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diff --git a/78317-0.txt b/78317-0.txt new file mode 100644 index 0000000..3d4d319 --- /dev/null +++ b/78317-0.txt @@ -0,0 +1,5565 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 78317 *** + + + + +Transcriber’s Notes: + + Underscores “_” before and after a word or phrase indicate _italics_ + in the original text. + Equal signs “=” before and after a word or phrase indicate =bold= + in the original text. + Small capitals have been converted to SOLID capitals. + Illustrations and footnotes have been moved so they do not break up + paragraphs. + Deprecated spellings have been preserved. + Typographical and punctuation errors have been silently corrected. + + + + +THE SOIL SOLUTION + + + + + Published by + The Chemical Publishing Co. + Easton, Penna. + Publishers of Scientific Books + + Engineering Chemistry Portland Cement + Agricultural Chemistry Qualitative Analysis + Household Chemistry Chemists’ Pocket Manual + Metallurgy, Etc. + + + + + The Soil Solution + + The Nutrient Medium for Plant Growth + + By + FRANK K. CAMERON + + In Charge, Physical and Chemical Investigations, + Bureau of Soils, + U. S. Department of Agriculture + + EASTON, PA.: + THE CHEMICAL PUBLISHING CO. + 1911 + + LONDON, ENGLAND: + WILLIAMS & NORGATE + 14 HENRIETTA STREET, COVENT GARDEN, W. C. + + COPYRIGHT, 1911, BY EDWARD HART + + + + +Preface. + + +It has long been the custom to regard soil chemistry from one of two +diametrically opposed points of view. Either, it has been considered +extremely simple, or complex and hopelessly difficult. In either case +the impression has generally prevailed that practical work in soil +chemistry consists in treating the soil with some solvent or other and +analyzing the resulting solution for “available” plant food elements; +in other words, that the chemist’s role in soil studies is merely that +of an analyst. + +Soil chemistry is complex, but not by any means hopelessly so. +Unfortunately, the complexity of most of the problems presented has +deterred the student of pure chemistry from attacking them, and +because they do not offer any material pecuniary rewards, they have +not appealed strongly to the investigator in applied chemistry. +Investigations in soil chemistry, for their own sake, or for the sole +purpose of increasing the sum total of human knowledge concerning the +phenomena taking place in the soil, have been comparatively rare. The +subject has generally been regarded from the analytical point of view +and as incidental to agronomic studies. + +One purpose of this little book is to show the investigator in +chemistry who is not limited by the condition that his work must bring +some personal financial return, that the soil and its problems offer +a field for his efforts quite worthy of ranking along-side the most +interesting branches of pure chemistry, as well as being of the very +highest importance to the development of the welfare of the human race. +Another purpose is to point out the line of attack upon the problems of +soil chemistry which at this time offers the largest opportunity for +results. In how far the details of the story in the following pages are +correct, time with its further investigations will tell. In a sense, +the correctness of the details is of secondary importance. It is of the +first importance, however, that there should be a general recognition +that soil phenomena are essentially dynamic in character, and that the +investigation of the properties of the soil solution and its relation +to crop production is a procedure certain to yield results of positive +value. + +Again, it is a purpose of this book to make available for students +of agriculture, a systematic outline of the work so far accomplished +in this particular field. It is to the students of to-day from whom +are to come the investigations of the near future that the book is +particularly addressed. Some of the details presented in the following +pages are matters on which opposed opinions are now held strongly +by different authorities, and to the unbiased minds of the coming +investigators must be left the decision as to how closely the truth +has been approximated in what is written to-day. The field of effort +covered by this book is one in which there is an increasing activity, +and new facts and deductions will inevitably bring modifications to +present opinions. To encourage this further acquisition of knowledge is +the main purpose of the book. + +The material brought together in this book has been presented to the +faculties and students of several of our Agricultural Colleges, in the +form of a short course of lectures. In large part, moreover, it has +been published in Volume XIV of the Journal of Physical Chemistry. To +make it accessible to and more easily read by one familiar with the +progress of technical soil investigations, it has been recast in its +present form. + +It has been assumed that the reader will have a fair working knowledge +of the concepts of modern chemistry. Nevertheless, an effort has been +made to avoid technical terms so far as this can be done without undue +sacrifice of lucidity of expression. Free references have been made to +the bulletins of the Bureau of Soils, U. S. Department of Agriculture, +because they are generally accessible to the American student, and +because in them will be found detailed discussions and bibliographical +material pertinent to the subjects outlined here. To his coworkers, +the author is indebted for many criticisms and suggestions; and more +especially in the making of the book is he indebted to Mr. S. C. Stuntz. + + Washington, D. C. + 1911. + + + + + Table of Contents. + PAGE + Preface iii + I. The Soil 1 + II. Soil Management or Control 4 + III. Soil Analysis and the Historical Methods of Soil + Investigation 8 + IV. The Plant-Food Theory of Fertilizers 16 + V. The Dynamic Nature of Soil Phenomena 18 + VI. The Film Water 24 + VII. The Mineral Constituents of the Soil Solution 31 + VIII. Absorption by Soils 59 + IX. The Relation of Plant Growth to Concentration 70 + X. The Balance Between Supply and Removal of Mineral + Plant Nutrients 75 + XI. The Organic Constituents of the Soil Solution 79 + XII. Fertilizers 105 + XIII. Alkali 110 + Index 127 + + + + +AN INTRODUCTION TO THE STUDY OF THE SOIL SOLUTION. + + + + +Chapter I. + +THE SOIL. + + +The soil, or that part of the land surface of the earth adapted to +the growth and support of crops, is a heterogeneous mixture composed +of solids, gases and a liquid, and containing living organisms. There +are present: mineral debris from rock degradation and decomposition; +organic matter from the degradation and decomposition of former plant +and animal tissues; the soil atmosphere, always richer in carbon +dioxide and water vapor and possibly other gases than the atmosphere +above the soil; living organisms, such as various kinds of bacteria and +fungi, with the products of their activities, notably the “nitrogen +carriers” and the enzymes; and finally the soil moisture, a solution +of products yielded by the above components and in equilibrium or +approaching equilibrium with the solids and gases with which it is in +contact. + +In its relation to crop plants,[1] that part of the soil of immediate +importance is the soil moisture. From this solution the plants, through +their roots, draw all the material involved in their growth, except +the carbon dioxide absorbed through their leaves. The soil solution is +the natural nutrient medium from which the plants absorb the mineral +constituents which have been shown to be absolutely essential to their +continued existence and development. And from this solution plants +sometimes absorb dissolved organic substances, but such absorptions +are probably adventitious and incidental to the growth of the plant in +a particular environment. While it appears certain that no organic +substance in the nutrient medium is necessary to the maintenance of +plant growth, nevertheless organic substances are probably always +present under natural conditions. They may or may not be absorbed by +the plant and may affect it beneficially or otherwise. + +[1] By crop plants are meant the ordinary green plants employed in +agriculture. As is well-known, the fungi as well as certain parasitic +and saprophytic non-green seed plants obtain their nutriment in a very +different way from ordinary green crop plants. + +The study of the soil solution is of the first importance in the +investigation of the relation of the soil to plant growth, and in the +following pages there is given an outline of our present knowledge of +the chemical principles involved, with such discussion of the physical +and biological factors as is essential to an orderly presentation of +the subject. + +To understand clearly the relations of the soil solution to the soil +as a whole and to the plant which it nourishes, it is desirable to +consider some attributes of soils in general. Every soil, no matter +of what type it may be, is a complex system. In it various processes +are continually in operation, excepting possibly in the extreme case +when it remains frozen for a time at some definite temperature. The +resultant or summation of these processes, whether expressed in +plant production or otherwise, will vary from time to time, both +quantitatively and in direction; for instance, as to the amount and +kinds of plant growth it produces. That is to say, any particular +soil area is seemingly an organic entity, functioning according to +its own inherent properties, but subject to the modifying influences +of environment, as by exceptional climatic extremes, flood, fire, and +especially by artificially imposed agencies of control. + +From the practical point of view the problem of the soil in its +relation to crop production is like the problem of the factory or of +any other industrial endeavor, in that it is a problem of management +or control. The soil possesses this distinction, however, that it +is both the raw material and the factory.[2] The processes involved +are physical, chemical and biological, are always numerous and +interdependent, and are never (speaking generally) exactly the same, +so that each soil possesses marked individuality. No matter how soils +may be classified, as for instance into provinces, series and types,[3] +the fact remains that the soil of the individual field has properties +which give it a crop-producing power, an adaptation to a specific +crop or crop rotation, or a responsiveness to cultural treatment, +which can not be anticipated in any other field. Consequently, there +is no possibility of reducing soil management or agriculture to the +state of an exact science. That is to say, scientific investigation of +the problems involved cannot be expected to yield absolute results, +although furnishing the best possible basis on which to form judgments. +Soil management, like other agricultural practices, is an art, more or +less well founded on scientific principles, perhaps, but susceptible of +much higher development as the scientific principles involved become +better understood. + +[2] According to S. W. Johnson—Some points of agricultural science, Am. +Jour. Sci. (2), =28=, 71-85 (1859)—“The soil (speaking in the widest +sense) is then not only the ultimate exhaustless source of mineral +(fixed) food, to vegetation, but it is the storehouse and conservatory +of this food, protecting its own resources from waste and from too +rapid use, and converting the highly soluble matters of animal exuviæ +as well as of artificial refuse (manures) into permanent supplies.” + +[3] For definitions, see Soil Survey Field Book, 1906, Bureau of +Soils, U. S. Dept. of Agriculture, pp. 15-24. On the ground that +experience has shown that genetic classifications are the ones which +have generally persisted and proved the most useful, objection might be +made to the classification just cited. But a careful inspection of the +results of the Soil Survey by the U. S. Department of Agriculture will +show that while not categorically stating the fact, to all intents and +purposes it has employed a genetic classification. This is exemplified +by the fact that its delineation of soil provinces corresponds quite +closely with the recognized physiographic provinces of the United +States. See map accompanying Soils of the United States, by Milton +Whitney, Bull. No. =55= Bureau of Soils, U. S. Dept. Agriculture, 1909. + + + + +Chapter II. + +SOIL MANAGEMENT OR CONTROL. + + +Aside from such devices as greenhouses, wind-breaks, etc., which have +a local application only, there are three general methods of soil +control: tillage methods, such as plowing and harrowing; rotation of +crops; and the use of soil amendments or “fertilizers.” + +The existing knowledge regarding tillage methods is generally +considered to be fairly satisfactory. The purposes are well understood, +namely, to break up and “fine” the soil,[4] to keep down weeds, and by +forming mulches to decrease the loss of water by evaporation. Not much +increase is being made in our theoretical knowledge of this subject, +although mechanical improvements in the implements of tillage are being +and will undoubtedly continue to be made. + +[4] Actually, to granulate the soil. “Fine” would seem to be a +misnomer, but its agricultural significance is well understood, and it +has the sanction of long usage in the literature. + +The existing knowledge concerning crop rotations is fairly extensive, +but it is almost entirely empirical. Some at least of the purposes +served by a rotation of crops are fairly well known, such as the +elimination of weeds or lower types of parasitic growth associated +with particular crops; the introduction of humus by a grass crop or a +green manure crop, especially by the _Leguminosae_ with their symbiotic +_Azobacteria_; the improvement in the structure or arrangement of the +soil particles by alternating deep-rooted and shallow-rooted crops; +the avoidance of continually growing a crop in the presence of its +own excreta, products of decay, etc.; and lastly, economic and market +considerations. + +The existing knowledge of fertilizers, in spite of a vast amount of +work and an enormous literature, is still very meagre and it also is +almost entirely empirical; and this because studies on the subject have +been dominated for three-quarters of a century by one theory almost to +the exclusion of any other. The exponents of this theory have generally +assumed that the action of fertilizers is on the plant rather than on +the soil, and is independent of other factors. That is, while it is +admitted that other factors influence plant growth, it has been held +that the effect of the fertilizer is not to modify the influence of +the other factors but to directly influence the plant by increasing +its food supply. As a consequence, it has also been generally assumed +that the influence of fertilizers is additive, that is, the increase +in yield of crop is proportional to the increase in fertilizer added, +and the increase in yield produced by adding two fertilizers is the +sum of the increases which would have been produced by each alone. In +this form the theory is essentially a quantitative one, and fertilizer +practice should be easily susceptible of control by chemical analyses. +But the large mass of data obtained from plot experiments shows that +fertilizer effects are not additive. Indeed, the addition of some one +or more fertilizer constituent is sometimes followed by a decreased +yield. For example, about 20 per cent. of the trials of fertilizers +on soils growing corn and reported by the American State Experiment +Stations show a decreased yield. And furthermore, in spite of the +quantitative character of the theory, and the numerous analyses +of soils and of plants which have been made, there is yet lacking +any authoritative method for determining in quantitative terms the +fertilizer needs of a soil. That analytical methods have a very +restricted value in indicating even qualitatively the fertilizer needs +of the soil is evidenced by the fact that within the past few years a +number of the State Experiment Stations have publicly announced their +unwillingness to undertake them.[5] + +[5] In this connection see: The texture of the soil, by L. H. Bailey, +Cornell University Agr. Expt. Sta., Bull. No. =119= (1896); Suggestions +regarding the examination of lands, by E. W. Hilgard, University of +California, College of Agriculture, Circ. No. =25=, (1906); Chemical +analysis of soils, by William P. Brooks, Massachusetts Agr. Expt. Sta. +Circ. No. =11=, (1907); Testing soils for fertilizer needs, by F. W. +Taylor, New Hampshire Agr. Expt. Sta., Circ. No. =2=, (1908); The uses +and limitations of soil analysis, by J. T. Willard, The Industrialist. +Kansas State Agricultural College, =34=, 291, (1908); Soil analysis, +by Wm. Frear, Pennsylvania Agr. Expt. Sta., Chem. Circ. No. =1=; How +to determine the fertilizer requirements of Ohio soils, by Chas. E. +Thorne, Ohio Agr. Expt. Sta., Circ. No. =79=, (1908); Concerning work +which the station can and cannot undertake for residents of the state, +by Joseph L. Hills, Vermont Agr. Expt. Sta., Circ. No. =3=, (1909). + +The common procedure has been to define some arbitrary percentage limit +in the soil, below which the soil is supposed to require fertilizers. +But the amount of fertilizer to be applied is suggested on the +indefinite basis of “experience.” Thus, Hilgard, in an interesting +discussion of this subject,[6] quotes Dyer as showing that “on +Rothamsted soils of known productiveness or manurial condition, it +appears that when the citric acid extraction yields as much as 0.005 +per cent. of potash and 0.010 per cent. of phosphoric acid, the supply +is adequate for normal crop production, so that the use of the above +substances as fertilizers would be, if not ineffective, at least not +a profitable investment.” Hilgard himself sets limits as determined +by strong hydrochloric acid digestion; thus a soil containing upwards +of 0.45 per cent. (K₂O) does not need this substance as a fertilizer, +while one containing below 0.25 per cent. does need it at once, and +intermediate percentages indicate that potash fertilizers would +probably be profitable; the corresponding upper and lower limits for +phosphoric acid are set at 0.10 per cent. and 0.05 per cent. But +Hilgard points out that various things, such as the content of lime, +or the texture of the soil, may materially alter these limits. In a +very interesting set of experiments in which white mustard was grown +in various soils, and these same soils diluted with various amounts of +dune sand which had previously been extracted with strong hydrochloric +acid, he found that the plants did best when the soils had been diluted +with four times their weight of the extracted sand. This was the case +even with a pulverulent sandy loam; and with a black adobe, the best +results were obtained when the diluted soil contained but 0.15 per +cent. potash (K₂O) and 0.04 per cent. phosphoric acid (P₂O₅). It also +appears that Hilgard regards soil analyses of value only in the case of +virgin soils or soils which have been out of cultivation, and in common +with other authorities, he fails to point out how to determine the +_amount_ of fertilizer needed by lands. + +[6] Soils by E. W. Hilgard, 1906, p. 339, _et seq._ + +It is clear, therefore, that the principles underlying the practice or +art of soil management and crop rotation are in a state of development +far from satisfactory, and scientific methods of soil control yet +wanting.[7] Recent activities in soil investigations, however, +justify the hope that much improvement is to be anticipated, and the +application of the modern methods of physical, chemical, and biological +research to the soil problem promises a sure and probably rapid advance +in this branch of applied science. + +[7] It should, of course, be borne in mind that soil factors are not +the only ones in crop production. Control by seed selection, breeding +of standard types of plants, etc., may be, and probably is, more highly +developed than control by soil factors. The same might possibly be +claimed for moisture supply in irrigated areas; but on the other hand, +such factors as the bacterial and lower life processes in the soil are +generally under little or no control, and as a rule the amount and +distribution of sunlight under none at all. A notable effort has been +made in the last case with shade-grown tobacco (see Bulletins Nos. 20 +and 39, Bureau of Soils, U. S. Dept. Agriculture) and a few cases are +known where shade-crops are employed, but not in general agriculture. + + + + +Chapter III. + +SOIL ANALYSIS AND THE HISTORICAL METHODS OF SOIL INVESTIGATION. + + +Owing to the labors of Davy, Boussingault, de Saussure, Liebig, Sachs, +Knop, Salm-Horstmar, and other scarcely less distinguished savants, +it has been clearly shown that _growing plants need certain mineral +elements in order to maintain their metabolic functions_, and that +_these mineral elements can be obtained, under normal conditions, from +the soil_. All subsequent investigation has confirmed these statements +and they can now be accepted as facts with as much assurance as any +known law of nature. + +The determination and formulation of these two fundamental facts came +at a time when analytical chemistry was being rapidly developed and was +finding wide and useful applications in numerous fields of activity. It +was natural, therefore, that analytical chemistry should be enlisted +in this new field of work, obviously of the first importance to the +welfare of mankind. It was early found, however, that the chemical +analysis of a soil fails to explain its relative productivity. In +other words the content of a soil with respect to potash, phosphoric +acid, or other mineral plant-food constituent, bears no necessary +relation to its crop-producing power. Many cases were found where one +soil “analyzed well” but did not produce as large a crop as another +soil which “analyzed poor.”[8] To meet this difficulty a subsidiary +hypothesis was brought forward, which rapidly gained general acceptance +although lacking experimental support. + +[8] See also, Die Aufnahme der Nährstoffe aus dem Boden durch die +Pflanzen, von J. König und E. Haselhoff, Landw. Jahrb., 23, 1009, 1030, +(1894). + +This hypothesis supposes that the mineral constituents of the soil +are present in two different chemical conditions or distinct kinds +of combinations, one of which readily gives up its constituents to +growing plants, while the other does not; and the constituents have, +therefore, been called respectively “available” and “non-available.” +It would appear from his writings that Liebig regarded this distinction +as applying to the “absorbed” or “adsorbed” mineral matter; that +is, on the one hand the material held in or upon the soil grains by +surface forces, and on the other the chemically combined constituents +in the minerals themselves. We know that Liebig was much impressed by +the absorption experiments of Way, and himself did much work in this +field.[9] But the great body of soil investigators has evidently held +to the opinion that there are two general classes of minerals in the +soil. Some have held that the “available” potassium is held in zeolites +or “zeolitic” minerals, an interesting example often cited being +glauconite or “green sand marl,” which sometimes contains phosphorus +as well as potassium;[10] in minerals which are easily broken down by +alkaline solutions, as by sodium carbonate solutions or ammonia; or +in minerals which are easily broken down by organic acids supposedly +excreted from the roots of growing plants, or formed by the decay of +plant tissue.[11] + +[9] Way was misled, as we now know, in considering the results of his +absorption experiments with soils as merely metathetical reactions; see +Absorption by soils, by Harrison E. Patten and William H. Waggaman, +Bull. No. =52=, Bureau of Soils, U. S. Dept. Agriculture, 1908. + +[10] The formation of zeolites in the soil has often been assumed, +but has not yet been proven; see Rocks, rock-weathering and soils, by +George P. Merrill, 1906, p. 363. + +[11] The classic experiments of Sachs, in producing etchings on marble +slabs, and the etchings observed occasionally on rock surfaces are the +proofs universally cited. The experiments of Czapek, who substituted +slabs of aluminum phosphate and other substances for the marble, and +those of Kossowitch, show that the action can be accounted for more +satisfactorily and reasonably as due to dissolved carbon dioxide. In +fact such etchings can be produced on marble slabs by laying platinum +wires upon them and covering with moist soil, or cotton, or mats of +filter-paper; see Bull. No. =22=, p. 14, and Bull. No. =30=, p. 41, +Bureau of Soils, U. S. Dept. Agriculture. + +With the advent of this idea of a distinction between the available and +non-available mineral plant-food elements in the soil, came attempts +to distinguish them by analytical methods. Of these attempts we now +have a bewildering array, most of them frankly empirical. For instance, +Hilgard, in his classical investigation of the cotton soils for the +Tenth Census, treated his soil samples with an excess of hydrochloric +acid, evaporated to dryness, extracted with water, and regarded the +extracted mineral constituents as available. In Germany, a method +similar to Hilgard’s is now in common use, while in France nitric acid +is preferred generally because it is supposed to have peculiar solvent +powers on soil phosphates. In the United States the “official method” +of the Association of Official Agricultural Chemists is to keep 10 +grams of the soil in contact with 100 cc. of a solution of hydrochloric +acid (specific gravity 1.115) at the boiling point of water for exactly +10 hours. In England the popular method is that proposed by Dyer, +namely, to treat the soil with a 1 per cent. citric acid solution, +this strength of solution being supposed at one time to represent the +average acidity of root sap. Maxwell, in Hawaii, and afterwards in +Australia, claimed good results for the extraction of the soil with a +1 per cent. solution of aspartic acid, this acid being employed on the +erroneous ground that the organic acids of the soil are amino acids, +and that these are the effective agents in dissolving the soil minerals +and rendering their constituents “available.” The Kentucky Agricultural +Experiment Station favors an N/5 nitric acid solution,[12] but does not +recommend its use for soils of other localities, while in a contiguous +state, the Tennessee Station favors the “official” method.[13] Many +other methods have been proposed, but the foregoing are typical and +sufficient to illustrate the present status of soil analysis. + +[12] Soils, by A. M. Peter and S. D. Averitt, Bull. No. 126, p. 66, +(1906). + +[13] The soils of Tennessee, by Charles A. Mooers, Bull. No. 78, p. 49, +(1906). + +It is clear that these several methods must give differing results. And +it is not clear that any one of them is to be preferred to the others +for any reasons than analytical convenience. There is no reason to +expect that the proportion of solvent to soil required in these methods +bears any relation whatever to the mechanism of absorption by plant +roots. And the attempts to simulate the properties of plant sap in +some of these solvents are obviously illogical, for the plant sap does +not come in contact with the soil grains, except through an accidental +destruction of the plant. + +Naturally, comparisons were attempted between the amounts of the +mineral constituents extracted from a soil by these various solvents +and the amounts taken up by crops growing on the soil. It was found, +however, that the amount of any given mineral constituent extracted +from the soil by a solvent is not, generally, the same as that taken up +by the plant. Moreover, the ratio of one constituent to another in the +extract bears no definite relation to the ratio of these constituents +in the plant. Nevertheless many efforts were made to establish +“factors.” For instance, the percentage of potash extracted from the +soil of a field by hydrochloric acid is some multiple of the percentage +removed by a wheat crop; it was sought to determine this multiple, +assuming it to be a definite ratio and a natural constant, and it was +designated as the potash factor. But there is a different factor for +phosphorus, another for calcium, and still others for each and every +constituent. The factors found for a soil from one area generally +do not hold for a soil from another area. Again, different factors +obviously must be used for different crops. And, finally, the whole +scheme becomes hopeless when it is realized that the same crop will +yield widely varying ash analyses, depending upon the cultural methods +employed, the judicious selection of seed, the amount and distribution +of rainfall and sunlight, and possibly other agencies, all of which +affect the growth and absorptive functions of the plant to as great an +extent as does the particular soil upon which it may be growing. + +Moreover, from the purely analytical point of view the situation is +no better. For instance, the addition of potassium in the amounts +usually employed in ordinary fertilizer practice generally does produce +a noticeable effect on the yield of crop. The average application +of potash (K₂O) is certainly less than 50 lbs. to the acre. It is +customary to consider the surface foot of soil as the region affected +by the fertilizer, and an acre foot in good moisture condition weighs +about 4,000,000 lbs. To be conservative, let it be assumed that 60 +lbs. of potash have been added to 3,000,000 lbs. of soil. The official +method of the Association of Official Agricultural Chemists calls for +the determination of the potash in 2 grams of soil, which on the basis +of the present assumption calls for the estimation of an added amount +of 0.00004 gram of potash or 0.002 per cent. Taking as an example +the report of the Association of Official Agricultural Chemists for +1895[14] there are given the following results obtained independently +by a number of analysts, on soils which had presumably been sampled by +the referee with all possible care: + +[14] Proceedings of the Twelfth Annual Convention of the Association of +Official Agricultural Chemists, Bull. No. 47, Division of Chemistry, U. +S. Dept. Agriculture, p. 36, (1896). + + POTASH CALCULATED AS PER CENT. OF THE FINE DRIED EARTH. + + ============================================================= + | 1 | 2 | 3 | 4 + Analyst +-----+------+-----+------+-----+-----+-----+-------- + | Per | | Per | | Per | | Per | + |cent.| Var.|cent.| Var.|cent.| Var.|cent.| Var. + --------+-----+------+-----+------+-----+-----+-----+-------- + A |0.359| 0.044|0.154|-0.002| — | — | — | — + B |0.345| 0.030|0.112|-0.044|0.380|0.051|0.104|-0.050 + C |0.354| 0.039|0.235| 0.079|0.396|0.067|0.225| 0.071 + D |0.260|-0.055| — | — | — | — | — | — + E |0.373| 0.058|0.179| 0.023|0.365|0.036|0.175| 0.021 + F |0.210|-0.105|0.130|-0.026|0.220|0.109|0.109|-0.045 + G |0.304|-0.011|0.125|-0.031|0.286|0.043|0.158| 0.004 + Mean |0.315| — |0.156| — |0.329| — |0.154| — + --------+-----+------+-----+------+-----+-----+-----+-------- + +Not only do the individual determinations show differences far in +excess of 0.002 per cent., but the differences between each individual +reading and the mean is greater than 0.002 per cent., so that it is +evident from these results that the analytical procedure fails to +recognize appreciable amounts of the so-called available plant foods. +Consequently the “acid digestion” of a soil fails of the purpose for +which it was designed, and it is one of the mysteries of chemical +history that so much time and energy have been devoted to such a +hopeless quest. + +This state of affairs is the more surprising when the limitations of +the analytical procedure are considered. The data tabulated above +indicate that the analyses were made with an exactness that justifies +a statement to three decimal places, that is, to three significant +figures; and in fact, as was shown, such is necessary if the figures +are to have any significance regarding fertilizer applications. It is +obvious that the analysis of a finely pulverized definite mineral or +rock is less subject to error than a sample of soil sifted through +a 2 mm. mesh. Yet the U. S. Geological Survey commonly reports its +analytical data to only hundredths of a per cent., that is, to +two decimal places. What variation may be expected in duplicate +determinations by the same analysts it is difficult to say, for such +duplicates are not commonly published.[15] In spite of the widespread +view that the chemical analysis of a soil is a statement of great +accuracy, it is improbable that as usually determined the potash +content is correct to three or even two significant figures; it is +also doubtful if the phosphoric acid content is correct to even one +significant figure, if the total amount is below 0.1 per cent. of the +soil. That these determinations have a higher accuracy than here stated +is not shown by an inspection of the literature including the fairly +numerous results reported in the annual Proceedings of the Association +of Official Agricultural Chemists. + +[15] See: On the interpretation of mineral analyses, by S. L. Penfield, +Amer. Jour. Sci., (4), 10, 33, (1900); The analysis of silicate and +carbonate rocks, by W. F. Hillebrand, Bull. No. 305. U. S. Geol. Surv., +1907; Manual of the chemical analysis of rocks, by H. S. Washington, +1904, p. 24; Über Genauigkeit von Gesteinanalysen, von M. Dittrich, +Neues Jahrbuch für Mineralogie und Palaeontologie, 2, 69, (1903). + +It was early felt by some investigators that soil analyses were +unsatisfactory for studying the relation of the soil to the food +requirements of a crop, and a second method was devised, namely, the +growing of a crop, and determining the amount of mineral constituents +removed from the soil by analyzing the ash of the crop. From the +point of view of practical soil management this procedure involves +the serious difficulty of being first obliged to get the crop before +determining what must be done to best get it. It apparently has the +scientific advantage of directness in determining the mineral needs +of the plant from the plant itself. If these needs were constant, the +advantage would be real, but as already mentioned, one and the same +plant may have a very different ash content as the result of different +cultural methods, different climatic and seasonal factors, as well +as different soils. Generally, a poor crop has a higher percentage +of ash content than a good crop, and sometimes the poor crop may +remove from the soil more in absolute amounts of some one or other of +the ash constituents than does the good crop. The ratio of the ash +constituents is by no means constant for any one crop, and of course +varies with different crops.