THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP
PRODUCTION.
The chemical constituents of soils have been incidentally mentioned
and discussed above, both in connection with the processes of
soil-formation, and with the minerals that mainly participate therein.
The manner of their occurrence and their relations to plant life, so
far as known, must now be considered more in detail.
HISTORICAL REVIEW OF SOIL INVESTIGATION.
While the obvious importance of the physical soil-conditions has long
ago rendered them subjects of close study by Schübler[106] Boussingault
and others, the chemistry of soils was very generally neglected for a
considerable period, after the hopes at first entertained by Liebig
that chemical analysis would furnish a direct indication and measure
of soil fertility, had been sorely disappointed in respect to the only
soils then investigated, viz., the long-cultivated ones of Europe. The
results of chemical analysis sometimes agreed, but as often pointedly
disagreed, with cultural experiences; so that after the middle of the
nineteenth century, but few thought it worth while to occupy their time
in chemical soil analysis.
[106] The early work of Schübler on soil physics, published at Leipzig
in 1838 under the title of “Grundsätze der Agrikulturchemie” and now
almost inaccessible outside of old libraries, is remarkable as having
anticipated very definitely much that has since been brought forward
and elaborated anew. He is really the father of agricultural physics.
_Popular Forecasts of Soil Values._—In newly-settled countries, and
still more in those yet to be settled, the questions of the immediate
productive capacity, and the future durability of the virgin land are
the burning ones, since they determine the future of thousands for
weal or woe. This need has long ago led to approximate estimates made
on the part of the settler, by the _observation of the native growth,
especially the tree growth_; and where this consists of familiar
species, normally developed, such estimates on the part of experienced
men, based on previous cultural experience, are generally very
accurate; so much so that in many of the newer states they have been
adopted in determining not only the market value, but also the tax rate
upon such lands, their productiveness, and probable durability being a
matter of common note.
Thus in the long-leaf pine uplands of the Cotton States, the
scattered settlements have fully demonstrated that after
two or three years cropping with corn, ranging from as much
as 25 bushels per acre the first year to ten and less the
third, fertilization is absolutely necessary to farther
paying cultivation. Should the short-leaved pine mingle with
the long-leaved, production may hold out for from five to
seven years. If oaks and hickory are superadded, as many
as twelve years of good production without fertilization
may be looked for by the farmer; and should the long-leaved
pine disappear altogether, the mingled growth of oaks and
short-leaved pine will encourage him to hope for from twelve
to fifteen years of fair production without fertilization.
Corresponding estimates based upon the tree growth and in part also
upon minor vegetation, are current in the richer lands also. The
“black-oak and hickory uplands,” the “post-oak flats,” “hickory
bottoms,” “gum bottoms,” “hackberry hammocks,” “post-oak prairie,”
“red-cedar prairie,” and scores of other similar designations, possess
a very definite meaning in the minds of farmers and are constantly used
as a trustworthy basis for bargain and sale, and for crop estimates.
Moreover, experienced men will even after many years’ cultivation be
able to distinguish these various kinds of lands from one another.
_Cogency of Conclusions based upon Native Growth._—Since the native
vegetation normally represents the results of secular or even
millennial adaptation of plants to climatic and soil-conditions, this
use of the native flora seems eminently rational. Moreover, it is
obvious that if we were able to interpret correctly the meaning of such
vegetation with respect not only to cultural conditions and crops, but
also as regards the exact physical and chemical nature of the soil, so
as to recognize the _causes_ of the observed vegetative preferences; we
should be enabled to project that recognition into those cases where
native vegetation is not present to serve as a guide; and we might
thus render the physical and chemical examination of soils as useful
practically, _everywhere_, as is, locally, the observation of the
native growths. To a certain extent, such knowledge would be useful
in determining the salient characters of cultivated soils, also; and
would be the more useful and definite in its practical indications the
more nearly the cultural history of the land is known, and the less the
latter has been changed by fertilization. For, so soon as the first
flush of production has passed, the question of how to fertilize most
effectually and cheaply demands solution.
It was from this standpoint, suggested by his early experience in the
Middle West and subsequently most impressively presented to him in the
prosecution of the geological and agricultural survey of Mississippi,
that the writer originally undertook, in 1857, the detailed study of
the physical characters and chemical composition of soils. It seemed
to him incredible that the well-defined and practically so important
distinctions based on natural vegetation, everywhere recognized and
continually acted upon by farmers and settlers, should not be traceable
to definite physical and chemical differences in the respective lands,
by competent, comprehensively-trained scientific observers, whose field
of vision should be broad enough to embrace concurrently the several
points of view—geological, physical, chemical and botanical—that must
be conjointly considered in forming one’s judgment of land. Such
trained observers should not merely do as well as the “untutored
farmer,” but a great deal better.
_“Ecological” studies._—Yet thus far we vainly seek in general
agricultural literature for any systematic or consistent studies of
these relations. We do find “ecological” lists of trees and other
plants, or “plant associations,” growing in certain regions or land
areas, described in some of the general terms which may refer equally
well to lands of profuse productiveness, or to such as will hardly
pay for taxes when cultivated. Or when the productive value is
mentioned, the probable cause of such value is barely alluded to, even
conjecturally, unless it be in describing the “plant formations” as
xerophytic, mesophytic or hydrophytic, upon the arbitrary assumption
that moisture is the only governing factor; wholly ignoring such
vitally important factors as the physical texture of the soil, its
depth, the nature of the substrata, and the (oftentimes abundantly
obvious) predominant chemical nature of the land. And on the other
hand, we find even public surveys proceeding upon the basis of physical
data alone, practically ignoring the botanical and chemical point of
view, and inferentially denying, or at least ignoring, their relevancy
to the practical problems of the farm.[107]
[107] Bull. 22, Bureau of Soils, U. S. Dept. of Agriculture.
_Early Soil Surveys of Kentucky, Arkansas and Mississippi._—Among
the few who during the middle of the past century maintained their
belief in the possibility of practically useful results from direct
soil investigation, were Drs. David Dale Owen and Robert Peter, who
prosecuted such work extensively in connection with the geological and
agricultural surveys of Kentucky and Arkansas; and the writer, who
carried out similar work in the states of Mississippi and Louisiana,
with results in many respects so definite that he has ever since
regarded this as a most fruitful study, and has later continued it in
California and the Pacific Northwest. This was done in the face of
almost uniform discouragement from agricultural chemists until within
the last two decades; with occasional severe criticisms of this work as
a waste of labor and of public funds.
