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.