[16] Finally, it is now known that the +amount of the several mineral nutrients which a soil must furnish to a +crop in the earlier stages of growth is greater than the crop contents +at maturity,[17] consequently an analysis of the ripe crop would not +indicate the plant’s drain upon the soil at all growing periods. So +that, while ash analyses have taught some important things concerning +plant growth, they have of necessity failed as guides or criteria of +the crop-producing power of a soil, its fertilizer requirements, or its +content of “available” plant-food. + +[16] For a brief but comprehensive discussion of ash analyses see, The +ash constituents of plants, etc., by B. Tollens, Expt. Sta. Rec., 13, +207-220, 305-317, (1901-02). + +[17] Über die Nährstoffaufnahme der Pflanzen in verschiedenen Zeiten +ihres Wachstums, von Wilfarth, Römer und Wimmer. Landw. Vers. Sta., 63, +1-70, (1905); Plant food removed from growing plants by rain or dew, by +J. A. Le Clerc and J. F. Breazeale, Year Book, U. S. Dept. Agriculture, +1908, p. 389-402. + +A third method of soil investigation, also essentially analytical in +character, is the plot or pot test. The difference between a plot or +pot experiment is mainly one of size, although it is claimed, and with +a certain amount of justice, that the plot experiment more nearly +approximates actual practice, and should be given a somewhat different +consideration than the more readily controlled pot experiment. Here +again it has to be considered that seasonal factors and factors other +than the soil play a relatively large part in the production of the +crop, so that conclusions regarding the productivity of a soil can +not be drawn from one season’s crop. Also, nowadays it is recognized +generally that continuous growing of one crop is an incorrect practice, +and a rotation should be followed and repeated several times before +conclusions regarding the productivity of the soil are justified. +If, however, the rotation has been well managed, the cultivation, +fertilizing and soil management generally been well done for sixteen, +twenty or more years, the soil has materially changed, and there can +be no assurance that the treatment then best for it, is that which was +best at the beginning of the experiment. Therefore the method throws no +certain light on the productive power of the soil, or the availability +of its mineral plant-food constituents. Although much has been learned +from plot experiments, and especially from the better controlled pot +experiments, they are inadequate to meet the fundamental problem +of the relation of the chemical characteristics of the soil to its +crop-producing powers. + + + + +Chapter IV. + +THE PLANT-FOOD THEORY OF FERTILIZERS. + + +The guiding principle in soil investigations for about three-quarters +of a century and until the past few years has been the assumption that +the principal function of the soil is to furnish mineral nutrients to +the plant, and that, to supply a lack in the soil, fertilizers are +added because of the mineral plant nutrients they contain. This theory +has apparently much to support it; actually, however, the evidence +usually cited accords better with a more comprehensive generalization +which will be formulated in a later chapter. It is attractively simple. +It will be shown later, however, that this very simplicity is an +argument against its validity. + +Those substances which experience has shown to be useful soil +amendments usually contain one or more of the constituents necessary +to plant metabolism, commonly phosphorus, potassium, nitrogen or +calcium. Fertilizers do not always produce increased yields of crops, +but it has been usual to consider bad results as due to other more or +less extraneous causes. Moreover, as will appear later, crop yield is +as strongly affected by some substances containing no mineral plant +nutrient as by ordinary fertilizers. Again, the plant-food theory +has been apparently confirmed by the popular misconception that crop +yields are decreasing. Government statistics, however, indicate very +positively that crop yields are increasing in Europe as well as in +America, more in areas where the acreage is stationary than in areas +where the acreage is increasing, and in areas where fertilizers are +not used as well as in areas where they are used. Analyses of European +soils which have been cropped for centuries show no characteristic +differences from the newer soils of the United States.[18] It is true +that, from bad management or other causes, individual fields where crop +production has fallen off are not uncommon. But that such a condition +is general or that it can be associated generally with a decreased +content in the soil of any particular mineral substance or substances, +is a conclusion not sustained by the available data. + +[18] A study of crop yields and soil composition in relation to soil +productivity, by Milton Whitney, Bull. No. 57, Bureau of Soils, U. S. +Dept. Agriculture, 1909. + +The plant-food theory of fertilizers must now be regarded as entirely +insufficient. Granting that it has been useful in the past and has +occasioned much valuable work, it seems to have reached the point +which another simple and temporarily useful theory, the phlogiston +theory of combustion, reached shortly before the plant-food theory of +fertilizers was evolved. Just as the phlogiston theory passed away when +the elementary nature of oxygen was established and Lavoisier taught +the scientific world to use the balance, so the plant-food theory of +fertilizers must pass with increasing knowledge of the relation of +soil to plant and the application of modern methods of research to the +problem. + + + + +Chapter V. + +THE DYNAMIC NATURE OF SOIL PHENOMENA. + + +In soil investigations, until recently, the assumption has been made, +more or less explicitly, that any given soil mass, as for instance a +field, remains fixed or in place indefinitely. It has been admitted, +of course, that some physical, chemical and biological processes +might be taking place in the soil, but these have been regarded as +relatively unimportant in their effects upon the soil mass _in toto_. +It has been assumed that the only important change taking place in the +soil is a loss of mineral plant nutrients, partly by leaching, partly +by removal in the garnered crops. In other words, the soil has been +regarded as a static system. This is a fundamental error. In studying +the soil as a medium for crop production, we must consider the plant +itself, or at least that part of the plant which enters the soil, +namely, the root; the solid particles of the soil; the soil water, +or the aqueous solution from which the plant draws all the materials +for its sustenance, excepting the carbon dioxide absorbed by its +aerial portions; the soil atmosphere; the biological processes taking +place. The one common characteristic of all these things is that they +are continually in a state of change; therefore the soil problem is +essentially dynamic. + +The root of a growing plant is always moving.[19] The amount of motion +may be small or large, depending upon the surrounding conditions or +attendant circumstances, but cessation of motion means the death of +the root. This becomes evident from a consideration of the mechanism of +root growth. The living root absorbs and excretes water and dissolved +substances through a restricted area just back of the root tip or the +tips of the root hairs. While absorption is taking place, however, +there is a deposition of denser material over the absorbing area, +or “root corking.” But coincident with the corking process, the tip +is pushed forward between the soil grains into the nutrient medium, +new cells are formed and a new absorbing surface continually brought +into functional activity. A failure of the plant root to move forward +in this way would mean a reabsorption of root effluvia with harmful +consequences to the plant, or a corking over of the root without +further formation of absorbing surface and with consequent cessation +of its functioning. This would mean the inevitable death of the root, +and, if general, of the whole plant. It is clear, therefore, that root +penetration and absorption of plant nutrients are essentially dynamic. + +[19] In order to penetrate the soil, a living root must be capable +of exerting large pressures, and indeed, the magnitude of these +pressures has been determined for some cases. See, for citations of +the literature, Pfeffer, Plant Physiology, translated by Ewart, 1903, +Vol. 2, p. 124 _et seq._ But it can not be doubted that, in general, +root movement is much facilitated and perhaps directed by movements +among the soil particles. As the absorbing tip of the root removes film +water from the adjacent soil grains, there is a necessary rearrangement +of these grains with a shrinking away from the tip, which then moves +forward by taking advantage of the movements among the soil grains. + +The solid components of the soil are always in motion. Every soil, no +matter how flat the area or how well protected by vegetal covering, +suffers some translocation of soil material through rains, as is +evidenced by suspended material in the run-off waters. On hillsides +this is shown by the soil accumulating on the “up” sides of fences, +especially stone fences. In the aggregate this movement is probably +quite large everywhere. It is manifestly so in the watersheds of many +of the world’s important rivers as shown by their muddy waters and +the formation of deltas, sometimes of great area and agricultural +importance. + +With the saturation or approach to saturation of the surface soil the +particles are more easily moved among themselves by an extraneous +force. It is very rarely that the surface of a field is a dead level. +Consequently when the soil is wetted, the gravitational force on the +individual soil grains produces a more or less pronounced “creeping” +effect down hill. On decided slopes this soil creep is believed to be +of great importance in connection with soil erosion.[20] + +As important as is the translocation of material by water, quite as +important probably is that produced by the winds. These are blowing all +the time, uphill as well as down, and their range of action is thus +far wider than is that of rain and flood. The effectiveness of the +wind as a translocating agency is seldom realized or even suspected by +the layman, although it is commonly known that the air always contains +some dust, and dust storms are familiar phenomena. That soil material +can be carried long distances is certain, however, as for instance +the sirocco dust, often carried from the Sahara over Europe.[21] Dust +carried high into the air by volcanic eruptions sometimes travels +enormous distances, as in the case of the eruption of Krakatoa, when +such material is reported to have traveled thousands of miles, and +volcanic debris from the eruptions at Soufrière fell upon ships several +hundred miles distant. Arctic explorers have reported the finding of +wind-borne soil materials over the polar ice, and mountaineers have +observed similar deposits on snow-capped peaks. Soil material on roofs +and similar inaccessible places has been observed many times, and +testifies to the continual activity of the wind. The burial of objects +even of considerable size by wind-borne soil gives like testimony. + +[20] Soil erosion is undoubtedly one of the greatest economic problems +of the time, and yet there is scarcely any subject about which there +are current so many popular misconceptions. In the rivers and to those +who use the rivers the water-borne soil material is an unmitigated +nuisance, save possibly to a few cultivators of low-lying lands who +for one reason or another, may flood their fields for the sake of the +silt deposited. To the upland farmer, however, erosion is not only a +necessity of natural conditions which can not be avoided entirely, but +under proper control it may be even a blessing. The scalded and gullied +hillsides, a trial and unnecessary disgrace to the owner, are probably +not the main sources of the material which finds its way to the river. +On the contrary, what are regarded usually as well-tilled fields supply +the greater part of the suspended material in the rivers. The problem +of erosion on the farm is not merely to check gullying and scalding, +and deepening of stream heads, but to so adjust both cropping system +and cultural methods as to secure a reasonable translocation of surface +soil material with a minimum contamination of the neighborhood streams. +See, Man and the earth, by Nathaniel Southgate Shaler, 1905. + +[21] For a comprehensive discussion of wind as a translocating agent, +see: The movement of soil material by the wind, by E. E. Free, Bureau +of Soils, Bull. No. 68, U. S. Dept. Agriculture. + +Measurements of the amount of action of wind in translocating soil +material are rare and probably have a qualitative value only. But +Udden[22] in what appears to be a conservative calculation, finds “the +capacity of the atmosphere [over the Mississippi Valley] to transport +dust is 1000 times as great as that of the [Mississippi] River.” The +wind seldom is carrying anything like so great a load as it is capable +of carrying. That is, the wind in its attack upon the land surface +does not ordinarily obtain so large an amount of material capable of +being wind-borne as it is possible for the wind to carry when suitable +material is artificially provided. It should be remembered that, +speaking generally, the velocity of the wind is lower just at the +surface of the ground than at heights above, and it is necessary to get +the soil material above the surface before the wind can exercise its +full efficiency as a carrying agent. + +[22] Erosion, transportation and sedimentation performed by the +atmosphere, by J. A. Udden, Jour. Geol., 2, 318-331 (1894). + +Moreover, wind-borne material is constantly being deposited as well +as being removed from the land surface. It is evident, however, that +this movement of soil material by winds is very great, and there +is no reason to believe that it is of any less importance in other +areas than in the Mississippi Valley. It is also evident that the +individual grains in any surface soil of any particular field or area +are continually and more or less rapidly changing, and the farmer is +not dealing to-day with just the same soil complex he faced a few years +back, or will face a few years hence. + +But besides the movements of the solid components of the soil by +translocating agencies, other movements are constantly taking place. +Whenever a moderately dry soil becomes wetted, it “swells up” until a +certain critical amount of moisture is present above which there is +a shrinking. But as a wet soil dries out again below the critical +amount, there is again a shrinking. As it is always either raining or +not raining, soils are always either getting wetted or are drying. +Consequently the individual grains are continually moving about among +themselves. A heavy object, such as stone, when left on the ground +gradually sinks into it.[23] Earthworms, burrowing animals and insects +are continually at work in most arable soils. The action of frost in +“heaving” a soil is familiar to everyone. Not so well known, however, +is the fact that the apparently superficial cracks which occur to a +greater or less extent in every soil, under drought conditions, are +in reality quite deep, extending well into the subsoil. By the edges +breaking off, and by wind- and water-borne material being carried in, +considerable surface soil is thus brought into the subsoil. Through +these various agencies, therefore, the solid components of the soil are +continually subject to much mixing; subsoil is becoming surface soil, +and to some extent _vice versa_. An important result of these various +processes is the bringing into the surface soil of degradation and +decomposition products from underlying rocks. The processes involved +are essentially dynamic.[24] + +[23] On the small vertical movements of a stone laid on the surface +of the ground, by Horace Darwin, Proceedings of the Royal Society of +London, 68, 253-261, (1901). On the other hand, geological literature +would probably furnish numerous references to the heaving out of +boulders, probably as the result of successive freezings and thawings +of the soil. The shape of the stone as well as the specific nature +of the movements of the soil particles evidently has an important +influence in determining whether the stone sinks into the soil or _vice +versa_. + +[24] It is clear that as the soil is continually changing through +physical agencies, the chemical analysis of it can not be expected to +furnish evidence as to the mineral constituents removed by crops or by +leaching. + +The soil solution is also a dynamic problem. When the rain falls on +the soil, a part, the “run-off,” flows over the surface and finds its +way into the regional drainage; a part immediately evaporates into the +air, and is designated as the “fly-off;” a third part, the “cut-off,” +enters the soil.[25] The cut-off water penetrates the soil by way of +the larger openings and interstices, and mainly under the influence +of gravity. For convenience this downward-moving water is designated +as “gravitational” water. It moves through the soil with comparative +rapidity and a portion reappears elsewhere as seepage water, springs, +etc. But with the return of fair-weather conditions at the surface, +there is increased evaporation and augmentation of the fly-off, and +there is developed a drag or “capillary pull” on the water below. +A large portion of the cut-off thus returns to the surface, mainly +through films over the surface of the soil grains and in the finest +interstices.[26] + +[25] This terminology has been suggested by Dr. W. J. McGee. + +[26] Leather, however, thinks the water returns from only a limited +depth, some 5-7 feet; see, The loss of water from soil during +dry weather, by J. Walter Leather, Memoirs of the Department of +Agriculture, Agricultural Research Institute, Pusa, India, Chemical +series, I, 79-116, (1908). Dr. George N. Coffey has called the author’s +attention to some observations in Western Kansas, where a prolonged +drought had dried the soil to a considerable depth. A fairly heavy rain +wetted the soil to less than two feet from the surface, and practically +all of this moisture had returned to the surface and evaporated +within a few days. Such special cases as these, however interesting +in themselves, are even less so than the normal cases in humid areas, +where a part of the water passes through the soil as seepage, the +larger portion returning to the surface, sometimes through distances of +many feet. + +The soil atmosphere is continually in motion, following with more or +less decided lag the barometric changes in the atmosphere above the +soil. Moreover, the chemical and physical processes continually taking +place in the soil involve the absorption or the formation of free +carbonic acid, and it seems probable that all rain water penetrating +the soil gives up some oxygen to the soil atmosphere. The bacteria +and lower life forms are necessarily undergoing changes continually. +In fact all components of the soil are continually undergoing, or are +involved in, changes of one kind or another. + +It is certain that investigation of the various motions and changes +taking place in the soil is quite as important as investigation of the +soil components, and that no clear idea of the chemistry of the soil +can be obtained without it. The development of a rational practice of +soil control is possible only when the soil is regarded from a dynamic +viewpoint. + + + + +Chapter VI. + +THE FILM WATER. + + +When a relatively small quantity of water is added to an absolutely +dry soil or other powdered solid, there is some shrinkage in the +apparent volume of the soil or powder. The water spreads over the +surfaces of the solid particles in a film, and a rise in temperature +shows that a noticeable energy change accompanies the formation of the +film.[27] With further increments of water the apparent volume of the +soil increases until a maximum is reached. The water content at which +this maximum volume of soil can be attained is a definite physical +characteristic for any given soil. What is popularly known as the +“optimum water content” corresponds to this critical content.[28] It +is the point at which further additions of water will not increase the +thickness of the moisture film on the soil grains, but will give free +water in the soil interstices. Just as the apparent volume of a given +mass of soil varies with the water content, and reaches a maximum at +a critical moisture content, so do all the physical properties vary +and have either a maximum or minimum value at this same critical +moisture content. Thus the apparent specific gravity of a soil reaches +a minimum, the force required to insert a penetrating tool becomes a +minimum, while the rate at which a soil warms up reaches a maximum,[29] +and the ease with which aeration takes place reaches a maximum. In +fine, this critical water content is that at which the soil can be +brought into the best possible physical condition for the growth of +crops. The practical significance of the optimum water content is far +greater than would be supposed from the attention given it hitherto +by students of the soil. It is the content of soil water which the +greenhouse man should strive to maintain, and which the irrigation +farmer should seek to provide, instead of the over-wetting so common to +the practice of both. In general farming it is that moisture content +at which the farmer will attain the best results in plowing and +cultivating, and attain these results most readily. + +[27] See, in this connection, Energy changes accompanying absorption, +by Harrison E. Patten, Trans. Am. Electrochem. Soc., 11, 387-407, +(1907); see also the recent valuable research, Les dégagements de +chaleur qui se produisent an contact de la terre sèche et de l’eau, +par A. Muntz et H. Gaudechon, Ann. sci. agron. (3), 4, II, 393-443, +(1909), where it is shown that probably a part of the heat is due to +chemical combination between the water and the other soil components. +To quote, “Ces diverses observations nous conduisent à penser, sans +nous en donner toutefois la preuve absolute, que la fixation de l’eau +sur les éléments terreux très fins et sur les matériaux organisés, est +tout au moins, en partie, attribuable à une combinaison chimique qui se +manifeste non seulement par un fort dégagement de chaleur, mais aussi +par la soustraction de l’eau à des substances aux-quelles elle semble +chimiquement liée.” + +[28] The moisture content and physical condition of soils, by Frank +K. Cameron and Francis E. Gallagher, Bull. No. 50, Bureau of Soils, +U. S. Dept. of Agriculture, 1908. See also Über physikalische +Bodenuntersuchung, von H. Rodewald, Schriften Naturwiss. Vereins +Schleswig-Holstein, 14, 397-399, (1909). + +[29] Heat transference in soils, by Harrison E. Patten, Bull. No. 59, +Bureau of Soils, U. S. Dept. Agriculture, 1909. + +With additions of water beyond the critical point, there is a presence +of free water in the soil interstices accompanied by important changes +in the soil structure. With continued additions, there is a more or +less rapid decrease in the apparent volume; there is a tendency for +the soil aggregates to break down and the “crumb structure” so greatly +desired by agriculturists is less and less readily obtained, and +working of the soil tends in some cases to produce that phenomenon +known as “puddling.” However desirable the property of puddling may +be to the potter or the brick maker, to the farmer it is a bane to be +avoided above all things. To overcome it requires his best skill, and +it usually takes several years of patient effort to restore a puddled +soil to good tilth. + +The force with which the film water is held against the soil grains +has not been determined as yet with any degree of precision, but +it is certainly very great. If a soil be saturated, that is, if so +much water be added that further additions will cause a flow of free +water, and the soil be then submitted to some mechanical device for +abstracting the water, the moisture content of the soil can be readily +diminished to the critical water content; but to diminish it further +by mechanical means is not easy. The tenacity with which film water +is held by the soil grains has been shown in several ways. In one of +these, for instance, a semi-permeable membrane was precipitated in the +walls of a porous clay cell, which was then filled with sugar solution +having an osmotic pressure of about 35 atmospheres. When this cell was +buried in a soil having a moisture content above the optimum, water +flowed into the cell. On the contrary, when the cell was buried in +another sample of the same soil having a moisture content well below +the optimum, there was a marked flow of water from the cell. It would +appear, therefore, that the attraction between the soil grains and the +film-forming water was certainly greater than the solution pressure of +the sugar.[30] Again, by whirling wetted soils in a rapidly revolving +centrifuge,[31] fitted with a filtering device in the periphery, +and developing a force equivalent on the average to 3,000 times the +attraction of gravitation, the soils could not be reduced below the +critical water content. From the results of Lagergren,[32] Young,[33] +and Lord Rayleigh,[34] it appears that the force holding a very thin +moisture film on the soil grains would be of an order of magnitude from +6,000 to 25,000 atmospheres. This force, however, must greatly decrease +with thickening of the film, as is shown by the fact that at the +critical moisture content a small further addition of water produces +no marked heat manifestation, though making a noticeable difference in +the physical properties of the soil. Therefore, while recognizing that +our knowledge of this force still lacks a desirable precision, it is +nevertheless clear that the force is very great. + +[30] The chemistry of the soil as related to crop production, by Milton +Whitney and Frank K. Cameron, Bull. No. 22, Bureau of Soils, U. S. +Dept. Agriculture, 1903, p. 54. + +[31] The moisture equivalent of soils, by Lyman J. Briggs and John W. +McLane, Bull. No. 45, Bureau of Soils, U. S. Dept. Agriculture, 1907. + +[32] Über die beim Benetzen fein verteilter Körper auftretende +Wärmetönung, von Lagergren, Bihang till K. sv. Vet.-Akad., Handl., 24, +Afd. II, No. 5, (1898). + +[33] Hydrostatics and elementary hydrokinetics, George M. Minchin, p. +311, 1892. + +[34] On the theory of surface forces, by Lord Rayleigh, Phil. Mag. (5), +30, 285-298, 456-475, (1890). + +The function of the film water in maintaining the soil structure is +undoubtedly important. A soil in good tilth, or good condition for +crop growth, shows a peculiar structural arrangement of the individual +soil grains or soil particles, which it is very difficult to describe +in precise terms, but which is readily recognized in practice. This +condition is usually described as a “crumb structure,” either because +of its appearance or because of the peculiar crumbly feeling which +a soil in this condition gives when rubbed between the fingers. The +individual grains of soil are gathered into groups or floccules. +While other causes may be more or less operative in particular cases, +it seems very probable that the film water is primarily the agency +holding together the grains in these floccules. The obvious explanation +is that the film is exerting a holding power because of its surface +tension. It follows, therefore, that anything which affects the surface +tension of water should affect the structure of the soil; that is, +the flocculation or granulation of the particles. But certain agents +which produce respectively flocculation or deflocculation, nevertheless +modify the surface tension of the solution in the same direction, and +in not widely varying degree. Similar difficulties arise in attempting +to correlate “crumbing” phenomena with the viscosity of the film +water,[35] and it must be admitted frankly that present views on this +subject are very unsatisfactory, and that more careful investigation is +urgently needed on this fundamental and important problem. Not only is +the absence of a satisfactory theory embarrassing in considering the +problems of soil structure and a rational control, but the difficulties +are no less in the equally important problems of the movement of film +moisture, and the distribution of moisture in a soil. + +[35] Equally unsuccessful is the attempt to correlate flocculating +agents with changes in the density of water. See, The condensation of +water by electrolytes, by F. K. Cameron and W. O. Robinson, Jour. Phys. +Chem., 14, 1-11, (1910). + +The movement of moisture into a soil from an illimitable supply is a +comparatively simple phenomenon, controlled by a rate law which may be +expressed by the equation _yⁿ_ = _kt_ when _y_ is the distance through +which the movement has taken place; _t_ is the time, and _k_ and _n_ +are characteristic constants for the particular soil and solution.[36] +This expression may be more readily recognized as a rate formula +when written _dy/at_ = A_yᵐ_, where A and _m_ are constants for the +particular system. The first form of the equation promises to be the +more useful. This formula also describes the rate of advance of a +dissolved substance into the soil. + +Owing to irregularities in the soil column this equation is more +readily studied with capillary tubes or with such absorbents as +filter-paper or blotting paper. The following tables will, however, +give an idea as to its validity for soils. + + ALLUVIAL SOIL, GILA RIVER.[37] + + ===============+====================+================= + Time,_t_ min. | Height,_y_ inches | _k_ (_n_ = 1.86) + ---------------+--------------------+----------------- + 2 | 1.5 | 1.05 + 5 | 2.4 | 1.02 + 10 | 3.6 | 1.08 + 15 | 4.3 | 1.01 + 30 | 6.3 | 1.05 + 60 | 9.2 | 1.07 + ---------------+--------------------+----------------- + + DISTILLED WATER IN PENN. LOAM (_t_ = 21° C). + + ==========+==============+================ + Time,_t_ | Height,_y_ | _k_ + min. | cm. | (_n_ = 2.25) + ----------+--------------+---------------- + 1 | 1.15 | 1.37 + 2 | 1.54 | 1.33 + 3 | 1.85 | 1.33 + 4 | 2.08 | 1.30 + 5 | 2.28 | 1.28 + 7 | 2.59 | 1.21 + 10 | 2.97 | 1.16 + 15 | 3.47 | 1.10 + 20 | 3.90 | 1.07 + 30 | 4.67 | 1.06 + 40 | 5.39 | 1.11 + 50 | 5.90 | 1.09 + 60 | 6.47 | 1.12 + 75 | 7.20 | 1.13 + 90 | 8.03 | 1.21 + 105 | 8.72 | 1.25 + ----------+--------------+---------------- + +[36] See Bull. No. =30=, Bureau of Soils, U. S. Dept. Agriculture, p. +50 _et seq._; also, The flow of liquids through capillary spaces, by J. +M. Bell and F. K. Cameron, Jour. Phys. Chem., =10=, 659, (1906); See +also, Wo. Ostwald, 2 Supplementheft Zeitschrift Kolloidchemie, 1908, 20. + +[37] Computed from observations by Loughridge, Report Agr. Expt. Sta., +University California, 1893-94, p. 93. + + INDIGO CARMINE IN PENN. LOAM SOIL (_t_ = 21° C.). + + Solution contained 2 grains dye per liter. + =========+============+==============+================+============= + Time,_t_| Height,_y_ | _k_ for water| Height colored | _k_ for dye + min. | wet cm. | (_n_ = 2.25)| cm. | (_n_ = 2.25) + ---------+------------+--------------+----------------+------------- + 1 | 1.28 | 1.75 | 0.64 | 0.37 + 2 | 1.67 | 1.59 | 0.90 | 0.39 + 3 | 2.05 | 1.68 | .. | .. + 4 | 2.26 | 1.56 | .. | .. + 5 | 2.49 | 1.56 | 1.02 | 0.21 + 7 | 2.74 | 1.38 | .. | .. + 10 | 3.20 | 1.40 | .. | .. + 15 | 3.72 | 1.29 | .. | .. + 20 | 4.28 | 1.32 | 1.92 | 0.22 + 30 | 5.10 | 1.31 | .. | .. + 40 | 5.77 | 1.29 | 2.69 | 0.23 + 50 | 6.41 | 1.26 | 3.20 | 0.28 + 60 | 6.90 | 1.29 | .. | .. + 75 | 7.46 | 1.23 | .. | .. + 90 | 8.74 | 1.46 | 3.59 | 0.20 + 105 | 9.00 | 1.33 | .. | .. + ---------+------------+--------------+----------------+------------- + +It has also been shown repeatedly by experiment that the movement of +moisture is relatively rapid when the moisture content of the soil +is above the optimum, but that the movement is exceedingly slow when +the soil has a lower water content than the optimum; that is, the +point at which the water is entirely in the form of film water. For +instance, if a moderately wet sample of soil be brought into intimate +contact with an air-dry sample of the same soil, there will, at first, +be a relatively rapid movement of the moisture, but as soon as the +wetted portion has been brought to the “optimum” condition, no further +movement can be detected, although the experiment has been tried of +leaving such samples together for months and with a difference of +water content amounting, in the case of clay soils, to 15 or 20 per +cent. Since the drought limit, or the soil moisture content at which +plants wilt, is, for most soils, considerably below the optimum water +content, the movement of film water is obviously a problem of the first +importance from a practical point of view as well as of the highest +theoretical interest. + +The movement of water vapor, or its distillation from place to +place in the soil, is another problem often confused with the +above. Its importance is not yet clear, although according to some +investigators[38] it would appear that the addition of soluble +fertilizer salts by causing a lowering of the vapor pressure of the +water induces a distillation to that region from other regions of the +soil as well as from the atmosphere above. This brings up the problem +of the diffusion of water and other vapors through the soil. It has +been shown that the soil “plug” retards the rate at which diffusion +takes place but induces no other effect in the ordinary phenomenon of +free diffusion. This fact is obviously of the first importance in the +theory of mulches, but requires no further consideration here.[39] + +[38] Sur la diffusion des engrais salins dans le terre, par Muntz et +Gaudechon, Comptes rendus, =148=, 253-258, (1909). + +[39] See, Contribution to our knowledge of the aeration of soils, and +Studies of the movement of soil moisture, by Edgar Buckingham, Bulls. +Nos. =25=, 1904, and =33=, 1907, Bureau of Soils, U. S. Dept. of +Agriculture. + + + + +Chapter VII. + +THE MINERAL CONSTITUENTS OF THE SOIL SOLUTION.[40] + + +The mineral constituents of the soil are products of the +disintegration, degradation and decomposition of rocks. The +decomposition products are mainly silica in the form of quartz, +ferruginous material consisting of more or less hydrated ferric +oxide and alumina, and hydrated aluminum silicate. The ferruginous +material, being deposited or formed in the soil in a very finely +divided condition, frequently coats the soil fragments to such an +extent as completely to mask their true character. But if a soil be +thoroughly shaken with water, and especially in the presence of some +deflocculating agent such as a slight excess of ammonia, as in the +ordinary preparation of a soil sample for mechanical analysis[41] +the coating material is generally removed quite readily, and the +mineral particles appear as fragments and splinters of the ordinary +rock-forming minerals. Sometimes these fragments are more or less +worn and rounded at the edges, showing mechanical abrasion or solvent +action; sometimes they show evidences of partial alteration and +decomposition; but surfaces of the unaltered mineral individuals always +are found. These unaltered minerals occur as fragments of all sizes, +and are to be found in the sands, silts, and presumably in the clays. +As might be anticipated, the minerals other than quartz generally show +a tendency to segregate in the finer mechanical separations of the +soil. The presence of these unaltered mineral fragments in the clays +has so far defied direct experimental proof because of the limitations +of the microscope, but from chemical reasoning and _a priori_ +considerations there can be but little doubt that they exist in the +clays as in the coarser separations.