_Investigation of Cultivated Soils._—All this opposition was largely
due to the prejudices engendered by the futile attempts to deduce
practically useful results from the chemical analysis of _soils long
cultivated_, without first studying the less complex phenomena of
_virgin_ soils; and these prejudices persisted longest in the United
States, even though in Europe the reaction against the hasty rejection
of chemical soil work had begun some time before; as is evidenced by
the methods employed at the Rothamsted Experimental Farm in England,
the Agricultural College of France, the Russian agronomic surveys, and
at several points in Germany. In none of these cases, however, more
than the purely chemical or physico-chemical standpoint was assumed;
although in Russia at least, virgin soils were easily obtainable and
their native growth verifiable; and were actually in part made the
subject of chemical investigation.
In the course of their work, Owen and Peter always carefully recorded
the native vegetation of the soils collected; but neither seems to
have formulated definitely the idea that such vegetation might be made
the basis of direct correlation of soil-composition with cultural
experience. Owen repeatedly expressed to the writer his conviction that
such a correlation could be definitely established by close study; but
early death prevented his personal elaboration of the results of his
work. Peter likewise stoutly maintained to the last his conviction that
soil analysis was the key to the forecasting of cultural possibilities;
but not being a botanist he did not see his way to put such forecasts
into definite form.
_Change of Views._
In the United States as well, the ancient prejudices have now gradually
given way before the urgent call for more definite information than
could otherwise possibly be given to farmers by the experiment
stations, most of whose cultural experiments, made without any definite
knowledge of the nature of the soil under trial, were found to be
of little value outside of their own experimental fields. Even the
multiplication of culture stations in several states, unaccompanied
by soil research, is found to be a delusive repetition of the same
inconclusive, random experimenting, since it takes into consideration
only the climatic differences, but leaves out of consideration the
potent factors of soil quality and soil variations. At most these were
usually mentioned by them in such indefinite terms as “a clay loam,”
“a coarse sandy soil,” “gray sediment land,” and the like; frequently
not even with a statement of the depth and character of the subsoil and
substrata, much less of their geological derivation or correlations.
Thus any one not happening to be personally acquainted with the land
in question would be wholly without definite data to correlate the
results with his own case. It is quite obvious that even if only to
make possible the identification of new lands with others that have
already fallen under cultural experience, and can therefore afford
useful indications to the new settler, a close physical and chemical
characterization of lands should be made the special object of study by
the experiment stations and public surveys, particularly in the newer
states.
_Advantages for Soil Study offered by Virgin Lands._—Among the special
advantages, then, offered by virgin soils for the study of the
correlations of soils and crops, the usual existence of a native flora,
representing the results of secular adaptation, is of fundamental
importance. As it is at this time still historically known of most
lands west of the Alleghenies what was their original timber growth,
it is clear that their original condition can still be ascertained
by comparison with uncultivated lands of similar growth, usually not
very far away; and as their cultural history also is largely within
the memory of the living generation, the behavior of such lands under
cultivation is known or verifiable. Foremost among the data thus
ascertainable is the _duration of satisfactory crop production, and
its average amount_. To ascertain these surviving data by inquiry
among the farming population should be among the foremost duties of
those connected with soil surveys; and persons temperamentally unable
to enlist the farmer’s sympathy and interest in such inquiries must
be considered seriously handicapped, no matter what their scientific
qualifications may be. In no quest is it more literally true that there
is no one from whom the earnest inquirer may not learn something worth
knowing.
_Practical Utility of Chemical Soil-Analysis; Permanent Value_ vs.
_Immediate Productiveness_.—In many existing treatises so much
emphasis is given to the alleged proofs of the inutility of chemical
soil examination in particular, that a special controversion of these
arguments seems necessary, in connection with a detailed statement
of what can, and in part has been, done in that direction. Hence the
often-repeated allusion, in the sequel, to points bearing on this
question. Hence, also, the detailed discussion of many points which in
most agricultural publications are given only passing notice.
_In all these discussions the difference between_ the ascertainment of
the _permanent-productive value of soils_, as against that of their
_immediate producing capacity_, must be strictly kept in view. The
former interests vitally the permanent settler or farmer; the latter
concerns the immediate outlook for crop production, the “Düngerzustand”
of the Germans. The methods for the ascertainment of these two factors
are wholly distinct, even though the results and their causes are
in most cases intimately correlated. The failure to observe this
distinction accounts for a great deal of the obloquy and reproach that
has in the past so often been heaped upon chemical soil-analysis and
its advocates.
PHYSICAL AND CHEMICAL CONDITIONS OF PLANT GROWTH.
While it is true that plants cannot form their substance or develop
healthy growth in the absence or scarcity of the chemical ingredients
mentioned on page xxxi of this volume, it is also true that they cannot
use these unless the physical conditions of normal vegetation are first
fulfilled. Both sets of conditions are intrinsically equally important
and exacting as to their fulfilment; and the farmers’ task is to bring
about this concurrence to the utmost extent possible. The chemical
ingredients of plant-food can, however, be artificially supplied in
the form of fertilizers, should they be deficient in the soil; but as
has been shown in the preceding pages, it is not always possible to
correct, within the limits of farm economy, physical defects existing
in the land. Hence, however important is the natural richness of the
soil in plant-food, _the first care should always be given to the
ascertainment of the proper physical conditions in the soil, subsoil
and substrata_. Without these, oftentimes, no amount of cultivation,
fertilization and irrigation is effective in assuring profitable
cultural results.
_Condition of the Plant-food Ingredients in the Soil._—But even
the abundant _presence_ of the plant-food ingredients, as shown by
analysis, will not avail, unless at least an adequate portion of
the same exists in a form or forms accessible to plants. Of course
this condition would seem to be best fulfilled by the ingredients in
question being in the _water-soluble_ condition. But in the first
place, plants are quite sensitive to an over-supply of soluble mineral
salts, as is evidenced by the injurious effects produced by the latter
in saline and alkali lands. Furthermore, substances in that form
would be very liable to be washed or leached out of the soil by heavy
rains or irrigation, and would be lost in the country drainage. It is
therefore clearly desirable that only a relatively small proportion
of the useful soil-ingredients should be in the water-soluble or
physically absorbed condition, but that a larger supply should be
present in forms not so easily soluble, yet accessible to the solvent
action which the acids of the soil and of the roots of plants are
capable of exercising. This _virtually_ available supply we may
designate as the _reserve food-store_.
