THE SOIL AS AN ECONOMIC FACTOR. 543 



accumulated in suiEcient quantity and relative proportion. Other 

 factors enter of course into rotation, such as change of the texture of 

 the soil produced by one crop making it better for another, the elimina- 

 tion of parasites, etc. 



A third method is that of fertilizing, and a most complex group of 

 phenomena is involved in this practice. The function of fertilizing is 

 probably three-fold at least. The added material may change the tex- 

 ture, structure or other physical conditions of the soil, whether it be by 

 mechanical mixture or by some other physical or physico-chemical 

 process, such as is probably involved in the flocculation of clays by cer- 

 tain solutions. Its most obvious purpose, and the one which is the 

 controlling idea among agriculturists at large, and perhaps to 

 entirely too great an extent among the supposed scientific workers in 

 this field, is the direct addition of plant food to the soil. It is not 

 intended by this latter statement to minimize the importance of this 

 function of fertilizers, but to insist in the most emphatic way that 

 more attention should be given to the third function, that is — the 

 changes in the solubility of the soil components already present induced 

 by the addition of fertilizing materials, especially the so-called mineral 

 fertilizers or salts. This is not the place for, nor would the space 

 allotted to this paper permit of, a satisfactory discussion of the subject. 

 SuflSce it to say, for the present, that it is thoroughly well known, 

 though not always generally recognized, that great changes in the 

 solubility of the soil components already present may be brought about 

 by the addition of foreign material, and in this sense the process of 

 fertilizing is one of retarding or expediting the natural weathering of 

 the soil components originally present. 



Within the last few years, it has come to be recognized that bacteria 

 or other microorganisms play a large part in the fertility of arable 

 soils ; that the development of some of these is desirable, that of others, 

 not. And while this side of the subject is in hardly more than the pre- 

 liminary stages of its development, yet a good deal is known as to the 

 several conditions which make for the presence of the desirable organ- 

 isms or vice versa, and these conditions are for the most part readily 

 controlled. Furthermore, the point has been reached when we can 

 actually inoculate the soil with desirable substances of this nature. 

 There is no branch of soil study which is offering greater promise of 

 advancement at the present time than this. And the opportunities it 

 offers for increasing our control over soil conditions augurs much for 

 improved agricultural practices. Here again it seems worth while to 

 call attention to the fact that these organisms are valuable not only for 

 themselves, or rather the immediate products of their activities, but 

 also quite as much probably for their influence upon the soil components 

 already present, either in breaking them down into simpler forms or in 