[42] + +[40] For a more detailed discussion and citations of the literature, +see The mineral constituents of the soil solution, by Frank K. Cameron +and James M. Bell, Bull. No. =30=, Bureau of Soils, U. S. Dept. +Agriculture, 1905. + +[41] Centrifugal methods of mechanical soil analysis, by L. J. Briggs, +F. O. Martin and J. R. Pearce, Bull. No. =24=, Bureau of Soils, U. S. +Dept. Agriculture, 1904. + +[42] See, The mineral composition of soil particles, by G. H. Failyer, +J. G. Smith and H. R. Wade, Bull. No. =54=, Bureau of Soils, U. S. +Dept. Agriculture, 1909. Recent improvements in microscope methods make +it possible to identify without serious trouble the mineral content of +silts with a diameter as low as 0.005 mm., and many even of the clay +particles have recently been determined with satisfactory accuracy. + +The minerals to be anticipated in the soil are those commonly occurring +in the rocks; but as a result of the action of mixing and transporting +agencies, a soil normally contains minerals from rocks other than those +from which it is primarily derived. + +It would hardly be fair to regard a beach sand, for instance, as a +normal soil. Yet it is surprising how many minerals other than quartz +can usually be found even in a beach sand. Opinions may differ as to +just what are the common rock-forming minerals, and perhaps no two +mineralogists or petrographers would give identical lists, but there +are a number of minerals which would appear undoubtedly in every list, +and these would be found generally in any soil. Again, it might happen +that in any given sample of soil, no pyroxene, for instance, could +be found; but experience shows that it would never happen in such a +case that no amphibole, chlorite, serpentine, or other ferro-magnesian +silicates would be present. However distinct these minerals cited may +be from each other morphologically or optically, they are much the +same in their chemical characteristics, their solubilities and their +reactions with water and such dilute solutions as exist in the soil. +Hence from the point of view of the soil chemist they may be considered +for all practical purposes varieties of one and the same mineral +species. Consequently an important result of researches on the minerals +of the soil is the generalization that soils are far more heterogeneous +than are rocks, and that _practically every soil contains all the +common rock-forming minerals_.[43] + +[43] See Bull. No. =30=, Bureau of Soils, U. S. Dept. Agriculture, +1905, p. 9. + +It is not difficult to account for the heterogeneity of the mineral +content of the soil. Many of our rocks are reconsolidated soils, and +the alternating formation of rock and soil from the same materials +is probably an agency, in some part at least, in the mixing of soil +material. The action of water in carrying off and transporting surface +material and in gullying and eroding sloping surfaces is probably a +large factor. But this agency, like the first, is rather restricted +and localized. Just as important as a mixing agency is the wind. This, +unlike water, works uphill as well as down, and is more or less in +action at all times, continually transporting soil material from place +to place. Wind-borne dust on roofs of dwellings, on rocky mountain tops +and similar places, where it could have been brought by no other agency +than the wind, is sometimes found supporting vegetation. Many chemical +and mineralogical analyses of wind-borne dust obtained from various +locations show it to have generally the same essential characteristics +as ordinary soils. + +Aside from the quartz and ferruginous materials mentioned above, +the major part of the soil minerals are silicates, ferro-silicates, +alumino-silicates, or ferro-alumino-silicates, of the common bases, +sodium, potassium, calcium, magnesium, and ferrous iron. Other +bases, such as lithium, barium, or the heavy metals may occasionally +be present in appreciable amounts as may other types of silicates, +or other mineral salts, but these may be regarded as more or less +incidental and rarely affecting in any essential way the general +character of the soil mass. These silicates or silico minerals are all +somewhat soluble in water, and being salts of weak acids with strong +bases, are greatly hydrolyzed. A convenient illustration is afforded +by the well-known rock and soil mineral, orthoclase. Assuming its type +formula, the reaction with water may be represented, + + K.AlSi₃O₈ + HOH ⇆ H.AlSi₃O₈ + KOH. + +Under ordinary soil conditions, with a relatively large proportion of +carbon dioxide in the soil atmosphere, the potash formed would be more +or less completely transformed to the bicarbonate, + + KOH + CO₂ + H₂O ⇆ KHCO₃ + H₂O. + +Confirmation of this view is afforded by the natural associations and +known alteration products of orthoclase. + +The acid of the formula H.AlSi₃O₈ is not known and is probably entirely +instable under ordinary conditions, and breaks down with the separation +of silica, to form the minerals pyrophyllite, kaolinite or kaolin, and +diaspore according to the following equations: + + H.AlSi₃O₈ - SiO₂ = H.AlSi₂O₆ (Pyrophyllite) + H.AlSi₃O₈ - 2SiO₂ = H.AlSiO₄ (Kaolinite) + H.AlSi₃O₈ - 3SiO₂ = H.AlO₂ (Diaspore). + +All three of these minerals and their corresponding salts have been +found in nature as alteration products of orthoclase. It is probable +that, under soil conditions, the principal metamorphic product of +feldspar is kaolin (or kaolinite when it is crystalline), hydrated +aluminum oxide being of much less importance[44] and pyrophyllite of +doubtful occurrence. A still more interesting case, perhaps, because +of the well recognized tendency of magnesium salts to form basic +compounds, is the alteration of pyroxene, amphibole and olivine with +the formation of a chlorite or serpentine, common associations in +nature, which may be represented + +[44] See Ueber die Bildung von Bauxit und verwandte Mineralien, von A. +Liebrich, Zeit. prakt. Geol., =1897=, 212-214. + + MgSiO₃ + HOH ⇆ MgSiO₃._n_Mg(OH)₂ + SiO₂. + + +It is tacitly assumed in the foregoing statements that the reaction +between a silicate mineral and water is a reversible reaction. This is +not definitely known to be the case, for the formation of the ordinary +silicate rock-forming minerals in the wet way at ordinary temperatures +has as yet been realized in only a few cases. The assumption has, +however, some experimental support. Minerals have been often made in +the wet way at somewhat elevated temperatures, especially interesting +cases in this connection being the formation of orthoclase by Friedel +and Sarasin[45] at slightly elevated temperatures, and the formation +of zeolites by Gonnard[46] and by Doroshevskii and Bardt,[47] and the +formation of apatite by Weinschenk.[48] Feldspars and zeolites are +common natural associations, it being generally conceded that zeolites +are alteration products of the feldspars through the action of water; +but Van Hise[49] has pointed out that under conditions of weathering +such as would obtain in the soil, the tendency is for the zeolites to +alter to feldspars. Wöhler’s classical experiment of recrystallizing +apophyllite from hot water[50] is significant, for only the products +of hydrolysis should be obtained if there is an irreversible reaction +between the mineral and water. Lemberg found that leucite (KAlSi₂O₆) +when treated with an aqueous solution containing 10 per cent. or +more of sodium chloride, was partially transformed to analcite +(NaAlSi₂O₆._n_H₂O), potassium chloride being formed at the same time. +The reverse reaction was also realized, that is, the partial conversion +of analcite to leucite by treatment with a solution of potassium +chloride, and similar transformations were carried out with the +feldspars.[51] Lemberg’s experiments are of especial value as they were +carried out at ordinary as well as at high temperatures. It appears +probable, therefore, that the hydrolysis of a silicate of the alkalis +or alkaline earths is a reversible reaction. It should be noted, +however, that Kahlenberg and Lincoln[52] have shown that probably, +in very dilute solutions of alkali silicates, the hydrolysis is +practically complete and the silica is nearly all present as colloidal +silica and not as silicic acid. Nevertheless at higher concentrations +silicates are formed, and there is abundant evidence in nature that the +alumino- or ferro-silicates are reacting with bases to form salts, for +example such as the micas.[53] If the hydrolysis were quite complete, +it would appear to follow that the reaction between water and the +silicate is irreversible. In that case it is difficult to see how any +silicate mineral could persist in the soil for any length of time, +and all soils should soon become sterile wastes composed essentially +of quartz, kaolin and ferruginous oxides. It has been suggested that +the original mineral particles are protected from decomposition by +the formation of a coating “gel.” That is, that silica, alumina, +ferruginous or other materials result from the decomposition of the +minerals in a jelly-like form on the surface of the soil grains, +protecting them from further action of the soil solution.[54] If +diffusion can take place through the gel, solution and hydrolysis of +the mineral would proceed, although the presence of the gel would +probably retard the rate of the reaction. If it be postulated, however, +that diffusion through the gel does not take place, the minerals of the +soil can have no influence on the composition of the soil solution, +which is an unthinkable alternative. The presence of such gels in the +soil has frequently been assumed, but satisfactory proof is generally +wanting. + +[45] Sur la reproduction par voie aqueuse du feldspath orthose, par +Friedel et Sarasin, Comptes rendus, =92=, 1374, (1881). + +[46] Note sur une observation de Fournet, concernant la production des +zéolites a froid, par F. Gonnard, Bull. Soc. min. France, =5=, 267-269, +(1882); Jahrb. Min., =1884=. I, Ref. 28. + +[47] Metathetical reactions with artificial zeolites, by A. +Doroshevskii and A. Bardt, Jour. Russ. Phys. Chem. Soc., =42=, 435-42 +(1910). Chem. Zentr., 1910, II, 68. + +[48] Beiträge zur Mineralsynthesis, von E. Weinschenk, Zeit. Kryst., +=17=, 489-504, (1890). + +[49] U. S. Geol. Surv. Monograph, =47=, A treatise on metamorphism, by +Charles R. Van Hise, 1904, p. 333. + +[50] Jahresb. Fortschr. Chemie Liebig and Kopp, =1847-48=, 1262; note. + +[51] Ueber Silicatumwandlungen, von J. Lemberg, Zeit. deutsch. geol. +Ges., =28=, 519-621, (1876); Inaug. diss. Dorpat, =1877=; Bied. +Centbl., =8=, 567-577, (1879). + +[52] Solutions of silicates of the alkalis, by L. Khlenberg and A. T. +Lincoln, Jour. Phys. Chem., =2=, 77-90, (1898). + +[53] Van Hise, loc. cit., p. 693. + +[54] A gel is a jelly-like substance, apparently continuous, which +forms either by the settling from suspension in a liquid of very +fine particles which then become aggregated; or, is formed by the +evaporation of a liquid containing fine particles in suspension until +the quantity of liquid remaining is just sufficient to serve as a +cementation medium holding the suspended particles together in a +semi-rigid mass. For an experimental demonstration of the formation of +such a gel, see, The effect of water on rock powders, by Allerton S. +Cushman, Bull. No. =92=, Bureau of Chemistry, U. S. Dept. Agriculture, +1905. + +In general, the same kind of considerations developed for orthoclase +hold for the other soil minerals. If minerals of this character be +pulverized or ground reasonably fine and then be shaken with distilled +water which has been previously boiled to eliminate the dissolved +carbon dioxide, the resulting solution will give an alkaline reaction +with such indicators as phenolphthalein or litmus.[55] If a soil be +shaken up thoroughly with water, the resulting solution filtered free +of suspended matter, as by passing through a Pasteur-Chamberland +bougie, and then boiled to eliminate the carbon dioxide, in the vast +majority of cases the solution will also give an alkaline reaction +with phenolphthalein or litmus. The waters of most of our springs, +ponds, creeks or rivers being natural soil solutions, give an alkaline +reaction after boiling. + +[55] In making such experiments in the laboratory or in lecture +demonstrations, it is well to have the mass of water large in +comparison with the mass of powdered mineral or rock; otherwise +secondary adsorption effects may occur and obscure the results of the +hydrolysis. + +But the mineral content of these natural waters varies greatly. These +waters are composed in part of the “run-off,” in part of a portion of +the “cut-off” waters, described above. This portion of the cut-off, +normally, in passing through the soil goes mainly through the larger +interstices. It is not long in contact with the individual soil +particles and floccules, and because diffusion of dissolved mineral +substances is quite slow, especially in dilute solutions, it takes up +but little mineral matter from such aqueous films as it may intercept. + +A different state of things exists with that portion of the cut-off +water which returns towards the surface by reason of capillary forces, +to form the great natural nutrient medium for plants. This water is +moving over the soil particles in films, and with slowness. It _is_ +long in contact with successive fragments of any particular mineral +and all the different minerals making up the soil. Consequently, it +tends towards a saturated solution with respect to the mineral mass; +and it follows that if every soil contains all the common rock-forming +minerals, every soil should give the same saturated solution, barring +the presence of disturbing factors.[56] Disturbing factors, however, +enter into all cases under field conditions, such for instance as the +presence of some uncommon or unusual mineral in appreciable amounts, +differences in temperature, surface effects, or extraneous substances. +These will be considered later, but another disturbing factor requires +immediate consideration. + +In every soil, varying proportions of the soluble mineral constituents +are present otherwise than as definite mineral species; that is, they +are present as solid solutions, or absorbed on the soil grains or +perhaps absorbed in some other manner. The concentration of the liquid +solution in contact with a solid solution or complex of absorbent and +absorbed material is dependent upon the relative masses of solution and +solid. Thus, the concentration of a solution with respect to phosphoric +acid, when brought into contact with so-called basic phosphates of lime +or iron, is dependent in a marked way upon the proportion of solution +to solid.[57] Consequently it is to be expected that an aqueous extract +of a soil will vary in concentration with the proportion of water +used; and that with the same proportion of water, different soils or +different samples of the same soil will yield different concentrations. + +[56] Feldspars certainly, and phosphorites possibly, are mineral +components of the soil; and these substances when ground sufficiently +fine have been added to soils with sometimes an increased production +of crop. Other minerals, such as leucite, have given similar results. +But also apparently pure quartz sand sometimes accomplishes the same +results, as for example, in the experiments of Hilgard cited above. +It has not been shown, however, that the addition of any of these +substances produces an appreciable change in the concentration of the +soil solution. + +[57] The action of water and aqueous solutions upon soil phosphates, by +Frank K. Cameron and James M. Bell, Bull. No. =41=, Bureau of Soils, U. +S. Dept. of Agriculture, 1907. + +How far absorbed mineral constituents affect the solubility of the +definite minerals in the soil or influence the concentration of the +soil solution, it is not possible to predict with any approach to +certainty. Those soils which hold the most moisture are generally the +best absorbers. Moreover, the soluble mineral constituents of the soil, +for instance potassium or phosphoric acid, are absorbed to a very high +degree from dilute solutions. Consequently it is to be expected that +variations in the concentration of the natural soil solution would be +less than in aqueous extracts, when there is employed a constant and +relatively large proportion of water to soil. These considerations +are of great theoretical importance since they appear to negative +the possibility of getting, with present experimental resources, any +_exact_ knowledge of the concentrations of the mineral constituents in +the soil solution when the soil is in condition to grow the common crop +plants. Moreover, they furnish a guide to the limitations which must be +recognized in attempting to postulate what these concentrations may be +on the basis of analytical data obtained from aqueous soil extracts. + +Many attempts have been made to extract the solution naturally existing +in the soil and to analyze it. The results obtained have not been very +satisfactory, owing mainly to the mechanical difficulties involved. As +pointed out above, the solution in a soil under suitable conditions for +crop growth is held by a force of great magnitude. Nevertheless, by +using powerful centrifuges, with saturated soil, it has been possible +to throw out the excess of solution over the critical water content +of the soil. In this way small quantities, generally a very few cubic +centimeters at a time, have been obtained. The analysis of a few cubic +centimeters of a very dilute solution is in itself difficult, involving +necessarily more or less uncertainty as to the absolute value of the +results. Nevertheless, the concentration of the soil solutions thus +obtained, with respect to phosphoric acid and potash, varied but little +for soils of various textures from sands to clays, and the variations +observed could not be correlated with the known crop-producing power +of the soils. The average concentrations of the soil solutions thus +obtained lies in the neighborhood of 6-8 parts per million (p.p.m.) of +solution for phosphoric acid (P₂O₅) and 25-30 parts per million for +potash (K₂O).[58] In the following table are given the results obtained +by analyzing solutions extracted from different samples of loams and +sands by means of a centrifuge. The crop growing on these soils and the +crop condition at the time the samples were collected are given in the +table, and the percentages of water in the samples when placed in the +centrifuge are also given. + +[58] In this connection it is interesting to note that recent +investigations on the proportions of phosphoric acid, potassium and +nitrates in cultural solutions best adapted to the growth of wheat, +give the same ratio of phosphoric acid to potassium as the figures just +cited show to exist normally in the soil solution. + + ANALYSIS OF SOIL SOLUTION REMOVED FROM FRESH SOILS + BY THE CENTRIFUGE. + + ==================+=======+==========+=========+================== + | | | |Parts per million + | | | | of solution + Soil | Crop | Condition|Per cent +--------+----+---- + | | of crop |moisture.| PO₄ | Ca | K + ------------------+-------+----------+---------+--------+----+---- + Leonardtown loam | Wheat | Good | 22.0 | 6 | 17 | 22 + Leonardtown loam | Wheat | Poor | 25.2 | 10 | 9 | 19 + Leonardtown loam | Wheat | Good | 17.6 | 8 | 22 | 38 + Sassafras loam | Clover| Good | 19.7 | 5 | 18 | 19 + Sassafras loam | Corn | Medium | 17.5 | 8 | 13 | 36 + Sassafras loam | Corn | Medium | 18.3 | 8 | 83 | 25 + Sassafras loam | Wheat | Good | 18.8 | 7 | 44 | 34 + Sassafras loam | Wheat | Poor | 20.0 | 7 | 27 | 24 + Sassafras loam | Corn | Good | 17.3 | 8 | 24 | 25 + Norfolk sand | Forest| Poor | 10.0 | 5 | 18 | 31 + Norfolk sand | Corn | Good | 11.9 | 11 | 36 | 31 + Norfolk sand | Wheat | Good | 10.7 | 18 | 45 | 31 + Norfolk sand | Wheat | Poor | 11.2 | 8 | 38 | 24 + Norfolk sand | Corn | Medium | 10.6 | 9 | 65 | 35 + ------------------+-------+----------+---------+--------+----+---- + +The concentrations of the solutions obtained from the samples do not +justify any correlation with the crop-producing power of the soils, nor +with the texture of the soils. The wide variation in the concentrations +with respect to calcium is probably due to the fact that all of the +samples came from fields which had been limed, some quite recently, +and that the content of carbon dioxide in the different samples +varied. It is of special interest to note that the content of calcium +in the solutions does not show any obvious relation to the content of +phosphoric acid.[59] + +[59] For the literature of the earlier work on the composition of +aqueous extracts of soils, see: How crops feed, by Samuel W. Johnson, +1890, p. 309 _et seq._; see also. On the analytical determination of +probably available “mineral” plant-food in soils, by Bernard Dyer, +Jour. Chem. Soc. =65=, 115-167, (1894); and Soils, by E. W. Hilgard, +1906, p. 327 _et seq._ + +An effort has been made to ascertain the mineral concentration of +soil solutions as they occur naturally in the field. Because of the +practical impossibility of extracting the actual soil solution, an +empirical method was employed. Areas were selected where good and +poor crops were growing near each other on the same soil types, and +preferably in the same field. Samples of soil from under these crops +were taken at several intervals during the growing season, quickly +removed to a nearby laboratory, shaken thoroughly with distilled water +in the proportion of one part of soil to five parts of water, allowed +to stand twenty minutes and the supernatant solution passed through a +Pasteur-Chamberland filter.[60] + +[60] Capillary studies and filtration of clays from soil solutions, by +Lyman J. Briggs and Macy H. Lapham, Bull. No. =19=, Bureau of Soils. +U. S. Dept. Agriculture, 1902; Colorimetric, turbidity and titration +methods used in soil investigations, by Oswald Schreiner and George H. +Failyer, Bull. No. =31=, Bureau of Soils, U. S. Dept. Agriculture, 1906. + +As has been pointed out above, the aqueous extract of a soil thus +arbitrarily prepared has no definite or causal relation to the +soil solution in the field. It is certain that the solutions would +not generally be the same. It should also be emphasized that such +a procedure can not, as some investigators have assumed, afford a +criterion between soluble and insoluble salts in the soil, else the +proportion of water to soil used above some minimum would be immaterial +as far as the amounts which go into solution are concerned. The +proportion of water to soil is not immaterial, however, considering the +chemical nature of the soil components and the results of experiment. +Consequently, it is clear that the concentration of the soil solution +is not simply the ratio of the amounts found in the aqueous extract, to +the percentage of moisture in the soil, but something quite different. + +Artificial solutions prepared in the manner described above should, +however, furnish evidence as to whether or not there are recognizable +differences in the soluble mineral constituents of good and poor +soils respectively; and if such differences exist, whether they are +consistent. That is to say, if the more productive soils also uniformly +yield aqueous extracts of a higher concentration, then it would be a +fair inference that their natural soil solutions are maintained at a +higher concentration than in the less productive soils. + +Results obtained for several localities and several crops, taken from +the original records, are given in the following tables.[61] + +[61] The chemistry of the soil as related to crop production, by Milton +Whitney and F. K. Cameron, Bull. No. =22=, Bureau of Soils, U. S. Dept. +Agriculture, 1903. + + WATER SOLUBLE CONSTITUENTS OF SOIL. + + Locality, Salem, N. J. Soil type, Norfolk sand. Crop, wheat. + Yield, good. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | Per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + March 10 | 0-12 | 13.2 | 12 | 5 | 12 + | 12-24 | 11.5 | 7 | 5 | 16 + June 8 | 1-24 | 4.3 | 4 | 14 | 13 + June 13 | 1-24 | 4.6 | 5 | 13 | 17 + June 19 | 1-24 | 9.6 | 2 | 14 | 24 + ---------+-------+----------+-------------+---------+--------------- + + Locality, Salem, N. J. Soil type, Norfolk sand. Crop, wheat. + Yield, poor. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | Per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + April 3 | 0-12 | 12.0 | 11 | 5 | 32 + | 12-24 | 12.0 | 10 | 3 | 22 + June 16 | 1-24 | 9.3 | 4 | 29 | 20 + ---------+-------+----------+-------------+---------+--------------- + + Locality, Salem, N. J. Soil type, Sassafras loam. Crop, wheat. + Yield, medium. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + March 10 | 0-12 | 23.2 | 19 | 10 | 8 + | 12-24 | 21.6 | 11 | 10 | 14 + March 14 | 0-12 | 22.3 | 18 | 8 | 18 + | 12-24 | 20.2 | 15 | 12 | 21 + | 24-36 | 20.3 | 18 | 17 | 16 + March 20 | 0-12 | 19.3 | 7 | 10 | 21 + | 12-24 | 18.6 | 4 | 11 | 21 + | 24-36 | 12.6 | 5 | 12 | 21 + June 16 | 1-24 | 22.5 | 4 | 14 | 23 + ---------+-------+----------+-------------+---------+--------------- + + Locality, Salem, N. J. Soil type, Sassafras loam. Crop, grass. + Yield, fair. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | Per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + March 10 | 0-12 | 25.0 | 13 | 28 | 18 + | 12-24 | 23.8 | 7 | 26 | 13 + | 24-36 | 19.9 | 16 | 8 | 15 + March 14 | 0-12 | 25.8 | 21 | 12 | 21 + | 12-24 | 23.1 | 8 | 12 | 15 + | 24-36 | 21.8 | 9 | 15 | 21 + March 31 | 0-12 | 23.0 | 11 | 23 | 43 + | 12-24 | 21.6 | 8 | 20 | 34 + April 2 | 0-12 | 24.8 | 8 | 16 | 41 + | 12-24 | 24.0 | 6 | 21 | 38 + | 24-36 | 21.4 | 3 | 11 | 25 + ---------+-------+----------+-------------+---------+--------------- + + Locality, Salem, N. J. Soil type, Sassafras loam. Crop, wheat. + Yield, good. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | per cent.| acid (PO₄ | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + March 17 | 0-12 | 22.0 | 8 | 6 | 10 + | 12-24 | 18.1 | 8 | 15 | 14 + March 17 | 0-12 | 18.3 | 10 | 15 | Lost + | 12-24 | 18.1 | 9 | 24 | 25 + March 24 | 0-12 | 24.7 | 14 | 12 | 30 + | 12-24 | 22.3 | 8 | 11 | 38 + March 26 | 0-12 | 23.4 | 4 | 16 | 16 + | 12-24 | 23.9 | 12 | 16 | 20 + | 24-36 | 22.4 | 8 | 3 | 21 + April 2 | 0-12 | 25.6 | 8 | 16 | 30 + | 12-24 | 24.4 | 8 | 17 | 47 + | 24-36 | 21.6 | 8 | 11 | 38 + June 5 | 0-12 | 5.2 | 14 | 51 | 23 + | 12-24 | 8.0 | 15 | 55 | 32 + June 8 | 1-24 | 10.6 | 2 | 20 | 13 + June 11 | 1-24 | 15.5 | 6 | 26 | 14 + June 13 | 1-24 | 8.2 | 6 | 19 | 22 + June 16 | 1-24 | 15.0 | 5 | 21 | 19 + June 17 | 1-24 | 10.6 | 7 | 63 | 17 + ---------+-------+----------+-------------+---------+--------------- + + Locality, Salem, N. J. Soil type, Sassafras loam. Crop, clover. + Yield, fair. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + March 20 | 0-12 | 20.8 | 5 | 15 | 32 + | 12-24 | 20.2 | 5 | 15 | 27 + | 24-36 | 18.6 | 5 | 12 | 36 + March 26 | 0-12 | 26.8 | 9 | 31 | 20 + | 12-24 | 22.9 | 8 | 20 | 18 + | 24-36 | 22.5 | 4 | 14 | 20 + June 6 | 0-12 | 8.1 | 8 | 16 | 17 + | 12-24 | 12.7 | 9 | 18 | 20 + ---------+-------+----------+-------------+---------+--------------- + + Locality, St. Marys, Md. Soil type, Leonardtown loam. Crop, wheat. + Yield, good. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + April 27 | 0-12 | 21.8 | 5 | 10 | 12 + | 12-24 | 21.3 | 4 | 7 | 10 + April 29 | 0-12 | 22.2 | 8 | 15 | 52 + | 12-24 | 21.8 | 4 | 11 | 38 + May 1 | 0-12 | 22.4 | 7 | 14 | 23 + | 12-24 | 21.8 | 7 | 8 | 30 + May 1 | 0-12 | 17.0 | 5 | 16 | 25 + | 12-24 | 21.0 | 5 | 7 | 19 + May 9 | 0-12 | 15.0 | 13 | 34 | 28 + | 12-24 | 15.9 | 9 | 17 | 26 + May 15 | 0-12 | 14.2 | 3 | 14 | 24 + | 12-24 | 19.9 | 4 | 13 | 25 + August 14| 0-24 | 15.0 | 6 | 11 | 13 + August 15| 0-24 | 15.7 | 5 | 3 | 17 + August 15| 0-24 | 16.4 | 8 | 15 | 15 + ---------+-------+----------+-------------+---------+--------------- + + Locality, St. Marys, Md. Soil type, Leonardtown loam. Crop, wheat. + Yield, poor. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + May 14 | 0-12 | 14.7 | 5 | 8 | 35 + | 12-24 | 19.9 | 4 | 4 | 30 + May 23 | 0-12 | 7.8 | 4 | 7 | 22 + | 12-24 | 14.9 | 4 | 11 | 23 + August 14| 0-24 | 16.0 | 4 | 4 | 16 + August 15| 0-24 | 19.5 | 6 | 4 | 13 + ---------+-------+----------+-------------+---------+--------------- + + Locality, St. Marys, Md. Soil type, Leonardtown loam. Crop, corn. + Yield, good. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + May 8 | 0-12 | 18.2 | 9 | 12 | 29 + | 12-24 | 18.9 | 10 | 7 | 26 + May 18 | 0-12 | 18.2 | 3 | 24 | 38 + | 12-24 | 18.8 | 6 | 19 | 28 + August 8 | 0-24 | 17.5 | 7 | 30 | 18 + ---------+-------+----------+-------------+---------+--------------- + + Locality, St. Marys, Md. Soil type, Leonardtown loam. Crop, corn. + Yield, poor. + =========+=======+==========+======================================= + | | | Parts per million of oven-dried soil + | Depth | Moisture +-------------+---------+--------------- + Date | inches| content | Phosphoric | Calcium | Potassium + | | per cent.| acid (PO₄) | (Ca) | (K) + ---------+-------+----------+-------------+---------+--------------- + May 23 | 0-12 | 16.6 | 5 | 12 | 22 + | 12-24 | 17.4 | 6 | 8 | 22 + August 8 | 0-24 | 19.9 | 9 | 25 | 20 + August 15| 0-24 | 21.6 | 7 | 15 | 13 + ---------+-------+----------+-------------+---------+--------------- + +It will be observed that the results given in the above tables are +expressed in parts per million of oven-dried soils, in order to have +some definite basis of comparison, and because it was anticipated +at the time the investigation was made that larger quantities of +dissolved minerals would be found under the better crops, and _vice +versa_. An inspection of the results, however, shows that no such +correlation can be made, nor in fact can any consistent correlation be +made between the dissolved material and crop, soil type, water content, +depth of soil or part of the growing season.[62] It appears, therefore, +that in so far as the field method of analyzing an arbitrarily prepared +aqueous extract is competent, there is no evidence that there are +important characteristic differences in the concentration of the +mineral constituents in different soil solutions in the field. + +[62] King, however, claims that the concentration of the soil solution +with respect to mineral plant nutrients, is higher in the soils of +the northern states than in the soils of the South Atlantic states. +See: Some results of investigations in soil management, by F. H. King, +Yearbook, U. S. Dept. Agriculture, 1903, p. 159-174. Bailey E. Brown +has obtained some preliminary results which suggest that there may +be seasonal variations with respect to some of the dissolved mineral +constituents. See, Annual Report of the Pennsylvania State Experiment +Station, 1908-9, pp. 31 _et seq._ + +The order of concentration of the soil solution can be approximated +from the given data, if the assumption be made that in the preparation +of the aqueous extract, soluble mineral constituents are of minor +importance, other than the constituents already dissolved in the soil +solution. The calculation is very laborious, is not exact, and on +account of the assumptions made the actual figures obtained are of no +especial value in any particular case. Remembering the method of making +up the solutions from which these results were obtained, it would be +sufficiently near the truth to assume an average moisture content of 20 +per cent., when the figures given here for the soil approximate those +which would be obtained for the soil solution. More exact calculations +have been made for a large number of such cases, and it has been +found from this method of estimation that the average composition +with respect to phosphoric acid would be about 6-8 parts per million, +and for potash about 25 parts per million, figures which agree with +the results obtained for the examination of solutions extracted from +saturated soils by means of the centrifuge. + +The results given in the foregoing tables were obtained under +great difficulties, and in some part the variations they show are +undoubtedly due to inevitable inaccuracies of analytical work done +under such circumstances. Some of the variations may also be due to +the disturbing influences in the soil referred to above. Experience +has shown, however, that the preparation of an aqueous extract of the +soil of any particular field is by no means a simple matter. Extracts +made from samples taken within a few feet of one another frequently +show variations of the same order as with samples from entirely +different fields, or even soil types. Differences in the preliminary +drying out of the sample before the addition of the water, seems to +result in the same order of differences as obtained between different +soils. In consequence of these facts, and of the further fact that an +arbitrary aqueous extract of a soil cannot be assumed to represent in +any definite way the natural soil solution, the results of the field +examination are inconclusive as to the concentration of the soil +solution _in situ_. It is more necessary, therefore, that other lines +of evidence should be sought as to the mineral characteristics and +concentration of the soil solution. Such a line of evidence is found in +certain percolation experiments.