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.