Finally, there is practically in all soils a certain proportion of
the _soil-minerals in their original form_, as they existed in the
rock-materials from which the soil was formed. These minerals being
usually in a more or less finely divided or pulverulent condition, they
are attacked much more rapidly by the chemically-acting “weathering”
agencies, viz., water, oxygen, carbonic and humus acids, than when
in solid masses; and thus, transformation of the inert rock-powder
into the other two classes of mineral soil-ingredients progresses in
naturally fertile soils with sufficient rapidity to produce, in a
single season, sensible and practically important results, known as the
effects of _fallowing_.
_The Reserve._—The nature of these processes has been discussed
in chapters 1 to 4; and it will be remembered that two of their
most prominent results are the _formation of clay_, and of
_zeolitic-compounds_, the latter being, as heretofore stated (pp. 36
ff) hydrous silicates of earths and alkalies, easily decomposable by
acids, and also capable of exchanging part or the whole of such basic
ingredients with solutions of others that may enter the soil. These
zeolitic compounds therefore fulfil two important functions in the
premises, viz.: a ready yielding-up of part of their ingredients to
acid solvents, and a tendency to fix, by exchange, a portion or the
whole of the soluble compounds that may be set free in, or brought
upon the land. The first-mentioned property is of direct avail in that
the soil-humus forms, and the roots of plants exude, acid solvents on
their surface, and can thus draw upon the reserve store of food; the
second tells in the direction of preventing the waste of water-soluble
manurial ingredients supplied to, or formed in the soil. (See above,
chapter 3, page 38).
The reserve food-store may then be placed under the following heads:
_Hydrous or “zeolitic” silicates_, from which dilute acids can take up
the bases potash, soda, lime and magnesia. These silicates may be in
either the gelatinous or powdery form; in the former case they may also
occlude water-soluble substances.
_Carbonates of lime and magnesia_, which are readily dissolved by
carbonated water as well as by the vegetable acids.
_Phosphates of lime and magnesia_, not very readily soluble in
carbonated water, but more readily attacked by the acids of the soil
and of plant roots; thus supplying phosphoric acid to plants. The more
finely divided they are the more readily they are dissolved; some soils
containing only crystalline needles of apatite (see chap. 5, p. 63)
only are nevertheless poor in available phosphoric acid.
The natural phosphates of iron and alumina are practically insoluble
in all solvents at the disposal of vegetation and though present in
considerable amounts in some soils, (see chapter 19, page 355), may be
considered as being permanently inert, and therefore not to be counted
among the soil resources for plant nutrition. As yet no artificial
process by which their phosphoric acid can be made available within the
soil, has been discovered.
_Water-soluble Ingredients._—As regards these it has already been
explained that they are largely retained in the condition of purely
physical adsorption, as in the case of charcoal or quartz sand, through
which sea water filters and is thereby partially deprived of its
salts. But these can be gradually withdrawn by washing with pure water
alone, and still more easily when stronger solvents are used. Since
the soil-water is always more or less charged with carbonic acid, and
the roots themselves secrete carbonic as well as stronger acids in
their absorption of mineral plant-food, there is no difficulty about
explaining the manner in which such physically condensed ingredients
are taken up.[108]
[108] Whitney (Bull. 22, U. S. Bureau of Soils) claims on the basis
of a large number of (three-minute) extractions of soils made with
distilled water, that these solutions are essentially of the same
composition in all soils; that all soils contain enough plant-food to
produce crops indefinitely; and that the differences in production
are due wholly to differences in the moisture supply, which he
claims is, aside from climate, the only governing factor in plant
growth. The tables of analytical results given in Bull. 22 fail to
sustain the first contention; the second is pointedly contradicted
both by practical experience, and by thousands of cumulative culture
experiments made by scientific observers; the third fails with the
second, except of course in so far as an adequate supply of moisture
is known to be an absolute condition both of plant growth, and the
utilization of plant-food. It is moreover well known that it is not
water alone, but water impregnated more or less with humic and carbonic
acids, that is the active solvent surrounding the plant root.
_Recognition of the Prominent Chemical Character of Soils._ In a former
chapter the soils formed from the several minerals and rocks have been
discussed in a general manner. We can as a rule obtain some insight
into the nature of any soil which we can trace to its parent rock or
rocks, if we are acquainted with the composition of the latter.
Similarly, but in a much more direct manner, we can obtain a
strong presumption as to the nature of any soil by determining the
undecomposed minerals present in it. In all ordinary cases the
presumption must be that the decomposed portion of the soil has been
derived from the minerals still found in it. Of course it may happen
in the case of lands derived from widely distinct and distant regions
that no such characteristic minerals can be found; this is very
commonly true of the soils forming the deltas of large rivers, in
which sometimes the only remaining recognizable mineral is quartz in
its several forms, with occasional grains of such hardy minerals as
tourmaline, garnet, etc. Apart from such cases, the hand lens or the
microscope permits us to recognize in most soils the minerals that have
mainly contributed to their formation, thus also gaining a clew to
their prominent chemical nature.
Such recognition sometimes involves, of course, a somewhat intimate
knowledge of mineralogy; yet a little practice will enable almost
any one to identify the more important soil-forming minerals, under
the lens or microscope, according to the degree of abrasion or
decomposition they may have undergone. The details of such researches
lie outside of the limits of this treatise, but some general directions
on the subject are given farther on.[109]
[109] See Appendix B.
_Acidity, Neutrality, Alkalinity._—A test never to be omitted is that
of the reaction of the soil on litmus or other test paper, to ascertain
its acid, neutral or alkaline reaction. Should the latter occur quickly
(by the prompt blueing of red litmus paper), “black alkali” would be
indicated; but a blueing after 20 to 30 minutes means merely that
a sufficiency of lime carbonate is present. An acid reaction (the
reddening of blue litmus paper) of course indicates a “sour” soil (see
chap. 8, page 122).
_Chemical Analysis of Soils._—When the observations mentioned above
give no very decisive results or inferences as to the soil’s chemical
character, the more elaborate processes of qualitative and quantitative
chemical analysis may be called in. It would seem at first sight that
these ought to yield very definite results to guide the cultivator; yet
such is by no means always the case. Both the previous history of the
land, and the method of analysis, influence materially the practical
utility of the results of chemical soil analysis.
