptera</i>, which only resort to the soil for the -purpose of undergoing pupation. The insect fauna is, -therefore, a closely inter-connected biological complex; -for a discussion and an enumeration of its representatives -reference may be made to papers by Cameron (1913, 1917), -and Morris (1921, 1922 <span class="smcapall">a</span>).</p> - -<p><i>Arachnida.</i>—The two principal classes represented in the -soil are the <i>Areinida</i>, or spiders, and the <i>Acarina</i>, or mites, -and ticks. The <i>Areinida</i>, which are well-known to be -carnivorous, are an unimportant constituent of the fauna. -<i>Acarina</i>, on the other hand, are abundant, and exhibit a<span class="pagenum" id="Page151">[151]</span> -wide range of feeding habits; most of the soil forms are -probably carnivorous, and either free-living or parasitic.</p> - -<h3><span class="smcap">Number of Organisms Present and their Distribution -in Depth.</span></h3> - -<p>In computing the number of invertebrates normally -present in a given type of soil, the method adopted consists -of making individual counts of all such organisms as occur -in each sample of a series taken over a period of twelve -months. This method considerably reduces errors due to -season and to the possible deviation of one or more samples -from the average. If the total number of these organisms -is known for the samples taken, it becomes a simple procedure -to arrive at their approximate numbers per acre.</p> - -<p class="tabhead" id="TabXIV">TABLE XIV.<br /> -<span class="fsize90">(Based on Morris, 1922 <span class="smcapall">A</span>.)</span></p> - -<table class="standard"> - -<tr class="bt bb"> -<th class="bl br"> </th> -<th class="br">Unmanured<br />Plot.</th> -<th class="br">Manured<br />Plot.</th> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Insects</span></td> -<td class="general br">2,474,700</td> -<td class="general br"> 7,727,300</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Larger Nematoda and Oligochæta Limicolæ</span></td> -<td class="general br">  794,600</td> -<td class="general br"> 3,600,400</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Myriapoda—</span></td> -<td class="general br"> </td> -<td class="general br"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Diplopoda</span></td> -<td class="general br">  596,000</td> -<td class="general br"> 1,367,000</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Chilopoda</span></td> -<td class="general br">  215,400</td> -<td class="general br">   208,700</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Symphyla</span></td> -<td class="general br bb">   64,000</td> -<td class="general br bb">   215,500</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padl4 padr2">Total</span></td> -<td class="general br">  875,400</td> -<td class="general br"> 1,791,200</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Oligochæta (Terricolæ)</span></td> -<td class="general br">  457,900</td> -<td class="general br"> 1,010,100</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Arachnida—</span></td> -<td class="general br"> </td> -<td class="general br"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Acarina</span></td> -<td class="general br">  215,400</td> -<td class="general br">   531,900</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Areinida</span></td> -<td class="general br bb">   20,200</td> -<td class="general br bb">    20,200</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padl4 padr2">Total</span></td> -<td class="general br">  235,600</td> -<td class="general br">   552,100</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Crustacea (Isopoda)</span></td> -<td class="general br">   33,700</td> -<td class="general br">    80,800</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Mollusca (Pulmonata)</span></td> -<td class="general br">   13,500</td> -<td class="general br">    33,700</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padl4 padr2">Total Invertebrata</span></td> -<td class="general br">4,885,400</td> -<td class="general br">14,795,600</td> -</tr> - -</table> - -<p><span class="pagenum" id="Page152">[152]</span></p> - -<div class="container w30em" id="Fig20"> - -<img src="images/illo160.png" alt="" /> - -<p class="caption long"><span class="smcap">Fig. 20.</span>—Distribution in -depth of the more important groups of soil invertebrates in -the manured and unmanured (or control) plots at Rothamsted. (From Morris, “Annals -of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University Press.)</p> - -</div><!--container--> - -<p><span class="pagenum" id="Page153">[153]</span></p> - -<p><a href="#TabXIV">Table XIV.</a> represents a numerical estimate of the invertebrate -fauna of two plots of arable land at Rothamsted. -The soil is clay with flints overlying chalk, and the land -in question has been devoted for eighty years to continuous -cropping with wheat; one plot (No. 3) receives an annual -dressing of farmyard manure at the rate of 14 tons per acre, -and the other plot (No. 2) receives no natural or artificial -fertilizer. The significant feature in a comparison of the -fauna of the two plots is the great numerical increase in -organisms due to the addition of manure. From the point -of view of distribution in depth, <a href="#Fig20">Fig. 20</a> clearly demonstrates -that the bulk of the fauna is concentrated in the -first three inches of the soil. With the exception of the -<i>Acarina</i> it is evident that the limits of vertical distribution -extend below the depth of nine inches investigated, although -the numbers of organisms likely to be present are inconsiderable. -The <i>Oligochæta</i>, or true earthworms, occur in -Rothamsted soil in numbers very much in excess of the -figures given by Darwin, who quoted observations by Hensen. -The latter authority calculated that there were 53,767 -earthworms in an acre of garden soil, and estimated that -about half that number would be present in an acre of corn -field. In the Rothamsted investigations their numbers -exceeded Hensen’s estimate over 16 times in unmanured -land, and over 36 times in manured land.</p> - -<p>In an area of pasture-land in Cheshire few insects occurred -below a depth of 2 inches, and they reached the -limit of their vertical distribution at or near 6 inches. Their -number (3,586,000 per acre) is considerably in excess of -that present in unmanured arable land at Rothamsted.</p> - -<p><span class="pagenum" id="Page154">[154]</span></p> - -<div class="container w30em" id="Fig21"> - -<img src="images/illo162.png" alt="" /> - -<p class="caption">1, <i>Collembola</i>; 2, <i>Thysanura</i>; 3, <i>Orthoptera</i>; 4, <i>Thysanoptera</i>; 5, <i>Hemiptera</i>; 6, <i>Lepidoptera</i>; -7, <i>Coleoptera</i>; 8, <i>Diptera</i>; 9, <i>Hymenoptera</i>.</p> - -<p class="caption long"><span class="smcap">Fig. 21.</span>—Number of individuals -in the different orders of insects in manured and unmanured -arable land at Rothamsted. (From Morris, “Annals of Applied Biology,” vol. ix., -nos. 3 and 4, Cambridge University Press.)</p> - -</div><!--container--> - -<p><span class="pagenum" id="Page155">[155]</span></p> - -<h3><span class="smcap">Dominance of Certain Species and Groups.</span></h3> - -<p>In <a href="#Fig21">Fig. 21</a> a numerical analysis is given of the different -orders of insects represented in Rothamsted soil. -The ascendency of the <i>Hymenoptera</i> and <i>Collembola</i> is -almost entirely due to the occurrence of three species in -large numbers, viz., the ant <i>Myrmica lævinodis</i> and the -<i>Collembola</i>, <i>Onychiurus ambulans</i> and <i>O. fimetarius</i>. In -the unmanured plot these two <i>Collembola</i> constituted -13 per cent. of the insects and the species of ant accounted -for nearly 28 per cent. In the manured plot they amounted -respectively to 27 per cent. and 36 per cent. of the insects -present. Next in order of numerical ascendency are larval -<i>Diptera</i>, mainly belonging to the families <i>Cecidomyidæ</i>, -<i>Chironomidæ</i>, and <i>Mycetophilidæ</i>. The <i>Diptera</i> are followed -by the <i>Coleoptera</i>, whose most abundant representatives -are larval <i>Elateridæ</i> (wireworms).</p> - -<div class="container w40em" id="Fig22"> - -<img src="images/illo163.png" alt="" /> - -<p class="caption">1, <i>Collembola</i>; 2, <i>Thysanura</i>; 3, <i>Orthoptera</i>; 4, <i>Thysanoptera</i>; -5, <i>Hemiptera</i>; 6, <i>Lepidoptera</i>; 7, <i>Coleoptera</i>; -8, <i>Diptera</i>; 9, <i>Hymenoptera</i>; 10, <i>Diplopoda</i>; 11, <i>Chilopoda</i>; 12, <i>Areinida</i>; 13, <i>Acarina</i>.</p> - -<p class="caption long"><span class="smcap">Fig. 22.</span>—Number of species of different orders of invertebrates present in the -manured and unmanured (or control) plots at Rothamsted. (From Morris, “Annals of -Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University Press.)</p> - -</div><!--container--> - -<p><span class="pagenum" id="Page156">[156]</span></p> - -<p>In point of view of number of species present (<a href="#Fig22">Fig. -22</a>), <i>Coleoptera</i> take the front rank; in the unmanured -plot they are very closely approached by <i>Collembola</i> and -<i>Diptera</i>.</p> - -<p>Passing from the insects, the next assemblage of organisms -in point of number of individuals are the smaller worms. -The difficulties attending the specific identification of these -organisms are great, and, in the present survey, the <i>Nematodes</i> -and all the smaller <i>Oligochætes</i> have not been separated.</p> - -<p>The abundance of the <i>Myriapoda</i> is mainly due to the -prevalence of <i>Diplopoda</i>, which are represented by four -species. The <i>Chilopoda</i> almost entirely consist of a single -species <i>Geophilus longicornis</i>.</p> - -<p>The dominant group of the <i>Arachnida</i> is the <i>Acarine</i> -family <i>Gamascidæ</i>, which are represented by about a dozen -species.</p> - -<h3><span class="smcap">Classification of Soil Invertebrates According to -Feeding Habits.</span></h3> - -<table class="standard"> - -<tr class="bt bb"> -<th class="bl br"> </th> -<th class="br">Phyto-<br />phagous.</th> -<th class="br">Sapro-<br />phagous.</th> -<th class="br">Carniv-<br />orous.</th> -<th class="br">Hetero-<br />phagous.</th> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Unmanured plot</span></td> -<td class="general br">14</td> -<td class="general br">48</td> -<td class="general br">13</td> -<td class="general br">20</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">Manured plot</span></td> -<td class="general br">13</td> -<td class="general br">58</td> -<td class="general br"> 9</td> -<td class="general br">20</td> -</tr> - -</table> - -<p>From the point of view of the fauna as a whole, the -zoological classification of the soil invertebrates is only -significant when the various groups are analysed according -to the feeding habits of their members. All animals are -directly or indirectly dependent upon plant life for their -nutrition. For the present purpose they are divided into -four categories, and the position of each class of animals -in the scheme is based upon the habits of its chief representatives -in the soil. Definite information on this subject, -however, is not always forthcoming, and it is only possible -to achieve approximate estimates. In the table above the<span class="pagenum" id="Page157">[157]</span> -percentages in number of individuals present in the two -plots investigated at Rothamsted are given under each -type of feeding habit.</p> - -<p>It must be borne in mind that these estimates only -apply to average conditions; the outbreak of a plant -pest in any one year must naturally materially alter the -proportions given. The phytophagous organisms are represented -by a certain number of the <i>Insecta</i> together -with the pulmonate <i>Mollusca</i>. Carnivorous forms which -are mainly beneficial from the agricultural standpoint, include -<i>Insecta</i>, together with the <i>Chilopoda</i>, many <i>Acarina</i> -and the <i>Areinida</i>. Saprophagous forms constitute the -dominant element of the soil fauna. More than 30 per cent. -of the <i>Insecta</i> exhibit this habit, which is also the dominant -one in the <i>Oligochæta</i>, <i>Symphyla</i>, and in many of the soil -<i>Nematodes</i>. Heterophagous species include all those of somewhat -plastic habits; for the most part they are saprophagous, -but, on the other hand, a considerable proportion -of the species attack growing plants or exhibit both habits. -Under this category are included a certain number of the -<i>Insecta</i>, the <i>Diplopoda</i>, <i>Isopoda</i>, and some <i>Acarina</i>.</p> - -<h3><span class="smcap">The Influence of Environmental Factors upon The -Invertebrates of the Soil.</span></h3> - -<p>Since animals are endowed with powers of independent -locomotion: they are not necessarily tied to their environment -to the same extent that plants are. The investigation -of the influence of environmental factors sooner or later -involves a study of the tropisms of the animals concerned. -Until these are adequately understood it is scarcely possible -to arrive at any exact conclusions relative to their behaviour -in the soil. Insects, for example, respond to the stimuli of -various, and often apparently insignificant forces, acting upon -their sensory organs. Such responses are known as chemotropism, -phototropism, hydrotropism, thermotropism, and so -forth according to the nature of the stimuli. Tropisms are -automatic and, so far as they distinguish sensations, are<span class="pagenum" id="Page158">[158]</span> -independent of any choice, and consequently of psychic -phenomena. Animal automatism, however, does not present -the rigidity of mechanical automatism. Differential sensibility, -vital rhythms, or periodicity, etc., are other important -aspects of animal behaviour.</p> - -<p>The environmental factors, affecting more especially the -insect population of the soil, have been discussed by Cameron -(1917) and Hamilton (1917), and certain broader aspects -of animal ecology by Adams (1915) and Shelford (1912). -These factors are so numerous and so inter-connected, that -it is only possible to refer to them briefly in the space available. -As might be expected, soils that are of a light and -open texture are the ones most frequented by soil insects, -nutritional and other factors being equal. Furthermore, it -has already been shown that in arable land insects and other -animals penetrate to a greater depth than in pastures. -This fact is primarily due to the greater looseness of the -soil occasioned by agricultural operations, which ensure -at the same time better drainage aeration, and greater -facilities for penetration. Hamilton found that soil insect -larvæ are very sensitive to evaporation, and especially so -if the temperature is 20° C. or over. In their natural habitat -the relative humidity of the air, in moist or wet soil, is not -far below saturation, and the temperature of the soil rarely -goes above 20°-23°C., and then only in exposed, dry, hard -soil in which these larvæ do not occur.