[63] + +[63] The absorption of phosphates and potassium by soils, by Oswald +Schreiner and George H. Failyer, Bull. No. =32=, Bureau of Soils, U. S. +Dept. Agriculture, 1906. + +If a solution of a soluble phosphate be percolated through a soil, a +part of the phosphate will be removed from the solution and absorbed +by the soil; that is, there will be a redistribution of the phosphate +between the soil and the water. As the process continues, however, +relatively less and less phosphate is absorbed by the soil and the +concentration of the percolate becomes more and more nearly that of +the added solution. This absorption takes place more or less closely +in accordance with the simple law that the absorption of phosphates by +the soil, per unit of solution which is percolating, is proportional +to the total amount of phosphate which the soil may yet take from that +solution if percolated indefinitely. This law is expressed by the +equation + + _dy_ + —————— = _K_(_A_ - _y_) + _dx_ + +where _y_ is the amount absorbed, _x_ amount of solution that has +passed, and _A_ is the total amount which can ultimately be absorbed +by that particular soil from that particular solution. _K_ is also a +characteristic constant. If the percolation be maintained at constant +rate, then _t_, time, can be substituted for _x_ and the equation +becomes + + _dy_ + ———— = _K_(_A_ - _y_), + _dt_ + +the ordinary rate equation for a mono-molecular reaction of the first +order, whether chemical or physical. + +With such absorptions as are involved in soils, a clay exposes a +greater amount of absorbing surface than does a loam or sand, and it +will show the greatest absorption towards any particular solution, +other things being equal. The curve showing the concentration of +percolate would lie lower for a clay than for a loam, or for a sand. +This is illustrated in the accompanying sketch diagram, where _y_ +represents concentration of percolate and _t_ represents time. + +[Illustration: Fig. 1.] + +If after percolation has proceeded for some time (in some experiments +for several weeks and until the soil contained 1 or 2 per cent. of +phosphoric acid) pure water be passed through the soil, then, as soon +as the previously used phosphate solution has been displaced, the +concentration of the percolate drops and continues practically constant +for an indefinite period. Moreover, no matter what the soil may be +as to texture or composition, the same concentration of percolate is +obtained, namely, 6-8 parts per million, the concentration which the +soils yielded prior to treatment with the phosphate solution. Similar +experiments when the soils were treated with salts of potassium have +given like results, although the curves obtained from passing pure +water through the soils do not lie quite so close together; but the +concentration of the percolate with respect to potassium generally +lies somewhere between 25 and 30 parts per million. + +The removal of a soluble constituent from the soil by percolating water +appears to be described by a rate equation similar to that given above +for absorption. If the rate of percolation be maintained constant this +formula is + + _dx_ + ————— = _K_(_B_ - _x_) + _dt_ + +where _x_ is the amount removed by the percolation, with time _t_, +_K_ is a constant characteristic for the particular system under +consideration, and _B_ is the total amount of the constituent which may +ultimately be leached out. In other words, the rate in any particular +soil will depend upon the amount of the constituent still absorbed in +that soil but has no necessary connection with the rate which would +hold for the same amount of the constituent in any other soil. + +Theoretically, two consequences follow from this law which require +consideration here. The rate at which a constituent is removed +gradually becomes less as percolation proceeds. If the soil contains +an amount of the constituent approaching the total amount which it +can absorb, as for instance is probably the case sometimes when large +applications of lime have been made to the soil, the concentration +of the percolating solution might be expected to change noticeably. +Generally, however, a soil contains nowhere near as much phosphoric +acid or potassium as it is capable of absorbing, so that the +concentration of the percolating water changes but very little with +respect to these constituents. It follows from the equation that +if percolation continues uninterrupted, the concentration of the +percolate, so far as it is determined by an absorbed constituent, must +get less and less until it becomes a vanishing quantity. This state +of affairs does not exist in the soil, however, for percolation by +pure water does not continue uninterrupted for any length of time. The +rise of the capillary water in the soil will, under normal conditions, +enable the soil to reabsorb more of the ordinary mineral constituents +than is removed by percolating waters. Further attention will be given +the matter in another chapter. + +[Illustration: Fig. 2.] + +Another but quite different line of evidence as to the probable +concentration of the soil solution is furnished by the investigation of +the solubility of certain phosphates.[64] It is popularly supposed that +when superphosphate containing mono-calcium phosphate, CaH₄(PO₄)₂.H₂O, +is added to a soil there is a more or less permanent increase of +readily soluble phosphoric acid in the soil, although a part “inverts” +to the somewhat less soluble dicalcium phosphate, Ca₂H₂(PO₄)₂·2H₂O. +Such probably is far from a correct view of what actually takes place. +The results obtained by studying the solubility of the different lime +phosphates in water at ordinary temperature (25° C.) can be expressed +in a diagram similar to the accompanying sketch, which is much +distorted for convenience in lettering. As the diagram indicates, when +the concentration of the solution increases with respect to phosphoric +acid, the lime is at first less and less soluble until the point +represented by _B_ is reached, then becomes more and more soluble until +the point _D_ is reached, from then on becoming less and less soluble, +until the solution reaches a syrupy consistency. In contact with all +solutions represented by points on the line _DE_ the stable solid +substance which can exist is mono-calcium phosphate, CaH₄(PO₄)₂.H₂O. +Along the line _CD_ the only solid which is stable and can continue to +persist is the dicalcium phosphate. From the point _C_ the composition +of the stable solid varies continuously with the concentration of the +liquid solution. Therefore, these solids form a series varying in +composition from pure dicalcium phosphate to pure calcium hydroxide. +One of these basic phosphates, as they would ordinarily be called, has +a less solubility than any other, as indicated by the point _B_. All +solutions to the right of the point _B_ have an acid reaction, while +all solutions to the left possess an alkaline reaction. It follows from +these facts that if we start with any lime phosphate corresponding +to some point to the right of _B_ and dilute it, or what amounts to +the same thing in case it has been added to the soil, if we leach it, +phosphoric acid will go into solution more rapidly than will lime until +the composition of the residue is that of the basic phosphate stable at +_B_. Similarly, if we start with a phosphate more basic, lime will be +removed more rapidly than phosphoric acid, until the residue has the +composition of the phosphate of lowest solubility. From this point, +with continued leaching, the lime and phosphoric acid will dissolve +in a definite ratio, which ratio is obviously that of the phosphate +of least solubility. That is to say, if the leaching process is slow, +as would be the case under soil conditions, the solution would have a +perfectly definite concentration with respect to lime and phosphoric +acid. What the ratio of lime to phosphoric acid may be, is of no +particular interest in this connection, but the order of concentration +of phosphoric acid is of interest. Owing to serious analytical +difficulties, this has not yet been determined with any great +precision, but by interpolating on the experimentally determined curve +_AC_, this concentration is found to be somewhere in the neighborhood +of 5-10 parts per million, figures close to those obtained for the +concentration of the soil solution with respect to phosphoric acid by +the previously described investigations. + +[64] For reference to the literature and detailed discussion see: The +action of water and aqueous solutions upon soil phosphates, by F. K. +Cameron and J. M. Bell, Bull. No. =41=, Bureau of Soils, U. S. Dept. +Agriculture, 1907. + +Under ordinary circumstances, however, it is not probable that lime +is the dominant base controlling the concentration of phosphoric +acid in the soil solution, since the great majority of agricultural +soils contain vastly more ferric oxide (more or less hydrated) than is +equivalent to any amount of phosphoric acid that will ever be brought +into the soil; and ferric phosphates are less soluble relatively than +lime phosphates. Investigation of the relation of ferric oxide to +solutions of phosphoric acid shows that the system is quite similar in +many respects to the basic lime phosphates and water just described. +When the ratio of iron to phosphoric acid in the solid is greater +than that required by the formula of the normal phosphate, FePO₄, the +aqueous solution will have an acid reaction and contain a mere trace +of iron and an amount of phosphoric acid determinedly the composition +of the solid and by the proportion of solid to water. The basic ferric +phosphates seem to be solid solutions which yield a very dilute aqueous +solution when brought into contact with water. What the concentration +will be under soil conditions is shown by the percolation experiments +cited above. + +The addition of other substances will in many cases affect more +or less the solubility of the soil minerals. If these substances +be electrolytes, they will generally, but not always, affect the +solubility of the minerals as would be anticipated from the hypothesis +of electrolytic dissociation. Thus, the addition of potassium sulphate +lessens the solubility and hydrolysis of a potash feldspar or a +potash mica. Contrary, however, to the indications of the hypothesis, +sodium nitrate decreases the solubility of a ferric phosphate. While +appreciable solubility effects take place with sufficiently high +concentrations, laboratory experiments indicate that the addition of +such substances, even in a liberal application of fertilizers, is not +sufficient to produce any great effect on the concentration of the +soil solution. Similarly, it has often been supposed that the ammonia, +and nitrous and nitric oxides of the atmosphere carried into the +soil by rain, or formed in the soil by bacterial action, affect the +solubility of the soil minerals, but it is highly improbable that the +concentration with respect to these agents ever becomes sufficiently +high, as laboratory investigations show to be necessary to affect +appreciably the solubility of the ordinary rock- or soil-forming +minerals. + +Rain brings from the atmosphere into the soil two agents, however, +which do markedly affect the solubility of the soil minerals, namely, +oxygen and carbon dioxide. The atmosphere within the soil contains +normally a somewhat smaller proportion of oxygen than does the air +above the soil. Rain in falling through the air absorbs or dissolves +relatively more oxygen than nitrogen. Therefore when the rain water +has penetrated the soil to any considerable depth there should be, +and probably is, a liberation of dissolved oxygen into the atmosphere +of the soil interstices. This dissolved oxygen in becoming liberated +or when dissolved in the film water appears to be especially active +towards the ferrous or ferro-magnesian silicates. These minerals are, +moreover, as a class probably the most soluble of the rock-forming +silicates. Consequently oxygen brought into the soil in this manner is +one of the most important agencies in breaking down and decomposing +such minerals as the amphiboles, pyroxenes, chlorites, certain +serpentines, phlogopites and biotites; at the same time there is +formed ferric oxide (more or less hydrated) and silica (probably as +quartz) and magnesium, potassium, calcium or sodium pass into solution, +probably as bicarbonates. That the concentration of the soil moisture +may thus be made temporarily abnormal is not impossible, though +scarcely probable. + +The soil atmosphere has normally a decidedly higher content of carbon +dioxide than the atmosphere above the soil. Consequently the soil water +is always more or less “charged” with carbon dioxide, and the presence +of the carbon dioxide decidedly augments the solvent powers of the +water towards a great many and different kinds of rock-forming or soil +minerals.[65] + +[65] For references to the literature see Bull. No. =30=, Bureau of +Soils, U. S. Dept. of Agriculture; also, The action of carbon dioxide +under pressure upon a few metal hydroxides at 0° C., by F. K. Cameron +and W. O. Robinson, Jour. phys. chem., =12=, 561-573, (1908); The +influence of colloids and fine suspensions on the solubility of gases +in water, Part I. Solubility of carbon dioxide and nitrous oxide, by +Alexander Findlay and Henry Jermain Maude Creighton, Trans. Chem. Soc., +=97=, 536-561, (1910). + +What the mechanism of the reaction may be is far from clear. The +obvious explanation, at least in the case of the ordinary silicates of +the alkalis or alkaline earths, is that by forming bicarbonates of the +hydrolyzed bases, the active mass of the reaction product with water +is decreased and hydrolysis thereby increased. But this explanation is +apparently insufficient to account for the effects sometimes observed. +It has been shown that the passage of carbon dioxide through solutions +of the silicates, will produce more or less slowly a precipitation of +silica, and there seems little reason to doubt that it does induce to +some degree a decomposition and consequent greater solubility of the +silicates of the alkalis and alkaline earths. It also increases to an +appreciable extent the solubility of the phosphates of iron, alumina, +and lime. Therefore, the variation in the content of carbon dioxide in +different soils, and its continual variation from time to time in any +one soil, must be expected to produce corresponding changes in the soil +solution with respect to such bases as potassium and lime, and also +with respect to phosphoric acid. This has been verified experimentally +with aqueous extracts of soils, the solutions being charged with carbon +dioxide while in contact with the soils.[66] It is not conceivable, +however, that any great difference can exist in the partial pressures +of carbon dioxide in different soils which are in a condition to +support crops, and therefore great absolute differences in the mineral +content of the soil solution are not to be anticipated, nor are they +actually observed. + +[66] See, for instance, the results obtained by Peter, Proceedings of +the 19th Annual Convention of the Association of American Agricultural +Colleges and Experiment Stations, Bull. No. =164=, Office of Experiment +Stations, U. S. Dept. Agriculture, 1906, p. 151 _et seq._ + +It has long been held that the organic substances in the soil have an +important solvent effect on the minerals. This assumption seems quite +unwarranted in the light of our present knowledge, although it is +not to be denied that occasionally there may be present in the soil +some soluble organic substance which influences the mineral content. +Generally it has been assumed that the effective organic substances +influencing the solubility of the minerals are organic acids, of which +a number have found their way into past and even current literature, +and which have been designated as humic, ulmic, crenic, apocrenic, +azohumic acids, etc. Their existence has been predicated upon two +facts: First, humus is soluble in alkaline solutions but is more or +less completely reprecipitated on the addition of an excess of a +strong mineral acid, a phenomenon also characteristic of many organic +acids. But many other organic substances than acids are also soluble +in the presence of alkalis and insoluble in the presence of an excess +of strong mineral acids. Second, organic-copper complexes have been +obtained from humus constituents, and supposed to be copper salts of +various humus acids. The descriptions of these complexes so far given +do not show that they met the usual criteria for definite compounds, +but indicate on the contrary that they were the results of absorption +or possibly adsorption phenomena. Consequently the existence of +“humic” acids is purely hypothetical and without experimental or other +scientific verification, and calls for no further consideration here. + +It is a widespread and popular notion that substances with a slight +solubility also dissolve slowly, and that consequently the solubility +of the minerals in the soil water must necessarily be a very slow +process. This is, however, a misapprehension. It has been shown with a +number of the common rock-forming minerals, that if they be powdered +and then stirred into a relatively small volume of water, they dissolve +very rapidly at first, and in a very short time, generally a few +minutes, the solution is nearly saturated with respect to the mineral. +Complete saturation, however, may require many days. The general shape +of curve expressing the rate of solubility is shown in the accompanying +figure.[67] For soils, this fact has been verified repeatedly, in the +following way: A cell fitted with parallel electrodes is placed in +circuit with a slide-wire[68] or Wheatstone bridge in such a manner +that the resistance of the cell contents can be quickly determined. +Distilled water is then placed in the cell and its resistance found. +Generally this will be upwards of 100,000 ohms. The soil or rock +powder under examination is then added to the cell, being rapidly +stirred into the water contained therein. The resistance drops to +about 5,000 ohms within a short space of time, usually three or four +minutes. A further slight drop in the resistance generally takes place, +but it requires days, and sometimes even months to become more than +barely appreciable. In this manner it has been shown that the soil +and many of the common soil minerals dissolve quite rapidly if they +are sufficiently fine to offer a large surface to the action of the +water. It would seem to follow, therefore, that in the case of the soil +solution the concentration with respect to these constituents derived +from the soil minerals, will be rapidly restored whenever disturbed +through absorption by plants, leaching, or otherwise. + +[67] See, for example, Umwandlung des Feldspars in Sericit +(Kaliglimmer) von Carl Benedick, Bull. Geol. Inst. Upsala, =7=, +278-286, (1904). + +[68] See Electrical instruments for determining the moisture, +temperature and soluble salt content of soils, by L. J. Briggs, Bull. +No. 15, and the electric bridge for the determination of soluble salts +in soils, by R. O. E. Davis and H. Bryan, Bull. No. =61=, Bureau of +Soils, U. S. Dept. Agriculture. + +[Illustration: Fig. 3.] + +That the minerals of the soil, or a powdered mineral or rock-powder, +will dissolve continually as the concentration of the solution in +contact with it is disturbed by abstraction of a dissolved mineral +substance, has been shown by numerous experimenters. An apparently +obvious way to test this point would be to treat the soil sample +with successive portions of water, and to analyze the successive +portions for the dissolved mineral substances. This method, however, +involves serious experimental difficulties, owing to the smaller +sized mineral particles being suspended in the mother liquor, thus +precluding satisfactory decantation and clogging filters. Moreover, +such a process in no case simulates field conditions. To meet these +difficulties, the soil or mineral powder has been placed between two +porous media, as in the space between two concentric cylinders of +unglazed porcelain, the space being closed by a rubber stopper. To the +interior cylinder is fitted a stopper carrying a tube of insoluble +metal, such as platinum or tin. This tube is bent into a goose-neck +form, and just below the stopper the tube is perforated with a small +opening. The whole apparatus is filled with water and set in a beaker, +also filled with water. The metal tube is made the cathode in an +electric circuit, a platinum or other suitable anode being introduced +into the beaker. In a few minutes the dissolved and hydrolyzed bases +pass into the cathode chamber, and as the water also accumulates in the +chamber by electrolytic endosmosis, a solution of the bases dissolved +from the soil minerals drops from the end of the metal goose-neck. By +adding water to the outer beaker from time to time, a steady stream +of alkaline solution has been obtained for months, and in no case yet +has a soil thus treated failed to continue to yield up the bases it +contains in its mineral particles. The acids, such as phosphoric acid +for example, are of course found in the water outside the porous cells, +and in the case of the phosphoric acid it also appears to continue +indefinitely to be withdrawn from the soil.[69] It thus appears that +as the products of solution and hydrolysis are removed, by such an +endosmotic device as that just described or by the roots of growing +plants, by leaching or otherwise, the soil minerals will continue to +dissolve. + +[69] For detailed description of the apparatus and experimental data, +see Bull. No. =30=, p. 27, _et seq._, Bureau of Soils, U. S. Dept. +Agriculture. + +The foregoing arguments as to the concentration of the soil solution +with respect to those constituents derived from the soil minerals, are +based on the generally recognized principle that a material system +left to itself tends towards a condition of stable equilibrium or +final rest, that is, a condition where such changes as are taking +place are so balanced that no change occurs in the system as a whole. +But the soil is a system continually subject to outside forces and +influences, and as pointed out above, is of necessity a dynamic +system. It is doubtful in the extreme if any soil in place is ever in +a state of final stable equilibrium. It would be natural, therefore, +to expect and to find that even if the solution in the soil were +dependent on the solubility of the soil minerals alone and were +continually tending towards a definite normal concentration, actually +this concentration would seldom if ever be realized. Most important +in this connection is the fact that the concentration of the soil +solution is always dependent in some degree upon the concentration of +the soluble constituents in the solid phases in other than definite +chemical combinations. Other factors affecting the concentration of +the mineral constituents in the soil solution are always existent, and +theoretically at least, can not be ignored. Nevertheless _a priori_ +reasoning as well as the experimental evidence at hand indicates +that the various processes taking place in the soil as a whole +continually tend to form and maintain a normal concentration of mineral +constituents in the soil solution. + + + + +Chapter VIII. + +ABSORPTION BY SOILS. + + +A property of soils, affecting profoundly the composition and +concentration of the soil solution, is absorption.[70] It is generally +recognized that soils are good absorbers for vapors, and this fact +finds practical expression in the common practice of burying things +with a disagreeable odor, such as animal carcasses, night-soil, etc. It +is also well-known that dissolved as well as suspended material can be +more or less completely removed from water by passing it through sand +or soil, and this fact finds important application in water supplies +for cities and towns, sewage disposal, etc. It was known as long ago +as Aristotle’s time that ordinary salt is partly removed from water by +passing through sand or soil. In recent times the practical as well +as theoretical importance of this phenomenon has led to considerable +study and experimental research, so that our knowledge of absorption +effects is now fairly extensive, though it can hardly be claimed that +it is satisfactory. The absorption of a dissolved substance from +solution by a soil may be one or more of at least three kinds of +phenomena. It may be a mechanical inclusion or trapping, distinguished +by the term _imbibition_, the most familiar and striking case being +the absorption of water itself by soil or sponge or similar medium. It +may be a partial taking up of the dissolved substance to form a new +compound or a _solid solution_,[71] as probably is the absorption of +phosphoric acid by lime or ferric oxide. Or it may be a condensation +or concentration of the dissolved substance on or about the surface +of the absorbing medium, a phenomenon known as _adsorption_. To prove +the existence of adsorption definitely and conclusively in any given +case is always difficult, if ever possible, but the existence of this +phenomenon is the most logical explanation of many observations, and is +generally admitted by chemists and physicists at the present time.[72] +It is by adsorption, probably, that potash and ammonia are held by the +soil when added in fertilizers. + +[70] For a detailed discussion and citations of the literature, see: +Absorption of vapors and gases by soils, by H. E. Patten and F. E. +Gallagher, Bull. No. =51=; and Absorption by soils, by H. E. Patten +and W. H. Waggaman, Bull. No. =52=, Bureau of Soils, U. S. Dept. +Agriculture, 1908. + +[71] That is, a homogeneous solid, which may be either crystalline or +amorphous. Probably the readiest criterion for distinguishing between +a definite compound and a solid solution, is that the former is stable +in contact with a liquid solution of its constituents over a measurable +range of concentrations, while the composition of the solid solution +changes with every change in the concentration of the liquid solution +in contact with it. + +[72] A clear and apparently indisputable case of adsorption has been +noted by Patten (Some surface factors affecting distribution, Trans. +Am. Electrochem. Soc., =10=, 67-74, (1906). On adding powdered quartz +to an aqueous solution of gentian violet, there is a distribution of +the dye between the water and the quartz. A microscopic examination of +the latter showed that the dye was concentrated in thin layers upon the +surface of the quartz grains, from which it could be washed with water, +no change in the quartz grains being noticeable. + +That absorption is dependent in some manner upon the solubility of the +dissolved substance in the particular solvent employed would seem to +be obvious. But what the relation may be, if it exists at all, is not +known. For instance, silk absorbs picric acid from solutions in water +and alcohol but not from solutions in benzene, although the solubility +of picric acid in benzene lies between its solubility in water and in +alcohol.[73] + +[73] Absorption of dilute acids by silk, by James Walker and James R. +Appleyard, Jour. Chem. Soc., =69=, 1334-1349, (1896). + +The absorption of any given dissolved substance from different solvents +is markedly different. Most soils absorb methylene blue from aqueous +solutions with great avidity, but washing out the absorbed dye with +water is an extremely tedious and unsatisfactory process, although the +dye can be readily and more or less completely removed from the soil +by alcohol. As might be anticipated from this, it is known that the +presence of one dissolved substance affects the absorption of another, +but in what way can not, generally, be anticipated, although it would +seem that the importance of this subject for manurial practice would +invite further research. + +From the same solution, different absorbents remove a dissolved +substance in different degrees. Speaking generally, paper absorbs dyes +more readily than do soils, while soils absorb bases more readily than +does paper. Hence the reddening of litmus paper when in contact with a +moist soil. Heavy soils or soils containing much hydrated ferric oxide +absorb bases more readily than do light soils, but this is probably +owing to relative amounts of surface exposed, for the same relation +holds true with respect to phosphoric acid. Soils rich in humus are +better absorbers than soils not so rich. But here again there is yet +doubt as to whether the explanation lies in the amount or in the kind +of surface acting. + +From the same solvent different dissolved substances are absorbed quite +differently by any given absorbent. This can be readily illustrated +again by dyes. If an aqueous solution of a mixture of methylene +blue and sodium eosine, for instance, be shaken up with a soil, or +percolated through a column of soil, the methylene blue is absorbed the +more quickly and completely and a partial separation of the two dyes +can be readily effected, the separation being more or less complete +according to the conditions of the experiment. In the same manner two +salts in solution can be separated partially at least.[74] Soils absorb +potassium more readily than sodium; magnesium more readily than lime; +and ammonia more readily than any of these bases.[75] + +[74] For a number of interesting examples, see, Ueber das Aufsteigen +von Salzlösungen in Filtrirpapier, von Emil Fischer und Edward +Schmidmer, Liebig’s Annalen der Chemie, =272=, 156-169, (1893). + +[75] The prompt absorption of a base by soils is shown by the following +experiment: To some freshly boiled distilled water add several drops +of alcoholic phenolphthalein, and then just enough base to produce +a decided red color. If the solution be now passed through a short +column of soil, cotton, shredded filter-paper or similar absorbent, the +percolate will be perfectly colorless. The red color will be restored, +however, by adding a little of the base to the percolate. + +The absorption from aqueous solutions of inorganic salts involves a +most interesting complication. Just as a mixture of two or more dyes +or salts in solution can be separated by the selective action of an +absorbent, so can an electrolyte itself be decomposed or resolved. +Thus, if a solution of potassium chloride be passed through a column +of soil, or cotton, or paper, or any similar absorbent, the filtrate +will not only be less concentrated than the original solution, but the +potassium will be found to have been absorbed to a greater extent than +the chlorine, that is, the percolate contains free hydrochloric acid. +The importance of this phenomenon for the conservation of the desirable +constituents of manurial salts, and the elimination or leaching out +of the less desirable constituents is obviously great. Equally great +perhaps, is the effect upon the reaction of the soil, whether it be +rendered temporarily alkaline or acid, an effect of the very greatest +importance for the growth of some of our common crop plants[76] and +for the lower soil organisms, such as the bacteria, molds, together +with ferments, enzymes, etc., many of which are very sensitive to the +reaction of the media in which they may be, and which in turn are of +undoubted importance in determining the fertility of the soil for +higher plants. + +[76] See, The toxic action of acids and salts on seedlings, by F. K. +Cameron and J. F. Breazeale, Jour. Phys. Chem., =8=, 1-13, (1904). It +is quite conceivable, for instance, that if the drainage conditions +were not exceptionally good under a heavy type of soil, it might be +rendered temporarily unfit for clover or alfalfa by a heavy application +of potassium salts or of sodium nitrate. The idea put forward by some +authorities that too long continued or over fertilizing renders soils +acid, may have better foundation than their theoretical reasoning would +seem to warrant. + +The absorption of a dissolved substance from solution by an absorbent +is in effect a distribution phenomenon and the simplest formula to +give quantitative expression to such a distribution is C/C¹ = K when C +is the concentration in the liquid phase and C¹ the concentration in +the solid phase, K being a characteristic constant for the particular +case under consideration. When a chemical reaction or a change of +state, chemical or physical, is involved in the absorption in either +dissolved substance or absorbent the formula becomes Cⁿ/C¹ = K when +_n_ is a function which may be very simple or very complex. Attempts +to develop a precise formula of this general type for the absorption +by some given soil, although such a formula would be desirable for +theoretical and practical reasons alike, have uniformly failed. A +sufficient reason for this failure seems to lie in the fact that most +dissolved substances produce an appreciable effect on the granulation +or flocculation of the soil particles, which is progressive with +the absorption so that a continual change of absorbing or effective +surface is taking place as the absorption proceeds.[77] Moreover, in +the case of an absorption, with the formation of a continuous film of +the dissolved substance, a new kind of absorbing surface is developed. +Hence _n_ is a function of so difficult a character as to defy thus far +any attempt at formulation.[78] + +[77] That mineral fertilizers have a decided influence on the +granulation of soils and the properties dependent thereon, and that +this is of practical importance, is gradually coming to be recognized; +see, for instance, Ein Beitrag zur Kenntnis der Wirkung künstlicher +Dünger auf die Durchlässigkeit des Bodens für Wasser, von Edwin +Blanck, Landw. Jahrb., =38=, 863-869, (1909), and the literature there +cited. Dr. R. O. E. Davis in a yet unpublished investigation has shown +that the addition of soluble salts produces decided effects upon the +soil-moisture relations which affect crop production. The critical +moisture content is displaced, the penetrability, permeability, +specific volume, vapor tension, etc., are affected in measurable +degree, and it appears that the physical functions of mineral +fertilizers are much greater in amount and importance than has been +popularly assumed. + +[78] The distribution of solute between water and soil, by F. K. +Cameron and H. E. Patten, Jour. Phys. Chem., =11=, 581-593, (1907). + +We cannot therefore predict in any quantitative way what will be +the distribution of a soluble substance such as salts in commercial +fertilizers, for instance, between the solid soil particles and the +soil solution. Empirical experiments show, however, that with the +amount of a soluble salt present under normal conditions in a humid +climate, or as used in fertilizer practice, the absorption of ammonia, +lime, potassium or phosphoric acid is relatively very great, and in a +general way in about the order named. + +Absorption is not an instantaneous process. However, the rate at +which a dissolved substance is absorbed is generally quite rapid. +That is, if a soil be stirred or mixed with an aqueous solution, the +absorption takes place very quickly, in the absence of any outside +disturbing influences. The law governing the rate of absorption by +soils has not therefore possessed any great practical interest and +has not been studied from a quantitative point of view, although it +is known qualitatively that the rate is increased by increasing the +concentration of the solution, or by increasing the amount of the +absorbent or at least its effective surface. Two rate equations are +of interest in this connection, and have been carefully studied. The +rate at which a salt or other dissolved substance will advance into an +absorbing soil from a solution is given by the same equation as that +describing the rate of advance of the water itself, _yⁿ_ = _kt_ where +_y_ is the distance and _t_ the time.[79] The constants _n_ and _k_ +for the slower moving dissolved substance are different from those for +the water. This equation has probably little importance for ordinary +agriculture, for absorption by the soil from a large (and relatively +illimitable) mass of solution is unusual. That it may have considerable +importance in seepage, irrigation, and some soil engineering problems, +seems quite likely. + +The rate at which a soil will absorb a dissolved substance from a +percolating solution is given by the equation + + _dx_ + ————— = K(A - _x_), + _dt_ + +as has been pointed out above.[80] More interesting and important, +however, is the fact that this same equation describes the rate at +which an absorbed substance is removed from the soil by leaching. In +the case of soils in humid areas _dx_/_dt_ rapidly becomes exceedingly +small as _x_ approaches A, that is, when the amount of soluble material +in the soil becomes small, and is practically constant under such +conditions, as has been pointed out above when describing the removal +of potassium and phosphoric acid from soils by percolating waters. This +formula has a special interest in considering the reclamation of alkali +lands by underdrainage, a problem to which reference will be made later. + +[79] See formula, page 28. + +[80] See formula, page 47. + +Both percolation experiments, as those cited above, and direct +absorption experiments made by shaking up soils with solutions of the +salts in question, show conclusively that the absorption phenomena +taking place in the soil are in harmony with the direct solubility +effects in tending to produce and maintain a solution of a normal +concentration as regards those constituents which it happens are also +derived from the soil minerals.