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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




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