</p> - -<p>The significance of the rate of evaporation as an environmental -factor was first emphasised by Shelford. According -to him the best and more accurate index of the varying -physical conditions affecting land animals, wholly or in part -exposed to the atmosphere, is the evaporating power of air. -By means of a porous cup-atmometer, as devised by Livingston, -Shelford has carried out an important series of experiments -on the reactions of various animals to atmospheres -of different evaporation capacities, and reference should be -made to his text-book.</p> - -<p>The importance of the organic matter present in the soil<span class="pagenum" id="Page159">[159]</span> -is well illustrated in the <a href="#TabXIV">table</a> on <a href="#Page152">p. 152</a>. The great increase -in the number of insects and other animals is partly due to -their direct introduction along with the manure, and partly -to their entry into the soil in response to chemotropic stimuli -exerted by fermentation. Organic matter influences the -fauna in other ways also; it increases the moisture content -of the soil, and it provides many species with an abundance -of food material. Also, the amount of carbon dioxide -present in the soil is partly dependent upon decaying organic -matter. Hamilton conducted experiments on the -behaviour of certain soil insects in relation to varying -amounts of carbon dioxide. Although his work is of too -limited a nature to be accepted without reserve, it lends -support to the conclusions of Adams who says: “The -animals which thrive in the soil are likely to be those which -tolerate a large amount of carbon dioxide, and are able to -use a relatively small amount of oxygen, at least for considerable -intervals, as when the soil is wet during prolonged -rains. The optimum soil habitat is therefore determined, -to a very important degree, by the proper ratio or balance -between the amount of available oxygen and the amount -of carbon dioxide which can be endured without injury.”</p> - -<p>Little is known concerning the occurrence of ammonia -in the soil atmosphere, but its presence in minute quantities -is probably an important chemotropic factor in relation to -saprophagous organisms which are the largest constituent of -the fauna. A great increase in Dipterous larvæ occurs on -the addition of farmyard manure, and this is noteworthy -in the light of Richardson’s experiments (1916), which indicate -that ammonia exercises a marked attraction for -<i>Diptera</i>, which spend some part of their existence in animal -excrement in some form or another.</p> - -<p>The nature of the vegetation supported by the soil is of -paramount importance in relation to phytophagous organisms, -and examples need scarcely be instanced of certain -species of soil insects being dependent upon the presence -of their specific food plants.</p> - -<p><span class="pagenum" id="Page160">[160]</span></p> - -<h3><span class="smcap">The Relation of Soil Invertebrates to Agriculture.</span></h3> - -<p>The relation of these organisms to agriculture may be -considered from three points of view: (<i>a</i>) their influence -upon the soil itself; (<i>b</i>) their relation to the nitrogen cycle; -and (<i>c</i>), their direct influence upon economic plants.</p> - -<p>(<i>a</i>) The behaviour of earthworms as a factor inducing -soil fertility is discussed by Darwin in his well-known -work on the subject, and their action may be briefly summarised -as follows. In feeding habits they are very largely -saprophagous, and consume decaying vegetable matter -including humus, which they swallow, together with large -quantities of soil. Earthworms come to the surface to -discharge their fæces (“worm casts”), and in this process -they are continually bringing up some of the deeper soil -to the air. Darwin estimated that earthworms annually -brought to the surface of the soil in their “casts” sufficient -earth to form a layer ·2 inch in depth, or 10 tons per acre. -Their action, along with the atmosphere, are the chief -agencies which produce the uniformity and looseness of texture -of the surface soil. By means of their burrows earthworms -facilitate the penetration of air and water into the -soil, while their habit of dragging leaves and other vegetable -material into these burrows increases the organic matter -present below the surface. These facts are generally agreed -upon, but it is a disputed point whether earthworms, by -devouring organic matter, aid the conversion of the latter -into plant food more rapidly than takes place solely through -the activities of micro-organisms.</p> - -<p>Soil insects and other arthropods, by their burrowing -activities, are also instrumental in loosening the soil texture -and thereby facilitating soil aeration and the percolation of -water. The action of termites in warmer countries is discussed -by Drummond in his “Tropical Africa,” who compares -the rôle of subterranean termites to that of earthworms. -The great abundance of ants renders them also significant -in this same respect, and very few species are direct enemies -of the agriculturist.</p> - -<p><span class="pagenum" id="Page161">[161]</span></p> - -<div class="container" id="Fig23"> - -<img src="images/illo169.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 23.</span>—Diagram showing the Relation of the Soil Invertebrata (other than -Protozoa) to the Nitrogen Cycle.</p> - -</div><!--container--> - -<p>(<i>b</i>) In their relation to the nitrogen cycle (<i>vide</i> <a href="#Page174">p. 174</a>), -the activities of the soil invertebrates may be expressed -diagrammatically, as a side-chain in the process (<a href="#Fig23">Fig. 23</a>). -The proteins, elaborated by plants, are utilised as nitrogenous -food by the phytophagous animals present. The waste -products of the latter, which contain the nitrogen not used -for growth or the replacement of loss by wear and tear, -are returned to the soil. Here they disintegrate, and are -ultimately converted into ammonium salts, mainly by -bacterial action. The dead bodies of these animals are also -broken down by various means, becoming eventually -chemically dissociated and available as plant food. Animal -(and plant) residues serve, however, as food for the large -number of saprophagous invertebrates present in the soil. -In this event the nitrogen contained in such residues becomes -“locked up,” as it were, for the time being in their -bodies. Both saprophagous and phytophagous animals -are preyed upon by carnivorous species, but ultimately<span class="pagenum" id="Page162">[162]</span> -the nitrogen is returned to the soil upon the death of those -organisms. The amount present in the bodies of the whole -invertebrate fauna has been calculated by Morris (1922) -upon analyses furnished by chemists at Rothamsted. It is -estimated that the fauna of manured land contains about -7349 grm., or 16·2 lb. of nitrogen per acre, and that of -untreated land, 3490 grm., or 7·5 lb. per acre. These amounts -are equal respectively to the nitrogen content of 103·6 lb. -and 48 lb. of nitrate of soda.</p> - -<p>The primary question affecting agriculture is, whether -any notable loss of nitrogen is occasioned by the presence of -these organisms in the soil. It has been mentioned that their -nitrogenous waste material, and their dead bodies, ultimately -undergo disintegration; any loss, if any, takes place during -the latter process. With the more complex compounds it -probably consists in the production of amino-acids and their -subsequent hydrolysis or oxidation. During this process -an appreciable loss of nitrogen in the gaseous form occurs. -This loss, which is discussed on p. 173 would represent the -net deficit occasioned by the incidence of invertebrates in -the soil. Against this loss must be placed the beneficial -action of such organisms as earthworms, which, in all probability, -more than counterbalances it.</p> - -<p>(<i>c</i>) Many soil insects, on account of their phytophagous -habits, are well-known to be some of the most serious -enemies of agriculture. Certain of these, and also other -classes of invertebrates, which are likewise directly injurious, -have been instanced in the earlier pages of this chapter. -Detailed information on this subject will be found in textbooks -of economic zoology, notably the volume by Reh -(1913).</p> - -<h3>LITERATURE REFERRED TO.</h3> - -<div class="litlist"> - -<p><span class="smcap">Adams, C. C.</span>, “An Ecological Study of Prairie and Forest Invertebrates,” -Bull. Illin. St. Lab. Nat. Hist., 1915, xi.</p> - -<p><span class="smcap">Cameron, A. E.</span>, “General Survey of the Insect Fauna of the Soil,” -Journ. Econ. Biol., 1913, viii. “Insect Association of a Local -Environmental Complex in the District of Holmes Chapel, -Cheshire,” Trans. Roy. Soc. Edin., 1917, lii.</p> - -<p><span class="pagenum" id="Page163">[163]</span></p> - -<p><span class="smcap">Darwin, C.</span>, “Vegetable Mould and Earthworms,” London, 1881.</p> - -<p><span class="smcap">Hamilton, C. C.</span>, “The Behaviour of some Soil Insects in Gradients -of Evaporating Power of Air, etc.,” Biol. Bull., 1917, -xxxii.</p> - -<p><span class="smcap">Morris, H. M.</span>, “Observations on the Insect Fauna of Permanent -Pasture in Cheshire,” Ann. App. Biol., 1921, vii. “On a -Method of Separating Insects and other Arthropods from Soil,” -Bull. Entom. Res., 1922, xiii. “The Insect and Other Invertebrate -Fauna of Arable Land at Rothamsted,” Ann. App. -Biol., 1922 <span class="smcapall">A</span>, ix.</p> - -<p><span class="smcap">Reh, L.</span>, In Sorauer’s “Pflanzenkrankheiten,” 1913, iii.</p> - -<p><span class="smcap">Richardson, C. H.</span>, “The Attraction of Diptera to Ammonia,” -Ann. Ent. Soc. Amer., 1916, ix.</p> - -<p><span class="smcap">Russell, E. J.</span>, “The Effect of Earthworms on Soil Productiveness,” -Journ. Agric. Sci., 1910, iii.</p> - -<p><span class="smcap">Shelford, V. E.</span>, “Animal Communities in Temperate America,” -Chicago. 1914, “The Importance of the Measure of Evaporation -in Economic Studies of Insects,” Journ. Econ. Entom., -1912, vii.</p> - -</div><!--litlist--> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page164">[164]</span></p> - -<h2 class="nobreak">CHAPTER X.<br /> -<span class="chaptitle"><span class="smcap">The Chemical Activities of the Soil Population and -their Relation to the Growing Plant.</span></span></h2> - -</div><!--chapter--> - -<p>In the preceding chapters it is shown that the soil is normally -inhabited by a very mixed population of organisms, varying -in size from the smallest bacteria up to nematodes and -others just visible to the unaided eye, on to larger animals, -and finally earthworms, which can be readily seen and -handled. These organisms all live in the soil, and therefore -must find in it the conditions necessary for their growth. -We have dealt in the first chapter with the supplies of -water, air, and heat, without which life is clearly impossible. -Equally necessary is the source of energy, for the organism -requires energy material as surely as the motor engine -requires petrol, and it ceases to function unless an adequate -supply is forthcoming.</p> - -<p>All the energy comes in the first instance from the sun, -if we exclude the unknown but probably small fraction -coming from radio-active elements. But this radiant energy -is not utilisable by the soil population, excepting surface -algæ; it has to be transformed into another kind. So far, -chlorophyll is the only known transformer; it fixes the energy -of sunlight and stores it up in bodies like hemicellulose, -sugar, starch, protein, etc. The transformation is imperfect; -even the heaviest yielding crops grown under glass, in conditions -made as favourable as our knowledge permits, -utilise only about 4 per cent. of the total energy available -during their period of growth; in natural conditions not -more than 0·4 per cent. is utilised. Such as it is, however,<span class="pagenum" id="Page165">[165]</span> -the energy fixed in the plant represents all, indeed more -than all, that the soil organisms can obtain.</p> - -<p>In the state of Nature, vegetation dies and is left on the -soil. Two things may then happen. It may become drawn -into the soil by earthworms and other agents; the energy -supply is thus distributed in the soil to serve the needs of -the varied soil population. This is the normal case, associated -with the normal soil population and the normal -flora. If, however, the mingling agents are absent, the -dead vegetation lies like a mat on the surface of the soil, -only partially decomposing, unsuitable for the growth of -most seedlings, and effectually preventing most of the -vegetation below from pushing a way through: thus there -comes to be no vegetation at all, or only a very restricted -and special flora. The soil population becomes also specialised. -Peats and acid grassland afford examples.</p> - -<p>On the neutral grass plots at Rothamsted, the dead -vegetation does not accumulate on the surface but is rapidly -decomposed or drawn into the soil, leaving the surface of -the earth bare and free for the growth of seedlings. On -the acid plots dead vegetation remains long on the surface, -blotting out all new growth excepting two or three grasses -which form underground runners capable of penetrating the -mat, and sorrel, the seedling roots of which seem to have -the power of boring through a fibrous layer of this sort. -It is possible to remove the mat entirely by bacterial action -alone, if sufficient lime be added periodically to make the -reaction neutral, but failing these repeated additions the -mat persists.</p> - -<p>We shall confine ourselves to the normal case where -earthworms bring the source of energy into the soil.