The most combustible portions of their food are, in consequence of the respiratory process, exhaled as water and carbonic acid gas; while the ash ingredients, and the larger share of the nitrogen, are accumulated in the excreta. In this way there is a concentration of constituents which, after they have served the nutritive function for the animal, become the proper food of the plant. Among the various ingredients of manures, two in particular have acquired a special significance in late years, viz., phosphoric acid and ammonia. These bodies are commercially the most valuable of all fertilizing substances, a necessary result of their scarcity; and in general, phosphoric acid is a smaller ingredient of cultivated soils than any other of the components of the ash of plants. Ammonia, especially in the form of carbonate, not only powerfully stimulates vege ! table growth, but it probably exerts a strong solvent effect on the minerals which compose the soil. Hence, guano and other animal manures which contain or yield much ammonia and phosphoric acid, are in such large demand among those who practise "high farming." But the exclusive use of fertilizers which supply to vegetation only a small portion of its ash ingredients, must sooner or later be found inadequate to produce profitable returns; must, in fact, reduce the soil to a minimum of feri tility.

The true system of manuring is to maintain in excess a supply of all forms of plant food, and indeed of all materials which experience proves to have a good effect on vegetation, whether this effect be chemical or physical. - When chemical analysis first demonstrated 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 held 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 cultivated plants. - Rotation of Crops. The greatest 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 agricultural 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 alternating 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 composition and texture are perpetually changing. The quantity of organic matter, especially, rapidly diminishes when the soil is under cultivation, and the soluble mineral matters are in most cases removed by cropping faster than they are supplied by weathering or disintegration.

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; exhausting crops, cereals, beets, turnips, carrots, and potatoes; very exhausting crops, tobacco, flax, hemp, 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 surface. 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 require a better soil than those which only produce foliage, because the rapidity of assimilation seems to increase when the reproductive function comes into activity.

Plants which ripen seed may require a richer soil, not because 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 unlike. In the light of the above statements, it is easy to see that when a soil refuses to yield remunerative crops of shallow-rooted and quick-growing wheat, it may still produce a luxuriant 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 intervention 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 process, 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.