[81] It is an interesting coincidence +that nitric acid (in combination with various bases of course) is very +little absorbed by most soils, and does vary in concentration, not +only in different soils but in the same soil, between wide limits, and +within short intervals of time.[82] The nitrates of the soil are not +derived from minerals, and should more properly be considered with the +organic constituents of the soil solution. + +[81] An extreme case is worth citing in this connection. Mr. W. H. +Heileman in studying the influence of various kinds of alkali upon +plant growth, added from 3-4 per cent. of sodium carbonate to soils +known to be otherwise free from alkali. Wheat seedlings grown in the +soils so treated showed no ill effects from the added salt. When +distilled water was percolated slowly through the soils, or shaken up +with them, the resulting solution contained the merest traces of the +alkali. + +The ordinary method of determining the lime requirement of a soil +by adding lime water until the solution shows an alkaline reaction, +is another obvious absorption phenomenon, and is not dependent, as +popularly supposed, upon the presence of acids in the soil. Soils which +by no possibility could contain any free acid, frequently absorb very +large amounts of lime in this manner. + +[82] Usually, in the growing season, the soil solution has a much +higher concentration with respect to nitrates in the morning than it +has in the evening. + +An important application of these views concerning absorption arises +in connection with certain widespread notions concerning soil acidity. +There is a popular belief that most soils are acid, that the soil +solution contains some free acid, mineral or organic, other than +dissolved carbon dioxide, and that a neutral or alkaline solution is +necessary to the successful production of most of our crops. This +belief is, however, unwarranted, for the vast majority of soils yield +an aqueous extract which is alkaline when boiled to expel carbon +dioxide, and some of our crops, for instance wheat, seem to thrive +better in a slightly acid medium. This popular fallacy seems to have +its origin in the fact that most soils when moistened and pressed +against blue litmus paper, redden it. This reddening may sometimes +be due to the actual presence of some acid, or to dissolved carbon +dioxide, but is undoubtedly due in the majority of cases to selective +absorption. Litmus is a red dye of an acid-like character, which forms +a soluble blue salt with the ordinary bases. But it has been shown +that most soils are better absorbents of bases than is paper, whereas +paper is a better absorbent of dye, speaking generally, than is a soil. +Consequently when moist soil is brought into contact with wetted blue +litmus paper the base is absorbed more readily by the soil, and the dye +by the paper, the latter therefore becoming reddened. + +The reddening of blue or “neutral” litmus paper can be accomplished +with various absorbents. By pressing the litmus paper between moistened +wads of absorbent cotton the reddening can be readily accomplished, +usually in the course of ten minutes to a half hour. That the +phenomenon is not due to any adhering acid on the cotton can be shown +in the following way: A litmus solution is carefully prepared so that +there is a very small excess of base present over that required to +give the blue color. A wad of absorbent cotton is carefully washed by +repeatedly sousing it in distilled water from which carbon dioxide has +been expelled by boiling. When the cotton has been thoroughly washed, +it is stirred thoroughly in a portion of distilled water, free from +carbon dioxide, then withdrawn by some appropriate instrument and +allowed to drain for a few minutes. The litmus is added in fairly large +quantity to the drainings, which should then have a blue color. Again +stir the cotton in the water, and more or less quickly, depending on +the amount and purity of the litmus preparation as well as the quantity +of cotton used, the solution will become red. The only criterion for +determining surely that a soil is acid, is to make an aqueous extract, +expel the dissolved carbon dioxide by boiling, or by passing through +the solution an inactive gas, such as nitrogen, and then to test the +reaction of the solution. Acid soils undoubtedly do exist, but they are +by no means common or widespread, and are to be regarded as exceptional +and abnormal. + +The phenomena of selective absorption suggest the important part which +surfaces play in modifying and changing chemical reactions.[83] For +instance, Becquerel[84] observed that a solution of copper nitrate or +cobalt chloride diffusing from a cracked test-tube placed in a solution +of sodium sulphide, led to the formation of the corresponding sulphide, +but in the crack the metal itself was precipitated. Experiments of +Graham[85] show that when a solution of silver nitrate is percolated +through charcoal, not only is there a selective absorption as is shown +by the percolate containing free acid, but there is a chemical reaction +involved, since the silver is deposited in metallic spangles in the +interstices of the absorbent. Graham has shown, and since his time +others, that often metals can be separated from solutions of their +salts by such absorbents as charcoal. Spring[86] has shown that at +bounding surfaces of dilute solutions, chemical action is increased. + +[83] For references to the literature see, Bull. No. =30=, Bureau of +Soils, U. S. Dept. Agriculture, p. 61 _et seq._ + +[84] Note sur les réductions métalliques produites dans les espaces +capillaires, par M. Becquerel, Comptes rendus, =82=, 354-356, (1876). + +[85] Effects of animal charcoal on solutions, by T. Graham, Quart. +Jour. Sci., I, 120-125, (1830). + +[86] Über eine Zunahme chemischer Energie an der freien Oberfläche +flüssiger Körper, von W. Spring, Zeit. physik. Chem., =4=, 658-662, +(1889). + +It has been shown that the amount and kind of surface has a marked +influence on the decomposition of hypochlorous acid, carbon dioxide, +phosphine, arsine, and other compounds. Meyer and his associates, as +well as a number of other investigators, have shown that the character +of the surface of the containing vessel greatly affects the combination +of hydrogen and oxygen. Many reactions have been investigated by van’t +Hoff, who concludes that both the nature and amount of surface exposed +have an influence. The inversion of sugar is affected by the nature +of the walls of the containing vessel, and its reduction by Fehling’s +solution is affected both by the walls of the vessel and the amount +of cuprous oxide formed in the reaction. Alteration in the character +as well as degree of a number of reactions by having them take place +in capillary spaces has been observed by Liebreich, Becquerel, +Lieving and other investigators. So-called “contact reactions,” as in +the production of sulphuric acid, are now familiar processes finding +commercial applications. And the solubility of some substances at +least, notably gypsum, has been shown to vary considerably with the +size and consequent shape of the particles in the solid substance in +contact with its solution.[87] + +[87] See especially, Beziehungen zwischen Oberflächenspannung und +Löslichkeit, von G. A. Hulett, Zeit. Phys. Chem., =37=, 385-406, +(1901). Löslichkeit und Löslichkeits Beeinflussung, von V. Rothmund, p. +109, (1907); Principles théoretiques des methodes d’analyse minerale, +par G. Chesneau, p. 16-25, (1906). + +It has been shown that some soils will at times produce the blue +coloration in alcoholic solutions of guiac, which is characteristic +of oxidases, and yellow aloin solutions are sometimes colored red. +Hydrogen peroxide is decomposed by some soils even after they have been +thoroughly ignited to get rid of all organic matter. But in how far +these effects may be due to surface influences can not be positively +stated; yet uncompleted investigations by Dr. M. X. Sullivan indicate +that some of these phenomena at least must be attributed to specific +influences (although probably of catalytic character) of particular +soil components, such possibly as manganous oxide or ferric oxide; but +the mechanism of the reactions is as yet largely speculative. + +The soil is composed in large part of very fine particles of rounded +shape, exposing relatively an enormous surface to the soil solution, +and normally this solution is mainly under capillary conditions, so +that we should expect that many reactions would take place quite +differently in the soil from the way they would in a beaker or flask. +This fact has been generally overlooked or ignored, and is probably +the explanation of many of the apparently anomalous results hitherto +reported in chemical investigations of soils. Abnormal solubilities, +precipitations, oxidations or reductions are frequently found in the +literature, and when their abnormality is noted at all, they are too +often and with slight show of reason ascribed to indefinite bacterial +action or more mysterious vital agencies. Many of them are undoubtedly +the results of surface actions. Unfortunately, aside from some few +studies of absorption phenomena, surface effects have received little +or no attention from soil investigators, although obviously one of the +most important and apparently fruitful fields, requiring immediate +attention. Enough is known to justify the statement that the chemistry +of the soil need not be, and probably is not, the chemistry of the +beaker. + + + + +Chapter IX. + +THE RELATION OF PLANT GROWTH TO CONCENTRATION. + + +That the concentration of the mineral constituents in the soil solution +under normal conditions is competent for plant support, is shown by +numerous experiments. Birner and Lucanus[88] in an experiment that +has long since become classic, found that they could raise wheat to +maturity in a well-water, the concentration of which was approximately +18 parts per million with respect to potassium, and 2 parts per million +with respect to phosphoric acid, while the corresponding concentrations +of the soil solution are normally about 25-30 parts per million of +potassium and 6-8 parts per million of phosphoric acid. Nevertheless +Birner and Lucanus report that the wheat grown in the well-water throve +even better than that grown at the same time in a rich garden mold. +Since then many investigators in numerous trials have obtained similar +results. Recently wheat, corn, and some of the common grasses have been +grown to a satisfactory maturity in tap water with a concentration +of about 7 parts per million of potassium and 0.5 parts per million +of phosphoric acid. And repeatedly wheat plants, grasses, cowpeas, +vetches, potatoes and other plants have grown in a satisfactory way in +solutions made by shaking up a soil in distilled water and separating +from the solid particles by means of filters of unglazed porcelain. + +[88] Wasserculturversuche mit Hafer, von Dr. Birner und Dr. Lucanus, +Landw. Vers.-Sta., =8=, 128-177, (1866). + +There can be no doubt, therefore, that the soil solution is normally +of a concentration amply sufficient to support ordinary crop plants, +and is maintained at a sufficient concentration, so far as mineral +plant nutrients are concerned. Undoubtedly, however, variations in +the concentration of the soil solution can, and often do, take place, +and the results of laboratory experiment indicate that they probably +produce effects on plants. + +It has been shown in water-culture experiments with wheat, that if a +given ratio of mineral nutrients be maintained, relatively small effect +is produced on the growing plants by varying the concentration over +a wide range, in one case from 75 parts per million to 750 parts per +million,[89] and this effect seems to be largely independent of the +nature of the particular mixture of solutes. But varying the relative +proportions of the mineral constituents has been shown by numerous +experiments to produce very marked changes in the growth of plants. +Not only does a control of the concentration and proportion of the +mineral constituents of a solution produce a more rapid, or a slower +growth, a greater or lesser total growth, but it produces differences +in the character of growth; as for instance, causing the tops to grow +relatively faster than the roots, or _vice versa_. However, many +effects of this type can be produced, and sometimes more readily, by +soluble organic substances, or mechanical agencies. The mechanism of +these effects is by no means clear, in many cases. That other causes +obtain than a sufficient supply of mineral nutrients will be shown +in the following chapters. Experiments with wheat seedlings in water +cultures, where the weights of the green tops were taken as the measure +of growth, showed that the most-favorable ratio was one of phosphoric +acid (PO₄) to three or four of potassium (K), about the ratio which has +been found to exist normally in the soil solution of humid areas of +the United States, namely, 6-8 parts per million of phosphoric acid to +25-30 parts per million of potassium. + +[89] Effect of the concentration of the nutrient solution upon wheat +cultures, by J. F. Breazeale, Science, n. s., =22=, 146-149, (1905). + +All growing plants require for their growth and development various +organic compounds containing carbon, hydrogen, oxygen and nitrogen. The +higher crop plants with which agricultural investigations appear to +be more immediately concerned, seem to have inherent power to produce +these needed substances within themselves. But it is becoming more and +more evident that the large problem of soil fertility, or the relation +of the soil to crop production, frequently if not generally involves +the growth and development of lower organisms including ferments and +bacteria. These may or may not in particular cases, favor the growth +of the desired higher plants. Many of these lower organisms require +certain organic compounds or thrive better if these are brought to +them in the soil solution, and indeed evidence is not lacking that +such may sometimes be the case even with the higher plants. Certainly +their growth can be much affected by the presence of different organic +substances in the nutrient solution. Enough work has been done in +this field of investigation to show that the concentration of the +soil solution or artificial nutrient solution with respect to the +organic compounds must generally be low; too high a concentration +always inhibits growth or even produces death; and there is probably +an optimum concentration, or one at which the plant will grow best; +but this optimum concentration varies with the specific nature of the +plant, the presence of other dissolved substances, mineral or organic, +and possibly with other factors. While a notable amount of work has +thus been done in a field of inquiry obviously of practical as well as +theoretical interest, almost no definite information has as yet been +obtained as to the concentration of organic substances in the soil +solution, or its effect upon plants under field conditions, excepting +in the case of the nitrates, the products of bacterial activities. The +concentration with respect to nitrates is known to vary greatly from +a few parts to several thousand parts per million, and this sometimes +within a few days or even hours. The great changes in concentration +with respect to nitrates, the rapidity of the changes, and the +correspondingly large effects on growing plants make this a subject +requiring special treatment by itself. This at present seems more +easily appreciated from a consideration of the bacteria involved, and +will not be discussed more fully here.[90] + +[90] See: The fixation of atmospheric nitrogen by bacteria, by J. +G. Lipman, Bull. No. =81=, Bureau of Chemistry, U. S. Dept. of +Agriculture, 1904; A review of investigations in soil bacteriology, +by Edward B. Voorhees and Jacob G. Lipman, Bull. No. =194=, Office of +Experiment Stations, U. S. Dept. of Agriculture, 1907; The physiology +of plants, by W. Pfeffer, translated by A. J. Ewart, vol. I, p. 388 +_et seq._, 1900; The effect of partial sterilization of soil on the +production of plant food, by Edward John Russell and Henry Brougham +Hutchinson, Jour. Agric. Sci., =3=, 111-144, (1909). + +Of the ash constituents of plants, there must be in the soil solution, +potassium, magnesium, phosphorus, sulphur and iron for any plant +growth, and for the higher crop plants, calcium must also be present. +Of these, iron is usually present in barely appreciable concentration +and more than this is not desirable, or is even harmful for common crop +plants. Under the normal conditions for soils in humid areas, sulphur +also is usually present in scarcely more than appreciable quantities +and there is no positive evidence to show that higher concentrations +are especially desirable, though this may be the case for certain +crops, such for instance as the onion. Phosphorus is usually present +to the extent of 5 or 6 parts per million of phosphoric acid (P₂O₅), +while it has repeatedly been shown that such crops as wheat can thrive +and make a good growth with a concentration a tenth of this. It appears +to be clear therefore that as far as food supply is concerned there is +normally an ample supply of phosphorus in the soil solution; but it +does not follow that increasing the concentration of the solution if +only temporarily would not result in favorable effects upon growing +plants. + +A consideration of the bases, however, introduces serious difficulties, +which will probably require much further research by the plant +physiologist as well as the soil chemist. It is impossible as yet to +determine the concentrations at which different plants will not grow. +It is even impossible to determine the concentrations at which they +will thrive best. It seems certain that different crop plants require +different amounts of these minerals, but whether or not they require +different concentrations of the constituents in the nutrient solution +for their several best growths is yet not clearly shown. It now seems +probable that to some extent at least these basic mineral nutrients can +replace one another for the plant’s metabolism. It has been shown in +the case of certain lower plant organisms that potassium can be more +or less successfully replaced by rubidium and caesium, and in the case +of some higher plants, possibly calcium, magnesium and potassium can +partially replace one another.[91] In spite of the fact that sodium +as well as potassium is a necessary constituent for the metabolism +of higher animals which feed upon plants, it is generally held that +sodium can not replace potassium in the processes of plant growth, +although Wheeler and his colleagues have advanced evidence to show that +a partial replacement is possible.[92] It seems evident, however, that +no generalizations can hold concerning the effect of the concentration +of any one base on plant growth which do not include recognition +of possible modifications due to the presence of other bases; and +the formulation of such generalizations must needs wait upon a more +thorough knowledge of the parts played by the several mineral nutrients +in the metabolism of different classes of plants. + +[91] For a more detailed discussion of this subject, and the functions +of the several ash constituents in plant nutrition, see: The physiology +of plants, by W. Pfeffer, translated by A. J. Ewart, vol. I, p. 410, +_et seq._, 1900. + +[92] The effect of the addition of sodium to deficient amounts of +potassium, upon the growth of plants in both water and sand culture, +by B. L. Hartwell, H. J. Wheeler and F. R. Pember, Report Rhode Island +Agricultural Experiment Station, 1906-7, p. 299-357. + +As to forms or chemical combinations in which the inorganic +constituents of the soil solution are best adapted to plant growth, +but little can yet be said other than that the different combinations +do have an importance. Some empirical information is available, such +as for instance, that potassium sulphate or carbonate is a better +fertilizer for some crops than is potassium chloride. It is known that +the mineral nutrients in the plant are partly in inorganic combinations +but largely in organic combinations. But the causal relationships are +yet to be worked out. And finally, although some meagre experimental +data have been obtained as to the effect of certain inorganic +constituents on the absorption of others, by particular plants, the +mechanism of absorption itself, including the selective powers of the +plant, is yet wanting an adequate explanation. + + + + +Chapter X. + +THE BALANCE BETWEEN SUPPLY AND REMOVAL OF MINERAL PLANT NUTRIENTS. + + +The mechanism of the solution and transport of mineral nutrients +developed in the preceding pages makes it of interest to determine +the relation between the possible or probable supply of mineral +plant nutrients and crop demands over large areas. The inquiry can +be formulated more specifically: Is the movement of mineral plant +nutrients towards the surface soil equal to or in excess of the removal +by drainage waters and garnered crops? Satisfactory data are yet +wanting for anything like exact computations, but approximate figures +are available which appear sufficient for the present purpose. + +The rainfall (R) can be considered as disposed in three portions, the +fly-off (_f_), the run-off (_r_), and the cut-off (_c_). Stating this +as an equation, + + R = _f_ + _r_ + _c_. + +The cut-off can be resolved into the portion (_a_) seeping through the +soil to ultimately join the run-off, and the portion (_b_) returning to +the surface to ultimately join the fly-off. Stated as equations, + + R = _f_ + _r_ + _a_ + _b_ + = _f_ + _b_ + (_r_ + _a_). + +In other words, the rainfall can also be considered as made up of the +fly-off, the capillary water of the soil and the drainage from the +area. According to Murray,[93] Geikie,[94] Newell,[95] and others, the +drainage water for humid areas, or such an area as the United States +as a whole, would be between 20 and 30 per cent. of the rainfall, the +major portion coming from seepage water rather than surface drainage. +Assuming the higher figure, and making the further very probable +assumption that the capillary water in the soil (_b_) is never less +than the fly-off or the water that evaporates during rain (_f_), +it follows from the equations given that the capillary water is at +least 35 per cent. of the rainfall. If we assume the lower value for +the drainage, then the capillary water is at least 40 per cent. of +the rainfall, and if we assume the extreme case—that the fly-off is +practically negligible—the capillary water becomes 80 per cent. of the +rainfall. It appears, therefore, that in all probability the proportion +of the cut-off water which returns to the surface as film water or +capillary water is always greater, and generally much greater, than the +portion which seeps through the soil to join the run-off. + +[93] On the total annual rainfall on the land of the globe, and the +relation of rainfall to the annual discharge of rivers, by Sir John +Murray, Scot. Geog. Mag., =3=, 65-77, (1887). + +[94] Textbook of Geology, by Sir Archibald Geikie, p. 484, (1903). + +[95] _In_ Principles and conditions of the movements of ground water, +by F. H. King, Ann. rept. U. S. Geol. Surv., =19=, II, 59-294, +(1897-98). + +From the available data, it appears that the average concentration of +the run-off waters of the United States is about 1.8 parts per million +of potassium (K) and about 0.6 parts per million of phosphoric acid +(PO₄),[96] while the concentration of the capillary groundwater is some +ten or twelve times greater. But even if these concentrations were the +same, it is altogether probable that very much the greater part of the +mineral plant nutrients dissolved by meteoric waters is continually, if +slowly, moving towards the surface of the soil. + +The average rainfall of the United States may be taken as approximately +30 inches.[97] If it be assumed that the discharge into the sea is +25 per cent., then the capillary cut-off water is at least 37.5, and +probably nearer 70 per cent. of the rainfall. King’s experimental +work[98] indicates that the higher figure is much nearer the truth. +Computing from the concentrations just cited, with the equations given +above, it is found that approximately 3,500,000 tons of potassium (K) +and 1,200,000 tons of phosphoric acid (PO₄) are carried into the sea +annually from the United States, while from 48,000,000 to 100,000,000 +tons of potassium and 18,000,000 to 40,000,000 tons of phosphoric acid +are being carried towards the surface of the soil. If it be assumed +that an average of one ton per acre of dry crop containing one per +cent. potash and 0.6 per cent. phosphoric acid[99] be removed from the +entire area of the United States, then the annual loss from this source +would be 24,000,000 tons of potassium and 14,000,000 tons of phosphoric +acid. Consequently, there is an ample margin between the losses by +cropping and seepage waters, and the supply of capillary waters. It is +true that cases exist where the production of vegetable matter is much +greater than a ton to the acre, productions of five tons or even more +being on record. But such cases occur only where the water supply is +also greater, either through natural rainfall or artificial irrigation; +and it should also be borne in mind that the production of so large a +mass of green crop involves a considerable drawing power on the water +in the soil in addition to the evaporation which would take place at +the surface under ordinary conditions. In other words, the plant would +then be playing no small part in drawing to itself its needed supplies +of water and dissolved mineral nutrients. + +[96] Estimated from data in Bull. No. 330, U. S. Geological Survey, The +data of geochemistry, by Frank Wigglesworth Clarke, 1908, p. 53-90. + +[97] The latest authoritative statement is that the average annual +rainfall of the United States is 29.4 inches; see: Water Resources, +by W. J. McGee, vol. 1, p. 39-49, and Distribution of rainfall, by +Henry Gannett, vol. 2, p. 10-12, Report of the National conservation +commission, Senate doc. No. 676, 60th Congress, 2d session, 1909. + +[98] King: loc. cit., p. 85. + +[99] Estimated from Wolff’s tables, How crops grow, by Samuel W. +Johnson, 1890, appendix. + +The question may be asked, if the processes outlined above are +generally operative, why accumulations of soluble mineral substances +are not usually found at the surface of the soil. As a matter of fact +such accumulations do occur normally when the evaporation at the +surface is relatively large, that is, under arid conditions. And under +humid conditions it appears to be a general rule that the surface +soil contains more readily soluble or absorbed mineral matter than do +subsoils.[100] No great accumulation occurs at the surface normally +under humid conditions because the rainfall is sufficiently distributed +throughout the year to enable the cut-off water to carry back promptly +into the lower soil levels any excessive amount of soluble material, +there to start anew its slower ascent towards the surface. + +[100] See, for instance: Investigations in soil management, by F. H. +King, Madison, Wis., 1904, p. 62 _et seq._ This tendency towards a +higher content of absorbed soluble mineral matter in the surface soil +has been amply confirmed by other experiments. It has been advanced +as an argument against the assumption that the hydrolysis of the soil +minerals is a reversible process. But as pointed out elsewhere in the +text, many of the soil minerals can be made in the wet way at more or +less elevated temperatures and the more rational explanation is simply +that at ordinary temperatures the rate of formation is exceedingly slow. + +Calculations such as those here presented are at the best open to many +objections, and it is wise to avoid giving them too much emphasis. So +far as the available data justify any conclusion, however, it appears +that the rise of capillary water is entirely capable of maintaining +a sufficient supply of mineral nutrients for crop requirements; and +furthermore, it is obvious that the problem of the supply of mineral +plant nutrients is dynamic and cannot be successfully attacked by +considerations which are essentially static. + + + + +Chapter XI. + +THE ORGANIC CONSTITUENTS OF THE SOIL SOLUTION. + + +The organic substances in the soil are tissue remains, to a large +extent of plants, and to a less extent of animals; and it is to be +expected that there may be found also in the soil the substances which +were in the organisms at the time of their death, and degradation and +decomposition products derived from these. Moreover, there are to +be anticipated numerous products of bacterial origin, secretions of +algae, fungi, etc., so that the organic complex in the soil may contain +numerous substances of widely different chemical characteristics. +Degradation products of proteins, fats, and carbohydrates, as well +as decomposition products may be expected in almost any soil. But it +does not follow that any particular organic substance (excluding, of +course, carbon dioxide or nitrates) is to be found in every soil. No +generalization regarding the organic substances in the soil can be made +such as that formulated for the inorganic compounds. It is probable +that further investigation will show certain organic substances or +classes of substances to be common to most soils, but it is reasonably +certain that many other organic substances will be found in only a few +soils, or occasionally, and these latter will be often a prominent +factor characterizing the particular soil in which they may occur. + +Although no broad generalization is justified regarding the composition +of the soil solution with respect to organic substances dissolved, +nevertheless the extension of the methods developed in the study of the +inorganic substances dissolved has led to a considerable knowledge of +the organic ones. + +In view of the facts shown in the preceding chapters, and at the +same time recognizing that good and poor soils respectively must +show differences in the soil solution if the fundamental thesis is +valid as to the relation of soils to crop production, experiments +have been made to investigate in a comparative way solutions obtained +from good and poor soils of the same type, locality, and physical +characteristics. For this purpose two samples of soil were taken from +adjacent fields which had been under observation for two years. The +soils were of the same type, Cecil clay, and were so similar in their +physical characteristics as to be distinguished with difficulty in +the laboratory. On one field a good crop of wheat was grown, followed +by a good crop of clover and tame grasses. On the other field, the +corresponding crops had been quite poor. The field yielding the good +crops had been plowed somewhat deeper, and had previously received a +moderate application of stable manure. Otherwise, so far as could be +learned, the cultural history of the fields had been the same. For +convenience, the sample from the first field will be designated “good,” +and from the other “poor.” + +Aqueous extracts from these soils were prepared, the same proportion +of distilled water to soil being taken in each case, and the time of +contact being the same. The solutions were freed from suspended matter +by being passed through Pasteur-Chamberland bougies under pressure. +Young wheat seedlings germinated at the same time, and selected +carefully for uniformity of size and apparent vigor, were grown in +these solutions for three days. At the expiration of this period the +seedlings in the extract from the good soil were about five inches in +height, and the roots were clear, clean and turgid. The plants in the +poor extract were scarcely three inches in height, and the roots were +assuming a slimy, unhealthy appearance and becoming flaccid at the +tips. The plants were then all removed, the roots washed carefully in +tap water; the plants which had been in the poor solution were placed +in the good solution, and those which had been in the good solution +were placed in the poor solution. At the end of four days further, +the poor plants had surpassed in height the ones which had previously +been in the good solution, and the roots had acquired the general +characteristics of healthy plants. These which had been originally in +the good solution and then transferred to the poor, had made little +additional growth, and the roots had become somewhat flaccid.