The cause of this uncertainty becomes obvious when we consider the
three groups of ingredients outlined above, viz., the insoluble or
unavailable, wholly _undecomposed rock minerals_; the “_reserve_,”
consisting of compounds not soluble in water but soluble in or
decomposable by weak acids; and the _water-soluble portion_, either
actually dissolved in the water held by the soil, or held by the soil
itself in (physical) absorption. While the latter portion is directly
and immediately available to plants, the amounts thus held are usually
quite small, and (outside of alkali lands) would rarely suffice for
the needs of a crop during a growing season.[110] This demand must be
materially supplemented by what can be made available from the soil
minerals and the “reserve” by weathering, conjoined with the direct
action of the acids secreted from the plant’s root-hairs upon the soil
particles to which they are attached. It is obvious that the greater
or less _abundance_ of the plant-food in the soil-material upon which
these processes may be brought to bear, must essentially influence the
adequacy of the plant-food thus supplied. Moreover, the greater or less
extent to which these sources may have been drawn upon previously in
the course of cultivation, will similarly influence that adequacy, on
account of the diminution of the readily available supply.
[110] The investigations of King (On the Influence of Soil Management
upon the Water Soluble Salts in Soils and the Yield of Crops, Madison,
1903) show that from some soils at least, a sufficiency of plant-food
ingredients for a season’s crop may be dissolved by distilled water
alone, if the soil be repeatedly leached and dried at 110°. Whether
such a supply can be expected under field conditions, remains to be
tested.
_Water-soluble and Acid-soluble Portions most Important._—It thus
seems that while the undecomposed rock minerals are indicative of the
nature of the soil, but not directly concerned in plant nutrition, the
most direct interest attaches to the _water-soluble portion_, and the
_acid-soluble_ reserve. Both of these can, of course, be withdrawn from
the soil by treatment with acids of greater or less strength; and it
would seem that if we knew just what is the kind and strength of the
acid solvent employed by each plant, we could so imitate their action
as to determine definitely whether or not the soil contains an adequate
or deficient supply of actually available food for the coming crop.
_We Cannot Imitate Plant-root Action._—In this, however, we encounter
serious difficulties. The acids secreted by the plant roots are not the
only solvents active in the dissolution of plant-food; as yet we know
the nature of only a few; and even these, instead of acting for a long
time (season) on a relatively small number of soil particles touched by
the root-hairs, can in our laboratories only be allowed to act for a
short time on the entire soil-mass. Clearly, the results thus obtained
cannot be a direct measure of the amount of plant-food which a plant
may take up in a given time; we can only gain comparative figures.
These, however, can be utilized by comparison with actual cultural
experience obtained in similar cases.
_Cultural experience_ must, of course, be the final test in all these
questions; and it is generally more fruitful to investigate the causes
underlying such actual practical experience, than to attempt to supply,
artificially, the supposed conditions of plant growth. The latter are
so complex and so difficult of control, that the results obtained
by synthetic, small-scale experiments are constantly liable to the
suspicion that they are partly or wholly due to other causes than those
purposely supplied by the experimenter.
_Analysis of Cultivated Soils._—It is also clear that
in view of the inevitable complexity of the conditions
governing vegetable growth, we should whenever feasible
proceed from the more simple to the more complex. The
failure to conform to this rule in soil investigation has
been the cause of an enormous waste of energy and work
bestowed, at the very outset, upon the most complex problem
of all, viz., the investigation of soils long cultivated
and manured; lands which, having been subject perhaps
for centuries to a great and wholly indefinite variety
of crops and cultural practices, had thereby become so
beset with artificial conditions that without a previous
knowledge of what constitutes the normal regime in natural
soils, the correlation of their chemical constitution, as
ascertainable by our present methods, with their production
under culture, became as complex a problem as that of
motions of three mutually gravitating points in space.
Neither can be solved by the ordinary processes of analysis,
chemical or mathematical. Nevertheless, though it was at
one time contended that the minute proportion of plant-food
ingredients withdrawn from soils by cultivation could not
be detected by quantitative analysis, numerous examples
have shown that with our present more delicate methods this
can in most cases be done, though not always after a single
year’s crop.
_Methods of Soil Analysis._—The more or less incisive
solvent agents used in extracting a soil for analysis will
of course produce results widely at variance with each
other. When fusion with carbonate of soda, or treatment
with fluohydric acid is resorted to, we obtain for each
soil-ingredient the sum of all the amounts contained in
each of the three categories—the unchanged minerals, the
zeolitic “reserve,” and the water-soluble portion. It was
early recognized that the results of such analyses bear no
intelligible relation to the productive capacity of soils;
for pulverized rocks of many kinds, or volcanic ashes
freshly ejected and notoriously incapable of supporting
plant growth, might be made to give exactly the same
composition. The amounts of plant-food ingredients thus
shown might be several hundreds or thousands of times
greater than what one crop would take from the soil, and
yet not an ear of grain could be produced on the material.
The only case in which any useful information could be thus
obtained would be that of the absence, or great scarcity, of
one or more of the plant-food ingredients.
The next step was to use in soil analysis acids of such
strength as to dissolve all the zeolitic (and water-soluble)
portion, leaving the unweathered soil minerals behind; it
being assumed that the prolonged action of the roots and
soil-solvents would in the end act similarly to the acids
employed, such as chlorhydric or nitric acids.
But here also the results of analysis very commonly failed
to correspond to cultural experience in the case of
_cultivated_ soils; which frequently failed utterly
to produce satisfactory crops even when the acid-analysis
had shown an abundance of plant-food ingredients. Upon
this evidence, this method of soil investigation was also
condemned as being of little or no practical utility; and
this has ever since been a widely prevalent view.
The preferable investigation of cultivated soils was due to the fact
that they are practically the only ones _available_ in the countries
where the study of agricultural science was then being prosecuted;
and the paucity of useful results there achieved discouraged the
undertaking of similar researches where, as in the United States,
the materials for the investigation of the simpler cases—those of
unchanged, natural or virgin soils—were readily accessible. It was
not apparent on the surface that the indefinitely varied conditions
introduced by long culture would inevitably cause this lack of definite
correlation between the immediate productive capacity of a soil and
the composition of its acid-soluble portion, and that yet the same
might not be true of natural, uncultivated soils, which have all been
subjected, alike, only to the natural processes of weathering, and to
the annual return of nearly the whole of the ingredients withdrawn by
plant growth.
