TEE SOIL AS AN ECONOMIC FACTOR. 54i 



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 com- 

 ponents 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, decom- 

 posed, 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 contradis- 

 tinction 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 (iontinued growth of a particular 

 kind of crop, and its periodic removal by cropping, it may happen 

 that some one or more of the necessaiy 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 con- 

 tinued far enough, the plants fail to thrive and the soil becomes barren, 

 or, as it is popularly phrased, exhausted. Much that is incorrect is 