</p> - -<p>Directly the energy is available, it begins to be utilised. -Two laws govern the change. The first is well-known to -biologists: it states that the total energy of the system remains -constant and can neither be increased nor diminished except -from outside; in other words, that energy can be neither -created nor destroyed. The second law is less familiar: it is<span class="pagenum" id="Page166">[166]</span> -that energy once transformed to heat by one organism -cannot be used again by another. It is not destroyed; -it remains intact, but is useless to the organism. One -cannot have an indefinite chain of organisms living on -each other’s excretory products; there was a certain -quantity of energy in the food eaten by the first, and no more -than this quantity can be got out whether one organism -obtains the whole or whether others share it.</p> - -<p>The outside value for the amount of energy fixed in the -soil is obtainable by combustion of the soil in a calorimeter, -but much of this is not available to the soil organisms. -The normal sedimentary soils of England still contain -decomposition products of the débris of plants and animals -originally deposited with them, but in the long course of -ages much of the extractable energy has been utilised. -The soil population is thus dependent on recently grown -vegetation, and it is therefore largely confined to the layer, -usually in this country about 6 inches thick, through which -the recently dead vegetation is distributed. Below this level -there may be sufficient air, water, temperature, etc., but -there is insufficient source of energy for any large population.</p> - -<p>Unfortunately there is no ready means for distinguishing -between the total and the actually available quantity of -energy in the soil. But it is not difficult, by adopting the -Rothamsted analytical method, to ascertain the approximate -amount of energy that has been transformed in a given -period. The Rothamsted plots are periodically analysed -and a balance sheet is drawn up showing how much of each -constituent has been added to and removed from the soil -in the intervening period. For two of the Broadbalk plots -the results are shown in <a href="#TabXV">Tables XV.</a>, <a href="#TabXVI">XVI.</a></p> - -<p>The dunged plot receives 14 tons farmyard manure per -annum, a quantity in excess of what would usually be given; -the unmanured plot, on the other hand, has received no -manure for many years and is abnormally poor. Normal -soils lie somewhere between these limits, but tending rather -to the value for the dunged than for the unmanured plot. -It will be seen that each acre of the dunged land loses on an -average 41,000 calories per day, while each acre of the -unmanured land loses on an average 2700 calories per day.</p> - -<p><span class="pagenum" id="Page167">[167]</span></p> - -<p class="tabhead" id="TabXV">TABLE XV.—MATERIAL BALANCE SHEET: BROADBALK SOIL, -ROTHAMSTED.<br /> -<span class="fsize90">(<span class="smcap">Lb. per Acre per Annum.</span>)</span></p> - -<table class="standard"> - -<tr class="bt bb"> -<th rowspan="2" class="bl br"> </th> -<th colspan="2" class="br">Farmyard<br />Manure<br />Added.</th> -<th colspan="2" class="br">No<br />Manure<br />Added.</th> -</tr> - -<tr class="bb"> -<th class="w4 br">C.</th> -<th class="w4 br">N.</th> -<th class="w4 br">C.</th> -<th class="w4 br">N.</th> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Added in farmyard manure</span></td> -<td class="general br">3600</td> -<td class="general br">200</td> -<td class="general br">nil</td> -<td class="general br">nil</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Added in stubble</span></td> -<td class="general br bb"> 300</td> -<td class="general br bb">  3</td> -<td class="general br bb">100</td> -<td class="general br bb">1</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl4 padr2">Total added</span></td> -<td class="general br">3900</td> -<td class="general br">203</td> -<td class="general br">100</td> -<td class="general br">1</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Taken from soil</span></td> -<td class="general br">nil</td> -<td class="general br">nil</td> -<td class="general br">200</td> -<td class="general br">nil</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Stored in soil</span></td> -<td class="general br bb"> 200</td> -<td class="general br bb"> 30</td> -<td class="general br bb">nil</td> -<td class="general br bb">nil</td> -</tr> - -<tr> -<td class="text bl br"><span class="padl4 padr2">Lost from soil</span></td> -<td class="general br">3700</td> -<td class="general br">170</td> -<td class="general br">300</td> -<td class="general br">nil<a id="FNanchor8" href="#Footnote8" class="fnanchor">[H]</a></td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padl2 padr2">Per cent.</span></td> -<td class="general br">  95</td> -<td class="general br"> 84</td> -<td class="general br">100</td> -<td class="general br">nil</td> -</tr> - -<tr> -<td colspan="5" class="text"><span class="padl6">Initial C : N ratio in farmyard manure, 18 : 1</span></td> -</tr> - -<tr> -<td colspan="5" class="text"><span class="padl6">Final C : N ratio in soil, 10 : 1.</span></td> -</tr> - -<tr> -<td colspan="5" class="text"><span class="padl6"><a id="Footnote8" href="#FNanchor8" class="label">[H]</a> -Gain of 6 lb. See <a href="#Page173">p. 173</a>.</span></td> -</tr> - -</table> - -<p class="tabhead" id="TabXVI">TABLE XVI.—ANNUAL ENERGY CHANGES IN SOIL: BROADBALK. -APPROXIMATE VALUES ONLY.<br /> -<span class="fsize90"><span class="smcap">Millions of Kilo Calories per Acre per Annum.</span></span></p> - -<table class="tabxvi"> - -<tr class="bt bb"> -<th class="bl br"> </th> -<th colspan="2" class="br">Farmyard<br />Manure<br />Added.</th> -<th colspan="2" class="br">No<br />Manure<br />Added.</th> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Added in manure</span></td> -<td class="numbers rght">14</td> -<td class="br"> </td> -<td class="numbers rght">nil</td> -<td class="br"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Added in stubble</span></td> -<td class="numbers rght bb">2</td> -<td class="br bb"> </td> -<td class="numbers rght bb">0·3</td> -<td class="br bb"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padl2 padr2">Total added</span></td> -<td class="numbers rght">16</td> -<td class="br"> </td> -<td class="numbers rght">0·3</td> -<td class="br"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Taken from soil</span></td> -<td class="numbers rght">nil</td> -<td class="br"> </td> -<td class="numbers rght">0·5</td> -<td class="numbers lft br">-1</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Stored in soil</span></td> -<td class="numbers rght bb">0·5</td> -<td class="numbers lft br bb">-1</td> -<td class="numbers rght bb">nil</td> -<td class="br bb"> </td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padl2 padr2">Dissipated per annum</span></td> -<td class="numbers rght">15</td> -<td class="br"> </td> -<td class="numbers rght">1</td> -<td class="br"> </td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Per day: calories</span></td> -<td colspan="2" class="general br">41,000</td> -<td colspan="2" class="general br">2700</td> -</tr> - -<tr> -<td class="text bl br"><span class="padr2">Equivalent to</span></td> -<td colspan="2" class="general br">12 men.</td> -<td colspan="2" class="general br"><sup>3</sup>⁄<sub>4</sub> man.</td> -</tr> - -<tr class="bb"> -<td class="text bl br"><span class="padr2">The human food grown provides for</span></td> -<td colspan="2" class="general br"> 2 men.</td> -<td colspan="2" class="general br"><sup>1</sup>⁄<sub>2</sub> man.</td> -</tr> - -</table> - -<p>These numbers are interesting when we reflect that the -human food produced on the dunged land yields only 7000<span class="pagenum" id="Page168">[168]</span> -calories per day, from which it is clear that our agricultural -efforts so far provide more energy for the soil population, -for which it was not intended, than for ourselves.</p> - -<p>The account is not complete; we have omitted all -reference to the oxidation of ammonia and of elements other -than carbon. Nature seems to be in an unexpectedly -economical mood in the soil, and all compounds which can -be oxidised with liberation of energy seem to have corresponding -organisms capable of utilising them. Even phenol, -benzene, hydrogen, and marsh gas can all be oxidised and -utilised as energy sources by some of the soil population.</p> - -<p>Even with this remarkable power the soil population -has insufficient energy to satisfy all its possibilities; our -present knowledge indicates that energy supply is, in this -country at any rate, the factor limiting the numbers of the -population. Increases in the water supply or the temperature -of the soil produce no consistent effect on the population, -but directly the energy supply is increased the numbers at -once rise.</p> - -<h3><span class="smcap">Material Changes.</span></h3> - -<p>These transformations of energy involve transformations -of matter. The original plant residues may be divided -roughly into substances forming the structure of the plant, -such as the hemicelluloses, the pentosans, gums, and the -contents of the cell—the protoplasm and the storage products, -protein; in addition, there are smaller quantities of -fats and waxes and other constituents. Some of the easily-decomposable -carbohydrates never reach the soil at all, -being broken down by intracellular respiration or attack of -micro-organisms. But much of the structure material—hemicelluloses, -pentosans, etc.—remains.</p> - -<p>Once the plant residues pass through the earthworm -bodies they become completely disintegrated and lose all -signs of structure.</p> - -<p>The only visible product so far known is humus, the -black sticky substance characteristic of soil and of manure.<span class="pagenum" id="Page169">[169]</span> -Two modes of formation have been suggested. Carbohydrates, -sugars, pentosans, etc., are known to yield furfuraldehyde -or hydroxymethylfurfuraldehyde on decomposition, -and it has been shown at Rothamsted that this readily -condenses to form a humus-like body, if not humus itself. -In the laboratory the reaction is effected in presence of acid, -but even amino-acids suffice. All the necessary conditions -occur in the soil, and humus formation may proceed in this -way.</p> - -<p>Some of the structure material—the lignin—contains -aromatic ring groupings. Fischer and Schrader have shown -that in alkaline conditions these ring substances absorb -oxygen and form something very like humus. It is quite -possible that humus formation also proceeds in the soil -in this way. Whether the two products are chemically -identical is not known.</p> - -<p>The scheme can be represented <span class="nowrap">thus:—</span></p> - -<div class="container w40em"> - -<img src="images/illo177.png" alt="" /> - -<div class="illotext"> - -<table class="tree1"> - -<tr> -<td colspan="3">Cell structure material</td> -</tr> - -<tr> -<td>Aliphatic (Hemicelluloses, Pentosans, etc.)</td> -<td> </td> -<td>Aromatic (Lignin, etc., in presence of oxygen and under aerobic conditions)</td> -</tr> - -<tr> -<td>Fatty acids</td> -<td>Furfuraldehyde or Hydroxymethylfurfuraldehyde (in presence of acid)</td> -<td> </td> -</tr> - -<tr> -<td>Calcium carbonate.</td> -<td> </td> -<td>Humus.</td> -</tr> - -</table> - -</div><!--illotext--> - -</div><!--container--> - -<p>The disintegration of the cell and the first stages in the -decomposition of the structure material are almost certainly -brought about by micro-organisms. Whether they -complete the process is not known: purely chemical agencies -could easily account for part.</p> - -<p>The decomposition of protein in the soil has not been<span class="pagenum" id="Page170">[170]</span> -studied in any detail. From what is known of the acid -hydrolysis and the putrefactive decompositions, however, it -is not difficult to draw up a scheme which, at any rate, -accords with the facts at present known. It is probable -that the protein gives rise to amino-acids, which then break -down by one of the known general reactions.</p> - -<p>Two types of non-nitrogenous products may be expected: -The aliphatic amino-acids give rise to ammonia and fatty -acids; these form calcium salts which break down to calcium -carbonate. The aromatic amino-acids—tyrosin, phenylalanine, -etc.—which would account for about 6 per cent. -of the nitrogen of vegetable proteins, would be expected to -give ammonia and phenolic substances. Now phenols are -poisonous to plants and if no method existed for their -removal the accumulation would ultimately render the soil -sterile. Matters would be even worse on cultivated soils, -since cows’ urine, which enters into the composition of -farmyard manure and is the chief constituent of liquid -manure, contains, according to Mooser, no less than 0·25 -to 0·77 grams of <i>p</i>-cresol per litre,<a id="FNanchor9" href="#Footnote9" class="fnanchor">[I]</a> a quantity three to ten -times that present in human urine. Fortunately this -contingency never arises, for the soil contains a remarkable -set of organisms capable of decomposing the phenols and -leaving the soil entirely suitable for plant growth. This -affords an interesting case of an organism—in this case -the plant—growing well in a medium in spite of some adverse -condition, not because it is specially adapted to meet -this condition, but because some wholly different agent removes -it.</p> - -<div class="footnote"> - -<p><a id="Footnote9" href="#FNanchor9" class="label">[I]</a> Mooser, Zeitschrift physiol. -Chem., 1909, lxiii., 176. No phenol was -found. It is possible that the <i>p</i>-cresol is not entirely derived from the protein, -but that some comes from the glucosides in the animals’ food.</p> - -</div><!--footnote--> - -<p>Other ring compounds, e.g. pyrrol, arise in smaller -quantity in the decomposition of protein, but their fate in -the soil is not known.</p> - -<p>We may summarise the probable changes of the protein -as <span class="nowrap">follows:—</span></p> - -<p><span class="pagenum" id="Page171">[171]</span></p> - -<div class="container w40em"> - -<img src="images/illo179.png" alt="" /> - -<div class="illotext"> - -<table class="tree2"> - -<tr> -<td colspan="3"> </td> -<td colspan="2">Protein.</td> -<td colspan="2"> </td> -</tr> - -<tr> -<td> </td> -<td colspan="2">Aliphatic<br />amino-acids</td> -<td colspan="2">Aromatic<br />amino-acids</td> -<td colspan="2">Other<br />compounds<br />(Pyrrol, etc.)