accumulated in sufficient 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 elimination 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 texture, 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 certain 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. Suffice 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 preliminary stages of its development, yet a good deal is known as to the several conditions which make for the presence of the desirable organisms 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