Following the failure of the treatment with strong acids to yield
with cultivated soils results definitely correlated with cultural
experience, numerous attempts were made to gain better indications
by the employment of weaker acid solvents. The pure arbitrariness of
such diluted solvents was equaled by the total indefiniteness and
irrelevance of the results with different soils. Only two rational
alternatives seem to remain, viz., either to push the extraction to
the full extent beyond which action becomes so slow as to clearly
exclude any farther effective action of plant acids; or else to use the
latter themselves at such strengths as by actual experiment is found
to exist in their root sap. The first alternative aims to ascertain
the _permanent productive values_ of soils; the latter to test
their _immediate productive capacity_. Both alternatives are purely
empirical, and derive their only claim to practical value from their
accordance with practical experience (see chapter 19).
THE SOLVENT ACTION OF WATER UPON SOILS.
The almost universal solvent power of pure water has already been
alluded to in chapter 2 (see p. 18), and illustrated by the analyses
of drain and river waters. While these convey a general idea of the
chief substances dissolved and carried off, the direct investigation
of the solutions actually obtainable from the soil by longer treatment
and with no more water than is compatible with the welfare of ordinary
crops, necessarily gives somewhat different results. For when drains
flow during or after heavy rains the water has not time to become
saturated. The following data afford a clearer insight into the actual
and possible solvent effects of water in the soil, and its possible
adequacy to plant nutrition unaided by acid solvents.
_Extraction of Soils with Pure Water._—Eichhorn and Wunder treated
soils from Bonn, and from Chemnitz (Saxony) respectively for ten days
and four weeks with about one-third of their weight of water; the
solutions thus obtained contain in 1,000,000 parts:
==========================+=======+===========
| Bonn. | Chemnitz.
--------------------------+-------+-----------
Silica | 48.0 | 25.7
Potash (K₂O) | 115.4 | 7.5
Soda (Na₂O) | 11.0 | 30.4
Lime (CaO) | 128.0 | 83.6
Magnesia (MgO) | 38.4 | 37.4
Peroxid of Iron (Fe₂O₃) | Trace | 11.7
Alumina (Al₂O₃) | ? | ?
Phosphoric acid (P₂O₅) | 31.0 | Trace
Sulfuric acid (SO₃) | 100.2 |
Chlorid of Sodium (NaCl) | 58.6 | 47.6
---------------------------+-------+-----------
These figures differ widely in most respects from those given for drain
and river waters. Potash especially is far more abundantly present in
the Bonn-soil solution than in the drain water, and so is phosphoric
acid; while lime is not widely different. Eichhorn therefore calculates
that with a reasonably adequate supply of water, these ingredients
would fully suffice for a full crop of wheat. The Chemnitz soil, on the
other hand, does not yield enough plant-food for more than a very small
crop upon the same assumptions.
_Continuous Solubility of Soil-ingredients._—It seems to be impossible
to exhaust a soil’s solubility by repeated or continuous leaching with
water. This was demonstrated in 1863 and 1864 by Ulbricht[111] and
by Schultze;[112] their general conclusions have quite lately been
corroborated by King,[113] as the result of extended and very careful
investigations.
Schultze experimented on a rich soil from Mecklenburg, by continuous
leaching with distilled water for six days, one liter passing every
twenty-four hours, with the following results.
[111] Vers. Stat. V. p. 207.
[112] Ibid. VI. p. 411.
[113] Proc. Ass’n Prom. Agr. Sci. 1904.
RICH SOIL FROM MECKLENBURG (Schultze.)
1,000,000 PARTS OF EXTRACTS CONTAINED:
==============+============+===========+==========+==========
|Total matter|Organic and|Inorganic.|Phosphoric
| dissolved. | volatile. | | acid.
--------------+------------+-----------+----------+----------
First extract | 535.0 | 340.0 | 195 | 5.6
Second “ | 120.0 | 57.0 | 63 | 8.2
Third “ | 261.0 | 101.0 | 160 | 8.8
Fourth “ | 203.0 | 83.0 | 120 | 7.5
Fifth “ | 260.0 | 82.0 | 178 | 6.9
Sixth “ | 200.0 | 77.0 | 123 | 4.4
| ------- | ----- | --- | ----
Total | 1,579.0 | 740.0 | 839 | 41.4
--------------+------------+-----------+----------+----------
It thus appears that while the first extraction removed the main
portion of the organic matter, the inorganic matters dissolved were
not greatly diminished in subsequent leachings; and that phosphoric
acid continued to come off to the last. The rich soil used in this case
gave results corresponding in general to these from the Bonn soil, in
the previous table. From a poorer soil similarly treated by Ulbricht,
described by him as a ferruginous sand from Dahme, the leaching of
which was continued for thirty days in periods of three days each, with
a total of forty times its weight of water, the results were as follows:
SOIL OF LOW PRODUCTION FROM DAHME (Ulbricht).
THE SEVERAL EXTRACTS CONTAINED IN 1,000,000 PARTS:
============+========+========+========+========+========+========
| First | Second | Third | Fourth | Fifth | Sixth
|Extract.|Extract.|Extract.|Extract.|Extract.|Extract.
------------+--------+--------+--------+--------+--------+--------
Potash | 7 | 6 | 7 | 7 | | 3
Soda | 41 | 11 | 26 | 17 | | 8
Lime | 96 | 70 | 55 | 48 | 62 |
Magnesia | 14 | 10 | 9 | 7 | 8 |
Phosphoric | | | | | |
acid | trace | 2 | trace | 1 | |
+ ----- + -- + ----- + -- | -- + --
Totals | 158 | 99 | 97 | 80 | 70 | 11
------------+--------+--------+--------+--------+--------+--------
It will be seen that there is a considerable difference both in the
total amounts of matters dissolved and in the phosphoric acid taken out
by the water, as compared with the rich soil treated by Schultze. The
uniformity of the amounts of potash removed at the successive leachings
is remarkable.
