198 AGRICULTURAL CHEMISTRY tain in excess a supply of all forms of plant food, and indeed of all materials which expe- rience proves to have a good effect on vege- tation, whether this effect be chemical or physical. When chemical analysis first de- monstrated that different classes of plants yield an ash of different composition, the idea of special manures had its origin. By special manures were meant mixtures containing just the quantity of each ash ingredient removed from the soil by an average yield of each crop. But investigation has demonstrated that there are in general no practical advantages in these attempts to feed the plant by ration. Lawes and Gilbert, of Rothamstead, England, believed they had established by a multitude of field experiments that ammonia is specially suited to the production of wheat, and phosphoric acid to the growth of turnips; but there are other authentic trials which as fully prove just the reverse. While on a certain soil, and under a certain set of circumstances, experience may without difficulty establish a rule, science is not yet far enough advanced to lay down a universally applicable principle concerning the special nutrition of the various classes of cul- tivated plants. Rotation of Crops. The great- est return from the soil is generally secured, not by continuously growing one plant, even though it command the highest market price, but by an alternation or rotation of crops. There is no difficulty in cultivating any agri- cultural plant successively for any number of years on the same ground, provided enough be expended in putting the soil into the right physical and chemical condition. But such a procedure is usually more expensive than al- ternating the crops. When a light virgin soil comes under the hand of the farmer, it yields good crops for a few years, but then falls to a low state of productiveness. At first it may have yielded wheat; when no longer able to support that crop, it may still give fair crops of barley ; the next year, if put to turnips or potatoes, it may seem to recover its fertility somewhat, and produce a good burden of roots; but now it will not yield again a good crop of wheat, though probably clover would flourish on it. The causes of such facts lie partly in the soil, and partly in the plants themselves. As for the soil, as already stated, its compo- sition and texture are perpetually changing. The quantity of organic matter, especially, rapidly diminishes when the soil is under cul- tivation, and the soluble mineral matters are in most cases removed by cropping faster than they are supplied by weathering or disintegra- tion. Practical men have classed cultivated plants according to their demands on the soil, as follows: Enriching crops, clover, lucern, and esparsette ; non-exhausting crops, peas and beans, also cereals when cut green ; exhaust- ing crops, cereals, beets, turnips, carrots, and Eotatoes ; very exhausting crops, tobacco, flax, emp, and hops. Among the causes of the different exhaustive effect of various plants are the following: 1. Different extent or structure of roots and leaves. The enriching crops expose to the air an enormous surface of foliage, and throw out very large, long, and numerous roots. The cereals have much less leaf and root sur- face. 2. Different rapidity of growth. Clover and root crops continue in foliage during the whole season, while the cereals ripen in July or August. 3. Periods or crises of growth; seed production. Plants which ripen seed re- quire a better soil than those which only pro- duce foliage, because the rapidity of assimila- tion seems to increase when the reproductive function comes into activity. Plants which ripen seed may require a richer soil, not be- cause they remove more from it, but because they need more in a given time. 4. Some crops are entirely removed from the soil, as flax ; while others leave the ground filled with an enormous mass of roots, as clover, or strewn with stalks and foliage, as the potato and beet. 5. The quantity of ash ingredients removed from the soil by different plants is widely un- like. In the light of the above statements, it is easy to see that when a soil refuses to yield re- munerative crops of shallow-rooted and quick- growing wheat, it may still produce a luxuri- ant growth of deep-rooted, large-leaved, and slow-growing clover. It is evident, too, that' when a clover ley is broken up and sown to wheat, this grain may yield well, because the decaying turf and roots are a ready source of every kind of plant food. This preparation of the soil for an exhausting crop, by the inter- vention of one of easy growth, is shown in the practice of green manuring, which is in fact a rotation of crops, but is also a fertilizing pro- cess, because the first crop is entirely sacrificed for the sake of the succeeding ones. Green manuring consists in ploughing under clover, buckwheat, spurry, or other crops, when in blossom, so that the soil shall be enriched by their decay. As these plants (the last named especially) will grow on poor soils, it is possi- ble by their help to reclaim the lightest sands, and bring them up to a fair degree of produc- tiveness in the course of a few years. Compo- sition of Crops, and their Value as Food. There are definite and unalterable relations between the character and habits of the animal and the composition and physical qualities of its food. In rearing and sustaining domestic animals, four distinct conditions occur, viz. : growth, or general development ; fattening, or increase of flesh and fat ; yielding milk ; and perform- ing labor. Different species of animals possess different degrees of aptitude in turning their food into one or other of these directions. Thus, the hog fattens most readily, the cow yields most milk, and the horse performs the greatest amount of labor. All these animals might be fed alike on a certain diet, and yet manifest their characteristic tendencies in a good degree, for the functions of all animals are the same to a certain point. That food, however, which best develops fat in the hog,