[101] + +[101] The success of this and of many of the following experiments +was due in large measure to the skill and patience of Mr. James E. +Breazeale. + +This experiment was repeated several times, not only with the soils +cited but with samples from adjacent good and poor spots in fields +on several soil types from widely separated areas; for instance, +Cecil clay from near Statesville, North Carolina; Sassafras loam +from Maryland; Windsor sand from Delaware; and similar results were +obtained. In other words, these water cultures produced plants which +showed much the same differences, in kind and degree, as had been +observed in the field. This was recognized as an important step +forward, for it indicated that _whatever was making a difference in +the crop-producing power of these soils in the field was transmitted +to their aqueous extracts_, and methods for studying the chemical +properties of solutions are far in advance of methods for studying +mixtures of solids. + +The soil extracts described above were subjected to a careful analysis +for their mineral constituents. They were found to be practically +identical in this respect. Further, the poor extract contained +decidedly more nitrates than the good—from three to four times as much. +It follows, therefore, that the difference in the soils which produced +a good and a poor crop respectively, was not due to a difference in +mineral plant nutrients, or other mineral differences probably, nor to +their respective content of nitrates. Consequently, the poor solution +was such, not because of the lack of anything, but because of the +presence of something inimical or “toxic” to plant growth; and further, +this something must be an organic substance or substances more or less +soluble in water. This conclusion was confirmed in the following way. + +Samples of the poor solution from the soil obtained near Statesville, +N. C., were diluted twice, five times, and ten times, and wheat +seedlings were grown in these solutions, using a sample of the good +solution as a check. It was found after several days growth that the +plants in the solution diluted tenfold were about as good, or perhaps +slightly better, than those grown in the check solution. In every +case diluting the poor solution had improved it for plant growth, +and the higher the dilution the greater the improvement, in spite +of the consequent dilution of the mineral plant nutrients. The only +explanation of these results which has yet suggested itself is that +the toxic organic substances present were less effective on dilution +until the concentration reached a point where they actually became +stimulative, as is common with toxins of every character. + +Another set of experiments confirmed the conclusion that the poor +solution contained some organic substance inhibitory to plant growth. +A number of water cultures was prepared from the aqueous extract of +the poor soil, and lime in various forms was added to the cultures. To +two of the cultures lime carbonate and lime sulphate respectively were +added in excess, so that there was in each case a powdered solid at +the bottom of the containing vessel. At the end of two days the wheat +seedlings which were growing in the vessels containing the powdered +solids had decidedly outstripped those growing in all the others, the +tops having the appearance of unusually good and healthy plants. The +roots were of a very remarkable character, being exceptionally long, +very turgid, clear, clean and translucent. + +At once, new experiments were carried out in which there were added +to the poor solution, precipitated ferric hydroxide freed from all +adhering salts, precipitated alumina, shredded filter-paper, absorbent +cotton, or carbon black. In every case the same result was obtained +as before, a much improved growth of top and a vastly better root +development. Since, by no possibility could these various added +substances have increased the concentration with respect to mineral +nutrients, another explanation must be sought. Aside from their +insolubility, the one property common to these various substances +was the large amount of surface they brought into contact with the +solution. The one obvious explanation of their effects on the growth of +the wheat seedlings, therefore, is that they withdrew or absorbed from +the solution some substance or substances deleterious to plant growth. +As diluting with respect to mineral nutrients could not possibly be +expected to improve the cultural value of the solution, the conclusion +seems evident that the effect produced by these various absorbents +was due to more or less complete removal from the solution of organic +substances inhibitory to plant growth. These experiments were then +repeated in a modified form by shaking the poor solution with such +absorbents as precipitated ferric oxide or carbon black and filtering +before adding the seedling plants. The solutions thus prepared proved +very satisfactory nutrient media, although the decided elongation of +the roots, always observed when the absorbents were in contact with the +solutions, was not so noticeable with these filtered solutions. + +The experiments just described were repeated with extracts from a +number of soils which were supporting or had recently supported poor +crops. The accumulated mass of evidence admits of no doubt that in many +cases the apparent lack of fertility of a soil is due to the presence +of some organic substance or substances soluble in soil water. This +point established, there was studied the effect of fertilizers when +added to aqueous extracts from poor soils. + +A large amount of experimenting has been done on this subject. It has +been found that the common commercial fertilizers, as well as many +other substances, when added to the soil extract containing growing +plants, sometimes improve the plants, sometimes the contrary. But, in +general, those particular substances which improve any given soil for +a crop also improve the aqueous extract of the soil for the growth of +the same crop plant: _i. e._, should a soil be known to respond well +to the application of superphosphates when planted to wheat, then the +probability is great that the aqueous extract of the soil will be +improved as a culture medium for the wheat plant by addition of calcium +phosphate. Particularly important in this connection are certain +experiments with organic fertilizers. + +A soil which had been found to be quite unproductive with regard to +wheat and ordinary tame grasses yielded, however, a much better growth +of plants if pyrogallol or better pyrogallol and lime were added to +the soil some days before planting. An aqueous extract of this soil +tested with young wheat seedlings produced but a poor growth, as did +the soil itself. But with the addition of pyrogallol or pyrogallol and +lime to the soil extract, and especially if the extract so treated +were allowed to stand for a few days with free access of air, there +was obtained a culture medium which yielded remarkably good results +with wheat seedlings. Not only was there an excellent and increased +development of tops, but the roots of the seedlings grown in the +solution treated with pyrogallol were unusually long, turgid, clear +and translucent. Here, then, there was obtained an increased amount +and improved character of growth by the addition of a substance which +contained only carbon, hydrogen and oxygen, and no recognized plant +food. Other organic substances, such for instance as tannin, gave +similar results. + +With the recognition that the presence of organic dissolved substances +in the nutrient medium produced effects on a growing plant of as great +or even greater magnitude than those produced by inorganic dissolved +substances, there was carried out a number of experiments to test +more specifically such substances as might reasonably be expected to +be present naturally in soils. The results thus obtained suggested +experiments with other related substances. The first substance to +suggest itself is stable manure. Taking it all in all, this substance +is probably the most efficient as well as the most generally used soil +amendment in the experience of mankind. The good effects produced by +this substance have in the past been generally considered as due to the +readily “available” potash, phosphoric acid and nitrogen it contains, +but thoughtful experimenters and agriculturists have long doubted +that this explanation is sufficient, since, after all, the mineral +constituents of stable manure are usually small in amount, and out of +all proportion to the effects resulting from its use. That some of the +results are due to an improvement in the physical condition of the soil +when manure is used has quite rightly been generally assumed; but to +its content of nitrogenous components its value has in the main been +ascribed. + +A well-fermented aqueous extract of stable manure was prepared, and +filtered free of suspended solids. Four equal volumes of this solution +were taken. Three of these portions were evaporated to dryness +in platinum dishes, and the residues incinerated. To the dishes +containing: the ash were added respectively nitric acid, sulphuric +acid, and hydrochloric acid in slight excess, and the dishes again +brought to dryness. Water cultures for wheat seedlings were then +prepared.[102] Into one was introduced the given volume of manure +extract; into another the ash from an equal volume of the extract which +had subsequently been treated with nitric acid; and cultures with the +ash which had been treated respectively with sulphuric and hydrochloric +acid were similarly prepared. After ten days growth, the plants from +the several cultures were compared. The plants from the cultures which +contained the sulphates and the chlorides were not materially different +from the plants grown in the check culture. The plants from the nitrate +culture had larger shoots, but shorter roots than the check plants. +But the plants grown in the culture to which the manure extract had +been added directly had by far larger and better shoots and the roots +were incomparably superior to those grown in any other culture, being +larger, thicker, better branched, clear, bright and translucent, and +very turgid, very like the roots obtained in cultures to which carbon +black or precipitated ferric oxide had been added. + +[102] Further studies on the properties of unproductive soils, B. E. +Livingston _et al._, Bull. =36=, 1907, and =48=, 1908, Bureau of Soils, +U. S. Dept. Agriculture. + +The results of this experiment, which has been repeated a number +of times, using manure extracts of various origins, leave no doubt +that it is the organic components of the manure which produce the +characteristic effects, for the ash culture contained all and even more +of the mineral constituents “available” in the original extract, and +the nitrate culture excluded any explanation based on the nitrogenous +content of the manure. This conclusion was supported by the results of +another experiment. + +To a manure extract was added alcohol, which precipitated most of the +organic dissolved substances but very little of the inorganic ones. +The precipitated organic matter was filtered off, dried carefully in a +water oven to eliminate the alcohol, and then taken up in sufficient +water to equal the original volume of manure extract. The nitrate +containing the major part of the salts was boiled vigorously to +eliminate the alcohol and water was then added to restore the original +concentration. A third solution was prepared by bringing together the +organic and inorganic substances which had previously been separated as +above described. The three solutions were used as water cultures for +wheat seedlings, a solution of the original manure extract being taken +for a check culture. The original manure extract and the reconstructed +manure extract gave plants of about equal development. The culture +containing the organic dissolved substances only, gave plants of +nearly, but not quite, equal development to those grown in the check +culture. But the plants grown in the solution containing the dissolved +minerals only, while fine plants and making what would ordinarily be +considered a good development, were decidedly smaller as regards their +aerial parts, and the roots were in no wise comparable to the roots +of the plants grown in the cultures containing the dissolved organic +substances. + +This last experiment has been repeated, with dissolved substances +prepared from another manure extract, but in this case the organic +and inorganic substances were separated by dialysis. This suggested +yet another experiment, in which it was sought to hasten the process +of dialysis, by introducing electrodes into the manure extract, each +electrode being surrounded by some porous membrane, either of parchment +paper, or unglazed porcelain. Not only were the mineral constituents +of the manure extract readily separated in this way, passing into +the electrode chambers, as did also to some slight extent organic +compounds, but also about the outer walls of the electrode chambers +there was marked segregation and deposition of organic materials. The +organic substances deposited at the cathode were found to stimulate +greatly the growth of wheat seedlings while those deposited at the +anode were found to retard the growth of seedlings. It seems probable, +therefore, that stable manure contains organic components which produce +as great or greater effects upon growing plants as do the inorganic +substances it contains: that on the whole these organic components +induce increased plant growth, but some of them, by themselves alone, +would retard plant growth. + +In a similar way green manures have been examined. If fresh clover, +alfalfa, or cowpeas, be macerated and an aqueous extract thus prepared, +it will in general be quite toxic to plants such as wheat; and if this +extract be allowed to stand and ferment or sour the resulting solution +will be totally unfit for the growth of seedling plants. But if the +clover, alfalfa, or cowpea vines be allowed to wilt thoroughly before +being macerated and extracted, or if they be macerated and incorporated +with soil and allowed to remain thus for ten days or a fortnight +before being extracted; then, the resulting solution will be quite +stimulating to such plants as wheat, corn or the grasses, when added +either to water or soil cultures. It would seem, therefore, that the +mineral constituents of the legumes commonly employed as green manures +are less important than the organic, in affecting the growth of crops +subsequently planted, and the inhibitory or toxic action of fresh green +manure seems to be recognized in the common practice of waiting some +days after turning under a green manure crop before seeding to a new +crop. + +The wilting of a green manure involves a darkening and some blackening +of the mass, with apparently some absorption of oxygen. This fact +has suggested a trial of other organic substances which show a +decided ability to absorb oxygen. Among such substances, pyrogallol +stands preëminent. It has been shown that when pyrogallol, or better +pyrogallol and lime, is added to certain soils, naturally low in +productive power, and allowed to stand for a few days, these soils are +readily brought into good condition and support good crops of wheat, +rye, or grasses. Pyrogallol in water cultures is rather toxic to wheat +plants, even in quite dilute solutions. But if the aqueous solution +of pyrogallol be allowed to stand exposed to the air, and better if +the solution be made slightly alkaline as by the addition of lime, +oxygen is absorbed, and a dark brown or blackened solution is soon +formed, which is stimulating to wheat seedlings. Many experiments have +indicated it to be a general rule that soluble organic substances +which are toxic to plant growth yield oxidation products which are +harmless or positively beneficial. + +The suggestion has been made that the well-known infertility of +subsoils, when freshly turned up, is caused by the presence of +alkaloids of the purine or codeine type, due to the activities of +anaerobic bacteria. Water cultures and pot cultures show that while +these substances do have a marked effect on plant growth, it is, +frequently, quite beneficial; strychnine for example, in certain +concentrations, produces a very decided stimulation in the growth of +wheat seedlings. It is clear that some other explanation will have to +be sought for the lack of fertility of subsoils. + +A number of the substances which may be expected for one reason +or another to be present in soils, have been investigated as to +their effect on plants. In this connection may be cited the work of +Livingston[103] and of Dachnowski,[104] who have studied the effect +on vegetation of the organic substances dissolved in bog waters. In +the following table are given the results obtained by growing wheat +seedlings in solutions containing some one of a number of substances +which might be expected to occur in a soil or to be derivatives of such +substances. It will be observed that in the case of these dissolved +organic substances, as has been repeatedly established with the +inorganic ones, in concentrations sufficiently dilute not to be toxic, +they generally show the opposite effect and appear to be stimulating. + +[103] Physiological Properties of Bog Water, by B. E. Livingston, Bot. +gaz., =39=, 348-355, (1905). + +[104] The toxic property of bog water and bog soil, by Alfred +Dachnowski, Bot. gaz., =46=, 130-143, (1908). + + TABLE I.—EFFECT OF VARIOUS ORGANIC COMPOUNDS UPON THE GROWTH OF + WHEAT PLANTS, WITH ESPECIAL REFERENCE TO THEIR TOXIC PROPERTIES[105] + +[105] Certain organic constituents of soils in relation to soil +fertility, by Oswald Schreiner and Howard S. Reed, assisted by J. J. +Skinner, Bull. No. =47=, Bureau of Soils, U. S. Dept. Agriculture, 1907. + + LEGEND: + A = Duration of experiment + B = Lowest concentration causing death + C = Lowest concentration causing injury + D = Concentration causing greatest stimulation + =======================+====+======+======+======+=================== + | | | | | + | | | | | + Compound | A | B | C | D | Remarks + | | | | | + -----------------------+----+------+------+------+------------------- + |days|p.p.m.|p.p.m.|p.p.m.| + | | | | | + _a_ Aspartic acid | 10 | 500 | 100| .... |Normal growth in + HOOC.CH₂.CH(NH₂).COOH | | | | |concentration + | | | | |below 100 p.p.m. + -----------------------+----+------+------+------+------------------- + _b_ Asparagine | 9 | | | |No injury below + NH₂.OC.CH₂.CH(NH₂).COOH| | | | |1,000 p.p.m. + -----------------------+----+------+------+------+------------------- + _c_ Glycocoll, | 9 | | | |Tops of all plants + CH₂(NH₂).COOH | | | | |good. Roots slightly + | | | | |injured at higher + | | | | |concentrations + -----------------------+----+------+------+------+------------------- + _d_ Alanine, | 10 | .... | 500 | 25 |Only roots were + CH₃.CH(NH₂).COOH | | | | |injured at + | | | | |500 p.p.m. + -----------------------+----+------+------+------+------------------- + _e_ Leucine | 9 | .... | .... | .... |No injurious action + CH₃.(CH₂)₃.CH(NH₂).COOH| | | | | + -----------------------+----+------+------+------+------------------- + _f_ Tyrosine, | 11 | .... | 10 | | + OH | | | | | + / | | | | | + C₆ H₄ | | | | | + \ | | | | | + CH₂.CH(NH₂).COOH | | | | | + -----------------------+----+------+------+------+------------------- + _g_ Choline, | 10 | | 500 | 1 |Roots affected more + CH₂CH₂OH | | | | | than tops + / | | | | | + (CH₃)₃N | | | | | + \ | | | | | + OH | | | | | + -----------------------+----+------+------+------+------------------- + | | | | | + _h_ Neurine, | 9 | 250 | 25 | | + CH:CH₂ | | | | | + / | | | | | + (CH₃)₃N | | | | | + \ | | | | | + OH | | | | | + -----------------------+----+------+------+------+------------------- + Neurine (neutralized) | 8 | 250 | 25 | | + -----------------------+----+------+------+------+------------------- + _i_ Betaine, | 9 | ... | ... | |No injury + CH₂.CO | | | | | + / / | | | | | + (CH₃)₃N / | | | | | + \ / | | | | | + O | | | | | + -----------------------+----+------+------+------+------------------- + _j_ Alloxan, | 10 |1,000 | 100 | | + NH.CO | | | | | + / \ | | | | | + CO CO | | | | | + \ / | | | | | + NH.CO | | | | | + -----------------------+----+------+------+------+------------------- + _k_ Guanine, | 12 | | | |Insoluble above 40 + NH.C.NH.CO.C.NH | | | | |p.p.m. No harmful + \\ || \ | | | | |effects. + \\ || CH | | | | | + \\ || // | | | | | + N————C. N | | | | | + -----------------------+----+------+------+------+------------------- + _l_ Xanthine | | | | |No injurious + | | | | |action. + CO.NH.CO.C.NH | | | | | + \ || \ | | | | | + \ || CH | | | | | + \ || // | | | | | + NH——C—N | | | | | + -----------------------+----+------+------+------+------------------- + _m_ Guanadine, | 9 | 100 | 1 | | + NH₂ | | | | | + / | | | | | + HN : C | | | | | + \ | | | | | + NH₂ | | | | | + -----------------------+----+------+------+------+------------------- + _n_ Skatol, | 9 | 200 | 50 | |Roots injured more + C.CH₃ | | | | |than tops + / \\ | | | | | + C₆H₄ CH | | | | | + \ / | | | | | + NH | | | | | + -----------------------+----+------+------+------+------------------- + | | | | | + _o_ Pyridine, C₅H₅N | 9 | .... | 50 | .... |In solutions of 50 + | | | | |p.p.m. and less + | | | | |the root growth + | | | | |was normal. + -----------------------+----+------+-------+------+------------------ + Picoline, C₅H₄N.CH₃ | 7 |1,000 | 500 | 100 | + -----------------------+----+------+-------+------+------------------ + | | | | | + Piperidin | 7 | 250 | 25 | | + CH₂ | | | | | + H₂C / \ CH₂ | | | | | + | | | | | | | + | | | | | | | + | | | | | | | + H₂C \ / CH₂ | | | | | + NH | | | | | + -----------------------+----+------+-------+------+------------------ + Piperidine | 7 | 100 | 25 | 1 | + (neutralized) | | | | | + -----------------------+----+------+-------+------+------------------ + / \ / \ | | | | | + | | | | | | | | + Quinolin, | | | | 6 | 500 | 5 | | + | | | | | | | | + \ / \ / | | | | | + N | | | | | + -----------------------+----+------+-------+------+------------------ + _p_ Ricin | 10 | | 40 | |Insoluble above 50 + | | | | | p.p.m. + -----------------------+----+------+-------+------+------------------ + _q_ Mucin | 10 | | 100 | |Not tested in + | | | | | concentrations + | | | | |higher than + | | | | |100 p.p.m. + -----------------------+----+------+-------+------+------------------ + | | | | | + _r_ Pyrocatechin, | 12 | 500 | 25 | 1 | + C₆H₄(OH)₂(1,2) | | | | | + -----------------------+----+------+------+------+------------------- + _s_ Arbutin, C₁₂H₁₆O₇ | 12 | 500 | 25 | 1 | + -----------------------+----+------+------+------+------------------- + _t_ Phloroglucin, | 13 | 500 | 25 | 1 | + C₆H₃(OH)₃(1,3,5) | | | | | + -----------------------+----+------+------+------+------------------- + _u_ Vanillin, | 9 | 500 | 1 | | + CHO | | | | | + / | | | | | + C₆H₃——O.CH₃ | | | | | + \ | | | | | + OH | | | | | + -----------------------+----+------+------+------+------------------- + Vanillic acid, | 7 | 100 | 25 | 5 | + COOH | | | | | + / | | | | | + C₆H₃—O.CH₃ | | | | | + \ | | | | | + OH | | | | | + -----------------------+----+------+------+------+------------------- + _v_ Quinic acid, | 10 | 500 | 100 | | + C₆H₇(OH)₄.COOH | | | | | + -----------------------+----+------+------+------+------------------- + O | | | | | + / | | | | | | + _w_ Quinone, C₆H₄ | | 9 | 100 | 1 | | + \ | | | | | | + O | | | | | + -----------------------+----+------+------+------+-------------------- + _x_ Cinnamic acid, | 8 | 100 | 25 | | + C₆H₅CH : CH.COOH | | | | | + -----------------------+----+------+------+------+------------------- + Sodium cinnamate | 12 | ... | 100 | |Roots were + | | | | |stimulated + | | | | |in lower + | | | | |concentrations + -----------------------+----+------+------+------+------------------- + _y_ Cumarin, | 8 | 100 | 1 | | + CH:CH.CO | | | | | + / / | | | | | + C₈H₄/ / | 8 | 100 | 1 | | + \ / | | | | | + O | | | | | + -----------------------+----+------+------+------+------------------- + | | | | |Insoluble above + _z_ Daphnetin | 12 | | 50 | |50 p.p.m. Roots + | | | | |somewhat injured + CH : CH.CO | | | | | + / / | | | | | + C₆H₂ ——— O | | | | | + \\ | | | | | + (OH)₂ | | | | | + ----------------------+----+------+------+------+------------------- + _aa_ Esculin, C₁₅H₁₆O₉ | 13 | 500 | 1 | | + ----------------------+----+------+------+------+------------------- + _bb_ Piperonal | | | | | + (heliotropine)— | | | | | + CHO | | | | | + / | | | | | + C₆H₅——O | | | | | + \ \ | | | | | + \ \ | | | | | + O——CH₂ | 7 | 100 | 1 | ... | + ------------------------+----+------+------+------+------------------- + _cc_ Borneol, C₁₀H₁₇(OH)| 10 | 100 | 1 | ... | + _dd_ Camphor, C₁₀H₁₆O | 8 | 300 | 5 | ... | + _ee_ Turpentine, C₁₀H₁₆ | 8 | 500 | 10 | ... | + ------------------------+----+------+------+------+------------------- + +_a._ Aspartic acid has been found in young sugar-cane and in seedlings +of the bean and pumpkin. + +_b._ Asparagine was first found in asparagus; but has since been shown +to be relatively abundant in many species. + +_c._ Glycocoll is one of the simpler and more common degradation +products of proteins. + +_d._ Alanine is a common degradation product of proteins and is related +chemically to phenylalanine, and to tyrosine, which has been found in +many plants. + +_e._ Leucine, an amino-acid of a paraffine series and a decomposition +product of proteids, has been found in certain mushrooms, vetches, +lupine, gourds, potatoes, corn, etc. + +_f._ Tyrosine is an important decomposition product of proteids, is +widely distributed and found in many plants and fungi. + +_g._ Choline is a derivative of certain lecithins and is found in many +seeds and growing plants. + +_h._ Neurine is a substance closely related to choline, and probably +formed from it. + +_i._ Betaine is closely related to both choline and neurine, and is +found in many seeds and plants. + +_j._ Alloxan is closely related chemically to convicine, which latter +is found in beets and certain beans. + +_k._ Guanine is a widely distributed nitrogenous body, and has +been found in the seeds of vetch, alfalfa, clover, gourds, barley, +sugar-beets and sugar-cane. + +_l._ Xanthine, a substance closely related to guanine, has been found +in a number of plants. + +_m._ Guanidine, a substance chemically related to guanine, has been +found in a number of plants of different species. + +_n._ Skatol is a derivative of proteids and is a common product of the +activities of some varieties of bacteria. + +_o._ Pyridine has been shown to exist in soils, as such probably, by +Shorey, who obtained it from certain soils in Hawaii. + +_p._ Ricin is found in the castor-oil plant. + +_q._ Mucin has been found in yams. + +_r._ Pyrocatechin has been found in the bark of various trees, the +berries of the Virginia creeper, the sap of sugar-beets and in several +varieties of willows. + +_s._ Arbutin has been found in many plants, especially in some of the +grasses. + +_t._ Phloroglucin is easily derived from a number of plant constituents. + +_u._ Vanillin forms readily from a glucoside, which is very widely +distributed in many plants, and by some authorities is supposed to be a +product of the decomposition of wood tissues. + +_v._ Quinic acid, which is found with quinine in the cinchona bark, +also occurs in beet leaves, certain hays, cranberry leaves, and +occasionally in other plants. + +_w._ Quinone has been shown to result from the action of a certain +fungus, _Streptothrix chromogena_, common in soils. + +_x._ Cinnamic acid is found in certain barks, and forms esters which +have been found in the leaves of various plants. + +_y._ Cumarin has been found in a large number of plants, including the +grasses, beets, sweet clover, etc. + +_z._ Daphnetin occurs in some species of _Daphne_ and is closely +related to cumarin. + +_aa._ Esculin, as well as the corresponding esculetin, has been found +occasionally in a number of plants. + +_bb._ Heliotropine, or piperonal, has the odor of heliotrope and is +found in flowers. + +_cc._ Borneol occurs in needles of different varieties of pine, fir, +spruce and hemlock, golden rod and thyme. + +_dd._ Camphor is closely related chemically to borneol and is secreted +by a number of plants; it is found in the wood of _Cinnamomum_, +cinnamon root, in the leaves of sassafras, spikenard, rosemary, +rosewood, etc. + +_ee._ Turpentine is a constituent of many plants and coniferous trees. + +Finally, a number of organic substances has been isolated from soils. +Their composition, and in several cases their constitutions have been +determined. The effects of these on plants, when they are present +in the cultural media have been studied. Thus, Shorey[106] was able +to isolate picoline carboxylic acid (C₇H₇NO₂) from certain soils in +Hawaii, and this same substance has since been found in several soils +of the United States. In aqueous solutions it is quite toxic to wheat +seedlings. Since then a number of other definite organic compounds have +been isolated from soils belonging to at least eight different classes +of organic substances, including:[107] + + Hentriacontane, C₃₁H₆₄. + Monohydroxystearic acid, CH₃(CH₂)₆CHOH(CH₂)₉COOH. + Dihydroxystearic acid, CH₂(CH₂)₇CHOH.CHOH.(CH₂)₇ COOH. + Agroceric acid, C₂₁H₄₂O₃. + Paraffinic acid, C₂₄H₄₈O₂. + Lignoceric acid, C₂₄H₄₈O₂. + Phytosterol, C₂₆H₄₄O.H₂O. + Pentosan, C₅H₈O₄. + Agrosterol, C₂₆H₄₄O.H₂0. + Picoline carboxylic acid, C₇H₇O₂N. + Histidine, C₆H₉O₂N₃. + Arginine, C₆H₁₄O₂,N₄. + Cytosine, C₄H₅ON₃.H₂O. + Xanthine, C₅H₄O₂N₄. + Hypoxanthine, C₅H₄ON₄. + Glycerides, resin acids, etc. + +[106] Organic nitrogen in Hawaiian soils, by E. C. Shorey, report of +Hawaii Experiment Station, 1906, 37-59. + +[107] Chemical Nature of Soil Organic Matter, by Oswald Schreiner +and Edmund C. Shorey, Bull. 74, Bureau of Soils, U. S. Department of +Agriculture, 1910. + +Some of these, picoline carboxylic acid, dihydroxystearic acid and +the pentosan just cited, are toxic to growing plants; others are not. +The origin and mode of production of these substances in the soil +is, generally speaking, uncertain and obscure, and is yet one of the +important fundamental problems confronting the soil chemist. + +It is important to note that the organic substances thus far isolated +from soils are of widely varying types, and with very different +chemical characteristics. As pointed out above, almost any type of +organic substance is likely to be found in soils, and the effects of +any of them on growing plants can hardly be predicted from _a priori_ +considerations. + +It has been found that as a general rule the continued growth of +one crop in any soil results in a low crop production. Pot cultures +have given even more pronounced results in the same direction. The +explanation long accepted is that the soil has, as a result of +continued cropping, become deficient in some one or more of the +“available” mineral nutrients. Pot experiments, where the garnered crop +was returned to the soil and still a diminished yield was obtained, +throw doubt on this explanation. Still further doubt results from +water-cultures which, by growing a crop in them, become “poor” for +subsequent crops, although there is maintained in them an ample +supply of mineral plant nutrients, and they are easily renovated by +good absorbers. These facts find a more satisfactory explanation as +being due to the production in the nutrient medium of deleterious +organic substances originating in the growing plant itself. This idea +seems to have been advanced first by De Candolle, in 1832,[108] to +account for the beneficial results obtained by employing a rotation of +crops. It appears to have been held by Liebig at one time, although +he subsequently abandoned it in favor of the view that the benefits +of a crop rotation are due to the several crops requiring different +proportions of mineral nutrients, and that the disturbance of the +balance in the soil produced by one crop is not unfavorable to the +growth of some other crop. Although lacking direct experimental +confirmation, this latter view of Liebig’s has long prevailed among +agricultural investigators, partly by reason of his authority, partly +by reason of the dominance of the plant-food theory of fertilizers, +and partly by reason of the fact that the ideas of De Candolle as +originally advanced included certain errors soon detected. The trend of +recent investigations has been distinctly in favor of a modified form +of the view of De Candolle. It has been recognized that other factors +enter into crop rotations, such as the elimination of associated weeds, +various kinds of animal, insect and plant parasites, preparation of +the soil by a deep-rooted crop for a shallow-rooted following crop, +etc. It has come to be recognized that there are natural associations +of plants, and natural rotations of vegetation certainly determined +by other than plant food factors. Thus, in the eastern United States, +wheat is followed by ragweed naturally, while across the fence +cocklebur and wild sunflower come in after the corn, the difference +in vegetation being as sharply marked after the removal of the crops +as when they still occupied the land. Analyses of the ragweed, for +instance, although it is a shallower rooted crop than wheat, show +that it takes from the soil as much of the mineral nutrients as +does the preceding[109] wheat crop. The investigation of Lawes and +Gilbert[110] on fairy rings showed that the continual widening of the +rings can not be satisfactorily explained by the comparison of the +mineral constituents in the soil within and without the rings. Work +at Woburn[111] on the effect of grass on apple trees finds no other +plausible explanation than that the growing grass produces in the +soil organic substances detrimental to young apple trees. A number of +similar cases have been recorded. + +[108] See in this connection, Further studies on the properties of +unproductive soils, by B. E. Livingston, Bull. No. =36=, Bureau of +soils, Dept. of Agric., 1907, p. 7-9. + +[109] Mr. J. G. Smith has made a comparison between the potash and +phosphoric acid content of the wheat and following crop of ragweed +grown on a farm in Fairfax Co., Va. His unpublished results, with some +others found in the literature, are given in the following table: + + ======================+======+==========+=========================== + |Potash|Phosphoric| + Material | K₂O |acid, P₂O₅| Analyst + | % | % | + ----------------------+------+----------+--------------------------- + Wheat | 0.76 | 0.52 |Smith + Young ragweed | 1.78 | 0.73 |Smith + Ragweed in seed | 1.28 | 0.35 |Smith + Ragweed in seed and | | | + accompanying plants | 1.18 | 0.39 |Smith + Winter wheat in flower| 1.796| 0.51 |Wolff’s tables in Johnson’s + | | | “How Crops Grow,” p. 376. + Ragweed | 1.79 | 0.41 |DeRoode,in Bull. 19, W. Va. + | | | Agr. Exp. Sta., 1891 + Ragweed | 1.809| 0.54 |Burney, 2d. Ann. rept. + | | | S. C. Stat., 1889, p. 146 + ----------------------+------+----------+--------------------------- + +On the whole, ragweed seems to require and take from the soil about +as much mineral matter as does wheat. It is stated by some of the +dairy farmers near Washington, who cut the mixture of ragweed, other +weeds and grass following wheat, for a hay crop, that the weight of +the ragweed crop is generally heavier than that of the wheat crop. +Therefore the ragweed actually removes more mineral matter from the +field than does the wheat. These facts lend no support to the popular +notion that wheat “exhausts” the soil of its “available” mineral plant +nutrients. For analyses of a number of common American weeds, see +Analyses of the ashes of certain weeds, by Francis P. Dunnington: Am. +Chem. Jour., =2=, 24-27, (1880). + +[110] Note on the occurrence of “fairy rings,” by J. H. Gilbert: Jour. +Linn. Soc, =15=, 17-24, (1875). + +[111] Second, third and fifth reports of the Woburn Experimental Fruit +Farm, =1900=, =1903=, =1905=. + +Finally, although less work has been done in this direction with higher +plants than with other organisms, it is now recognized as a general law +of all living organisms that they function less readily as the products +of their activities accumulate.[112] These products may, however, be +inimical, neutral or even stimulating to other organisms. + +[112] It may not be amiss to point out here that this general law holds +for all dynamic phenomena. In chemistry, for instance, the general law +is well recognized that the rate of reaction diminishes with increase +in the active mass of the reaction products. It can be shown that the +principle applies to plant growth. Young plants will withdraw potassium +more rapidly than chlorine from solutions of potassium chloride; that +is, the solution soon contains free hydrochloric acid. Conversely +the plants cause a solution of sodium nitrate to become alkaline. +Therefore, if the above principle holds, then the initial addition of +small amounts of hydrochloric acid to a solution of potassium chloride +should slow up the absorption of potassium by seedling wheat plants, +or the addition of sodium hydroxide the absorption of nitrogen from a +solution of sodium nitrate. Mr. J. J. Skinner has tested this idea with +the following results, growing carefully selected wheat seedlings, for +3 days in solutions of pure potassium chloride, solutions of potassium +chloride containing initially enough excess of hydrochloric acid to +be of an N/₅,₀₀₀ concentration with respect to the acid, solutions of +sodium nitrate, and solutions of sodium nitrate containing initially an +excess of sodium hydroxide. + +Solutions of KCl containing 80 p.p.m. K₂O. + +1 K₂O absorbed 40.0 p.p.m. 2 K₂O absorbed 40.0 p.p.m. 3 K₂O absorbed +36.3 p.p.m. + +Solutions of KCl (80 p.p.m. K₂O) and HCl (N/₅,₀₀₀). + +4 K₂O absorbed 26.7 p.p.m. 5 K₂O absorbed 29.5 p.p.m. 6 K₂O absorbed +26.7 p.p.m. + +Solutions of NaNO₃ containing 80 p.p.m. NH₃. + +7 NH₃ absorbed 30.2 p.p.m. S NH₃ absorbed 30.2 p.p.m. 9 NH₃ absorbed +32.5 p.p.m. + +Solutions of NaNO₃ (80 p.p.m. NH₃) and NaOH (N/₅,₀₀₀). + +10 NH₃ absorbed 27.8 p.p.m. 11 NH₃ absorbed 34.3 p.p.m. 12 NH₃ absorbed +27.8 p.p.m. + +This problem has been investigated critically by direct experimentation, +growing wheat, and other seedlings in water and agar cultures.