_King’s Results._—The same general features are again strikingly
illustrated by King’s results, as given in the following table. King’s
first leachings were always made by shaking up the soil with ten times
its dry weight of water for three minutes, then after subsidence
filtering the solutions through a Chamberland (porcelain biscuit)
filter, and then (without evaporation) determining the ingredients
dissolved, by very delicate, mostly colorimetric methods. Subsequent
leachings were made by packing the soil around the filters and washing
with five times the weight of water, taking about fifteen minutes
each time; but drying the soil at 120 degrees C. between successive
leachings.
WATER EXTRACTION OF SOILS OF LOW AND HIGH PRODUCTION,
BY F. H. KING.
PARTS PER MILLION.
=======================+=======+=======+=========+===========
|Potash,| Lime, |Magnesia,| Nitric
Extractions | K₂O. | CaO. | MgO. | Acid,
| | | | N₂O₅.
-----------------------+-------+-------+---------+-----------
SOILS OF LOW PRODUCTION.
Sassafras sandy 1 | 12.62| 74.39| 17.82 | 18.03
soil. 11 | 218.25| 135.35| 147.45 | 21.76
-----------------------+-------+-------+---------+-----------
Norfolk, North 1 | 21.17| 58.30| 22.91 | 30.64
Carolina sandy soil 11 | 166.08| 162.98| 125.00 | 27.11
Average. | 192.60| 149.20| 136.23 | 24.44
SOILS OF HIGH PRODUCTION.
Janesville, Wis. 1 | 25.35| 135.30| 51.72 | 55.10
Loam. 11 | 313.70|1120.30| 500.60 | 51.42
-----------------------+-------+-------+---------+-----------
Hagerst’wn, Pa. 1 | 21.73| 165.25| 76.88 | 25.72
Clay loam. 11 | 301.55| 967.80| 463.15 | 96.04
Average. | 307.60|1044.05| 487.88 | 73.73
-----------------------+-------+-------+---------+-----------
=======================+==========+========+========+========+======
|Phosphoric|Sulfuric|Carbonic|Chlorin,|Silica
Extractions | Acid, | Acid, | Acid, | Cl₂. | SiO₂.
| P₂O₅. | SO₃. | CO₂. | |
-----------------------+----------+--------+--------+--------+------
SOILS OF LOW PRODUCTION.
Sassafras sandy 1 | 7.41 | 53.84 | 13.94 | 1 | 5.60
soil. 11 | 64.16 | 203.96 | 221.33 | 2 | 70.20
-----------------------+----------+--------+--------+--------+------
Norfolk, North 1 | 10.15 | 42.82 | 20.42 | 1 | 8.24
Carolina sandy soil 11 | 80.34 | 172.42 | 148.52 | 2 |122.20
Average. | 72.25 | 126.13 | 184.93 | 2.- |146.20
SOILS OF HIGH PRODUCTION.
Janesville, Wis. 1 | 16.96 | 125.43 | 29.31 | 2.67 | 40.28
Loam. 11 | 418.85 | 592.75 | 472.95 | 0.00 |414.50
-----------------------+----------+--------+--------+--------+------
Hagerst’wn, Pa. 1 | 11.51 | 187.59 | 97.09 | 1.67 | 21.17
Clay loam. 11 | 136.21 | 502.82 | 620.00 | 0.00 |283.80
Average. | 277.03 | 547.79 | 546.48 | 0.00 |349.15
-----------------------+----------+--------+--------+--------+------
King’s observations show strikingly both the continuous solubility of
the soil, and the differences between the solutions derived from soils
of low and high productiveness; wholly negativing the contention of
Whitney that the solutions from different soils are of practically the
same composition.[114] King also calls attention to the fact, shown in
other experiments made in the extraction of soils without intermediate
dryings, that the amounts extracted were very much less in subsequent
than in the first extraction; doubtless because the evaporation from
the soil particles had carried a large proportion of soluble matters
to the surface, whence it was readily abstracted by the first touch
of the solvent water. At each drying not only are the soluble matters
again drawn to the surface, but heating a soil even to 100° renders
additional amounts of soil ingredients soluble both in water and in
acids. It can scarcely be doubted that the intense heating which desert
soils undergo during the warm season is similarly effective; and thus
the great productiveness of these soils under irrigation, and the
marvelously rapid development of the native vegetation when rains
moisten the parched soil, is in part at least accounted for by this
immediate availability of a large supply of plant-food.
[114] Bulletin No. 22, Bureau of Soils, U. S. D. A.
_Composition of Janesville loam._—In connection with the above data
given by King, it is interesting to note the composition of the soil
in the above table yielding the highest proportions of soluble matter,
when analyzed according to the method practiced by the writer (see
chap. 19, p. 343). This analysis was made under the supervision of
Professor Jaffa in the laboratory of the California Experiment Station
by Assistant Charles A. Triebel.
_Loam Soil from Janesville, Wisconsin_; sample sent by
Prof. F. H. King, Madison, Wis.
This soil is a light friable loam, resembling the northern
Loess in color and texture; it is highly productive. It is
underlaid at 5 feet by the drift gravel of that region,
enclosing much calcareous material, which evidently has had
a large share in the formation of this soil, just as is the
case in southern Michigan.
The soil, when dried at 110° C, consisted of
CHEMICAL ANALYSIS OF FINE EARTH.
Insoluble matter 69.35
Soluble silica 10.89
Potash (KO₂) .59
Soda (Na₂O) .04
Lime (CaO) .83
Magnesia (MgO) .51
Br. ox. of Manganese (Mn₃O₄) .08
Peroxid of Iron (Fe₂O₃) 3.60
Alumina (Al₂O₃) 5.26
Phosphoric acid (P₂O₃) .06
Sulfuric acid (SO₃) .10
Water and organic matter 8.72
------
Total 100.03
It will be noted that in accordance with the interpretation
of analyses of soils as given in the next chapter, this is
a high-class soil in every respect, except that its content
of phosphoric acid is only just above the lower limit of
sufficiency. But as is also shown below, in presence of a
large supply of lime even lower percentages of phosphoric
acid are adequate for long-continued production (see chap.
19, pp. 354, 365) by rendering the substance more freely
available; and that this is true in this case is shown by
the result of King’s leachings, in which this soil yields
a maximum of 419 parts per million as against 80 and 64
parts in the poor soils, which at the same time yield only
one fourth as much of lime. Unfortunately we have no full
analyses of these other soils for comparison; although
they have served as a basis of comparison for years in the
Washington Bureau of Soils.
