Page:The American Cyclopædia (1879) Volume I.djvu/221

 AGRICULTURAL CHEMISTRY 197 ner, viz. : to make trials, on separate plots of ground, of the effect of adding to the soil those ingredients which are most likely to be want- ing. The improvement of the soil involves numerous changes, both in its physical and chemical characters. The correction of the physical qualities of the soil usually effects a marked improvement in its chemical condition. It is at any rate indispensable to the full suc- cess ot chemical improvements (manures) that the soil be first brought to those degrees of division, porosity, dryness, and depth, that are most favorable to vegetable growth. Besides rendering the soil so dry, warm, deep, and penetrable, that the plant finds a genial root- ing place, these operations more or less facili- tate the solution and elaboration of the food of the plant, since the soil is thereby divided, and more thoroughly subjected to the action of water and air. Theory of Manuring. When the soil is deficient in those ingredients which favor the growth of the plant, the deficiencies may be supplied by manures. The principles on which manuring depends are the following : 1. Plants require various kinds of solid min- eral matters, and derive the same exclusively from the soil. 2. Some plants which in the natural state derive the gaseous elements of their organic structure, viz., carbon, hydrogen, nitrogen, and oxygen, from the atmosphere, must be supplied with more or less of these matters from the soil, in agricultural produc- tion. 3. Different plants require different proportions of these substances in order to lux- uriant growth. 4. Different plants require different quantities of these substances to ma- ture a full crop. 5. Different plants, from pe- culiarities of structure, draw differently on the same stores of nutriment. 6. Different soils abound or are deficient, to a greater or less degree, in one or many needful ingredients of the plant. 7. The same soil has a different composition in different years, caused by the removal of matters in the crops, or by the in- crease of available food from weathering (till- age). The substances usually classed together as manures may have three distinct functions : 1st. They may chiefly serve to improve the physical characters of the soil. Such are some manures that are applied in large quantity, as lime, marl, and organic matters. 2d. They may act partly as solvents or absorbents, and thus indirectly supply the plant with food; e. g., lime, gypsum, salts of ammonia. 3d. Finally, they may enter the plant as direct nu- trition. If manures acted merely as direct nutrition, it would be possible to judge of the manuring value of any substance by comparing its composition with that of the ashes of culti- vated plants; but since many fertilizers pro- duce all the above-mentioned effects, the ques- tion becomes a more complicated one. Not- withstanding the facts which practice has ac- cumulated concerning the action of a great variety of fertilizing substances, and the close scientific study of their effects, we are yet in the infancy of our knowledge respecting them. In agricultural periodicals are reports of thou- sands of experiments on the value of manures ; we find, however, the most conflicting state- ments, and a chaos of results. There are in- stances of nearly every proposed fertilizer in- creasing crops, and as many instances of failure. Farmers, however, continue to experiment as if there were a possibility of proving that for each kind of crop, or each variety of soil, there is a specific and unfailing fertilizer. The principles above stated, taken together with the fact that the physical adaptation of soils to crops is indefinitely varied and constantly changing, de- monstrate that there can be no fertilizing pa- nacea. They likewise make evident that what is this year a good application for a certain crop and soil, may have no action next year ; and that what is now inefficacious, may prove highly useful at some future time. The most generally useful manures are those which con- tain the largest number of ingredients, and present them in the greatest variety of forms. Stable manure occupies the first rank among fertilizers, because it contains everything that is needful for the nutrition of plants. It is in fact the debris of a previous vegetation, and contains all the ingredients of plants, though in proportions altered from the original ones, and, indeed, advantageously altered. The hay, roots, and grain which mature cattle receive every day as food, are in part digested and assimilated; but since full-grown animals do not increase in weight, unless fattened, they excrete daily as much as they ingest. The most combustible portions of their food are, in consequence of the respiratory process, ex- haled 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 con- stituents 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 ma- nures 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 sup- ply 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 fer- tility. The true system of manuring is to main-