[113] +It has been shown that wheat renders the culture media unsuitable +for subsequent wheat crops, though it can be reclaimed or renovated +by treatment with such absorbents as carbon black, or by other +methods.[114] Wheat did about as well when grown in a medium which had +previously supported a growth of cowpeas as when planted in a fresh +medium; poorer results were obtained after oats; no crop produced such +poor results in the succeeding wheat crop as did wheat itself. + +[113] Some factors in soil fertility, by Oswald Schreiner and Howard S. +Reed, Bull. No. =40=, Bureau of Soils, U. S. Dept. Agriculture, 1907. + +[114] Soil fatigue caused by organic compounds, by Oswald Schreiner and +M. X. Sullivan: Jour. Biol. Chem., =6=, 39-50, (1909). + +It is yet a matter of dispute as to whether the substances thus added +to nutrient media are truly excretory products of the plant, sloughed +off or otherwise eliminated from the surface of the roots, or further +elaborated by bacterial or other agencies before becoming effective. +These are important problems for the plant physiologist and the soil +chemist alike. It is beyond dispute, however, by reason of a large and +increasing weight of evidence, much of it direct experiment, that, as +a result of the growing of plants, soils and the soil water do contain +organic substances; harmful to the plant or organism eliminating them; +harmful, innocuous, or even stimulating to other plants or organisms. + +For the elimination from the soil of toxic or inhibitory organic +substances, whether excreted by roots or otherwise produced, several +methods are more or less effective. When, as is sometimes the case, +the substance is volatile, it may be removed by heating, distilling +with steam, or passing a current of air through the soil or cultural +medium. These methods, while effective in the laboratory and possibly +applicable to greenhouse conditions, are naturally inapplicable to +field conditions. In this last case the obvious procedure is to +increase as much as possible the absorptive powers of the soil; to +secure the best possible drainage; and with these, the best possible +aeration of the soil. + +It has been found that, in general, a cultural medium which has +been rendered unfit for the continued growth of a crop, is readily +renovated by treatment with oxidizing agents, and is sometimes rendered +even better than ever by such treatment, which would suggest that +the oxidation products from plant effluvia may be even beneficial +to the plant. To this end the growing plant seems itself to be an +active agent, apparently attempting automatically to protect itself +against the products of its own activities. It has been pointed out by +Molisch[115] that root secretions have an oxidizing power, apparently +of an enzymotic character. Some doubt of the validity of Molisch’s +work has been raised by Czapek, Pfeffer, and others; nevertheless it +is now accepted that while intercellular autoxidation or reduction +processes may take place in living roots, the higher plants, such +as our common crop plants, also show a more or less well-developed +extracellular oxidizing power in the neighborhood of the root tips and +root hairs.[116] That this oxidizing power displayed by growing roots +is enzymotic is indicated by the fact that artificial culture media +frequently display it also after plants have been grown in them for a +short while.[117] + +[115] Über Wurzelausscheidungen und deren Einwirkung auf organische +Substanzen, von Hans Molisch. Sitzungsber. Akad. Wiss. Wien, Math. nat. +K1., =96=, 84-109 (1888). + +[116] The rôle of oxidation in soil fertility, by Oswald Schreiner +and Howard S. Reed: Bull. No. =56=, Bureau of Soils, U. S. Dept. +Agriculture, 1909. + +[117] From considerations as yet highly speculative, a different type +of oxidation by roots might be anticipated. It is recognized that in +the absorption of mineral nutrients by plants a certain amount of +selection enters. For example, a plant with its roots in a solution of +potassium chloride, absorbs more potassium than chlorine, relatively, +and free hydrochloric acid is left in the solution. Obviously in the +absorption, work is done, and a possible explanation is that water is +decomposed at the absorbing surface of the root, with the liberation of +oxygen. Theoretically, it ought not to be difficult to investigate this +by a study of the energy changes during absorption, but growing plants +do not lend themselves readily to such experimentation. + +It has been shown that the oxidizing action of growing roots is +generally promoted by having the cultural medium slightly alkaline +or neutral rather than acid. It is also promoted by the addition +of various mineral salts, notably by nitrates, phosphates, or +lime salts. Potassium salts promote the oxidation but slightly, +and in some experiments have even produced a slight decrease. The +corresponding sodium and ammonium salts are more favorable than those +of potassium.[118] It appears altogether probable, therefore, that the +mineral salts in commercial fertilizers may have some importance in +this connection. + +Whatever may be the role of mineral fertilizers towards organic +substances toxic to growing plants, it is certain that they have +an importance and one that is probably specific, as indicated by +some recent investigations.[119] Culture solutions containing the +constituents potassium, nitric acid and phosphoric acid were prepared +in such manner that they covered the range of all possible ratios +of these constituents in intervals of ten per cent. in each. Into +one set of these solutions was introduced dihydroxystearic acid, +into another set cumarin, and into a third set, vanillin, and into a +fourth set, quinone. The growth of wheat seedlings in these several +sets showed indubitably that these several organic substances which +are all deterrent to the growth of wheat, were modified in their +influence by the presence of the mineral salts; but that nitrates +were more efficient than the other minerals in the case of the +solutions containing dihydroxystearic acid or vanillin; phosphates +were most efficient in the case of the solutions containing cumarin, +and potassium most efficient in solutions containing quinone. As the +organic substances used in these experiments, either in themselves or +as typifying classes of compounds, may be anticipated in soils under +natural conditions, it is again apparent that mineral fertilizers have +a function in addition to the traditional one of increasing the supply +of mineral nutrients. + +[118] Action of fertilizing salts on plant enzymes, by M. X. Sullivan, +Jour. biol. chem., =6=, (1909), proceed. XLIV. + +[119] Private communication by Dr. Oswald Schreiner and Mr. J. J. +Skinner. + +The fact that the oxidizing power of roots is more marked when grown +in aqueous extracts of soils in good tilth than in extracts made from +soils in poor tilth, shows that cultural methods are no less important +in field practice than are fertilizers in promoting this important +activity of plants. There is little reason to doubt that oxidizing +agencies other than plant roots (bacterial for instance) are more or +less active in every arable soil, and numerous investigations, among +which Russell’s researches[120] are conspicuous, leave little doubt +that oxidation processes are promoted by good tilth. It is apparent, +therefore, that by the activities of the plant itself as well as other +agencies, the general tendency in soils is the destruction of or +rendering innocuous harmful plant effluvia or other organic substances, +and to this end are effective each of the three methods of soil control +generally practiced, namely, tillage, crop rotation and fertilizers. + +Among the organic components of the soil none have greater importance +and interest than those containing nitrogen or as they are frequently +called the nitrogen carriers. Conspicuous among these are the nitrates. +While it is now generally conceded that ammonia and other nitrogen +compounds can be taken up by higher plants and elaborated by them under +special conditions, it nevertheless remains true that plants draw their +needed supplies of nitrogen from the soil solution, mainly in the +form of nitrates. The problems presented by these nitrogen carriers +are mainly bacterial[121] and physiological, but certain features +are of direct importance to the soil chemist and to a study of the +soil solution. It is now known generally that there are many kinds of +nitrifying and denitrifying bacteria in soils, and that probably every +arable soil contains several species, or varieties at least of both +kinds. With good tilth and consequent aerobic conditions, nitrifying +processes prevail, and with poor tilth or in subsoils, anaerobic +conditions and denitrifying processes prevail. Warmth, moisture, the +reaction of the soil, and perhaps other factors markedly affect the +activity of the organisms of the soil solution. Another important +factor is that the absorptive powers of the higher plants are markedly +affected by sunlight, so that, especially on bright and clear days, +there is generally a higher concentration of nitrates in the soil +solution in the morning than in the evening. This fact would seem to +affect seriously the value of some recent and extensive investigations +where it has been sought to classify soils by their content of +water-dissolved nitrates. Nitric acid is more readily leached from +soils than are most other acid radicals. Consequently nitrates, like +other organic components of the soil solution, and unlike inorganic +components, tend to vary greatly in concentration. + +[120] Oxidation in soils, and its connection with fertility, by +Edward J. Russell: Jour. Agric. Sci., I, 261-279, (1905); Pt. II. The +influence of partial sterilization, by Francis V. Darbishire and Edward +J. Russell, =2=, 305-326, (1907). + +[121] The fixation of atmospheric nitrogen by bacteria, by J. G. +Lipman, Bull. =81=, Bureau of Chemistry, U. S. Dept. of Agriculture, +1904, p. 146-160; A review of investigations in soil bacteriology, +by Edward B. Voorhees and Jacob G. Lipman, Bull, =194=, Office of +Experiment Stations, U. S. Dept. of Agriculture, 1907. + + + + +Chapter XII. + +FERTILIZERS. + + +It is generally recognized that the great practical problem confronting +the soil chemist is the proper use of soil amendments or fertilizers. +The farmers of the United States now spend annually for fertilizers +upwards of $100,000,000. It is estimated by various authorities that +a large fraction, perhaps as much as three-fourths, of the material +represented by this expenditure is misapplied for lack of intelligent +direction. Yet all of this enormous mass of fertilizers can be used +to advantage. Great as it is, it is relatively small beside the total +which will, and must, be used in a not distant future, with the growth +and development of intensive methods of cultivation consequent upon +the rapid settling of the country, the practical disappearance of new +lands and the increase in money value of the old lands. The commercial +importance of the problem, therefore, makes it desirable that special +emphasis should be given to fertilizers from the point of view +developed in the preceding chapters. It should be recalled that the use +of fertilizers constitutes one of the three great general methods of +soil control, and further that while tillage methods, crop rotations, +and fertilizer applications can be used to supplement one another, no +one of these methods can be expected to take satisfactorily the place +of another. + +Crop production is dependent upon a large number of factors. Upon the +rainfall, both as to the amount and distribution; upon the sunlight, as +to amount and distribution; upon the chemical and physical properties +of the soil; soil bacteria and other biologic agents; enzymes in the +soil; biological factors in the plant, and probably many other things. +Opinions do and will continue to differ as to what these factors are, +but at least every one agrees that they are many. + +Attempting to formulate these factors develops fundamental +difficulties, since it is not positively known how far the variables +are dependent or independent, and we have no idea as to the nature of +the function or functions. The weight of existing evidence favors the +view that all the factors are dependent variables, although numerous +attempts have been made from time to time to show that some one factor, +such as the rainfall for instance, or the mean annual temperature, or +available plant-food, is _practically_ an independent factor. Although +it should be rather easy to determine experimentally the nature of the +function, if any of these various factors were independent, this has +never been done, and this fact is itself a strong argument that all the +factors in crop production are dependent on one another. + +When there is introduced into the equation a factor for any one of +the methods of soil control, _i. e._, tillage, crop rotation, or +fertilizers, it becomes even more apparent that the function is +determined by dependent variables, for the new factor always more or +less affects several if not all of those already cited. For instance, +fertilizers certainly affect the chemical properties of the soil, its +physical properties, the soil bacteria, perhaps the plant-food supply, +the oxidation of plant effluvia and other factors. It is obvious, +therefore, that a satisfactory theory of fertilizer action can not be a +simple one but must of necessity be complex; and the same statement is +no less true as regards tillage and crop rotation. + +The recognition of the fact that the action of fertilizers is a complex +function depending upon many factors and groups of factors which vary +among themselves and with each individual soil, carries with it the +conviction that an exact or quantitative fertilizer practice, while +theoretically possible, is probably unattainable since methods for the +solution of such complex functions are generally wanting. It is not +surprising, therefore, that the empirical experience of the past has +failed to develop a quantitative practice. However disappointing this +may seem at first sight, the prospect is not altogether hopeless, for +this point of view indicates a systematic scheme for experimentally +determining a qualitative, but nevertheless rational, fertilizer +practice. The dominance of the plant-food theory of fertilizers in +the past, shutting off, as it has, a rational attack of the problem, +is causing the annual waste of millions of dollars in misapplied +fertilizers, and it is of scarcely less economic than scientific +importance to investigate and extend our knowledge of the effect +of soil amendments upon the many factors in crop production. With a +knowledge of the effect of fertilizers upon the physical, chemical +and biological factors in crop production, and of the nature of the +interdependence of these factors, will come the ability to manage +intelligently the individual field for the particular crop. This +knowledge can only come by attacking the problem from the dynamic +viewpoint, and so far as the soil factors are concerned, they can +apparently be studied best as they affect the properties of the soil +solution. + +While it seems certain that some fertilizer effects are directly upon +the soil and secondarily upon the plants, it cannot be doubted that in +others, the phenomena are more directly concerned with the absorption +by and the metabolism within the plant and until these plant processes +are better understood, nothing approaching a satisfactory practice can +be anticipated. Why and how plants exercise the selective powers they +appear to possess are fundamental questions yet to be answered. The +important effects sometimes produced by adding to the nutrient medium +such substances as manganese salts which are not necessary to the +growth of the plant, can no more be neglected than the study of the +phosphorus needs. The presence in the soil universally of substances +other than the recognized mineral nutrients,[122] may very well have a +significance for plant production hitherto unsuspected, for the fact +that an organism can continue to function in the absence of a substance +is no argument, much less proof, that it would not function better with +that substance present. Recent investigations, showing that animal +organisms are sometimes more resistant to certain toxins and diseases +under starvation conditions or when ingesting substances unnecessary +to normal development, suggest the possibility at least of similar +phenomena with plants. It is at any rate clear that the practical +problem of the best production of plants from soils is not merely one +of providing a relatively large supply of potassium, phosphorus and +nitrogen. + +[122] See, for instance, Barium in soils, by G. H. Failyer, Bull. No. +=71=, Bureau of Soils, U. S. Dept. of Agriculture, 1910. + +In this connection it is well to consider what constitutes a commercial +fertilizer. It must be a substance the addition of which either +directly or indirectly affects the properties of the soil or the +growing plant; it must be obtainable in large quantities and from a +source or sources of supply not readily exhausted; and it must be +cheap. Of the many substances filling the first condition, all those +which fulfill also the other conditions are used as fertilizers, with +the exception of common salt and human excrement. In spite of the fact +that it does not contain a conventional plant-food, sodium chloride +appears to produce results quite similar to those produced by the usual +fertilizer salts. Its use has been followed generally by an increased +yield of crop, but occasionally by a decreased one, and it appears not +improbable that further investigation would show sodium chloride to +have a considerable value as a fertilizer. Human excrement or night +soil, and the sewage and garbage refuse of our large cities are not +commercial fertilizers, although having undoubtedly a high agricultural +value. Objection has been urged to them that they are “filthy” and +liable to contain dangerous pathogenic organisms. Both objections could +be met. It seems a more rational explanation that the agricultural +methods of this country have not yet become sufficiently intensive to +necessitate the conservation of such materials or to justify their +commercial exploitation. + +New products will come into use from time to time, as in the case of +calcium cyanamid and basic calcium nitrate. But it is worthy of note +that these substances have become available not so much because of +their agricultural value, but incidentally to the efforts of inventors +and manufacturers to produce cheap nitric acid for the preparation of +high explosives.[123] There seems no reason to doubt that an ample +supply of desirable substances will always be available for fertilizer +purposes. The immediate practical problem for the future is not the +seeking of new fertilizers but the rational use of those at hand. + +[123] In this connection it may be of interest to call attention to +the fact that the Twelfth Census shows less than a fifth of the sodium +nitrate brought into the United States goes into the fertilizer trade. +Moreover, the production of ammonium salts by the extensive coke and +gas plants of the country has been practically _nil_ not because of any +inherent difficulties in making them or because the cost of production +has been high, but because the market demands in this country have been +too small. + + + + +Chapter XIII. + +ALKALI. + + +In the preceding chapters there have been considered the phenomena +which obtain under humid conditions. Under exceptional conditions of +prolonged drought there occurs an accumulation of soluble mineral +substances at or near the surface of the soil. This phenomenon is +pronounced in arid and semi-arid regions,[124] and the accumulations +of soluble salts occurring in such regions is known in the United +States as “alkali,” in India as “reh,” in Africa as “brak,” and in +other countries by various local designations. The study of the extreme +conditions producing alkali has added materially to the present +knowledge of the processes taking place in soil of humid areas. +Moreover, alkali-infested areas are themselves becoming of so much +importance with the growing needs for further new lands, that it seems +wise to give here an outline of the chemical principles involved in +their soil solutions.[125] + +Alkali is sometimes a single salt, but usually a mixture of some +two or more of the chlorides, sulphates, carbonates, bicarbonates, +and occasionally the nitrates, phosphates and borates, of sodium, +magnesium, potassium, and calcium, and occasionally strontium and +lithium. In the United States, when the carbonate of sodium is +present to an appreciable extent, the salt mixture is known as _black +alkali,_ in contradistinction to _white alkali_, which latter does +not contain sodium carbonate.[126] Generally, but not always, soils +containing alkali also contain accumulations of the less soluble +salts, calcium carbonate, or calcium sulphate, or a mixture of the +two. These substances, sometimes cementing the less soluble mineral +components of the soil, sometimes almost pure, are found in layers more +or less continuous, and from a fraction of an inch to several feet in +thickness, in a position approximately parallel to and at a moderate +depth below the surface of the soil. In such cases these layers form a +“hard-pan” and frequently the treatment of this type of hard-pan is the +most difficult and vexing problem in the management of alkali-bearing +soils. + +[124] Occasional occurrence of alkali in humid regions, by Frank K. +Cameron, Bull. No. =17=, Bureau of Soils, U. S. Dept. Agriculture, +1901, p. 36-38. This phenomenon should not be confused with the surface +deposition of various kinds of saline material from springs, which is +fairly common in both humid and arid regions, the world over. + +[125] Alkali soils of the United States, by Clarence W. Dorsey, Bull. +No. =35=, Bureau of Soils, U. S. Dept. Agriculture, 1906. + +[126] Black alkali is so called because the caustic solution containing +sodium carbonate, in rising to the surface of the soil, dissolves +and carries with it organic matter which is subsequently left on +the surface in more or less blackish deposits, often ring-like in +appearance. It is by no means uncommon, however, to find deposits of +“black alkali” which are not black at all, and it is quite common to +find “white alkali” so dark in color as to suggest the presence of +sodium carbonate, although the latter be absent. + +The origin of alkali is often uncertain. In some cases the geological +evidences in the area make it certain that the alkali came from the +desiccation of former bodies of sea water which had become isolated +from the ocean. In other cases the alkali appears to come from the +desiccation of lakes which are the depositories of the drainage of a +surrounding area, and which have no outlet to the sea. In still other +cases it has been supposed that the alkali is derived from wind-borne +sea-spray. Various explanations of a more or less special character +with regard to particular localities or circumstances are to be found +in the literature.[127] + +The chemical principles involved in the desiccation of a body of +sea water are now pretty well understood, owing mainly to the +investigations of van’t Hoff, Meyerhoffer, and their coworkers.[128] +The salts in sea water and those constituting “white alkali” are mainly +the chlorides and sulphates of sodium, potassium and magnesium. +Calcium is also present, appearing in deep deposits as anhydrite, and +at the surface as gypsum. + +[127] An interesting case is the Billings Area, Montana, where the +alkali seems to be derived from the oxidation, solution and subsequent +hydrolysis of the pyrites and marcasite of the neighboring Pierre +shales. The sulphuric acid thus formed, leaching through shales and +sandstones, takes up various bases and the predominating salts in the +alkali of this area are the sulphates of sodium and magnesium. + +[128] Zur Bildung der ozeanischen Salzablagerungen, von J. H. van’t +Hoff, Braunschweig, 1905-09. For a detailed discussion of these results +with reference to alkali deposits see: Calcium sulphate in aqueous +solutions, by Frank K. Cameron and James M. Bell, Bull. No. =33=, +Bureau of Soils, U. S. Dept. Agriculture, 1906. + +From the results of this work it is possible to predict the order +in which the different salts or minerals will separate from the +evaporating solution. At ordinary temperature (25° C) the first salt to +be deposited from the dilute solution is _gypsum_ (CaSO₄.2H₂O) followed +by _halite_ or _sodium chloride_ (NaCl) in quantity. Sodium chloride +continues to separate at all higher concentrations. Next will be +deposited _kainite_ (MgSO₄KCl.3H₂O). At the concentration then reached, +the stable sulphate of calcium is _anhydrite_ (CaSO₄), which continues +to separate from solution as desiccation proceeds. Consequently, if the +gypsum previously deposited is yet in contact with the solution, it +tends to be transformed to anhydrite and at all higher concentrations +the deposition of anhydrite may be expected. As evaporation proceeds a +point is reached where _kainite_ and _kieserite_ (MgSO₄.H₂O) separate. +Further evaporation brings a concentration at which _kieserite_ and +_carnallite_ (MgCl₂.KCl.6H₂O) are precipitated, and as the process +proceeds, finally the point is reached where _kieserite_, _carnallite_ +and _bischofite_ (MgCl₂.6H₂O) all three separate with sodium chloride. +The final products separating at a higher temperature, 83° C., are +the same four solids, sodium chloride, kieserite, carnallite and +bischofite.[129] The alternate layers of anhydrite and sodium chloride +noticeable in some desiccated sea beds is probably the result of +alterations in temperature, anhydrite being less soluble, and sodium +chloride somewhat more soluble in hot than in cold water. During warm +weather there would be a greater tendency for anhydrite to separate and +in colder weather for sodium chloride to be precipitated. Anhydrite at +the surface would gradually absorb water vapor from the atmosphere and +be transformed to gypsum.[130] + +[129] It will be interesting to compare with the above the following +brief description of the Stassfurt salt deposits, taken from Ries’s +Economic Geology of the United States, (1905), p. 127. “At the bottom +is the main bed of rock salt which is broken up into layers 2-5 inches +thick by layers of anhydrite. Above this come 200 feet of rock salt, +with which are mixed layers of magnesium chloride and polyhalite.... +Resting on this is 180 feet of rock salt, with alternating layers of +sulphates chiefly kieserite, the sulphate of magnesia. These layers are +about 1 foot thick. Lastly, and uppermost, is a 135-foot bed consisting +of a series of reddish layers of rock salts of magnesia and potassium, +kainite ... kieserite ... carnallite ... tachhydrite ... as well as +masses of snow-white boracite.” + +[130] As examples, some of the gypsum deposits of Kansas may be cited, +according to Haworth, Mineral resources of Kansas, 1897, p. 61, and the +classical case at Bex, Switzerland, described by J. G. F. Charpentier, +Uber die Salz-Lagerstätte von Bex: Ann. Phys. Chim., =3=, 75-80, +(1825), and by G. Bischof, Elements of chemical and physical geology, +London, 1854-58, Vol. I, p. 350-1. + +Besides the principal salts just described, there may separate at +one concentration or another other various double salts including +_langbeinite_ (2MgSO₄.K₂SO₄), _polyhalite_ (K₂SO₄.MgSO₄.2CaSO₄.2H₂O), +_glauberite_ (CaSO₄.Na₂SO₄), _syngenite_ (CaSO₄.K₂SO₄.H₂O), _potassium +pentasulphate_ (K₂SO₄.5CaSO₄.H₂O), _krugite_ (4CaSO₄.K₂SO₄.MgSO₄.2H₂O), +and possibly others. These are all stable over very restricted ranges +of concentration, however, and if formed, probably seldom persist, but +pass over to more stable salts as the desiccation proceeds, and have +little more than a passing theoretical interest. + +The addition of carbonates to the system introduces some further +modifications.[131] In this case lime carbonate is the first salt to +be precipitated, followed probably by the same order of deposition +as outlined above. As the mother liquor becomes more concentrated, +it apparently loses its alkaline character, for the addition of +an alcoholic solution of phenolphthalein does not produce the +characteristic red color. That the solution does actually contain +dissolved carbonates is shown by the appearance of the red color on +diluting a portion of the mother liquor with distilled water. An +interesting example in nature is furnished by the Great Salt Lake, +Utah. A test of the water of this lake in 1899 gave no alkaline +reaction with phenolphthalein, but the reaction appeared promptly when +distilled water was added, and further examination showed the water +to contain about 0.012 per cent. sodium carbonate.[132] Slosson has +reported similar cases in Wyoming.[133] + +[131] The action of water and aqueous solutions upon soil carbonates, +by Frank K. Cameron and James M. Bell, Bull. No. =49=, Bureau of Soils, +U. S. Dept. Agriculture, 1907. + +[132] Application of the theory of solutions to study of soils, by F. +K. Cameron, Report No. =64=, Field Operations of the Bureau of Soils, +1899, p. 149. + +[133] Alkali lakes and deposits, by W. C. Knight and E. E. Slosson, +Bull. No. =49=, Wyoming Agr. Expt. Station, 1901, p. 108. + +One “black alkali” system has been studied with some approach towards +completeness.[134] In this case magnesium and potassium salts are not +present, the system being composed of water, carbon dioxide, chlorides, +sulphates, sodium and calcium salts, with the condition imposed, that +the bases are present in amounts more than equivalent to the sulphuric +and hydrochloric acids. On desiccation at 25° C calcium carbonate first +appears followed by gypsum and then sodium sulphate decahydrate. Next +appears a double salt (2CaSO₄.3Na₂SO₄) followed by anhydrous sodium +sulphate, the Glauber’s salt which formerly crystallized being no +longer stable. Sodium chloride then precipitates and the concentration +finally reaches a point where gypsum is no longer stable, and the +final group of salts in contact with the evaporating solution under +conditions of stable equilibrium consists of calcium carbonate, the +double sulphate of soda and lime, anhydrous sodium sulphate and sodium +chloride. + +[134] The solubility of certain salts present in alkali soils, by Frank +K. Cameron, J. M. Bell and W. O. Robinson, Jour. Phys. Chem., =11=, +396-420, (1907). + +The desiccation of a lake which serves as the final repository +of a regional drainage involves essentially the principles just +discussed.[135] The constituents involved are the same. A serious +problem involved in the consideration of this source of “alkali” is +the high ratio of chlorine to the other constituents, in view of its +very low ratio in the rocks from which it comes. The explanation +undoubtedly involves the fact that the carbonates and sulphates are +constantly being removed as calcium salts from a body of water which is +more or less continuously receiving the drainage of any considerable +watershed, and is at the same time subject to a relatively high rate of +evaporation. The chlorine forming only very soluble salts under such +conditions would be segregated and concentrated in the residual mother +liquor. Most difficult is it to account for the relatively high ratio +of sodium to potassium in alkali from such an origin. Some light is +thrown on the subject by the progressive changes in concentration of a +lake water which receives a regional drainage under arid conditions. +To this end are given the following results of analyses of the waters +of Utah Lake, made at different times[136] over an interval of twenty +years, and showing that there is a segregation of chlorine and sodium +taking place, although in this case the lake has an outlet in the +Jordan River. + +[135] It has been suggested that the fact that shales or similar +geological deposits are frequently to be found near alkali areas, +indicates that the shales are the principal sources of the alkali. It +is supposed that the constituents of the alkali salts were formed by +the action of water on the shale minerals at or about the time the +shales were deposited, and carried down with the latter. Subsequently +the alkali has been leached out to appear at the surface of soils, +generally at a lower level than are the shales. + +[136] The water of Utah Lake, by F. K. Cameron: Jour. Am. Chem. Soc., +=27=, 113-116, (1905). + + ANALYSES OF THE WATER OF UTAH LAKE. + RESULTS IN PARTS PER MILLION + + =========+========+=========+=======+=========+========= + | Clarke | Cameron | Brown | Seidell | Brown + | 1883 | 1899 | 1903 | 1904[137] | 1904[138] + ---------+--------+---------+-------+---------+--------- + Ca | 55.8 | 67.6 | 80 | 67.7 | 67 + Sr | — | — | — | 1.7 | — + Mg | 18.6 | 13.8 | 92 | 73.5 | 86 + ---------+--------+---------+-------+---------+--------- + Na | 17.7 | 233.7 | 247 | 207.2 | 230 + K | ? | ? | 30 | 25.8 | 22 + ---------+--------+---------+-------+---------+--------- + Li | — | — | — | 0.7 | — + SO₄ | 130.6 | 236.7 | 365 | 332.9 | 378 + Cl | 12.4 | 316.5 | 336 | 288.5 | 337 + HCO₃ | — | — | 266 | 205.5 | 194 + CO₃ | 60.9 | 23.7 | — | 24.0 | 11 + SiO₂ | 10.0 | — | — | 22.6 | 28 + | ————— | ————— | ————— | ————— | ————— + Total | 306.0 | 892.0 | 1416 | 1250.1 | 1353 + ---------+--------+---------+-------+---------+--------- + +[137] Sample collected May 18. Lake unusually high. + +[138] Sample collected Aug. 31. Lake still high for that season of the +year. + +The third general origin of alkali supposes that wind-borne sea-spray +carries into the air salts which are left in very fine particles on the +evaporation of the water, or are deposited on the ordinary atmospheric +dust and carried over the land; and that this dust is precipitated here +and there as may be determined by the various meteorological conditions +which it encounters. All the land surface is supposed to be receiving +more or less of it from time to time, but in arid regions the rainfall +and drainage is not sufficient to return to the sea as much as is +received therefrom.