The Soil Solution

Chapter I.. The Soil.

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, 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.

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. 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, 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.

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.”

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.

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, 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.

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.

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