</td> -</tr> - -<tr> -<td colspan="2">Fatty acids and<br />hydroxy acids</td> -<td colspan="2">Ammonia</td> -<td colspan="2">Phenolic<br />compounds</td> -<td> </td> -</tr> - -<tr> -<td colspan="2"> </td> -<td colspan="2">Nitrite</td> -<td colspan="3"> </td> -</tr> - -<tr> -<td colspan="2">Calcium<br />carbonate</td> -<td colspan="2">Nitrate</td> -<td colspan="2">CO<sub>2</sub></td> -<td> </td> -</tr> - -</table> - -</div><!--illotext--> - -</div><!--container--> - -<p>It must be admitted that the evidence is indirect. The -rate of oxidation of ammonia by bacteria in the soil is -more rapid than the rate of formation, so that ammonia is -practically never found in the soil in more than minimal -amounts (1 or 2 parts per 1,000,000); indeed, the only -evidence of its formation was for a long time the fact that -no compound other than ammonia could be oxidised by the -nitrifying organism. It has, however, since been shown -at Rothamsted that ammonia accumulates in soils in which -the nitrifying organism has been killed.</p> - -<p>Nothing is known of the mechanism of the oxidation of -ammonia beyond the fact that it is biological; the reaction -is not easily effected chemically at ordinary temperatures. -Possibly the organism assimilates ammonia at one end of a -chain of metabolic processes and excretes nitrates at the other. -Or, the reaction may be simply a straight oxidation for energy -purposes, the ammonia changing to hydroxylamine and then -to nitrous and nitric acids.</p> - -<p>The nitrate does not remain long in the soil. Some is -taken up by the plant and some is washed out from the soil. -Part, however, either of the nitrate itself or of one of its -precursors is converted into an insoluble form: probably it -is changed into protein by the action of micro-organisms; -it then goes through the whole process once more.</p> - -<p>These are the general outlines; they present no particular -chemical difficulties. When we come to details, -however, there is much that cannot be understood.</p> - -<p><span class="pagenum" id="Page172">[172]</span></p> - -<p>First of all, there is the slow rate at which complex -nitrogen compounds disappear from the soil in comparison -with the rate of oxidation of the carbon. Thus, in the -original plant residues, there is some forty times as much -carbon as nitrogen: before they have been long in the soil -there is only ten times as much carbon as nitrogen; this -seems to be the stable position. What is the reason for this -preferential oxidation of the carbon? No explanation can -yet be given.</p> - -<div class="container w45em" id="Fig24"> - -<img src="images/illo180.png" alt="" /> - -<p class="caption"><span class="smcap">Fig. 24.</span></p> - -<div class="illotext w20em"> - -<p>X-axis: 1887-8 1890-1 1900-1 1910-11</p> - -<p>Y-axis: ℔ per acre</p> - -</div><!--illotext--> - -</div><!--container--> - -<p>An equally difficult problem arises in connection with -the length of time the process will continue. Decomposition -of the nitrogen compounds never seems to be complete<span class="pagenum" id="Page173">[173]</span> -in the soil; it dribbles on interminably. In the year 1870 -Lawes and Gilbert cut off a block of soil from its surroundings -and undermined it so that the drainage water could be -collected and analysed. The soil has been kept free from -vegetation or addition of nitrogen compounds from that time -till now; yet it has never failed to yield nitrates, and the -annual yield falls off only very slowly (<a href="#Fig24">Fig. 24</a>). This -same peculiarity is seen in the yield of crops on unmanured -land: it decreases, but very gradually; even after eighty -years the process is far from complete, and there is no sign -that it will ever come to an end.</p> - -<p class="tabhead" id="TabXVII">TABLE XVII.—APPROXIMATE LOSS OF NITROGEN FROM -CULTIVATED SOILS: BROADBALK WHEAT FIELD, -ROTHAMSTED, FORTY-NINE YEARS (1865-1914.)</p> - -<table class="nloss"> - -<tr class="bt bb"> -<th class="bl br"> </th> -<th colspan="2" class="br">Rich Soil:<br />Plot 2.<br />Lb. per Acre.</th> -<th colspan="2" class="br">Poor Soil:<br />Plot 3.<br />Lb. per Acre.</th> -</tr> - -<tr> -<td class="text bl br">Nitrogen in soil in 1865</td> -<td colspan="2" class="percents">·175 per cent. = 4340</td> -<td colspan="2" class="percents">·105 per cent. = 2720</td> -</tr> - -<tr> -<td class="text bl br">Nitrogen added in manure, rain (5 lb. per annum), and seed (2 lb. per annum)</td> -<td class="numbers bb">10,140</td> -<td class="note bb"> </td> -<td class="numbers bb">340</td> -<td class="note bb"> </td> -</tr> - -<tr> -<td class="text bl br">Nitrogen expected in 1914</td> -<td class="numbers">14,480</td> -<td class="note"> </td> -<td class="numbers">3060</td> -<td class="note"> </td> -</tr> - -<tr> -<td class="text bl br">Nitrogen found in 1914</td> -<td colspan="2" class="percents bb">·259 per cent. = 5950</td> -<td colspan="2" class="percents bb">·095 per cent. = 2590</td> -</tr> - -<tr> -<td class="text bl br">Loss from soil</td> -<td class="numbers">8530</td> -<td rowspan="2" class="note bb"> </td> -<td class="numbers">470</td> -<td rowspan="2" class="note bb"> </td> -</tr> - -<tr> -<td class="text bl br">Nitrogen accounted for in crops</td> -<td class="numbers bb">2500</td> -<td class="numbers bb">750</td> -</tr> - -<tr> -<td class="text bl br">Balance, being dead loss</td> -<td class="numbers">6030</td> -<td class="note"> </td> -<td class="numbers">-280</td> -<td class="note"><a id="FNanchor10" href="#Footnote10" class="fnanchor">[J]</a></td> -</tr> - -<tr class="bb"> -<td class="text bl br">Annual dead loss</td> -<td class="numbers">123</td> -<td class="note"> </td> -<td class="numbers">-  6</td> -<td class="note"><a href="#Footnote10" class="fnanchor">[J]</a></td> -</tr> - -<tr> -<td colspan="5" class="text"><a id="Footnote10" href="#FNanchor10" class="label">[J]</a> Gains. -Possibly the result of bacterial action.</td> -</tr> - -</table> - -<p>A further remarkable fact connected with the decomposition -of the nitrogen compounds is that it seems invariably -to be accompanied by an evolution of gaseous -nitrogen. Apparently there are two cases. Under anaerobic -conditions many of the soil organisms have the power of -obtaining their necessary oxygen from nitrates, thereby -causing a change in the molecule which leads in some cases -to liberation of gaseous nitrogen; but the same result seems<span class="pagenum" id="Page174">[174]</span> -to be attained in aerobic conditions, especially when carbon -is being rapidly oxidised.</p> - -<p>It is possible that the reaction is the same, and that -in spite of the general aerobic conditions there is locally an -anaerobic atmosphere. But it is also possible that some -direct oxidation of protein or amino-acids may yield gaseous -nitrogen. However it is brought about it affects a considerable -proportion of the entire stock of nitrogen, and it becomes -more serious as cultivation is intensified. Thus, on -the Broadbalk plot receiving farmyard manure the loss is -particularly heavy; on the unmanured plot it cannot be -detected. The nitrogen balance-sheet is shown in <a href="#TabXVII">Table -XVII.</a></p> - -<p>The oxidation of carbonaceous matter, however, is not -invariably accompanied by a net loss of nitrogen; in other -circumstances there is a net gain. In natural conditions -there seems always to have been some leguminous vegetation -growing; the gain may, therefore, be ascribed to the activity -of the nodule organism. In pot experiments, however, -it has been found possible, by adding sugar to the soil, to -obtain gains of nitrogen where there is no leguminous vegetation, -and this is attributed to the activity of Azotobacter.</p> - -<p>The nitrogen cycle as observed in the soil is as <span class="nowrap">follows:—</span></p> - -<div class="container w40em"> - -<img src="images/illo182.png" alt="Cycle" /> - -<div class="illotext"> - -<table class="ncycle"> - -<tr> -<td colspan="4">Protein</td> -</tr> - -<tr> -<td rowspan="3">By certain organisms and by growing plants</td> -<td>Ammonia</td> -<td rowspan="2">Mechanism uncertain</td> -<td rowspan="3">By Azotobacter, Clostridium, nodule organisms, etc.</td> -</tr> - -<tr> -<td>Nitrite</td> -</tr> - -<tr> -<td>Nitrate</td> -<td>Gaseous Nitrogen</td> -</tr> - -<tr> -<td colspan="4">By denitrifying organisms</td> -</tr> - -</table> - -</div><!--illotext--> - -</div><!--container--> - -<p><span class="pagenum" id="Page175">[175]</span></p> - -<p>There has been but little study of the process of decomposition -of the other compounds in plants. Part, if not all, -of the sulphur is known to appear as sulphate, and some -of the phosphorus as phosphate. It is certain that the -plant constituents decompose, for there is no sign of their -accumulation in the soil. They may exert transitory effects, -but there is nothing to show permanent continuance. The -toxic conditions which cause trouble in working with pure -cultures of organisms in specific cultures media do not, so far -as is known, arise in the soil. All attempts to find bacterio-toxins -or plant toxins in normal soils have failed. The product -toxic to one organism seems to be a useful nutrient -to another, and so the mixed population keeps the soil -healthy for all its members.</p> - -<p>There is little precise knowledge as to the part played -by the different members of the soil population in bringing -about these changes.</p> - -<p>We know in a general way that earthworms effect the -distribution of the plant residues in the soil, and serve to -disintegrate them; there is no evidence, however, that -they play any indispensable part in the decomposition. -Many root and other fragments do not go through this -process; observation shows that fungi can force a way in, -and they may be followed by nematodes which continue the -disintegration. Possibly some of the flagellates help, and -certainly the bacteria do. After that nothing is certain. -We cannot, with certainty, assign any particular reaction -in the decomposition to any specific organism, with the -exception of the oxidation of the phenolic substances, the -conversion of ammonia to nitrite and nitrate, and the fixation -of nitrogen. With these exceptions many organisms -seem capable of bringing about the reactions, and indeed -some of the reactions may be purely chemical and independent -of biological agencies.</p> - -<p>The relationships between the soil population and soil -fertility are readily stated in general outline, but they -are by no means clear cut when one comes to details;<span class="pagenum" id="Page176">[176]</span> -fertility is a complex property, and some of its factors are -independent of soil micro-organisms.</p> - -<p>The general relationship between plants and soil organisms -is one of complete mutual interdependence. The -growing plant fixes the sun’s energy and converts it into a -form utilisable by the soil organisms; without the plant -they could not exist. The plant is equally dependent on -the soil organisms in at least two directions: their scavenging -action removes the dead vegetation which would, if -accumulated on the surface of the soil, effectively prevent -most plants from growing. Further, the plant is dependent -on the soil population for supplies of nitrates. Nothing -is known about the relative efficiencies of the various soil -organisms as scavengers. Numerous fungi and bacteria are -effective producers of ammonia, the precursor of nitrates; -it is not known, however, whether flagellates and such -higher forms as nematodes act in this way.</p> - -<p>This widespread power of producing ammonia makes it -impossible in our present knowledge to regard any particular -group of organisms as <i>par excellence</i> promoters of fertility. -Indeed, it is safest not to attempt to do so. The primary -purpose of the activities of a soil organism is to obtain -energy and cell material for itself; any benefit to the plant -is purely incidental. For cell material it must have nitrogen -and phosphorus; here it competes with the plant. -If it produces more ammonia than it utilises—in other words, -if it is driven to nitrogen compounds for its energy, then the -plant benefits. If, on the other hand, it absorbs more -ammonia than it produces, as happens when it derives its -energy from non-nitrogenous substances, the plant suffers. -Thus, addition of peptone to the soil or an increase in bacterial -numbers effected without addition of external energy -(e.g. by partial sterilisation) leads to increased ammonia -supply, and, therefore, to increased fertility. But addition -of sugar to the soil causes so great an increase of numbers -of bacteria and other organisms that considerable absorption<span class="pagenum" id="Page177">[177]</span> -of ammonia and nitrate occurs, and fertility is for a time -depressed.</p> - -<p>Both actions proceed in soils partially sterilised by -organic substances, such as phenol, which are utilised -by some of the soil organisms; there is first a great rise -in numbers of these particular organisms with a depression -of ammonia and nitrate, then a drop to the new level, -higher than the old one, and an increased production of -ammonia and nitrate resulting from the partial sterilisation -effects.</p> - -<p>We must then regard the soil population as concerned -entirely to maintain itself, and only incidently benefiting -the plant, sometimes, indeed, injuring it; always essential, -yet always taking its toll, and sometimes a heavy toll, of -the plant nutrients it produces.</p> - -<p>This effect makes it difficult to deduce simple quantitative -relationships between bacterial activity and soil fertility, -and the difficulty is increased by the fact that bacteria and -plants may both be injured or benefited by the same -causes, so that high bacterial numbers in a fertile soil would -not necessarily be the cause, but might be simply the result -of fertility.</p> - -<p>The circumstance that certain soil organisms—bacteria, -algæ, and fungi—themselves assimilate ammonia and nitrate -may account for the remarkable slowness of nitrate accumulation, -to which reference has already been made. The -protein formed from the assimilated nitrogen remains in the -bodies of the organisms, living or dead, till decomposition -sets in. It is not difficult to picture a cycle of events in -which much of the nitrate formed is at once reabsorbed -by other organisms, and only little is actually thrown off -into the soil. Such a process might continue almost -interminably so long as any carbonaceous material remained.</p> - -<p>Finally, we come to the very interesting problem—is it -possible to control the population of the soil?