_Solubility of Soil Phosphates in Water._—The solubility of the
phosphate contents of soils has been elaborately investigated by
Th. Schloesing fils.[115] He found in the case of a number of soils
investigated by him that the amount of phosphoric acid P₂O₅ in the
soil-solution ranged from less than one millionth (or one milligram
per liter of water) in a poor soil, to over three milligrams in a rich
one. He also found that for one and the same soil the amount so found
was constant, if about a week’s time were allowed for saturation. He
calculates that while in general the amount of phosphoric acid capable
of being supplied to the crop during a growing season of twenty-eight
to thirty weeks would suffice for but few crops, the supply so afforded
is in no case a negligible quantity, frequently amounting to more than
half of the crop-requirements. Experiments with various crops prove
that these dilute solutions are utilized by all of them, sometimes to
the extent of completely consuming the content of the solution. The
much smaller content of phosphoric acid in drain waters is accounted
for by the lack of time for full saturation during the time that the
flow lasts. Whitney, (Bureau of Soils, Bulletin 22) has extracted
the soil-solution by means of the centrifuge from several soils; the
contents of phosphoric acid thus found are in general of the same order
as those shown in the preceding table by King, but much in excess of
Schloesing’s figures; notwithstanding the fact that Whitney’s soils had
been in contact with water for only twenty-four hours. The cause of
this wide discrepancy is not clear.
[115] Ann. de la Sci. Agron., 2de série tome I, pp. 416-349; 1899.
_Practical Conclusions from Water Extraction._—As regards the
practically useful conclusions to be drawn from the extraction of soils
with pure water, the data given above, and especially the results
obtained by King, seem to prove that there is a more or less definite
correlation between the immediate productiveness of soils and the
amount and kinds of ingredients dissolved; especially in the case of
phosphoric acid, the adequacy of the supply of which for immediate
production is assumed to be thus demonstrable by many French chemists.
Moreover, a number of King’s results, tabulated in curves, exhibit
a remarkable general parallelism of the curves showing totals of
plant-food extracted by water, and actual crop production. This is the
more remarkable since it is known to be, not pure water, but such as is
more or less impregnated with carbonic acid at least, that is actually
active in soil-solution and plant-nutrition. The farther development of
this method may, it would seem, lead to definite conclusions at least
in respect to the immediate productive capacity of cultivated, and
perhaps also of virgin soils. But it is not likely to give any definite
clew as to the _durability_ of such lands.
ASCERTAINMENT OF THE IMMEDIATE PLANT-FOOD REQUIREMENTS OF CULTIVATED
SOILS BY PHYSIOLOGICAL TESTS. PHYSIOLOGICAL SOIL-ANALYSIS.
As has already been stated, the quantitative analysis of cultivated
soils by means of strong acids affords a presumptive insight into their
immediate productiveness, and the kind of fertilizer needed to improve
it, only in case of the extreme deficiency of one or several of the
chiefly important plant-foods. The limits of deficiency of these in
virgin soils have been discussed above; but since in cultivated soils
amounts of soluble plant-food so small as to be beyond the limits
of ordinary analytical determinations, when distributed through an
acre-foot of soil may, when rightly applied, nevertheless produce very
decided effects, the indications thus obtainable are not absolute. Thus
a dressing of 150 lbs. of Chile saltpeter, containing only about 24
lbs. of nitrogen, is capable of causing the production of a full crop
of wheat where otherwise, even under favorable physical conditions,
only a fraction of a crop would have been harvested; _provided_, that
all the other requisite ingredients were present to a sufficient
extent and in available form. Yet the amount of nitrogen thus added
would amount, in one acre-foot of soil to only .0008%, say eight
ten-thousandths of one percent; which, with the amounts of substance
usually employed in soil analysis, would be an unweighable quantity,
and might easily be overlooked.
Since the amounts of potash and phosphoric acid actually taken out of
the soil by one crop are in general of the same order of magnitude as
the above, what is taken out by one or two crops will usually fall
within the limits of analytical errors, especially of those incurred
in sampling the soil. Yet that the changes caused by a number of
successive crops can be proved, even by the ordinary methods, has been
abundantly verified. For it seems that the losses of soil ingredients
in cultivated lands exceed considerably those calculated from the
actual drain represented by the crops.
_Plot Tests._—There is, however, an obvious and apparently simple
method by which every farmer might make his own fertilizer tests, on
a small and inexpensive scale, the results of which may afterwards be
put in effect on his entire land. It is to apply in proper proportions
on plots (of say from one twentieth to one fortieth of an acre), the
several plant-food ingredients usually supplied in fertilizers, singly
as well as conjointly with each other, leaving check unfertilized plots
around as well as among them. By comparison with these, the cultural
results should at once determine which of the fertilizers can most
advantageously be applied to the land. Such tests when carried out with
all the proper precautions are often very decisive and practically
successful. But they so frequently suffer from seasonal influences
(such as scanty or excessive rainfall, cold or heat, etc.), inequality
of soil conditions, failure to apply the fertilizers at the right time,
or in the right way, the depredations of insects and birds, and other
causes, that it generally takes several seasons’ trial to obtain any
definite results. On level lands of uniform nature and depth, they are
most likely to be successful; while on undulating or hill lands it is
not only very difficult to secure uniformity of soil and subsoil on
areas of sufficient size, but also to prevent the washing of fertilized
soil, or fertilizer in solution, from one plot to the other by the
influence of heavy rains or irrigation; thus wholly vitiating the
experiments. In very many cases, especially in the arid region, the
results of such trials have been practically _nil_, for the reason that
physical defects of the soil, and not lack of plant-food, were the
cause of unsatisfactory production.
[Illustration: Scheme for Plot-tests of Fertilizers.]
A full examination of physical conditions, as outlined in previous
chapters, should _in all cases precede_ the application of fertilizers;
such examination will at the same time serve to determine the greater
or less uniformity of soil-conditions, which is of first importance
to the cogency of fertilizer tests. As a matter of fact, few farmers
possess the necessary qualifications to carry out such tests
successfully, since their execution requires a certain familiarity
not only with the principles and methods of experimentation, but also
the faculty and practice of close and reasoning observation; which,
unfortunately, is not as yet a part of instruction in our schools.
The experience so often had in co-operative work between experiment
stations and farmers is cogent on this point.
