One explanation of the different capacity possessed by different crops for absorbing plant-food from the soil is to be found in the difference of their root systems. Every agriculturist knows that crops in this respect differ very widely. Crops having deep roots will naturally have a larger surface of soil from which to draw their food-supplies than crops having shallower roots. Such crops as red clover, wheat, and mangels are able to draw their food-supplies from the subsoil to an extent not possessed by shallower-rooted crops, such as barley, turnips, and grass. Crops having surface-roots, on the other hand, have often greater capacity for assimilating nitrogen,--this ingredient, as has already been pointed out, being chiefly located in the surface-soil. The tendency of growing shallow-rooted crops will therefore be towards impoverishing the surface-soil; whereas the occasional growth of a deep-rooted crop brings the plant-food in the subsoil into requisition. In this connection it may be well to draw attention to the singular capacity possessed by certain crops for absorbing nitrogen. Of these the case of clover is the most striking, and has long puzzled agriculturists. The discovery, which has been repeatedly referred to in these pages, that the leguminous order of crops, to which clover belongs, have the power of absorbing the free nitrogen of the air through the agency of micro-organic life in the plant and in the soil, has furnished an explanation of this long-debated problem. _Period of Growth._ A further reason is the difference in the period of a crop's growth. A crop which grows quickly, and consequently occupies the ground during a comparatively short period, will naturally require a richer soil, and therefore a more liberal treatment with manure, than one whose growth is more gradual.