</p> - -<p>The problem may seem superfluous in view of the<span class="pagenum" id="Page178">[178]</span> -difficulties just mentioned. Some aspects of it, however, -are fairly clearly defined.</p> - -<p>In the first instance, some organisms appear to be -wholly harmful to the plant; among them are parasitic -eelworms and fungi, and bacteria causing disease.</p> - -<p>Control of these organisms can be brought about by -partial sterilisation, and of all methods heat is the most -effective, but it is costly, and attempts are now being -made to replace it by chemical treatment. The results are -promising, but the investigation is laborious; the organisms -show specific relationships, and in finding a sufficiently -potent and convenient poison it is necessary in each case -to make an investigation into the relationship between -chemical constitution and toxicity to the particular organism -concerned. Formaldehyde is usually potent against fungi, -and the cresols, and particularly their chlor- and chloronitro-derivatives, -are potent against animals (eelworms, etc.).</p> - -<p>One group of organisms is wholly beneficial, those -associated with leguminous plants. Attempts have been -made to increase their activities by inoculating the soil -with more vigorous strains. The practical difficulties -still remain very considerable, but there is hope that they -may be overcome.</p> - -<p>It is also possible to shift the balance of the soil population -in certain directions. Special groups of soil organisms -can be caused to multiply temporarily, if not permanently, -by satisfying their particular requirements. Thus, when a -soil has been heated above 100° C. it becomes specially -suited to the growth of fungi, and quite unsuited to certain -bacteria such as the nitrifying organisms and others; if -this heated soil is infected with a normal soil population -the fungi develop to a remarkable extent. The nodule -organisms appear to be stimulated by addition of farmyard -manure and of phosphates, and the phenol-destroying -organisms by successive small additions of phenol.</p> - -<p>Finally, quite apart from the control of disease organisms, -it is possible to alter the soil population considerably by<span class="pagenum" id="Page179">[179]</span> -partial sterilisation, using a temperature of only about -60° C., or a poison like toluene that favours few of the -soil organisms. This problem has already been discussed -in <a href="#Page1">Chapter I.</a></p> - -<p>The control of the soil population is still only in its -infancy, but it already promises useful developments. It -cannot, however, be too strongly insisted that the only -sure basis of control is knowledge, and we cannot hope to -push control further till we have learned much more about the -soil population than we know at present.</p> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum"><a id="Page180"></a>[180-<br />181] -<a id="Page181"></a></span></p> - -<h2 class="nobreak">AUTHOR INDEX.</h2> - -</div><!--chapter--> - -<ul class="index"> - -<li class="newletter"><span class="smcap">Adametz</span>, <a href="#Page118">118</a>.</li> -<li>Adams, <a href="#Page158">158</a>.</li> -<li>Aiyer, <a href="#Page113">113</a>.</li> -<li>Appel, <a href="#Page133">133</a>.</li> -<li>Artari, <a href="#Page107">107</a>.</li> -<li>Ashby, <a href="#Page41">42</a>.</li> - -<li class="newletter"><span class="smcap">Barthel</span>, <a href="#Page24">24</a>.</li> -<li>Beijerinck, <a href="#Page6">6</a>, <a href="#Page37">37</a>, <a href="#Page41">41</a>, <a href="#Page42">42</a>, -<a href="#Page46">46</a>, <a href="#Page107">107</a>.</li> -<li>Berthelot, <a href="#Page5">5</a>, <a href="#Page6">6</a>, <a href="#Page41">41</a>.</li> -<li>Bewley, <a href="#Page47">47</a>, <a href="#Page51">51</a>, <a href="#Page132">132</a>.</li> -<li>Bezssonoff, <a href="#Page69">69</a>.</li> -<li>Boas, <a href="#Page138">138</a>.</li> -<li>Bokorny, <a href="#Page138">138</a>.</li> -<li>Bonazzi, <a href="#Page45">45</a>.</li> -<li>Boresch, <a href="#Page107">107</a>.</li> -<li>Boussingault, <a href="#Page3">3</a>.</li> -<li>Bredemann, <a href="#Page24">24</a>.</li> -<li>Bristol, <a href="#Page106">106</a>.</li> -<li>Brizi, <a href="#Page113">113</a>.</li> -<li>Brown and Halversen, <a href="#Page127">127</a>.</li> -<li>Burgess, <a href="#Page41">41</a>.</li> -<li>Burrill, <a href="#Page48">48</a>.</li> -<li>Bussey, Peters and Ulrich, <a href="#Page132">132</a>.</li> -<li>Butkevitch, <a href="#Page138">138</a>, <a href="#Page139">139</a>.</li> -<li>Butler, <a href="#Page132">132</a>.</li> - -<li class="newletter"><span class="smcap">Cameron</span>, <a href="#Page150">150</a>, <a href="#Page158">158</a>.</li> -<li>Chodat, <a href="#Page107">107</a>.</li> -<li>Christensen, <a href="#Page46">46</a>.</li> -<li>Clayton, <a href="#Page28">28</a>, <a href="#Page43">43</a>.</li> -<li>Coleman, <a href="#Page127">127</a>, <a href="#Page136">136</a>.</li> -<li>Conn, <a href="#Page23">23</a>, <a href="#Page54">54</a>, <a href="#Page61">61</a>, <a href="#Page123">123</a>.</li> -<li>Cramer, <a href="#Page39">39</a>.</li> -<li>Crump, <a href="#Page57">57</a>, <a href="#Page79">79</a>, <a href="#Page80">80</a>.</li> -<li>Cunningham, <a href="#Page69">69</a>.</li> -<li>Cutler, <a href="#Page57">57</a>, <a href="#Page58">58</a>, <a href="#Page78">78</a>, <a href="#Page80">80</a>.</li> - -<li class="newletter"><span class="smcap">Dale</span>, <a href="#Page118">118</a>, <a href="#Page121">121</a>.</li> -<li>Darwin, <a href="#Page153">153</a>.</li> -<li>Dascewska, <a href="#Page134">134</a>.</li> -<li>De Bruyn, <a href="#Page132">132</a>.</li> -<li>van Delden, <a href="#Page42">42</a>.</li> -<li>Delf, <a href="#Page87">87</a>.</li> -<li>Doryland, <a href="#Page33">33</a>, <a href="#Page40">40</a>.</li> -<li>Dox, <a href="#Page138">138</a>.</li> -<li>Dox and Neidig, <a href="#Page134">134</a>.</li> -<li>Drummond, <a href="#Page160">160</a>.</li> -<li>Duggar and Davis, <a href="#Page135">135</a>.</li> -<li>Duvaine, <a href="#Page20">20</a>.</li> - -<li class="newletter"><span class="smcap">Ehrenberg</span>, <a href="#Page138">138</a>.</li> -<li>Ehrlich and Jacobsen, <a href="#Page138">138</a>.</li> -<li>Esmarch, <a href="#Page102">102</a>, <a href="#Page103">103</a>.</li> - -<li class="newletter"><span class="smcap">Fabricius</span>, <a href="#Page61">61</a>.</li> -<li>Feilitzen, <a href="#Page61">61</a>.</li> -<li>Fischer, <a href="#Page118">118</a>, <a href="#Page126">126</a>.</li> -<li>Forte, <a href="#Page101">101</a>.</li> -<li>Frank, <a href="#Page132">132</a>.</li> -<li>Fritsch, <a href="#Page112">112</a>.</li> - -<li class="newletter"><span class="smcap">Gainey</span>, <a href="#Page46">46</a>.</li> -<li>Gillespie and Hurst, <a href="#Page140">140</a>.</li> -<li>Goddard, <a href="#Page118">118</a>, <a href="#Page120">120</a>, <a href="#Page121">121</a>.</li> -<li>Golding, <a href="#Page49">49</a>.</li> -<li>Goodey, <a href="#Page68">68</a>, <a href="#Page73">73</a>, <a href="#Page79">79</a>, <a href="#Page105">105</a>.</li> -<li>Greaves, <a href="#Page42">42</a>, <a href="#Page61">61</a>.</li> -<li>Green, <a href="#Page37">37</a>.</li> -<li>Grintzesco, <a href="#Page107">107</a>.</li> -<li>Groenewege, <a href="#Page42">42</a>.</li> - -<li class="newletter"><span class="smcap">Hagem</span>, <a href="#Page118">118</a>, <a href="#Page121">121</a>, <a href="#Page136">136</a>, -<a href="#Page138">138</a>, <a href="#Page139">139</a>.</li> -<li>Hamilton, <a href="#Page158">158</a>, <a href="#Page159">159</a>.</li> -<li>Hansen, <a href="#Page48">48</a>.</li> -<li>Hanzawa, <a href="#Page43">43</a>.</li> -<li>Harrison, <a href="#Page113">113</a>.</li> -<li>Heinze, <a href="#Page139">139</a>.</li> -<li>Hellriegel, <a href="#Page5">5</a>, <a href="#Page6">6</a>, <a href="#Page46">46</a>.</li> -<li>Hensen, <a href="#Page100">100</a>.</li> -<li>Hesselmann, <a href="#Page36">36</a>.</li> -<li>Hill, <a href="#Page94">94</a>.</li> -<li>Hiltner, <a href="#Page23">23</a>.</li> -<li>Hopkins, <a href="#Page36">36</a>.</li> -<li>Hutchinson, C. M., <a href="#Page42">42</a>.</li> -<li>Hutchinson, H. B., <a href="#Page27">27</a>, <a href="#Page43">43</a>, <a href="#Page47">47</a>, <a href="#Page51">51</a>, -<a href="#Page57">57</a>, <a href="#Page105">105</a>.</li> - -<li class="newletter"><span class="smcap">van Iterson</span><span class="pagenum" id="Page182">[182]</span>, -<a href="#Page133">133</a>.</li> - -<li class="newletter"><span class="smcap">Jensen</span>, <a href="#Page118">118</a>, <a href="#Page132">132</a>.</li> -<li>Jewson, <a href="#Page123">123</a>.</li> -<li>Joffe, <a href="#Page37">37</a>.</li> -<li>Jones, D. H. and Murdock, <a href="#Page127">127</a>.</li> -<li>Jones, L. R., <a href="#Page140">140</a>.</li> - -<li class="newletter"><span class="smcap">Kappen</span>, <a href="#Page136">136</a>.</li> -<li>Karrer, <a href="#Page105">105</a>.</li> -<li>Kaserer, <a href="#Page27">27</a>.</li> -<li>Klöcker, <a href="#Page134">134</a>.</li> -<li>Koch, A., <a href="#Page44">44</a>, <a href="#Page134">134</a>.</li> -<li>Koch, R., <a href="#Page20">20</a>, <a href="#Page53">53</a>.</li> -<li>Kofoid, <a href="#Page88">88</a>.</li> -<li>Kohshi, <a href="#Page134">134</a>.</li> -<li>Kopeloff, <a href="#Page118">118</a>, <a href="#Page136">136</a>.</li> -<li>Kossowitsch, <a href="#Page111">111</a>.</li> -<li>Krainskii, <a href="#Page44">44</a>.</li> -<li>Krzeminiewski, <a href="#Page43">43</a>.</li> -<li>Kufferath, <a href="#Page107">107</a>.</li> - -<li class="newletter"><span class="smcap">Latham</span>, <a href="#Page135">135</a>.</li> -<li>Laurent, <a href="#Page136">136</a>.</li> -<li>Lawes and Gilbert, <a href="#Page5">5</a>.</li> -<li>Lebedeff, <a href="#Page27">27</a>.</li> -<li>Leeuwenhoeck, <a href="#Page20">20</a>.</li> -<li>Lendner, <a href="#Page118">118</a>.</li> -<li>Lipman, C. B., <a href="#Page41">41</a>, <a href="#Page42">42</a>, <a href="#Page44">44</a>, <a href="#Page54">54</a>.</li> -<li>Lipman, J. G., Blair, Owen, and McLean, <a href="#Page94">94</a>.</li> -<li>Löhnis, <a href="#Page22">22</a>, <a href="#Page43">43</a>, <a href="#Page69">69</a>, <a href="#Page136">136</a>.</li> - -<li><span class="smcap">Magnus</span>, <a href="#Page107">107</a>.</li> -<li>Malpighi, <a href="#Page46">46</a>.</li> -<li>Marchal, <a href="#Page34">34</a>, <a href="#Page136">136</a>.</li> -<li>Martin, <a href="#Page73">73</a>.</li> -<li>Martin and Lewin, <a href="#Page69">69</a>.</li> -<li>McBeth, <a href="#Page28">28</a>, <a href="#Page134">134</a>.</li> -<li>McBeth and Scales, <a href="#Page118">118</a>, <a href="#Page140">140</a>.</li> -<li>McLean and Wilson, <a href="#Page118">118</a>, <a href="#Page136">136</a>.</li> -<li>Mockeridge, <a href="#Page43">43</a>.</li> -<li>Moore, G. T., <a href="#Page105">105</a>.</li> -<li>Morris, <a href="#Page150">150</a>, <a href="#Page151">151</a>, <a href="#Page162">162</a>.</li> -<li>Muntz and Coudon, <a href="#Page118">118</a>, <a href="#Page136">136</a>.</li> - -<li class="newletter"><span class="smcap">Nabokich</span>, <a href="#Page27">27</a>.</li> -<li>Nagaoka, <a href="#Page38">38</a>.</li> -<li>Nakano, <a href="#Page107">107</a>.</li> -<li>Nasir, <a href="#Page94">94</a>, <a href="#Page95">95</a>.</li> -<li>Neller, <a href="#Page137">137</a>.</li> - -<li class="newletter"><span class="smcap">Omelianski</span>, <a href="#Page27">27</a>, <a href="#Page42">42</a>.</li> -<li>Orla-Jensen, <a href="#Page26">26</a>, <a href="#Page35">35</a>.</li> -<li>Otto, <a href="#Page133">133</a>.</li> -<li>Oudemans and Koning, <a href="#Page118">118</a>.</li> - -<li class="newletter"><span class="smcap">Pasteur</span>, <a href="#Page3">3</a>, <a href="#Page20">20</a>.</li> -<li>Perey, <a href="#Page94">94</a>.</li> -<li>Perotti, <a href="#Page136">136</a>.</li> -<li>Petersen, <a href="#Page104">104</a>.</li> -<li>Pillai, <a href="#Page43">43</a>.</li> -<li>Potter and Snyder, <a href="#Page137">137</a>, <a href="#Page138">138</a>.</li> -<li>Povah, <a href="#Page138">138</a>.</li> -<li>Pratt, <a href="#Page132">132</a>.</li> -<li>Prescott, <a href="#Page61">61</a>.</li> -<li>Pringsheim, <a href="#Page107">107</a>.</li> - -<li class="newletter"><span class="smcap">Ramann</span>, <a href="#Page118">118</a>.</li> -<li>Rathbun, <a href="#Page120">120</a>.</li> -<li>Reh, <a href="#Page162">162</a>.</li> -<li>Remy, <a href="#Page118">118</a>.</li> -<li>Richards, <a href="#Page112">112</a>.</li> -<li>Richardson, <a href="#Page159">159</a>.</li> -<li>Ritter, <a href="#Page138">138</a>.</li> -<li>Robbins, W. J., <a href="#Page109">109</a>.</li> -<li>Robbins, W. W., <a href="#Page105">105</a>.</li> -<li>Roussy, <a href="#Page134">134</a>.</li> -<li>Russell, <a href="#Page112">112</a>.</li> -<li>Russell and Hutchinson, <a href="#Page57">57</a>, <a href="#Page66">66</a>, <a href="#Page94">94</a>.</li> - -<li class="newletter"><span class="smcap">Salunskov</span>, <a href="#Page42">42</a>.</li> -<li>Sandon, <a href="#Page57">57</a>, <a href="#Page75">75</a>.</li> -<li>Scales, <a href="#Page28">28</a>, <a href="#Page134">134</a>.</li> -<li>Schellenberg, <a href="#Page133">133</a>.</li> -<li>Schindler, <a href="#Page107">107</a>.</li> -<li>Schloesing, <a href="#Page3">3</a>, <a href="#Page4">4</a>, <a href="#Page34">34</a>.</li> -<li>Schmitz, <a href="#Page134">134</a>.</li> -<li>Schramm, <a href="#Page111">111</a>.</li> -<li>Servettaz, <a href="#Page109">109</a>.</li> -<li>Seydel, <a href="#Page44">44</a>.</li> -<li>Sherman, <a href="#Page69">69</a>.</li> -<li>Shibata, <a href="#Page136">136</a>.</li> -<li>Söhngen, <a href="#Page26">26</a>, <a href="#Page27">27</a>, <a href="#Page134">134</a>.</li> - -<li class="newletter"><span class="smcap">Takahashi</span>, <a href="#Page118">118</a>.</li> -<li>Taylor, <a href="#Page120">120</a>.</li> -<li>Ternetz, <a href="#Page135">135</a>.</li> -<li>Treub, <a href="#Page112">112</a>.</li> -<li>Truffaut, <a href="#Page69">69</a>.</li> - -<li class="newletter"><span class="smcap">Verkade</span> and Söhngen, <a href="#Page134">134</a>.</li> -<li>Von Ubisch, <a href="#Page109">109</a>.</li> - -<li class="newletter"><span class="smcap">Waksman</span>, <a href="#Page37">37</a>, <a href="#Page118">118</a>, -<a href="#Page120">120</a>, -<a href="#Page121">121</a>, <a href="#Page123">123</a>, <a href="#Page125">125</a>, <a href="#Page126">126</a>, -<a href="#Page134">134</a>, <a href="#Page136">136</a>.</li> -<li>Waksman and Cook, <a href="#Page136">136</a>.</li> -<li>Wann, <a href="#Page111">111</a>.</li> -<li>Warington, <a href="#Page4">4</a>, <a href="#Page34">34</a>.</li> -<li>Waynick, <a href="#Page42">42</a>.</li> -<li>Welwitsch, <a href="#Page112">112</a>.