Again, there are numerous instances of soils naturally sterile, which, after application of 400 lbs. of guano, manifested a wonderful productiveness. Now, the largest of the active ingredients of guano never amounts to 20 per cent.; so that to trace its action, or distinguish between two soils, one barren and the other made fertile by guano, the chemist must be able to estimate 100 parts in 2,000,000, or a fraction so small as 1/20,0000. The only method of chemical examination that promises to be useful is the following: A large quantity of soil, say 10 or even 100 lbs., is digested and exhausted with water saturated at ordinary temperatures with carbonic acid. In this way we dissolve all its "presently available plant food." The analysis of this dissolved portion might be expected to give insight into the value of the soil so far as dependent on chemical composition. Dr. Peter, chemist to the Kentucky geological survey, has recorded in his report some results obtained in this way, except that instead of exhausting 10 lbs. of soil, he used but 1,000 grains. The amount of dissolved matters in his trials in no case exceeded 7 grains, while it usually fell below 2 grains, quantities too small for accurate analysis.

For practical purposes there are, however, other and in general simpler means of ascertaining the ability of a soil to supply food for remunerative crops. Thus the character and amount of vegetation which it naturally produces generally suffice to indicate with certainty the value of a new soil in this respect. In nearly all cases of unproductive soils, the difficulty is less of a chemical than of a physical nature. The great deserts are sterile, not because they cannot yield the soil food required by vegetation, but because they are destitute of water. Wherever a spring arises in them, there is formed a spot of verdure, notwithstanding the incessant sunshine and parching winds. Some soils, however, with every external sign of fertility, are nevertheless barren, because deficient in some one or several of the indispensable constituents of the ash of plants. To ascertain and remedy these deficiencies, it is best to proceed in a synthetical rather than in the analytical mannor, 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-in--. 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 success of 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 rooting place, these operations more or less facilitate 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 mineral 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 production. 3. Different plants require different proportions of these substances in order to luxuriant growth. 4. Different plants require different quantities of these substances to mature a full crop. 5. Different plants, from peculiarities 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 increase of available food from weathering (tillage). 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 nutrition. 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 cultivated plants; but since many fertilizers produce all the above-mentioned effects, the question becomes a more complicated one. Not-withstanding the facts which practice has accumulated 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.

* Bloorl-producing; so called from the function of these bodies in animal nutrition.

In agricultural periodicals are reports of thousands of experiments on the value of manures; we find, however, the most conflicting statements, and a chaos of results. There are in ; stances of nearly every proposed fertilizer increasing 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, demonstrate that there can be no fertilizing panacea. 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 contain 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.