will depend in turn the composition and nature of the soil solution. The soil, then, is composed of something more than the solid components alone, and this point of view is well worth accentuating, because it is not always recognized, and because upon it is dependent much of the modern development in soil studies.
The solid components of the soil are derived from two general sources. The mineral components are derived from the decomposition of the subsoil, or the underlying rocks. Or at least from rocks, although perhaps at some distance, when the soil has been carried to its present position by water, wind or other like agencies. Besides these mineral components of the soil, there are always more or less detritus and organic remains from the decomposition and breaking down of organic tissues from the present and former vegetation.
It is true that some of the components of the soil are so very little soluble as to be called in popular language insoluble. But there is, in reality, no such thing as an insoluble substance, and even those components of the soil which most nearly approach this condition are, to some extent at least, soluble in the ground water. And it is from these slightly soluble minerals and organic substances that the ground solution obtains its dissolved material, which serves in turn as food for the plant when presented in this available form. By the action of atmospheric agencies, oxidation, etc., by the solvent action of the water, by the action of dissolved gases, such as carbon dioxide for instance, and by the action of numerous microorganisms which exist in practically all soils, both the mineral and organic matters are broken down, decomposed, and part of them made more soluble. This is essentially the process known as weathering, and it is going on all the time. It may be checked, or it may be augmented, but it cannot be stopped entirely. It is nature's method for bringing the mineral nutriments in the soil into such forms as to be 'available' for plant nourishment, in contradistinction to that potential plant food which is present, but in such forms as to be unavailable. The distinction between available and non-available plant food, while now sufficiently obvious, was not recognized in the earlier studies, and marks in fact one of the first great steps forward in soil investigations.
Plants do not take from the soil solution the various dissolved substances which it contains in the same relative proportions in which they are present. Therefore by the continued growth of a particular kind of crop, and its periodic removal by cropping, it may happen that some one or more of the necessary plant nutrients in the soil may be removed more rapidly than others or than the normal weathering of the soil can furnish it to the soil solution. If this process is continued far enough, the plants fail to thrive and the soil becomes barren, or, as it is popularly phrased, exhausted. Much that is incorrect is