[139] + +[139] For a recent interesting and valuable discussion of this subject +with reference to a particular area, see: The origin of the salt +deposits of Rajputana, by Sir Thomas H. Holland and W. A. K. Christie, +Records of the Geological Survey of India, =38=, 154-186, (1909). + +It is very probable that wind-borne salts from the sea are being +carried over and to some extent being deposited on all the land +surfaces of the earth. To what extent this process is taking place, and +whether it is sufficient to account for the alkali of any particular +region, available data fail to answer satisfactorily. Probably it is +always associated with one of the origins of alkali already discussed +and is in itself generally of secondary importance. + +An argument frequently advanced against the validity of the hypothesis +that wind-borne sea-spray is the origin of alkali is that the relative +proportions of the several constituents in “alkali” are seldom if +ever those obtaining in sea water. This argument does not take into +consideration, however, that the several salts in the spray probably +separate into crystals of widely different size and specific gravities, +and there may well be taking place a selective or sorting action by +the wind. More important, undoubtedly, is the selective action taking +place in the soil itself; it can only be an accidental coincidence +that the constituents of alkali in any particular occurrence should +have the same quantitative relations as in the material from which it +originated, no matter what may have been the nature of its origin. + +In the field, alkali is found in a bewildering array of forms and +types. Quite different combinations of constituents may be found in +the same field within a few rods or even a few feet, and each case +appears to have a distinct origin, to be in fact a law unto itself. +Each alkali deposit represents generally the resultant from a mixture +of salt which has been dissolved and reprecipitated a number of times, +and which while dissolved has been seeping through the soil under +gravitational forces, or has been moving through the soil as film +water under capillary stresses. In either event the salt mixture has +been subject to the power for selective absorption peculiar to the +particular soil mass through which it has been moving. Re-solution +is seldom an instantaneous process, and different rates of solution +necessarily involve some separation of salts. Finally the alkali +deposit is usually so mixed with other soil material that there cannot +be recognized the characteristic solid phases (such, for instance, +as the double sulphates of calcium and another base) which serve +as guides in laboratory studies and in certain salt mines. Even if +the characteristic salts are deposited in surface soils, it is very +doubtful, owing to their hygroscopicity, if any but gypsum, halite and +Glauber’s salt can persist for any length of time. The alternations +of temperature from night to day characteristic of arid regions, with +precipitation of dews, might easily be expected to make noticeable +and rapid changes in the characteristics of any given alkali or salt +mixture. + +It is not surprising, therefore, that attempts to account for the +genesis and present appearance of an alkali deposit by comparison +with artificial depositions of salt mixtures, as worked out in the +laboratory, have generally been disappointing. On the other hand, +laboratory studies have been quite fruitful in elucidating the +phenomena taking place on the leaching of alkali from a soil, or +so-called “alkali reclamation.” + +Whatever the origin of the alkali, its segregation at or near the +surface of the soil is everywhere much the same; that is, there is a +translocation and segregation of soluble salts in the below-surface +seepage waters, determined mainly by the topographic features, but +partly by the texture and structural properties of the soil and +subsoil, with a subsequent rise as capillary water consequent upon +evaporation at the surface. Precipitation of the solutes may take place +at the surface; more commonly it takes place a few inches below, +owing to the fact that under conditions of rapid evaporation, there is +ordinarily a discontinuance in the capillary columns or the film water +at a point below the surface of the soil, the water diffusing thence +into the above-surface atmosphere as the vapor phase. + +The composition of alkali is varied. In the vast majority of cases, the +world over, the predominating compound is sodium chloride. When calcium +carbonate is a conspicuous component of the soil, as a hard-pan or +otherwise, sodium carbonate or black alkali is also generally present, +or apt to appear when the land is irrigated. When calcium sulphate or +gypsum is likewise present, there is less probability of appreciable +amounts of black alkali, and where gypsum predominates or the calcium +carbonate is present in relatively inappreciable amounts, black alkali +is generally absent, and sodium sulphate is an important constituent +of the alkali. Relative rates of diffusion, selective absorption, and +sometimes other factors are prominent, however, and the character of +the alkali in different spots within a few yards of one another may +differ greatly. One of the most interesting manifestations of alkali is +the occasional occurrence of a predominating amount of calcium chloride +which, as a result of its unusually high hygroscopicity, renders the +soil damper, and therefore darker in color than the surrounding soil, +and frequently causes even experts to suspect the presence of black +alkali. Its true nature can, of course, be determined by a simple +chemical examination. + +The effect of alkali on the physical properties of the soil is often +very marked, aside from the cementing action or hard-pan formation +by the carbonate or sulphate of lime. Black alkali, by dissolving +and segregating the organic matter at the surface, removes from +the lower soil layers the “humus” compounds which are of enormous +importance to the maintenance of a soil structure favorable to plant +growth. Moreover, black alkali is one of the best of deflocculating +agents, and consequently soils where it is a noticeable component, +frequently puddle with great readiness and are reclaimed with the +utmost difficulty. Most of the other constituents of alkali, however, +are flocculating or “crumbing” agencies, and if not present in too +large amounts tend to increase the readiness with which the soil can +be brought into good tilth. In this latter case, by separating in the +solid phase, or in forming a viscous soil solution, near the saturation +point, they sometimes produce a condition in the soil simulating +puddling, and where it occurs below the surface, called an alkali +hard-pan. + +The management of soils infested with alkali is possible in accordance +with a few well established principles. Substantial progress has been +made in selecting and breeding plants and strains of plants adapted +to such soils. Extreme cases are the use of the so-called Australian +salt-bushes as forage crops, and the growing of date-palms which +through generations of breeding in the oases of the Sahara can thrive +in lands so salty as to destroy most of the halophilous plants. More +interesting is the unwitting development of the farmers of Utah of +strains of wheat and alfalfa which easily withstand three or four +times as high a salt content in the soil as do corresponding crops +in other alkali regions, such as New Mexico and Arizona.[140] Black +alkali, or one in which sodium carbonate is a prominent constituent, is +especially destructive to vegetation, not alone on account of a toxic +action on plants, but because in any considerable concentration it has +a corrosive action on the plant tissue. Not only on this account but +also because of its unfortunate effects on the physical properties +of the soil, black alkali has received unusual attention from soil +investigators. Hilgard[141] has repeatedly urged the use of gypsum as +an “antidote” to black alkali, assuming that under conditions of good +drainage and aeration a reaction takes place in accordance with the +following equation, + + Na₂CO₃ + CaSO₄ = CaCO₃ + Na₂SO₄. + +[140] Some mutual relations between alkali soils and vegetation, +by Thomas H. Kearney and Frank K. Cameron, Report No. =71=, U. S. +Dept. Agriculture, 1902; The date-palm and its utilization in the +Southwestern states, by Walter T. Swingle, Bull. =53=, Bureau of Plant +Industry, U. S. Dept. Agriculture, 1904; The comparative tolerance of +various plants for the salts common in alkali soils, by T. H. Kearney +and L. L. Harter, Bull. =113=, Bureau of Plant Industry, U. S. Dept. +Agriculture, 1907; Tolerance of alkali by various cultures, by R. H. +Loughridge, Bull. =133=, California Agr. Expt. Sta., 1901. + +[141] Soils, by E. W. Hilgard. 1906, p. 457-458. + +Furthermore, it has been shown that calcium salts and especially +calcium sulphate exercise a marked ameliorating effect on the action +of other salts upon growing vegetation.[142] On the other hand, the +reaction indicated by the equation just given does not run to an end +with complete precipitation of the carbonate, and the total amount +of alkali is increased in the soil by the addition of the gypsum. +Unfortunately, Hilgard’s suggestion has not yet acquired the sanction +of satisfactory field demonstration, although it would seem to merit +more consideration than has been given it. Inasmuch as lime is +generally a prominent constituent of soils containing black alkali, it +is possible that the maintenance of good drainage and aeration in the +soil is itself the best corrective of black alkali. + +[142] With the salts occurring in alkali, it is a generality that +the effects produced on higher green plants are relatively less with +mixtures than with an equivalent amount of a single salt. It has +recently been shown, however, that the contrary is true for at least +some kinds of bacterial flora. See, On the lack of antagonism between +certain salts, by C. B. Lipman, Bot. Gaz., =49=, 41-50, (1910). + +The best use of alkali soils involves irrigation, and it is in the +application of irrigation waters that management of alkali soils finds +its most highly developed and most important expression. With light +sandy soils it has sometimes been found practicable to add sufficient +water to carry the alkali down into the soil to such a depth that the +crop is well advanced toward maturity before the alkali again rises in +sufficient amounts to prove seriously detrimental to the more advanced +crops which are generally far more “alkali resistant” than the young +seedlings or the germinating seeds. In some cases this procedure can +be practiced for a number of years without greatly increasing the +seriousness of the alkali conditions, and it may be justified, for a +time at least, by economic considerations. Ultimately, however, and +more quickly with heavy than with light soils, increasing amounts +of alkali must be brought into the surface soil, and this method of +irrigating should not be considered as anything more than a temporary +expedient. The only procedure which should be seriously considered +as a permanent system on an alkali soil, no matter what the texture, +is the installation of underground drains, for which purpose, so far, +cylindrical tile drains commend themselves as giving the best results. +With a well established system of tile drains, the alkali and all +excess of soluble salts can be removed from the soil above the drains; +and alkali rising from the soil below can, at least very largely, be +prevented from rising to the upper soil layers. The reclamation of an +alkali tract by underdrainage is not, however, a necessarily quick +operation. Generally it must be a matter of several years persistent +and careful effort, but once attained should readily be maintained. The +reclamation of an alkali tract by flooding and underdrainage involves +the reverse process to the crystallization of salt from a brine. If +the water in percolating through the soil were long enough in contact +with the salts present to become a saturated solution in equilibrium +with them, then the composition of the resulting solution or drainage +water would depend upon the particular solid phases or salts which are +present in the soil, but not on the amounts of these salts; and the +relative proportions of the mineral constituents in the drainage water +should remain constant until some one of the solid phases in the soil +permanently disappears. + +In practice, however, the water passes through the soil at different +rates from time to time, the flow from the tiles being copious after a +flooding but gradually diminishing as time goes on. One or both of two +processes can therefore take place. The water may dissolve some of the +salts without at any time or place becoming saturated. As the different +salts have different rates of solution as well as different absolute +solubilities, it would be expected that not only the concentration of +the drainage water, but the composition of the dissolved salts would +change from time to time. On the other hand, a part of the water may be +imagined to percolate slowly through the finer openings, thus forming +a saturated solution with respect to the alkali salts which solution, +however, will be diluted on entrance to the drains by a part of the +water going through the larger soil openings and dissolving but little +salt in its passage. In this case, it would be anticipated that the +concentration of the drainage water would increase as the amount of +flow diminished but the composition of the dissolved salts would remain +practically constant until some one or more of the alkali salts was +completely removed. There are, unfortunately, but few experimental +data by which these can be tested. In the accompanying table are given +the results of an investigation on the reclamation of an alkali tract +near Salt Lake City, Utah, where observations on the composition of +the drainage water were made at frequent intervals for more than three +years.[143] + +[143] See, Calcium sulphate in aqueous solution, by Frank K. Cameron +and James M. Bell, Bull. No. =33=, 1906, p. 10 and 70, and Reclamation +of alkali land in Salt Lake Valley, Utah, by Clarence W. Dorsey, Bull. +No. =43=, 1907, p. 13, Bureau of Soils, U. S. Dept. Agriculture. + +At first sight these results might appear to show that the composition +of the salts was remaining reasonably constant. This conclusion +must be received with caution, however. Variations do occur in the +constituents which are present in smaller amount, but the variations +are not systematic and may plausibly be explained by dilution of +saturated solution by unsaturated solution on entering the drains. +Confining attention therefore to the constituents occurring in larger +proportions, namely, sodium chloride, sodium sulphate and sodium +bicarbonate (including the normal carbonate) it should be remembered +that the percentage of sodium in these three salts does not vary much, +and the “constancy” may be more apparent than real. Indeed a close +inspection of the results indicates that while the sodium is remaining +practically unchanged, there is some decrease in the chlorine and a +corresponding increase in the sulph-ion. From this it would follow that +the sodium chloride was being washed out of the soil more rapidly, +proportionately, than sodium sulphate; and it would also appear that +the solution entering the drains was not in final equilibrium with the +salts in the soil. + + COMPOSITION OF THE SALTS IN THE DRAINAGE WATER FROM + THE SWAN TRACT, UTAH + + =================+=========+=========+=========+========= + Date | Ca | Mg | Na | K + |per cent.|per cent.|per cent.|per cent. + -----------------+---------+---------+---------+--------- + 1902—September | 0.38 | 0.50 | 33.74 | 2.04 + October | 0.23 | 0.78 | 34.73 | 1.49 + November | 0.19 | 0.74 | 34.42 | 1.40 + 1903—May | 0.38 | 0.61 | 34.48 | 0.84 + June | 0.45 | 0.85 | 34.18 | 1.09 + July | 0.50 | 0.80 | 34.06 | 1.25 + August | 0.35 | 0.90 | 34.40 | 1.12 + September | 0.49 | 0.72 | 34.54 | 1.24 + October | 0.47 | 1.02 | 33.43 | 1.52 + 1904—January | 0.15 | 0.75 | 33.93 | 1.26 + February | 0.34 | 0.78 | 34.59 | 0.70 + March | 0.29 | 0.77 | 34.57 | 1.28 + April | 0.29 | 0.70 | 34.28 | 1.37 + May | 0.71 | 0.74 | 26.92 | 4.01 + June | 0.37 | 0.70 | 32.60 | 3.55 + August | 0.37 | 0.86 | 33.85 | 2.13 + September | 0.42 | 0.79 | 34.10 | 1.35 + October | 1.04 | 0.60 | 33.01 | 1.86 + December | 1.25 | 0.70 | 32.62 | 1.69 + 1905—February | 0.32 | 0.67 | 33.59 | 0.99 + March | 0.31 | 0.66 | 33.46 | 1.30 + April | 0.35 | 0.65 | 34.20 | 1.01 + May | 0.45 | 0.86 | 33.43 | 1.20 + June | 0.40 | 0.94 | 34.05 | 1.32 + July | 0.32 | 0.69 | 33.67 | 1.30 + August | 0.35 | 1.04 | 33.12 | 1.58 + September | 0.42 | 0.82 | 33.39 | 1.26 + 1906—January | 0.55 | 0.84 | 33.12 | 1.11 + =================+=========+=========+=========+========= + | SO₄ | Cl | HCO₃ | CO₃ + Date |per cent.|per cent.|per cent.|per cent. + -----------------+---------+---------+---------+--------- + 1902—September | 18.62 | 37.76 | 6.49 | 0.48 + October | 19.14 | 39.52 | 5.06 | 0.29 + November | 18.61 | 40.46 | 3.95 | 0.23 + 1903—May | 29.90 | 38.19 | 4.30 | 0.25 + June | 17.52 | 41.00 | 4.23 | 0.42 + July | 18.24 | 40.24 | 4.67 | 0.30 + August | 17.15 | 42.37 | 3.48 | 0.16 + September | 17.31 | 42.02 | 3.36 | 0.33 + October | 16.08 | 43.28 | 3.33 | 0.30 + 1904—January | 20.08 | 36.64 | 6.94 | 0.25 + February | 18.95 | 40.15 | 4.49 | —— + March | 16.31 | 42.28 | 3.81 | 0.19 + April | 20.93 | 38.04 | 3.33 | 1.06 + May | 21.26 | 40.93 | 4.05 | 1.38 + June | 19.94 | 37.42 | 4.05 | 1.37 + August | 17.12 | 41.31 | 3.20 | 1.16 + September | 19.01 | 39.85 | 4.11 | 0.37 + October | 21.42 | 36.63 | 4.68 | 0.76 + December | 19.89 | 37.44 | 6.18 | 0.22 + 1905—February | 22.30 | 33.32 | 8.45 | 0.36 + March | 21.60 | 33.86 | 8.46 | 0.35 + April | 20.03 | 36.99 | 6.22 | 0.55 + May | 20.59 | 36.04 | 6.96 | 0.47 + June | 20.89 | 35.85 | 5.71 | 0.84 + July | 21.17 | 34.94 | 7.23 | 0.68 + August | 21.58 | 35.92 | 5.72 | 0.99 + September | 21.18 | 34.85 | 7.41 | 0.67 + 1906—January | 21.10 | 34.35 | 8.57 | 0.36 + -----------------+---------+---------+---------+--------- + +How long drainage must continue before there is a radical change in the +composition of the seepage water cannot be predicted, and unfortunately +data regarding this point are not available. It is certain that in +time some one or more of the salts in the soil would be removed and +the nature of the drainage water would be changed. Alterations in the +composition of the drainage water furnish the readiest as well as the +best guides as to the changes and the nature of the changes taking +place in the soil during the process of reclamation. As a practical +matter it should be borne in mind that the persistence of the several +salts of the alkali mixture does not mean necessarily that they are +evenly distributed in the soil; while yet determining the composition +of the water entering the drain, they may have disappeared from the +upper soil layers which then may hold a solution of quite different +character, suited to the support of crops. In the case just cited the +soil contained, before drainage operations were commenced, upwards of +2.7 per cent. of readily soluble salts and would not support any growth +other than salt-bushes and similar halophilous plants. Four years later +the soil contained less than 0.3 per cent. soluble salts and yielded +a very satisfactory crop of alfalfa. In such cases, however, the land +cannot be considered as finally reclaimed until a material change in +the composition of the drainage water shows that there has been a +complete removal of some of the solid salts from that portion of the +soil feeding the drains. + +The rate at which alkali can be leached from a soil is dependent in a +large measure upon the absorptive properties of the soil, and to some +extent upon the nature of the salts composing the alkali. The leaching +is more rapid from sandy than from clay soils, and white alkali is +leached more readily than is black. In general, however, the same laws +hold here as in any leaching of a solute from an absorbent, and it has +been shown that even in the case of black alkali, the rate of removal +under a constant leaching follows the law + + _dx_ + ————— = K (A - _x_).[144] + _dt_ + +[144] The removal of “black alkali” by leaching, by F. K. Cameron and +H. E. Patten, Jour. Am. Chem. Soc., =28=, 1639, (1906). + +In practice, the water does not percolate through the soil under a +constant “head,” but the flow is intermittent, so that the value of the +above formula is mainly academic. On the other hand, if the drainage +between floodings is thorough, this procedure should be more efficient +than any other for causing a rapid removal of the alkali salts, if, as +is generally the case, a limited quantity of water is available. + +Finally, it remains to be pointed out that the use of excessive amounts +of water on alkali tracts is quite as unfortunate in its effects as the +use of too little. If water be added to an undrained soil or in excess +of the capacity of the drains to remove it, incalculable harm may be +done by enormously increasing in the surface soil the amount of salts +brought up from the lower layers as the capillary stream rises to the +surface in consequence of evaporation there. Should the wetting of the +soil proceed so far as to establish good capillary connection with the +permanent ground water, the harm may be sufficient to offset in a few +weeks or months expensive reclamation efforts of years. The harm to the +tract where the water is added may be far less than the harm done to +other areas. A large proportion of existing alkali deposits or “spots” +results from the evaporation of seepage waters coming sometimes from +considerable distances. The over-wetting of a soil means the production +of seepage waters which are to appear at the surface somewhere +else, generally at a lower level, and frequently means the more or +less complete ruin of the soils of the lower level. The experience +of India, Africa and our own arid states in the increase of alkali +spots following the introduction of irrigation, added to our present +theoretical knowledge, should make the planning of an irrigation +project without adequate drainage provisions, a stupidity, and its +accomplishment a public crime. Quite as important is the development +of a public opinion that the individual cultivator who deliberately or +carelessly uses excessive amounts of water on his tract is a serious +enemy to the body politic, and should be treated as such. + + + + +INDEX. + + + Absorbents, Influence on soil extracts, 38 + Absorption by soils, 9, 59, 65 + formula, 62 + of dyes, 60, 61 + rate, 63 + selective, 61 + Acid digestion of soils, 11, 12 + Adsorption, 9, 60 + Alkali, 110, 118 + Effect on soils, 118 + Order of deposition, 112 + Reclamation, 117, 121 + Source, 111, 117 + Antagonism between salts, 120 + Apophyllite, Crystallization from water, 35 + Apple trees, Effect of grass on, 98 + Appleyard, James R. _See_ Walker, James, and + Appleyard, James R. + Ash analyses, 11, 13 + Association of Official Agricultural Chemists’ analyses, quoted, 12 + cited, 12 + “official method”, 10, 12 + “Available” and “non-available” plant-food elements, 8 + Averitt, S. D. _See_ Peter, Alfred M., and Averitt, S. D. + + Bacteria in soils, 103 + Bailey, Liberty H., cited, 5 + Balance between supply and removal of mineral plant nutrients, 75 + Barium in soils, 107 + Bardt, A. _See_ Doroshevskii, A. and Bardt, A. + Becquerel, Antoine C., cited, 67 + quoted, 68 + Bell, James M., and Cameron, Frank K., cited, 28 + Bell, James M. _See also_ Cameron, Frank K., and Bell, J. M.; + Cameron, Frank K., Bell, J. M., + and Robinson, W. O. + Benedick, Carl, cited, 55 + Birner, H., and Lucanus, B., cited, 70 + Bischof, Gustav, cited, 113 + Black alkali, 110, 114, 119, 124 + Blanck, Edward, cited, 63 + Breazeale, James F., acknowledgments, 80 + cited, 71 + _See also_ Cameron, Frank K., and Breazeale, J. F.; + LeClerc, J. A. and Breazeale, J. F. + Briggs, Lyman J., cited, 55 + and Lapham, Macy H., cited, 41 + and McLane, John W., cited, 26 + Martin, F. O., and Pearce, J. R., cited, 31 + Brooks, William P., cited, 5 + Brown, Bailey E., cited, 46 + quoted, 46, 115 + Bryan, H. _See_ Davis, R. O. E., and Bryan, H. + Buckingham, Edgar, cited, 30 + Burney, W. B., quoted, 98 + + Cameron, Frank K., cited, 110, 114, 115 + _See also_ Bell, James M., and Cameron, Frank K.; + Kearney, Thomas H. and Cameron, Frank K.; + Whitney, Milton, and Cameron, Frank K. + and Bell, James M., cited, 31, 38, 50, 113, 122 + and Breazeale, James F., cited, 62 + and Gallagher, Francis E., cited, 24 + and Patten, Harrison E., cited, 63, 124 + and Robinson, William O., cited, 27, 53 + Bell, James M., and Robinson, William O., cited, 114 + Calcium nitrate, basic, 108 + Carbon dioxide in the soil, 53 + Charpentier, Jean G. F., cited, 113 + Chemical analysis of soils. _See_ Soil analysis—Chemical. + Chesneau, G., cited, 68 + Christie, W. A. K. _See_ Holland, Sir Thomas H., + and Christie, W. A. K. + Clarke, Frank Wigglesworth, cited, 76, 115 + Coffey, George N., quoted, 23 + Concentration of mineral constituents, 39 + Concentration, Plant growth and, 70 + Cracking of soil, 22 + Creep, 19 + Creighton, Henry J. M. _See_ Findlay, Alexander, + and Creighton, Henry J. M. + Critical moisture content, 24 + Crop control methods, 7, 105 + plants defined, 1 + producing power and aqueous extract, 81 + rotation, Natural, 97 + Objects of, 4 + yields increasing, 16 + Crumb structure of soils, 25 + Crumbing, 27, 119 + Cushman, Allerton S., cited, 36 + “Cut-off”, 22, 75 + Cyanamid, 108 + Czapek, Friedrich, Experiments on root etchings, 9 + Criticism of Molisch, 101 + + Dachnowski, Alfred, cited, 88 + Darbishire, Francis V., and Russell, Edward J., cited, 103 + Darwin, Horace, cited, 22 + Davis, R. O. E., quoted, 63 + and Bryan, H., cited, 55 + De Candolle, Augustin P., cited, 97 + Degradation of rocks, 1 + De Roode, Rudolph J. J., quoted, 98 + Diaspore, 34 + Dittrich, Max., cited, 13 + Doroshevskii, A., and Bardt, A., cited, 35 + Dorsey, Clarence W., cited, 110, 122 + Drainage waters, Composition, 124 + Drought limits defined, 29 + Dunnington, Francis P., cited, 98 + Dust, 20 + Dyer, Bernard, cited, 40 + method of soil analysis, 10 + quoted, 6 + Dynamic nature of soil phenomena, 18 + + Earthworms, 22 + European soils, analyses, 16 + Erosion, 20 + Etchings, Root, 9 + Ewart, A. J., cited, 18, 72, 73 + Excreta, Toxic, 99, 100, 103 + + “Factors”, 11 + Failyer, George H., cited, 107 + _See also_ Schreiner, Oswald, and Failyer, George H. Smith, + Joseph G., and Wade, H. R., cited, 32 + Fairy rings, 98 + Feldspars, 35, 38, 55 + Fertilizers, 4, 83, 105 + Film water, 24 + tenacity, Experiments, 25 + Findlay, Alexander, and Creighton, Henry J. M., cited, 53 + Fine a soil, to, 4 + Fischer, Emil, and Schmidmer, Edward, cited, 61 + “Fly-off”, 22, 75 + Frear, William, cited, 5 + Free, Edward Elway, cited, 20 + Friedel, Charles and Sarasin, Edmond, cited, 34 + + Gallagher, Francis Edward. _See_ Cameron, Frank K. + and Gallagher, Francis E. + Gannett, Henry, cited, 76 + Gaudechon, H. _See_ Muntz, A., and Gaudechon, H. + Geikie, _Sir_ Archibald, cited, 75 + Gels, 36 + Gilbert, Joseph H., cited, 98 + Gonnard, F., cited, 35 + “Good” and “poor” soils compared, 80 + Graham, Thomas, cited, 67 + Granulate a soil, to, 4 + Grass, Effect on apple trees, 98 + Gravitational water, 23 + Great Salt Lake, Reaction of water, 113 + Green manure, Effect on soil extracts, 87 + Gypsum on alkali soils, 119 + + Hardpan, 111 + Harter, Leonard L. _See_ Kearney, Thomas H., and Harter, L. L. + Hartwell, Burt L., Wheeler, H. J., and Pember, F. R., cited, 74 + Haselhoff, Emil. _See_ König, Joseph, and Haselhoff, E. + Haworth, Erasmus, cited, 113 + Heileman, William H., quoted, 65 + Heterogeneity of soils, 1, 21, 32, 79 + Hilgard, Eugene W., cited, 5, 6, 38, 40, 119 + Method of soil analysis, 10 + Hillebrand, William F., cited, 13 + Hills, Joseph L., cited, 5 + Holland, Sir Thomas H., and Christie, W. A. K., cited, 116 + Hulett, George A., cited, 68 + Humic acids, 55 + Humus, 61 + Hutchinson, Henry B. _See_ Russell, Edward J., + and Hutchinson, Henry B. + Hydrolysis, 33 + + Imbibition, 59 + Irrigation, 120 + + Johnson, Samuel W., cited, 40, 77 + quoted, 2 + + Kahlenberg, Louis, and Lincoln, Azariah T., cited, 35 + Kaolinite, 34 + Kearney, Thomas H., and Cameron, Frank K., cited, 119 + and Harter, Leonard L., cited, 119 + Kentucky agricultural experiment station, + Method of soil analysis, 10 + King, Franklin H., cited, 75, 76, 77 + quoted, 46, 76 + Knight, Wilbur C., and Slosson, Edwin E., cited, 114 + König, Joseph, and Haselhoff, E., cited, 8 + Kossovich, Petr. S., Experiments on root etchings, 9 + + Lagergren, Sten, cited, 26 + Lake desiccation, 114 + Lapham, Macy H. _See_ Briggs, Lyman J., and Lapham, Macy H. + Lawes, John B., and Gilbert, Joseph H. _See_ Gilbert, Joseph H. + Leather, J. Walter, cited, 23 + Le Clerc, J. Arthur, and Breazeale, James F., cited, 14 + Lemberg, Johann T., cited, 35 + Liebig, Justus, cited, 8, 97 + Liebrich, A., cited, 34 + Liebreich, quoted, 68 + Lieving, quoted, 68 + Lincoln, Azariah T. _See_ Kahlenberg, Louis, + and Lincoln, Azariah T. + Lipman, Jacob G., cited, 72, 103 + _See also_ Voorhees, Edward B., and Lipman, Jacob G. + Lipman, C. B., cited, 120 + Litmus, Absorption of, 66 + as indicator, 66 + Livingston, Burton E., cited, 85, 88, 97 + Loughridge, Robert H., cited, 28, 119 + Lucanus, B. _See_ Birner, H., and Lucanus, B. + + McGee, W. J., quoted, 22, 76 + McLane, John W. _See_ Briggs, Lyman J., and McLane, John W. + Manure, Stable, Effect on soil extracts, 84 + Martin, F. Oskar. _See_ Briggs, Lyman J., Martin, F. O., + and Pearce, J. R. + Maxwell, Walter, Method of soil analysis, 10 + Mechanical analysis, 31 + Merrill, George P., cited, 9 + Meyerhoffer, Wilhelm, cited, 111 + Meyer, Victor, cited, 67 + Minchin, George M., cited, 26 + Mineral constituents of soil solution, 31, 37 + Mineral plant nutrients, balance between supply and removal, 75 + Mississippi River, Soil-carrying power, 21 + Mixing of soils, 33 + Moisture content, 24 + Moisture movement into soil, 28 + Molisch, Hans, cited, 101 + Mooers, Charles A., cited, 10 + Motion in soils, 19 + Movement of soils, 20 + Muntz, A., and Gaudechon, H., cited, 30 + quoted, 24 + Murray, _Sir_ John, cited, 75 + + Newell, Frederick H., cited, 75 + Night-soil, 108 + Nitrates in agriculture, 108 + in soil solution, 103 + Nitrogen carriers, 103 + + “Official method” of soil analysis, 10 + Optimum moisture content, 24 + Organic compounds, Effect on plants, 82 + Organic constituents of soil solution, 54, 79 + Orthoclase, Alteration of, 33 + Ostwald, Wo., cited, 28 + Oxidizing power of roots, 101 + Oxygen in the soil, 53 + Oxystearic acid, Toxic to plants, 96 + + Patten, Harrison E., cited, 24, 25, 60 + _See_ Cameron, Frank K., and Patten, Harrison E. + and Waggaman, William H., cited, 9, 59 + and Gallagher, F. E., cited, 59 + Pearce, Julia R. _See_ Briggs, Lyman J., Martin, F. O., + and Pearce, J. R. + Pember, F. R. _See_ Hartwell, Burt L., Wheeler, H. J., + and Pember, F. R. + Penfield, Samuel L., cited, 13 + Percolation experiments, 47 + Peter, Alfred, cited, 54 + and Averitt, S. D., cited, 10 + Pfeffer, Wilhelm F. P., cited, 18, 72, 73, 101 + Phlogiston theory, 17 + Phosphates, 50 + Picoline carboxylic acid, toxic to plants, 96 + Plant-food theory, 16 + Plant growth and concentration, 70 + Plant nutrients, Supply and removal, 75 + Plot experiments, 14 + “Poor” and “good” soils compared, 80 + Pot experiments, 14 + Puddling, 25 + Pyrogallol, 87 + Pyrophyllite, 34 + + Ragweed, 97, 98 + Rainfall, 22, 75 + Rajputana, Salt deposits, 116 + Rayleigh, Lord, cited, 26 + Reed, Howard S. _See_ Schreiner, Oswald, and Reed, Howard S.; + Schreiner, Oswald, Reed, Howard S., + and Skinner, J. J. + Removal of plant nutrients, Supply and, 75 + Reversible reactions, 34 + Ries, Heinrich, quoted, 112 + River waters, Concentration of, 76 + Robinson, William O. _See_ Cameron, Frank K., + and Robinson, William O.; + Cameron, Frank K., Bell, James M., + and Robinson, W. O. + Rodewald, H., cited, 24 + Römer, Hermann. _See_ Wilfarth, Hermann, Römer, Hermann, + and Wimmer, G. + Root etchings, 9 + Root growth mechanism, 19 + Roots of growing plants, 18 + Rotation of crops, 97 + Rothmund, V., cited, 68 + “Run-off”, 22, 75 + Russell, Edward J., cited, 103 + _See also_ Darbishire, Francis V., and Russell, Edward J. + and Hutchinson, Henry B, cited, 72 + + Sachs, Julius, Experiments on root etchings, 9 + Salt as fertilizer, Common, 108 + Sarasin, Edmond. _See_ Friedel, Charles, + and Sarasin, Edmond, 34 + Schmidmer, Edward. _See_, Fischer, Emil, and Schmidmer, Edward. + Schreiner, Oswald, quoted, 102 + and Failyer, George H., cited, 41, 47 + and Reed, Howard S., cited, 100, 101 + and Shorey, Edmund C., cited, 95 + and Sullivan, M. X., cited, 100 + Reed, Howard S., and Skinner. J. J., quoted, 89 + Sea water, Desiccation of, 111 + Seedlings, Growth of, 74, 80, 82, 84, 86, 88, 100, 102 + Seedlings, Toxic action of acids and salts, 62 + Seidell, Atherton, quoted, 115 + Shaler, Nathaniel S., cited, 20 + Shorey, Edmund C., cited, 95 + _See also_ Schreiner, Oswald, and Shorey, E. C. + Shrinking of soils, 22 + Skinner, J. J., quoted, 99, 102 + Skinner, J. J. _See also_ Schreiner, Oswald, Reed, Howard S., + and Skinner, J. J. + Slosson, Edwin E. _See_ Knight, Wilbur C., + and Slosson, Edwin E. + Smith, Joseph G., quoted, 98 + _See also_ Failyer, George H., Smith, Joseph G., + and Wade, H. R. + Sodium chloride as fertilizer, 108 + Soil, the, 1 + Soil amendments, 105 + analysis, Chemical, 8, 22 + Methods, 10 + atmosphere, 23 + bacteria, 23, 103 + control, 4 + methods, 4 + erosion, 20 + fatigue, 100 + heaving, 22 + individuality, 2 + management, 2, 3, 4 + minerals, Chief, 32 + moisture defined, 1 + not a static system, 18 + phenomena, Dynamic nature of, 18 + shrinking, 22 + solution defined, 1 + Analyses, 39 + Importance of, 2 + Organic constituent of, 79 + Survey Field Book, cited, 3 + translocation by water, 20 + wind, 21 + Soils, Composition of, 1 + Mineral constituents of, 32 + Moisture content, 24 + Water extracts of, 39 + Solid solution defined, 59 + Solubility of minerals, 52, 55 + Spring, Walthère, cited, 67 + Structure, 27 + Subsoils, Infertility of, 88 + Sullivan, Michael X., cited, 102 + quoted, 68 + _See also_ Schreiner, Oswald, and Sullivan, M. X. + Supply and removal of plant nutrients, 75 + Surface effects, 67 + Surface tension, 27 + Swan tract, Utah, 123 + Swingle, Walter T., cited, 119 + + Taylor, Frederick W., cited, 5 + Tennessee agricultural experiment station, + Methods of soil analysis, 10 + Thorne, Charles E., cited, 5 + Tillage methods, 4 + Objects of, 4 + Tollens, Bernhard C. G., cited, 14 + Toxic excreta of roots, 99, 100, 103 + + Udden, Johan August, quoted, 21 + U. S. Dept. of Agriculture, Bureau of Soils. + _See_ Soil Survey Field Book. + U. S. Geological Survey, cited, 13 + Underdrainage, 121 + Utah Lake water analyses, 115 + + Van Hise, Charles R., cited, 35, 36 + van’t Hoff, Jakob H., cited, 67, 111 + Voorhees, Edward B., and Lipman, Jacob G., cited, 72, 103 + + Wade, Harold R. _See_ Failyer, George H., Smith, Joseph G., + and Wade, H. R. + Waggaman, William H. _See_ Patten, Harrison E., + and Waggaman, William H. + Walker, James, and Appleyard, James R., cited, 60 + Washington, Henry S., cited, 13 + Water, Movement into soils, 28 + vapor, Movement in soils, 29 + Way, John T., cited, 9 + Weeds, Analyses of, 98 + Weinschenk, E., cited, 35 + Wheeler, Homer J., cited, 74 + Wheeler, Homer J. _See also_ Hartwell, Burt L., Wheeler, H. J., + and Pember, F. R. + White alkali, 110, 111 + Whitney, Milton, cited, 16 + and Cameron, Frank K., cited, 26, 42 + Wilfarth, Hermann, Römer, Hermann, and Wimmer, G., cited, 14 + Willard, Julius T., cited, 5 + Wimmer, G. _See_ Wilfarth, Hermann, Römer, Hermann, + and Wimmer, G. + Wind, 20 + Carrying power of, 21 + Wind-borne soil material, 21, 33 + Wöhler, Friedrich, cited, 35 + Wolff, Emil T. von, tables, cited, 77 + Woburn, Experiments at, 98 + + Young, Thomas, cited, 26 + + Zeolites, 9, 34, 35 + + + + +Scientific Books + + +Published by + +THE CHEMICAL PUBLISHING COMPANY + +EASTON, PA. + + =BENEDICT=—Elementary Organic Analysis. Small 8vo. + Pages VI + 82. 15 Illustrations $1.00 + + =BERGEY=—Handbook of Practical Hygiene. + Small 8vo. Pages 164 $1.50 + + =BILTZ=—The Practical Methods of Determining + Molecular Weights. (Translated by Jones). + Small 8vo. 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