_Loosening of the Surface._—The first thing needful, of course, is to allow the water free opportunity to soak into the soil, so as to moisten the land as deeply as possible. That to this end the surface should be kept loose and pervious by tillage, breaking up crusts that may have been formed by the beating of rains, has already been discussed. In the case of heavy clay soils, however, this alone is not always sufficient. The most effectual way to loosen the land to greater depths than can be reached by tillage, is by means of underdrains laid at the greatest depth that is practically admissible. _Effects of Underdrains._—That drain tiles laid for the express purpose of carrying off surplus water should help to conserve soil moisture, seems at first sight to be a paradox. Yet the explanation of the fact, which has been demonstrated by long experience, is not difficult. The effect is most striking in clay soils, for sandy soils are commonly naturally underdrained already. In discussing the changes of volume which soils undergo in wetting and drying, the fundamental points in the premises have already been mentioned (see chap. 7, p. 112). Clay soils in drying shrink considerably, and re-expand on wetting, but rather slowly; moreover, some clays crumble when wetted _after_ drying, while others, very plastic when wet, crumble on drying (see chap. 7, p. 116). It follows that while a clay subsoil when kept permanently wet, will form a uniform, pasty, difficultly penetrable mass: when subjected to frequent alternate wetting and drying, it becomes fissured and crumbly, so as to resemble in its texture a tilled soil. This frequent alternation of wetting and drying is precisely what, in the course of time, is brought about by underdrains; rendering clay subsoils pervious both to air and water. The consequence is that even heavy rains can be fully absorbed by the soil mass lying above the drains, the surplus draining off readily in a short time. Roots therefore can not only penetrate, but exercise their vegetative functions perfectly at the full depth of the drains. They are still at liberty to penetrate as much deeper as their demands for moisture may require; but the depth of four to four and a half feet is already so much greater than in the humid region would usually be reached by them in undrained clay soils, that commonly the moisture successively retained within that mass is as much as is required by them during the growing season. At the same time, their feeding roots are so far below the surface, that ordinary short droughts do not reach them at all; while the underdrains prevent any injurious stagnation of water around them. It need hardly be added that the entire task of cultivation is also greatly facilitated; not only because drained soils can be plowed within a few hours after the cessation of rains, as against the same number of days that would have to elapse in the undrained areas; but because tillage is easier, and