</li> -<li>Werkenthin, <a href="#Page120">120</a>, <a href="#Page121">121</a>.</li> -<li>West, <a href="#Page88">88</a>, <a href="#Page105">105</a>.</li> -<li>Whiting, <a href="#Page36">36</a>.</li> -<li>Wilfarth, <a href="#Page46">46</a>.</li> -<li>Winogradsky, <a href="#Page4">4</a>, <a href="#Page6">6</a>, <a href="#Page34">34</a>, <a href="#Page41">41</a>, -<a href="#Page44">44</a>.</li> - -</ul> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> - -<p><span class="pagenum" id="Page183">[183]</span></p> - -<h2 class="nobreak">SUBJECT INDEX.</h2> - -</div><!--chapter--> - -<ul class="index"> - -<li class="newletter"><i>Absidia</i>, <a href="#Page121">121</a>.</li> - -<li id="IndRef12"><i>Acarina</i>, <a href="#Page150">150</a>, <a href="#Page151">151</a>, <a href="#Page157">157</a>.</li> - -<li>Acid formation by Fungi, <a href="#Page139">139</a>.</li> - -<li>Acidity of soil, <a href="#Page17">17</a>; effect on Actinomyces, <a href="#Page140">140</a>; relation to nitrification, -<a href="#Page36">36</a>.</li> - -<li><i>Actinomycetes</i>, <a href="#Page119">119</a>, <a href="#Page134">134</a>, <a href="#Page139">139</a>.</li> - -<li>Aeration of soil, effect on bacteria of, <a href="#Page61">61</a>.</li> - -<li><i>Agriotes</i>, <a href="#Page150">150</a>.</li> - -<li>Air supply in soil, <a href="#Page17">17</a>.</li> - -<li id="IndRef6">Algæ, agents causing disappearance of nitrate from soil, <a href="#Page12">12</a>; associations of, in soil, -<a href="#Page105">105</a>, <a href="#Page106">106</a>; blue green, <a href="#Page102">102 <i>sqq.</i></a> (see also -<a href="#IndRef1">Cyanophyceæ</a> and <a href="#IndRef2">Myxophyceæ</a>); colonisation of new ground by, -<a href="#Page112">112</a>; conditions of growth for, <a href="#Page101">101</a>, <a href="#Page104">104</a>, -<a href="#Page107">107</a>, <a href="#Page108">108</a>; distribution of, <a href="#Page102">102</a>, <a href="#Page104">104</a>, -<a href="#Page106">106</a>, <a href="#Page109">109</a>; economic significance of, <a href="#Page100">100</a>, -<a href="#Page102">102</a>; filamentous, <a href="#Page106">106</a>; flora of soil, <a href="#Page101">101</a>, -<a href="#Page112">112</a>; formation of humus substances, <a href="#Page112">112</a>; fragmentation of filaments, -<a href="#Page107">107</a>, <a href="#Page110">110</a>; frequency of occurrence, <a href="#Page102">102 <i>sqq.</i></a>; glucose, -effect of, on growth, <a href="#Page108">108</a>, <a href="#Page109">109</a>; green, <a href="#Page104">104 <i>sqq.</i></a> (see -also Chlorophyceæ); importance in cultivation of rice, <a href="#Page113">113</a>; numbers in soil of, <a href="#Page109">109</a>, -<a href="#Page110">110</a>; nutrition of, <a href="#Page107">107</a>, <a href="#Page108">108</a>, <a href="#Page110">110</a>; -producers, of organic substance, <a href="#Page100">100</a>; pure cultures of, <a href="#Page107">107</a>, -<a href="#Page111">111</a>; relation to gaseous interchange in soil, <a href="#Page113">113</a>; relation to soil moisture, -<a href="#Page112">112</a>; seasonal changes in numbers of, <a href="#Page88">88</a>; subterranean, <a href="#Page105">105</a>.</li> - -<li>Alkaloids, as source of nitrogen for fungi, <a href="#Page138">138</a>.</li> - -<li><i>Alternaria</i>, <a href="#Page119">119</a>.</li> - -<li>Amino-acids, formation of, by algæ, <a href="#Page108">108</a>.</li> - -<li>Amino-compounds, decomposition of, by fungi, <a href="#Page136">136</a>, <a href="#Page138">138</a>.</li> - -<li>Ammonia, assimilation of, by bacteria, <a href="#Page33">33</a>, <a href="#Page40">40</a>, <a href="#Page45">45</a>; effect of -partial sterilisation on soil content of, <a href="#Page66">66</a>; formation in soil, <a href="#Page170">170</a>; formation in soil -by bacteria, <a href="#Page32">32 <i>sqq.</i></a>; formation in soil by fungi, <a href="#Page135">135 <i>sqq.</i></a>, -<a href="#Page141">141</a>; influence of physical conditions on formation of, <a href="#Page137">137</a>; property of attracting -Diptera, <a href="#Page159">159</a>; utilisation by higher plants, <a href="#Page36">36</a>.</li> - -<li>Ammonium sulphate, effect on fungi, <a href="#Page121">121</a>, <a href="#Page126">126</a>, <a href="#Page127">127</a>.</li> - -<li><i>Anabæna</i>, <a href="#Page102">102</a>, <a href="#Page112">112</a>.</li> - -<li><i>Annelida</i>, <a href="#Page149">149</a>.</li> - -<li>Antagonism of salts in soil, <a href="#Page60">60</a>.</li> - -<li>Ants, <a href="#Page153">153</a>.</li> - -<li><i>Arachnida</i>, <a href="#Page150">150</a>, <a href="#Page151">151</a>.</li> - -<li>Arctic soil, bacterial flora of, <a href="#Page24">24</a>.</li> - -<li id="IndRef17"><i>Areinida</i>, <a href="#Page150">150</a>, <a href="#Page151">151</a>, <a href="#Page157">157</a>.</li> - -<li><i>Armillaria</i>, <a href="#Page132">132</a>.</li> - -<li><i>Ascomycetes</i>, <a href="#Page119">119</a>.</li> - -<li><i>Aspergillaceæ</i>, <a href="#Page136">136</a>.</li> - -<li><i>Aspergillus</i>, <a href="#Page119">119</a>, <a href="#Page120">120</a>, <a href="#Page135">135</a>, -<a href="#Page136">136</a>, <a href="#Page138">138</a>, <a href="#Page139">139</a>.</li> - -<li>Azotobacter, <a href="#Page6">6</a>, <a href="#Page41">41</a>, <a href="#Page95">95</a>, <a href="#Page96">96</a>; assimilation -of nitrates by, <a href="#Page45">45</a>; decreasing efficiency in liquid culture, <a href="#Page44">44</a>; indicator of soil -acidity, <a href="#Page44">44</a>.</li> - -<li class="newletter" id="IndRef7"><span class="smcap">Bacillariaceæ</span>, <a href="#Page100">100</a> (see also -<a href="#IndRef3">Diatom</a>).</li> - -<li><i>Bacillus amylobacter</i>, distribution of, <a href="#Page24">24</a>.</li> - -<li><i>Bacillus radicicola</i>, <a href="#Page24">24</a>, <a href="#Page46">46 <i>sqq.</i></a>; inoculation of soil with, -<a href="#Page50">50</a>; life cycle of, <a href="#Page47">47</a>.</li> - -<li>Bacteria, association with algæ in nitrogen fixation, <a href="#Page111">111</a>; anærobic respiration of, -<a href="#Page37">37</a>; effect of arsenic on, <a href="#Page61">61</a>; cellulose destroying, <a href="#Page134">134</a>; changes -in morphology in culture, <a href="#Page22">22</a>, <a href="#Page47">47</a>; classification of main groups, -<a href="#Page23">23</a>, <a href="#Page25">25</a>; composition of cells of, <a href="#Page39">39</a>; inverse relationship with -protozoa, <a href="#Page10">10</a>, <a href="#Page79">79</a>, <a href="#Page82">82 <i>sqq.</i></a>; isolation from soil, -<a href="#Page21">21</a>; methods of describing, <a href="#Page21">21</a>; method of estimating numbers of, <a href="#Page53">53 -<i>sqq.</i></a>, <a href="#Page80">80</a>; nitrogen fixation by, <a href="#Page110">110</a>, <a href="#Page111">111</a>; numbers in -relation to algæ, <a href="#Page110">110</a>; numbers in soil, <a href="#Page52">52 <i>sqq.</i></a>; oxidation of hydrogen by, -<a href="#Page27">27</a>, <a href="#Page37">37</a>; effect of partial sterilisation on, <a href="#Page8">8</a>, -<a href="#Page9">9</a>, <a href="#Page66">66</a>, <a href="#Page67">67</a>; part played in soil fertility -by<span class="pagenum" id="Page184">[184]</span>, <a href="#Page7">7</a>; pure cultures, isolation by plating, -<a href="#Page20">20</a>; seasonal changes in numbers of, <a href="#Page59">59</a>, <a href="#Page87">87 <i>sqq.</i></a>; effect of -salts on, <a href="#Page60">60</a>; short time changes in numbers of, <a href="#Page11">11</a>, <a href="#Page57">57</a>, -<a href="#Page58">58</a>; effect of temperature on, <a href="#Page67">67</a>; uneven distribution of, <a href="#Page57">57</a>.</li> - -<li><i>Basidiomycetes</i>, <a href="#Page119">119</a>, <a href="#Page123">123</a>, <a href="#Page132">132</a>.</li> - -<li>Beets, attacked by <i>Phoma betæ</i>, <a href="#Page135">135</a>.</li> - -<li><i>Boletus</i>, <a href="#Page132">132</a>.</li> - -<li><i>Botrytis</i>, <a href="#Page122">122</a>.</li> - -<li>Bryophyta, <a href="#Page100">100</a>, <a href="#Page132">132</a>.</li> - -<li><i>Bumilleria</i>, <a href="#Page105">105</a>.</li> - -<li class="newletter">Calcium compounds in soil and fungi, <a href="#Page139">139</a>.</li> - -<li><i>Carabidæ</i>, <a href="#Page150">150</a>.</li> - -<li>Carbohydrates, decomposition by bacteria, <a href="#Page26">26 <i>sqq.</i></a>; decomposition by fungi, -<a href="#Page140">140</a>; decomposition in soil, <a href="#Page168">168</a>; effect on ammonia production in soil, -<a href="#Page33">33</a>; presence in algal sheath and bacteria, <a href="#Page111">111</a>.</li> - -<li>Carbon, changes in amount in soil, <a href="#Page167">167</a>; relationships of bacteria, <a href="#Page27">27</a>; -relationships of fungi, <a href="#Page133">133</a>; source of, for soil bacteria, <a href="#Page39">39</a>; sources of, for soil -fungi, <a href="#Page139">139</a>.</li> - -<li>Carbon dioxide, assimilation by algæ, <a href="#Page99">99</a>, <a href="#Page107">107</a>, <a href="#Page108">108</a>; -assimilation by soil bacteria, <a href="#Page35">35</a>, <a href="#Page36">36</a>, <a href="#Page40">40</a>.</li> - -<li>Carotin, in algæ, <a href="#Page100">100</a>; formed by <i>Spirochæta cytophaga</i>, <a href="#Page29">29</a>.</li> - -<li><i>Cecidomyidæ</i>, <a href="#Page155">155</a>.</li> - -<li>Cellulose, decomposition by bacteria, <a href="#Page27">27</a>, <i>sqq.</i>; decomposition by fungi, <a href="#Page133">133</a>, -<a href="#Page134">134</a>, <a href="#Page141">141</a>; relation of nitrogen supply to decomposition of, <a href="#Page30">30</a>; -decomposition in soil, <a href="#Page168">168</a>; as source of energy for nitrogen fixation, <a href="#Page43">43</a>.</li> - -<li>Centipedes, see <i><a href="#IndRef4">Chilopoda</a></i>.</li> - -<li><i>Cephalosporium</i>, <a href="#Page120">120</a>.</li> - -<li><i>Cephalothecium</i>, <a href="#Page136">136</a>.</li> - -<li id="IndRef4"><i>Chilopoda</i>, <a href="#Page157">157</a>.</li> - -<li><i>Chironomidæ</i>, <a href="#Page155">155</a>.</li> - -<li><i>Chlorella</i>, <a href="#Page108">108</a>.</li> - -<li><i>Chlorococcum</i>, <a href="#Page105">105</a>.</li> - -<li><i>Chlorophyceæ</i>, <a href="#Page100">100</a>.</li> - -<li>Chlorophyll, loss of, from algæ, <a href="#Page108">108</a>.</li> - -<li>Ciliates, classification of, <a href="#Page72">72</a>; cyst wall of, <a href="#Page73">73</a>.</li> - -<li>Citric acid, formation of, by fungi, <a href="#Page139">139</a>.</li> - -<li><i>Cladosporium</i>, <a href="#Page119">119</a>.</li> - -<li>Clamp connections in fungi, <a href="#Page119">119</a>.</li> - -<li>Classification, of algæ, <a href="#Page100">100</a>; of bacteria, <a href="#Page23">23</a>, <a href="#Page25">25</a>; of fungi, -<a href="#Page131">131</a>; of protozoa, <a href="#Page69">69 <i>sqq.</i></a></li> - -<li>Climate, effect of, on algæ, <a href="#Page101">101</a>.</li> - -<li><i>Clostridium</i>, <a href="#Page41">41</a>, <a href="#Page44">44</a>; as fixer of nitrogen, <a href="#Page6">6</a>.</li> - -<li><i>Coccomyxa</i>, <a href="#Page104">104</a>.</li> - -<li><i>Coleoptera</i>, <a href="#Page150">150</a>, <a href="#Page154">154</a>, <a href="#Page155">155</a>.</li> - -<li><i>Collembola</i>, <a href="#Page150">150</a>, <a href="#Page153">153</a>, <a href="#Page154">154</a>.</li> - -<li><i>Colletotrichum</i>, <a href="#Page131">131</a>.</li> - -<li>Commensals, <a href="#Page132">132</a>.</li> - -<li><i>Conjugatæ</i>, <a href="#Page100">100</a>.</li> - -<li><i>Cortinarius</i>, <a href="#Page132">132</a>.</li> - -<li>Cotton, destroyed by fungi, <a href="#Page134">134</a>.</li> - -<li>Counting, of algæ, <a href="#Page109">109</a>; of bacteria, <a href="#Page53">53 <i>sqq.</i></a>; of fungi, -<a href="#Page122">122</a>; of protozoa, <a href="#Page77">77</a>, <a href="#Page79">79</a>, <a href="#Page80">80</a>.</li> - -<li>Cresol, decomposition of, by bacteria, <a href="#Page22">22</a>, <a href="#Page24">24</a>, <a href="#Page31">31</a>.</li> - -<li>Criteria, physiological, of fungi, <a href="#Page128">128</a>.</li> - -<li>Crop growth, effect on fungi, <a href="#Page122">122</a>.</li> - -<li><i>Cryptomonadineæ</i>, <a href="#Page100">100</a>.</li> - -<li>Cucumber leaf spot, <a href="#Page131">131</a>.</li> - -<li>Cyanamide, decomposition of, by fungi, <a href="#Page136">136</a>.</li> - -<li id="IndRef1"><i>Cyanophyceæ</i>, <a href="#Page103">103</a> (see also <i><a href="#IndRef2">Myxophyceæ</a></i> and -<a href="#IndRef6">blue-green algæ</a>).</li> - -<li><i>Cylindrospermum</i>, <a href="#Page102">102</a>.</li> - -<li>Cysts, <a href="#Page68">68</a>, <a href="#Page73">73</a>, <a href="#Page74">74</a>.</li> - -<li class="newletter"><span class="smcap">Denitrification</span>, by bacteria, <a href="#Page37">37</a>; by fungi, -<a href="#Page136">136</a>.</li> - -<li>Desiccation, resistance to, by algæ, <a href="#Page106">106</a>.</li> - -<li>Dew, relation to algæ, <a href="#Page101">101</a>, <a href="#Page113">113</a>.</li> - -<li id="IndRef3">Diatoms, <a href="#Page104">104 <i>sqq.</i></a> (see also <i><a href="#IndRef7">Bacillariaceæ</a></i>).</li> - -<li>Dicyanamide, decomposition of, by fungi, <a href="#Page136">136</a>.</li> - -<li>Dipeptides, formation of, by algæ, <a href="#Page108">108</a>.</li> - -<li id="IndRef11"><i>Diplopoda</i>, <a href="#Page157">157</a>.</li> - -<li><i>Diptera</i>, <a href="#Page150">150</a>, <a href="#Page154">154</a>, <a href="#Page155">155</a>, -<a href="#Page159">159</a>.</li> - -<li>Disaccharides and fungi, <a href="#Page134">134</a>.</li> - -<li class="newletter"><span class="smcap">Earthworms</span>, abundance of, in soil, <a href="#Page153">153</a>; effect of, in soil, -<a href="#Page13">13</a>, <a href="#Page160">160</a>, <a href="#Page175">175</a>.</li> - -<li>Eel-worms, <a href="#Page149">149</a> (see also <i><a href="#IndRef8">Nematoda</a></i>).</li> - -<li><i>Elaphomyces</i>, <a href="#Page132">132</a>.</li> - -<li><i>Enchytræidæ</i>, <a href="#Page149">149</a>.</li> - -<li>Energy, laws of, <a href="#Page165">165</a>; relationships of soils, <a href="#Page166">166</a>; requirements of soil -organisms, <a href="#Page15">15</a>, <a href="#Page16">16</a>.</li> - -<li>Energy supply, relation of bacterial activities to, <a href="#Page25">25 <i>sqq.</i></a>, <a href="#Page40">40</a>, -<a href="#Page44">44</a>; sources of, for soil bacteria, <a href="#Page26">26 <i>sqq.</i></a>, <a href="#Page40">40</a>, -<a href="#Page43">43</a>; supplies of, for soil organisms, <a href="#Page111">111</a>, <a href="#Page164">164</a>, -<a href="#Page167">167</a>, <a href="#Page168">168</a>.</li> - -<li>Environmental conditions in soil, <a href="#Page16">16</a>.