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.