And such changes do naturally come over the soil. The oak, after thriving for long generations on a particular spot, gradually sickens; its entire race dies out,—and other races succeed it. The operation of natural causes has gradually removed from the soil that which favoured the oak, and has introduced or given the predominance to those substances which favour the beech or the pine. In the hands of the farmer the land grows sick of this crop,—it becomes tired of that. These facts are generally indications of a change in the chemical constitution of the soil. This alteration may proceed slowly and for many years, and the same crops may still grow upon it for a succession of rotations. At length the change is too great for the plant to bear; it sickens, yields an unhealthy crop, and becomes ultimately extinct. The plants we raise for food have similar likes and dislikes with those that are naturally produced. On some kinds of food they thrive,—fed with others, they sicken or die. The soil must therefore be prepared for their special growth. In an artificial rotation of crops, we only follow nature. One crop extracts from the soil a certain quantity of all the inorganic constituents of plants; but some of these in much larger proportions than others. A second crop carries off in preference a larger quantity of those substances which the former had left; and thus it is clearly seen, both why an abundant manuring may so alter the constitution of the soil, as to enable it to grow almost any crop; and why, at the same time, this soil may in succession yield more abundant crops and in greater number, if the kinds of plant sown and reaped be so varied as to extract from the soil, one after the other, the several different substances which the manure we have originally added is known to contain. The management and tilling of the soil, in fact, is a branch of practical chemistry, which, like the art of dyeing or of lead smelting, may advance to a certain degree of perfection, without the aid of pure science; but which can only have its processes explained, and be led on to shorter,—more simple,—more economical,—and more perfect processes, by the aid of scientific principles. CHAPTER VIII. Of the Improvement of the Soil—Mechanical and Chemical Methods—Draining—Subsoiling—Ploughing, and Mixing of Soils—Use of Lime, Marl, and Shell-sand—Manures —Vegetable, Animal, and Mineral Manures. The soil is possessed of certain existing and obvious qualities, and of certain other dormant capabilities; how are these qualities to be improved,—these dormant capabilities to be awakened?