</li> - -<li>Eremacausis, <a href="#Page2">2</a>.</li> - -<li>Ericales, <a href="#Page132">132</a>, <a href="#Page135">135</a>.</li> - -<li><i>Euglena</i>, <a href="#Page99">99</a>.</li> - -<li><i>Euglenaceæ</i>, <a href="#Page100">100</a>.</li> - -<li>Experimental error, in bacterial counts, <a href="#Page54">54</a>; in fungal counts, <a href="#Page124">124</a>.</li> - -<li class="newletter">Farmyard manure<span class="pagenum" id="Page185">[185]</span>, see <a href="#IndRef9">Manure</a>.</li> - -<li>Fats, used by fungi, <a href="#Page134">134</a>.</li> - -<li>Fatty acids used by fungi, <a href="#Page134">134</a>.</li> - -<li id="IndRef21">Fertility of soil, views on, <a href="#Page2">2</a>; effect of decomposition of plant residues on, -<a href="#Page1">1</a>, <a href="#Page165">165</a>; effect of organisms on, <a href="#Page175">175</a>.</li> - -<li>Filter paper, destruction of, by fungi, <a href="#Page133">133</a>; destruction of, by <i>Spirochæta cytophaga</i>, -<a href="#Page28">28</a>.</li> - -<li>Fixation of nitrogen, discovery of, by Berthelot, <a href="#Page5">5</a>; by bacteria, <a href="#Page40">40 <i>sqq.</i></a>; -by algæ, <a href="#Page110">110</a>, <a href="#Page111">111</a>; by mixtures of bacteria and algæ, <a href="#Page111">111</a>; by -fungi, <a href="#Page135">135 <i>sqq.</i></a> (see also <a href="#IndRef10">Nitrogen Fixation</a>).</li> - -<li><i>Flagellatæ</i>, <a href="#Page100">100</a>.</li> - -<li>Flax sickness and fungi, <a href="#Page122">122</a>.</li> - -<li>Formaldehyde, as agent for destroying fungi, <a href="#Page141">141</a>.</li> - -<li>Fungi, control of, in soil, <a href="#Page139">139 <i>sqq.</i></a>; counting of, <a href="#Page122">122</a>; distribution of, -in soil, <a href="#Page119">119 <i>sqq.</i></a>, <a href="#Page127">127</a>; fertilisers, effect of, on numbers of in soil, -<a href="#Page126">126</a>; as facultative parasites, <a href="#Page131">131</a>, <a href="#Page132">132</a>; fruiting bodies of, -<a href="#Page123">123</a>; destruction of hemicelluloses by, <a href="#Page133">133</a>; individual, <a href="#Page122">122</a>, -<a href="#Page123">123</a>; action on monosaccharides of, <a href="#Page134">134</a>; mineral relationships of, -<a href="#Page139">139</a>; mycorrhizal, <a href="#Page132">132</a>, <a href="#Page135">135</a>, <a href="#Page139">139</a>, -<a href="#Page140">140</a>; heterocyclic nitrogen compounds and, <a href="#Page138">138</a>; occurrence in soil, -<a href="#Page118">118</a>; qualitative study of, <a href="#Page118">118</a>; selective feeding of, <a href="#Page140">140</a>; -specific determination of, <a href="#Page119">119</a>.</li> - -<li><i>Fungi imperfecti</i>, <a href="#Page119">119</a>.</li> - -<li><i>Fusaria</i>, <a href="#Page134">134</a>.</li> - -<li><i>Fusarium</i>, <a href="#Page119">119</a>, <a href="#Page120">120</a>, <a href="#Page122">122</a>, -<a href="#Page128">128</a>, <a href="#Page133">133</a>, <a href="#Page136">136</a>.</li> - -<li class="newletter"><i>Gamascidæ</i>, <a href="#Page156">156</a>.</li> - -<li>Gases of swamp water (Paddy soils), <a href="#Page113">113</a>.</li> - -<li><i>Gastrodia</i>, <a href="#Page132">132</a>.</li> - -<li>Gelatinous envelope of algæ, <a href="#Page109">109</a>, <a href="#Page111">111</a>.</li> - -<li>Geographical distribution of azotobacter, <a href="#Page41">41</a>; of soil bacteria, <a href="#Page24">24</a>; of protozoa, -<a href="#Page75">75</a>, <a href="#Page76">76</a>; of soil fungi, <a href="#Page119">119</a>, <a href="#Page125">125</a>.</li> - -<li>Germination, of algal spores, <a href="#Page107">107</a>.</li> - -<li>Glucose, use of, by algae, <a href="#Page108">108</a>, <a href="#Page109">109</a>, <a href="#Page111">111</a>; use of, by moss -protonema, <a href="#Page109">109</a>.</li> - -<li>Glycocoll, formation of, by algæ, <a href="#Page108">108</a>.</li> - -<li><i>Granulobacter</i>, <a href="#Page42">42</a>.</li> - -<li>Greenland, bacteria in soil from, <a href="#Page24">24</a>.</li> - -<li>“Grunlandmoor,” fungi in, <a href="#Page126">126</a>.</li> - -<li class="newletter"><i>Hantzschia</i>, <a href="#Page105">105</a>.</li> - -<li><i>Hemiptera</i>, <a href="#Page154">154</a>.</li> - -<li><i>Heterokontæ</i>, <a href="#Page100">100</a>.</li> - -<li>“Hochmoor,” fungi in, <a href="#Page126">126</a>.</li> - -<li><i>Hormidium</i>, <a href="#Page104">104</a>.</li> - -<li>Humus, the food of plants, <a href="#Page1">1</a>; formation of, by fungi, <a href="#Page134">134</a>, -<a href="#Page141">141</a>; formation of, in soil, <a href="#Page168">168</a>; forest, <a href="#Page132">132</a>; fungal hyphæ as -constituent of forest humus, <a href="#Page132">132</a>.</li> - -<li id="IndRef13">Hydrogen ion concentration, in soil, <a href="#Page17">17</a>; effect on fungi of, -<a href="#Page124">124</a>.</li> - -<li><i>Hymenoptera</i>, <a href="#Page150">150</a>, <a href="#Page154">154</a>.</li> - -<li class="newletter"><i>Insecta</i>, <a href="#Page150">150</a>, <a href="#Page157">157</a>.</li> - -<li>Insects, numbers present in soil, <a href="#Page154">154</a>.</li> - -<li>Invertebrata, definition of, <a href="#Page147">147</a>; method of investigating, <a href="#Page148">148</a>; groups -represented, <a href="#Page149">149</a>; distribution in the soil, <a href="#Page151">151</a>; dominant species and groups, -<a href="#Page153">153</a>; environmental factors of, <a href="#Page157">157</a>; feeding habits, <a href="#Page156">156</a>; -relation to agriculture, <a href="#Page160">160</a>; relation to nitrogen cycle, <a href="#Page161">161</a>.</li> - -<li>Iron compounds, oxidation by fungi, <a href="#Page139">139</a>.</li> - -<li id="IndRef20"><i>Isopoda</i>, <a href="#Page150">150</a>, <a href="#Page151">151</a>.</li> - -<li class="newletter"><i>Leguminosæ</i>, association with bacteria, <a href="#Page46">46 <i>sqq.</i></a>; enrichment of ground by, -<a href="#Page5">5</a>.</li> - -<li><i>Lepidoptera</i>, <a href="#Page150">150</a>, <a href="#Page154">154</a>.</li> - -<li>Life cycles, of bacteria, <a href="#Page22">22</a>, <a href="#Page47">47</a>; of protozoa, <a href="#Page72">72 -<i>sqq.</i></a></li> - -<li>Lime, effect on fungi in soil, <a href="#Page121">121</a>, <a href="#Page126">126</a>.</li> - -<li><i>Lyngbya</i>, <a href="#Page112">112</a>.</li> - -<li class="newletter">Magnesium compounds, effect on fungi, <a href="#Page139">139</a>.</li> - -<li>Manganese compounds, effect on bacteria, <a href="#Page61">61</a>.</li> - -<li id="IndRef9">Manure, farmyard, effect on algæ, <a href="#Page109">109</a>, <a href="#Page110">110</a>; effect on numbers of -bacteria, <a href="#Page60">60</a>; effect on numbers of fungi, <a href="#Page126">126</a>; effect on numbers of insects, -<a href="#Page154">154</a>, <a href="#Page155">155</a>.</li> - -<li>Manure, Artificial, effect on fungi, <a href="#Page127">127</a>.</li> - -<li>Manure, town stable, occurrence of disease organisms in, <a href="#Page132">132</a>.</li> - -<li><i>Mastigophora</i>, classification of, <a href="#Page71">71</a>; species of, <a href="#Page71">71</a>.</li> - -<li>Media, containing nitrates, chemical analysis of, <a href="#Page111">111</a>; for counting soil bacteria, -<a href="#Page54">54</a>; for counting protozoa, <a href="#Page79">79</a>; for counting fungi, <a href="#Page119">119</a>, -<a href="#Page123">123</a>.</li> - -<li><i>Melanconium</i>, <a href="#Page134">134</a>.</li> - -<li><i>Melolontha</i>, <a href="#Page150">150</a>.</li> - -<li>Methane, oxidation of, by bacteria, <a href="#Page26">26</a>, <a href="#Page27">27</a>.</li> - -<li>Millipedes, see <i><a href="#IndRef11">Diplopoda</a></i>.</li> - -<li>Mites, see <i><a href="#IndRef12">Acarina</a></i>.</li> - -<li id="IndRef14"><i>Mollusca</i><span class="pagenum" id="Page186">[186]</span>, <a href="#Page149">149</a>, -<a href="#Page157">157</a>.</li> - -<li><i>Moniliaceæ</i>, <a href="#Page136">136</a>.</li> - -<li><i>Mucor</i>, <a href="#Page120">120</a>, <a href="#Page121">121</a>, <a href="#Page136">136</a>, -<a href="#Page138">138</a>.</li> - -<li><i>Mucorales</i>, <a href="#Page121">121</a>, <a href="#Page134">134</a>.</li> - -<li><i>Mucorineæ</i>, <a href="#Page118">118</a>.</li> - -<li><i>Mycetophilidæ</i>, <a href="#Page155">155</a>.</li> - -<li id="IndRef18">Mycorrhiza, <a href="#Page132">132</a>, <a href="#Page135">135</a>, <a href="#Page139">139</a>, -<a href="#Page140">140</a>.</li> - -<li><i>Myriapoda</i>, <a href="#Page150">150</a>, <a href="#Page156">156</a>.</li> - -<li id="IndRef2"><i>Myxophyceæ</i>, <a href="#Page100">100</a> (see also <i><a href="#IndRef1">Cyanophyceæ</a></i> and -<a href="#IndRef6">blue-green algæ</a>).</li> - -<li class="newletter"><span class="smcap">Naphthalene</span>, decomposition of, by bacteria, <a href="#Page31">31</a>.</li> - -<li><i>Naviculoideæ</i>, <a href="#Page100">100</a>.</li> - -<li id="IndRef8"><i>Nematoda</i>, <a href="#Page149">149</a>, <a href="#Page151">151</a>, <a href="#Page157">157</a>.</li> - -<li>Nitrate, assimilation by algæ, <a href="#Page105">105</a>, <a href="#Page108">108</a>, <a href="#Page111">111</a>; assimilation -by bacteria, <a href="#Page33">33</a>, <a href="#Page40">40</a>, <a href="#Page44">44</a>, <a href="#Page51">51</a>; assimilation -by fungi, <a href="#Page136">136</a>, <a href="#Page138">138</a>; removal from soil, <a href="#Page12">12</a>, -<a href="#Page112">112</a>, <a href="#Page171">171</a>; variations in amount in soil, <a href="#Page11">11</a>.</li> - -<li>Nitre-beds, <a href="#Page1">1</a>.</li> - -<li>Nitrification, and bacteria, <a href="#Page34">34</a>; chemical changes in, <a href="#Page171">171</a>; and fungi, -<a href="#Page136">136</a>; energy supply in, <a href="#Page35">35</a>; mechanism of, <a href="#Page1">1</a>, -<a href="#Page3">3</a>; and soil fertility, <a href="#Page1">1</a>, <a href="#Page3">3</a>.</li> - -<li>Nitrites and fungi, <a href="#Page136">136</a>; formation by bacteria, <a href="#Page34">34</a>.</li> - -<li><i>Nitrobacter</i>, <a href="#Page35">35</a>.</li> - -<li id="IndRef10">Nitrogen, changes in amount in soil, <a href="#Page167">167</a>; cycle in soil, <a href="#Page161">161</a>; -fixation by bacteria, <a href="#Page6">6</a>, <a href="#Page40">40 <i>sqq.</i></a>; fixation by fungi, <a href="#Page135">135</a>, -<a href="#Page136">136</a>, <a href="#Page141">141</a>; fixation of, in clover plant, <a href="#Page5">5</a>; increase by protozoa -of fixation of, <a href="#Page94">94</a>, <a href="#Page95">95</a> (<a href="#Fig18">fig.</a>); fixation sources of energy for, -<a href="#Page43">43</a>, <a href="#Page49">49</a>; gain of, in soil, <a href="#Page174">174</a>; in invertebrates, -<a href="#Page162">162</a>; loss of, by leaching, <a href="#Page112">112</a>; loss of, from cultivated soils, -<a href="#Page173">173</a>; relationships of fungi, <a href="#Page135">135</a>; relationships of algæ, -<a href="#Page110">110</a>-<a href="#Page112">112</a>; relationships of bacteria, <a href="#Page32">32 <i>sqq.</i></a>, -<a href="#Page40">40 <i>sqq.</i></a>; relationships of insects, <a href="#Page162">162</a>.</li> - -<li><i>Nitrosococcus</i>, <a href="#Page35">35</a>.</li> - -<li><i>Nitrosomonas</i>, <a href="#Page35">35</a>.</li> - -<li id="IndRef19">Nodule Organism of the Leguminosæ, <a href="#Page6">6</a>, <a href="#Page46">46 <i>sqq.</i></a></li> - -<li><i>Nostocaceæ</i>, <a href="#Page100">100</a>, <a href="#Page101">101</a>, <a href="#Page102">102</a>, -<a href="#Page107">107</a>.</li> - -<li class="newletter"><i>Oligochæta</i>, <a href="#Page149">149</a>, <a href="#Page151">151</a>, <a href="#Page153">153</a>, -<a href="#Page157">157</a>.</li> - -<li><i>Oospora</i>, <a href="#Page120">120</a>.</li> - -<li><i>Orcheomyces</i>, <a href="#Page132">132</a>.</li> - -<li>Orchid cultivation and fungi, <a href="#Page132">132</a>, <a href="#Page140">140</a>.</li> - -<li><i>Orthoptera</i>, <a href="#Page154">154</a>.</li> - -<li><i>Oscillatoriaceæ</i>, <a href="#Page100">100</a>, <a href="#Page102">102</a>.</li> - -<li>Osmotic pressure, influencing effect of salts on bacteria, <a href="#Page50">50</a>.</li> - -<li>Oxalic acid, formation of, by fungi, <a href="#Page139">139</a>.</li> - -<li>Oxidations effected by soil organisms; by bacteria, <a href="#Page26">26</a> <i>et seq.</i>; by fungi, -<a href="#Page139">139</a>.</li> - -<li>Oxygen, absorption by soils, <a href="#Page4">4</a>.</li> - -<li class="newletter" id="IndRef16">Partial sterilisation of soil, <a href="#Page8">8</a>, <a href="#Page66">66 <i>sqq.</i></a>, -<a href="#Page96">96</a>, <a href="#Page178">178</a>; influence of organic antiseptics, <a href="#Page177">177</a>; limiting factor -in, <a href="#Page67">67</a>, <a href="#Page68">68</a>.</li> - -<li>Pectin, effect of, on fungi, <a href="#Page134">134</a>.</li> - -<li><i>Pedras negras</i>, <a href="#Page112">112</a>.</li> - -<li><i>Penicillia</i>, <a href="#Page134">134</a>.</li> - -<li>Pentosans, effect of, on fungi, <a href="#Page134">134</a>.</li> - -<li>Peptones, decomposition of, by fungi, <a href="#Page136">136</a>, <a href="#Page138">138</a>; source of nitrogen for algæ, -<a href="#Page108">108</a>.</li> - -<li>Periodicity, of protozoa in soil, <a href="#Page90">90 <i>sqq.</i></a> (<a href="#Fig16">fig.</a>), <a href="#Page92">92</a> -(<a href="#Fig17">fig.</a>), <a href="#Page93">93</a>.</li> - -<li>Phenol, decomposition of, by bacteria, <a href="#Page24">24</a>, <a href="#Page25">25</a>, <a href="#Page31">31</a>.</li> - -<li>Phenylalanine, formation of, by algæ, <a href="#Page108">108</a>.</li> - -<li><i>Phoma</i>, <a href="#Page132">132</a>.</li> - -<li><i>Phormidium</i>, <a href="#Page106">106</a>.</li> - -<li>Phosphates, availability of, influenced by bacteria, <a href="#Page52">52</a>; by fungi, <a href="#Page139">139</a>; effect on -bacteria, <a href="#Page46">46</a>, <a href="#Page51">51</a>, <a href="#Page60">60</a>.</li> - -<li>Photosynthesis, <a href="#Page99">99</a>, <a href="#Page100">100</a>, <a href="#Page107">107</a>, <a href="#Page110">110</a>, -<a href="#Page113">113</a>.</li> - -<li>Phycocyanin, <a href="#Page100">100</a>.</li> - -<li>Physical conditions in soil, <a href="#Page16">16</a>.</li> - -<li>Physiological criteria, of bacteria, <a href="#Page22">22</a>; of fungi, <a href="#Page128">128</a>.</li> - -<li><i>Phycomycetes</i>, <a href="#Page119">119</a>.</li> - -<li><i>Phytophthora</i>, <a href="#Page132">132</a>.</li> - -<li>Plant disease, and fungi, <a href="#Page139">139</a>.</li> - -<li>Plant residues, decomposition of, in soil, <a href="#Page168">168</a>; influence of soil reaction on, -<a href="#Page165">165</a>; relation to soil fertility, <a href="#Page1">1</a>, <a href="#Page165">165</a>.