Page:Popular Science Monthly Volume 60.djvu/550

542 popularly believed on this subject of exhaustion. There is probably no such thing, in a strict interpretation of the term, as an exhausted soil, for it is pretty thoroughly established that although a condition of the soil may have been brought about, as just described, in which it is not suited to the growth of a particular crop, it may be very well suited indeed to some other crop, and generally is suited to some other crop, and so the terra 'exhaustion' in this sense is really a relative one only. It has likewise been pretty thoroughly established now that plants, like animals, need a varied, and, as it is sometimes called, 'balanced' ration; that there is a particular combination or proportion of various constituents in the nutrients which is best adapted to a plant at any particular stage of its growth. For example, it is well understood that for the development of a grain crop, especially at the time when the grain is forming, a certain amount of magnesia is necessary. But if the ratio of the magnesia to the other necessary mineral elements is above a certain figure, the soil solutions become extremely toxic, with the results that the crops fail and the soil is barren. It has been found that the absolute amount of the magnesia which may be present and probably causing the barrenness of a given area is extremely small—so small, indeed, as to be difficult of accurate estimation. But if the ratio of other elements to the magnesia, for instance lime, be raised sufficiently high, the plants will do remarkably well, and, up to a certain limit, the more magnesia, the better they will do, so long as the ratio of the magnesia to the lime does not become too high. These views will explain what was meant when it was said above that the fertility or nonfertility of the soil is relative, and it is as a matter of fact, the explanation of the earlier remark that the expression 'a poor soil, a poor people, etc.,' may be misleading, if not positively untrue. For it is very probable that there is no soil which is cultivatable at all which could not be regarded as a fertile soil for some crop.

It is possible to modify the conditions as to the fertility in any given soil by tillage, which changes the physical condition of the soil and removes interfering growth from the crop we wish to cultivate; which, on the one hand, promotes capillary rise of water from the lower depths of the soil, or, on the other hand, cuts it off from the surface to prevent its too rapid escape; and which may improve the physical condition of the soil and further promote the natural rate of decomposition or weathering. Again it has been found in modern times that the rotation of crops, or change of crops, will aid the soil. And the reason for this probably lies in large part in the fact, as noted before, that one crop requires a different proportion of constituents in the soil from another, and if it is changed in the proportions of the constituents present, it is still adapted to the succeeding crop, giving time for those which became deficient through the growth of the first crop, to be again