Those desiring to do such work, however, can make use of something like
the plan given above; it being understood that in the case of clay
soils, the unplanted paths left between the plots should be at least
two feet in width; in the case of sandy soils the distance should be
not less than three feet, and more if the plots are located on a slope.
The crop from each plot should if possible be weighed as a whole; but
if the plot be large and the crop measurably uniform, an aliquot part,
such as one fourth, may be weighed instead. In regular experimentation
the crops are weighed both in the green (freshly cut) condition, and
after drying. Since the dry matter is the real basis of value in the
case of most field crops, its weight is the most important; as the
water-content of green crops may vary considerably. But in the case
of vegetables as well as fruit crops, not only must the weight of the
fresh crop be determined, but it should be sorted into the “marketable”
and “unmarketable” sizes and qualities. Failure to do this may vitiate
the entire experiment for practical purposes.
_Pot Culture Tests._—The uncertainty attending plot culture tests
on account of the difficulty of controlling seasonal and other
external conditions, has resulted in the extended adoption of indoor
culture tests, usually conducted in zinc or “galvanized” cylinders
of a size sufficient to contain from twelve to twenty or more
pounds of soil. These are kept in a green-house whose temperature
and moisture-condition can be regulated at will, and where the
soil-moisture is wholly under control. For investigations of the
effects of various kinds of plant-food upon vegetable development, this
method has served most satisfactorily and effectually, and striking
photographs of results thus obtained are seen on all hands: for which
reason, to save space, they have not been introduced into this volume.
It seems at first sight that the same method should serve admirably to
determine the manure-requirements of soils under controlled conditions.
It must, however, be remembered that the field conditions as regards
subsoil, evaporation, ascent of moisture from below, penetration and
spread of roots, etc., in other words, all the physical conditions
so vitally concerned in crop production, except the temperature and
moisture-condition of the soil, are wholly left out of consideration in
this method. Hence the application of the results so obtained to actual
field conditions can only be made with great caution, and are often
widely discrepant with actual experience.
The method has of late been carried to an extreme by the
U. S. Bureau of Soils in the proposition to supplant
the large soil-pots heretofore used by small paraffined
wire-cloth baskets, 3 × 3 inches in size, in which the
soil to be tested is sown with seeds which are allowed to
develop only for three to five weeks; it being claimed
that the development occurring during that time is quite
sufficient to indicate what will be the ultimate outcome
in crop production. But practical experience has long ago
demonstrated that these early stages of growth cannot be
relied upon to show the crop results to be expected. Yet if
this minute scale of pot-culture should, on further test,
prove to give truthful forecasts even in a mere majority of
cases, the facility with which it may be carried out will
entitle it to favorable consideration. A great deal more
proof is needed on this point than the confident claims of
the Bureau indicate.
CHEMICAL TESTS OF IMMEDIATE PRODUCTIVENESS.
_Testing chemical soil-character by crop analysis._—Another method for
the determination of immediate soil requirements has been elaborated
by E. Godlewski.[116] The principle upon which this method rests is
that plants growing in a soil deficient in available plant-food of
any one kind will in their ash show a corresponding deficiency, or at
least a minimum proportion of the same; and that in many cases, the
nature of the deficiency manifests itself in the form or development of
the plant, so clearly as to render chemical analysis unnecessary (see
below, chapter 22).
[116] Zeitschr. Landw. Vers. Oesterr., 1901.
To a certain extent the latter idea has been and is constantly being
utilized in practice. It is essentially involved in the habit of
judging of land by its natural vegetation; and by agricultural chemists
and intelligent farmers, when they check excessive growth of stems and
leaf (indicating excess of nitrogen) by the use of lime or phosphates;
or prescribe the use of nitrogenous manures when a superabundance of
small, unmarketable fruit is produced. From the coincidence of such
indications with the results of the analyses of soils and ashes,
very definite and permanently valuable indications as to the proper
fertilization and other treatment of the land may be deduced.
Godlewski insists strongly, and with a good deal of
plausibility, upon the importance of making such trials in
the open field and not merely in pots. While this is true,
it is also true that such field experiments suffer from the
same liability to imperfection as the “plot fertilizer-test”
plan just described; viz., that the season may exert a much
more powerful influence than the fertilization, and the
tests may lead to wholly erroneous conclusions unless the
experiments are continued for a number of years, and under
skilled supervision. But when once the normal ratio between
the ash ingredients for a particular soil and climatic
region have been ascertained, the data will be of lasting
benefit to agriculture there, and perhaps, other things
being equal, to the world at large.
H. Vanderyst has discussed the entire subject of physiological soil
analysis elaborately in the Revue Génerale Agronomique of Louvain,
1902-3 (Exp’t St. Record, April 1904, Vol. 8, page 757) and shows in
detail the conditions under which it may be successful. Among these he
reckons as full a knowledge of the chemical characteristics of a soil
as can be obtained by chemical analysis.
_Chemical Tests of Immediately Available Plant-food._—It is scarcely
doubtful that plants differ considerably in the energy of their action
upon the “reserve” soil ingredients; hence no one solvent used by
the analyst could represent correctly the action of plant-roots in
general upon the soil, even if we could give that action the same
time (a growing season) and opportunity afforded them in nature by
the root-surface. We are forced to proceed empirically; and among the
numerous solvents suggested for the purpose of soil extraction, that of
Dyer, already mentioned, viz., a one per cent solution of citric acid,
making allowance for such neutralization as may occur in the soil, has
seemed to the writer to give results most largely in agreement with
cultural experience. Walter Maxwell has recommended aspartic acid in
lieu of citric, as approaching nearer to practical results, at least
with sugar-cane.
According to the investigations of Dyer, on Rothamstead soils of
known productiveness or manurial condition, it appears that when the
citric-acid extraction yields as much as .005% of potash and .010% 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. These figures
refer to the ordinary field crops of England and to soils originally
fertile and well supplied with lime. It can readily be foreseen that
under other climatic and soil conditions, different figures may have to
be established. So far as the writer’s experience goes, however, the
above figures are very nearly valid for the arid climates as well; only
the figures obtained for arid soils are usually far in excess of the
above minimum postulates. Figures for lime and nitrogen are given in
chapters 8 and 19. But the results obtained with the highly ferruginous
soils of Hawaii show that under such conditions, figures far exceeding
the minimum ones established by Dyer nevertheless coexist with need of
phosphate fertilization.