</li> - -<li>Plasticity of fungi, <a href="#Page119">119</a>.</li> - -<li><i>Plectonema</i>, <a href="#Page106">106</a>.</li> - -<li>Potassium salts, effect on bacteria, <a href="#Page60">60</a>; influence of bacteria on the availability of, -<a href="#Page52">52</a>.</li> - -<li>Protein, decomposition of, in soil, <a href="#Page169">169</a>; decomposition by bacteria, <a href="#Page32">32</a>; -decomposition by fungi, <a href="#Page138">138</a>, <a href="#Page140">140</a>.</li> - -<li><i>Protococcales</i>, <a href="#Page100">100</a>.</li> - -<li><i>Protoderma viride</i>, <a href="#Page105">105</a>.</li> - -<li>Protonema of mosses, <a href="#Page100">100</a>, <a href="#Page105">105</a>, <a href="#Page106">106</a>, -<a href="#Page109">109</a>.</li> - -<li>Protophyta, chlorophyll-bearing, <a href="#Page100">100</a>.</li> - -<li>Protozoa, inoculation into soil of, <a href="#Page85">85 <i>sqq.</i></a>; isolation from soil, <a href="#Page69">69</a>; -classification of, <a href="#Page69">69 <i>sqq.</i></a>; life histories of, <a href="#Page72">72 <i>sqq.</i></a>; species of, in -soil, <a href="#Page70">70 <i>sqq.</i></a>; distribution of, in soil, <a href="#Page74">74 <i>sqq.</i></a>; retention of, by soil, -<a href="#Page78">78</a> (<a href="#Fig9">fig.</a>); size of, <a href="#Page90">90</a>; reproductive rates, -<a href="#Page93">93</a>; inverse relation with bacteria<span class="pagenum" id="Page187">[187]</span>, <a href="#Page79">79 -<i>sqq.</i></a>; presence of trophic forms in soil, <a href="#Page9">9</a>; numbers of, in soil, <a href="#Page90">90</a>, -<a href="#Page96">96</a>, <a href="#Page97">97</a>; fluctuations in numbers of, <a href="#Page10">10</a>, <a href="#Page81">81</a> -(<a href="#Fig10">fig.</a>), <a href="#Page82">82</a>; external conditions, effect on, <a href="#Page82">82</a>; seasonal changes, -effect on, <a href="#Page87">87 <i>sqq.</i></a>; weight of, <a href="#Page90">90</a>.</li> - -<li><i>Pteridophyta</i>, <a href="#Page132">132</a>.</li> - -<li><i>Pythium</i>, <a href="#Page132">132</a>.</li> - -<li class="newletter">Reaction of soil, <a href="#Page17">17</a>.</li> - -<li>Reaction of soil, effect on bacteria, <a href="#Page36">36</a>, <a href="#Page37">37</a>, <a href="#Page46">46</a>, -<a href="#Page48">48</a>, <a href="#Page61">61</a>; effect on protozoa, <a href="#Page93">93</a>, <a href="#Page94">94</a> (see -also <a href="#IndRef13">hydrogen ion concentration</a>).</li> - -<li>Relationships of Fungi, commensal, <a href="#Page132">132</a>; mycorrhizal, <a href="#Page132">132</a>; symbiotic, -<a href="#Page132">132</a>.</li> - -<li><i>Rhizopoda</i>; classification of, <a href="#Page70">70</a>, <a href="#Page71">71</a>; species of, <a href="#Page70">70</a>, -<a href="#Page71">71</a>.</li> - -<li>Rhythm, supposed in ammonification by fungi, <a href="#Page137">137</a>.</li> - -<li><i>Rhizoctonia</i>, <a href="#Page132">132</a>.</li> - -<li><i>Rhizopus</i>, <a href="#Page119">119</a>, <a href="#Page120">120</a>.</li> - -<li>Rice plant, aeration of roots, <a href="#Page113">113</a>; physiological disease of, <a href="#Page113">113</a>.</li> - -<li>Rock Phosphate as base for nitrifying organisms, <a href="#Page36">36</a>.</li> - -<li>Rothamsted, Broadbalk plot 2 (Farmyard Manure) algæ, <a href="#Page109">109</a>; fungi, <a href="#Page125">125</a>, -<a href="#Page127">127</a>; Insects, <a href="#Page152">152</a>.</li> - -<li>Rothamsted, Broadbalk plot 3 (Unmanured) algæ, <a href="#Page109">109</a>; fungi, <a href="#Page120">120</a>, -<a href="#Page122">122</a>, <a href="#Page127">127</a>; Insects, <a href="#Page152">152</a>.</li> - -<li>Rothamsted, Broadbalk Plots 10, 11, and 13; <a href="#Page122">122</a>, <a href="#Page127">127</a>.</li> - -<li>Rothamsted, Barnfield Plot 1-0 (Farmyard Manure), Protozoa, <a href="#Page80">80</a>.</li> - -<li>Rothamsted, unmanured grass plot, <a href="#Page120">120</a>.</li> - -<li><i>Russula</i>, <a href="#Page132">132</a>.</li> - -<li>Rusts, <a href="#Page119">119</a>.</li> - -<li class="newletter"><i>Saccharomyces</i>, <a href="#Page120">120</a>.</li> - -<li>Saprophytes, facultative, <a href="#Page131">131</a>.</li> - -<li>Saprophytism and algæ, <a href="#Page108">108</a>, <a href="#Page110">110</a>.</li> - -<li><i>Scenedesmus</i>, <a href="#Page108">108</a>.</li> - -<li>Seasonal fluctuations in numbers of soil organisms, <a href="#Page12">12</a>, <a href="#Page87">87 <i>et seq.</i></a>, -<a href="#Page125">125</a>.</li> - -<li>Selective media, use of, in isolation of soil bacteria, <a href="#Page21">21</a>.</li> - -<li>Serological tests, separation of varieties of <i>B. radicicola</i> by, <a href="#Page48">48</a>.</li> - -<li>Slugs, see <i><a href="#IndRef14">Mollusca</a></i>.</li> - -<li>Smuts, <a href="#Page119">119</a>.</li> - -<li>Snails, see <i><a href="#IndRef14">Mollusca</a></i>.</li> - -<li>Soil; comparison of, by volume, <a href="#Page17">17</a>; effect of depth below surface on algæ, <a href="#Page101">101</a>, -<a href="#Page104">104</a>, <a href="#Page109">109</a>, <a href="#Page110">110</a>, <a href="#Page113">113</a>; effect of depth -below surface on insects, <a href="#Page151">151</a>; effect of depth below surface on fungi, <a href="#Page121">121</a>, -<a href="#Page126">126</a>, <a href="#Page127">127</a>; effect of various treatments on fungi, <a href="#Page126">126</a>, -<a href="#Page127">127</a>, <a href="#Page132">132</a>; environmental factors in, <a href="#Page16">16</a>; inoculation of, for -leguminous plants, <a href="#Page50">50</a>; moisture (see <a href="#IndRef15">Water supply</a>); population, control of, -<a href="#Page177">177 <i>sqq.</i></a>; population, methods of investigation, <a href="#Page10">10</a>, <a href="#Page15">15</a>; -sterilisation and fungi, <a href="#Page137">137</a>, <a href="#Page138">138</a>, <a href="#Page141">141</a> (see -<a href="#IndRef16">Partial Sterilisation</a>); stored, survival of algæ in, <a href="#Page107">107</a>; type and fungi, -<a href="#Page121">121</a>, <a href="#Page126">126</a>, <a href="#Page127">127</a>.</li> - -<li>Soil conditions, effect on bacteria, <a href="#Page33">33</a>, <a href="#Page36">36</a>, <a href="#Page37">37</a>, -<a href="#Page40">40</a>, <a href="#Page46">46</a>, <a href="#Page48">48</a>, <a href="#Page50">50</a>, <a href="#Page59">59 -<i>sqq.</i></a>; effect on protozoa, <a href="#Page82">82</a>.</li> - -<li>Soil fertility, see <a href="#IndRef21">Fertility of soil</a>.</li> - -<li><i>Spicaria</i>, <a href="#Page120">120</a>.</li> - -<li>Spiders, see <i><a href="#IndRef17">Areinida</a></i>.</li> - -<li><i>Spirochæta cytophaga</i>, <a href="#Page28">28</a>, <a href="#Page43">43</a>.</li> - -<li>Spore forming bacteria in soil, <a href="#Page23">23</a>, <a href="#Page34">34</a>.</li> - -<li>Spore, fungus, inhibition of formation, <a href="#Page123">123</a>; presence in air of, <a href="#Page118">118</a>.</li> - -<li>Standardisation of cultural methods for soil bacteria, <a href="#Page54">54 <i>sqq.</i></a></li> - -<li>Starch, decomposition of, by fungi, <a href="#Page134">134</a>.</li> - -<li><i>Stichococcus</i>, <a href="#Page108">108</a>.</li> - -<li>Straw; effect on nitrate production in soil, <a href="#Page33">33</a>; manure, <a href="#Page29">29</a>; rotting of, -<a href="#Page30">30</a>.</li> - -<li>Sulphur oxidation, by bacteria, <a href="#Page37">37</a>; by fungi, <a href="#Page139">139</a>.</li> - -<li>Symbiosis, of Azotobacter with other organisms, <a href="#Page42">42</a>, <a href="#Page43">43</a>, see also -<a href="#IndRef18">Mycorrhiza</a> and <a href="#IndRef19">Nodule organism</a>.</li> - -<li><i>Symphyla</i>, <a href="#Page150">150</a>, <a href="#Page151">151</a>, <a href="#Page157">157</a>.</li> - -<li><i>Symploca</i>, <a href="#Page112">112</a>.</li> - -<li class="newletter"><i>Tachinidæ</i>, <a href="#Page150">150</a>.</li> - -<li>Tannins, used by fungi, <a href="#Page134">134</a>.</li> - -<li>Temperature of soil and fungi, <a href="#Page127">127</a>, <a href="#Page140">140</a>.</li> - -<li>Termites, <a href="#Page160">160</a>.</li> - -<li><i>Testacella</i>, <a href="#Page149">149</a>.</li> - -<li><i>Thiospirillum</i>, <a href="#Page37">37</a>.</li> - -<li><i>Thysanura</i>, <a href="#Page154">154</a>.</li> - -<li><i>Thysanoptera</i>, <a href="#Page154">154</a>.</li> - -<li><i>Tipula</i>, <a href="#Page150">150</a>.</li> - -<li>Toluene, decomposition by soil bacteria, <a href="#Page31">31</a>.</li> - -<li><i>Tolypothrix</i>, <a href="#Page112">112</a>.</li> - -<li><i>Trichoderma</i>, <a href="#Page119">119</a>, <a href="#Page120">120</a>, <a href="#Page122">122</a>, -<a href="#Page134">134</a>.</li> - -<li><i>Trochiscia</i>, <a href="#Page105">105</a>.</li> - -<li>Tropisms, <a href="#Page157">157</a>.</li> - -<li class="newletter"><i>Ulothrix</i>, <a href="#Page105">105</a>.</li> - -<li><i>Ulotrichales</i>, <a href="#Page100">100</a>.</li> - -<li>Urea, by fungi, <a href="#Page136">136</a>, <a href="#Page138">138</a>.</li> - -<li>Uric acid, utilisation of, by fungi, <a href="#Page138">138</a>.</li> - -<li class="newletter"><i>Vaucheria</i><span class="pagenum" id="Page188">[188]</span>, <a href="#Page104">104</a>, -<a href="#Page106">106</a>.</li> - -<li>Vitality, retention of, by algæ and moss protonema, <a href="#Page105">105</a>, <a href="#Page107">107</a>.</li> - -<li class="newletter" id="IndRef15">Water; supply in soil, <a href="#Page17">17</a>; and algæ, <a href="#Page112">112</a>; -bacteria, <a href="#Page50">50</a>, <a href="#Page61">61</a>, <a href="#Page82">82</a>; fungi, <a href="#Page127">127</a>; -protozoa, <a href="#Page82">82</a>.</li> - -<li>Wireworms, <a href="#Page155">155</a>.</li> - -<li>Wood, decay of, <a href="#Page134">134</a>.</li> - -<li>Woodlice, <a href="#Page150">150</a>; (see also <i><a href="#IndRef20">Isopoda</a></i>).</li> - -<li class="newletter"><span class="smcap">Yeasts</span>, <a href="#Page138">138</a>.</li> - -<li class="newletter"><i>Zygnema</i>, <a href="#Page104">104</a>.</li> - -<li><i>Zygorrhynchus mœlleri</i>, <a href="#Page119">119</a>, <a href="#Page120">120</a>, <a href="#Page121">121</a>.</li> - -</ul> - -<hr class="chap x-ebookmaker-drop" /> - -<p class="center fsize90 blankbefore6">PRINTED IN GREAT BRITAIN BY THE UNIVERSITY PRESS, ABERDEEN</p> - -<hr class="full" /> - -<div class="tnbot" id="TN"> - -<h2>Transcriber’s Notes</h2> - -<p>Inconsistent and archaic or unusual spelling, capitalisation, italicisation, hyphenation, etc. have been retained, unless -mentioned below. The names and classifications of the organisms as used in the book do not always conform to modern names and -classifications; these have not been changed.</p> - -<p>Depending on the hard- and software used and their settings, not all elements may display as intended.</p> - -<p>Page 14, table, lower right hand cell: the data given add up to 8, not to 9.</p> - -<p>Page 47, ·9 × ·18 in size: the source document does not include the units; presumably the sizes are in microns.</p> - -<p>Page 118, endnote 8c (2×): this note does not exist.</p> - -<p>Subject Index, entry Zygorrhynchus mœlleri: also refers to Zygorrhynchus vuilleminii.</p> - -<p class="blankbefore75">Changes made:</p> - -<p>Footnotes, tables and illustrations have been moved out of text paragraphs.</p> - -<p>Several minor obvious typographical, punctuation and spelling errors (including accents) have been corrected silently. In -several cases spelling differences (mainly of proper names) between the text and the index and endnotes have been standardised. -In the indexes and in tables some ditto marks have been replaced with the dittoed text. Some page references -have been corrected to indicate the correct page number.</p> - -<p>Text in <span class="illotext">a dashed box</span> is not present as such in the source document, but has been -transcribed from the illustration for legibility and ease of reference. Some tables have been re-arranged or split to -fit the available width.</p> - -<p>Page 28, 29: MacBeth changed to McBeth as elsewhere (in the Author Index the entries MacBeth and McBeth have been merged).</p> - -<p>Page 32, formula: 30 changed to 3O.</p> - -<p>Page 58: From Barnfeild, ... changed to From Barnfield, ....</p> - -<p>Page 85: closing bracket deleted after ... Table VII. and Fig. 13.</p> - -<p>Page 90, Table VIII, column 5: 350·000 and 150·000 changed to 350,000 and 150,000.</p> - -<p>Page 97: No creature lies or dies to itself, ... changed to No creature lives or dies to itself, ...</p> - -<p>Page 104: Danske Aerofile Alghe changed to Danske Aërofile Alger.</p> - -<p>Page 114: Recherche sulla Malattia del Riso ... changed to Ricerche sulla Malattia del Riso ....</p> - -<p>Page 115: ... sur de polymorphisme ... changed to ... sur le polymorphisme ....</p> - -<p>Page 116: literature notes 38 (Robbins) and 48 (Schindler) changed to 33 and 34 respectively.</p> - -<p>Page 120: Zygorrhynchus vuillemini changed to Zygorrhynchus vuilleminii as elsewhere.</p> - -<p>Page 126: references to Waksman<span class="fnanchor">[24]</span> and <span class="fnanchor">[24<i>e</i>]</span> changed to -<span class="fnanchor">[25]</span> and <span class="fnanchor">[25<i>e</i>]</span>.</p> - -<p>Page 129: ... preparée de la pres de Russum ... changed to ... préparée de la terre humeuse du Spanderswoud, près de -Bussum ....</p> - -<p>Page 134: reference to Kohshi<span class="fnanchor">[24]</span> changed to <span class="fnanchor">[34]</span>.</p> - -<p>Page 143: Sämenbildung changed to Säurenbildung (entry 5); Wurzelbranderregern im Baden changed to Wurzelbranderregern im -Boden (entry 11).</p> - -<p>Page 144: ... Umwandlung von Aminosamen in Oxysämen ... changed to ... Umwandlung von Aminosäuren in Oxysäuren ....; -... Wirkungen der Schimmelze ... changed to ... Wirkungen der Schimmelpilze ....; Hydrogen-iron concentration changed to -Hydrogen-ion concentration.</p> - -<p>Page 145: Ztschr. f. Garungs. Physiol. changed to Ztschr. f. Gärungsphysiol.</p> - -<p>Page 146: einige Pilze gegen Hemizellulosen changed to einiger Pilze gegen Hemicellulosen.</p> - -<p>Page 157: Such responses are known chemotropism ... changed to Such -responses are known as chemotropism ....</p> - -<p>Page 170: ... alphatic amino-acids ... changed to ... aliphatic amino-acids ....</p> - -</div><!--TN--> - -<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK THE MICRO-ORGANISMS OF THE SOIL ***</div> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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