In average crops of the usually cultivated plants, those portions which are removed from the field as the valuable part of the crops do not carry off more than 200 to 600 lbs. of ash ingredients per acre yearly, while the soil taken to the depth of one foot weighs three to four millions of pounds per acre. That part of the soil which is soluble in the water of rain represents its available plant food. Large quantities of water pass through the vegetation of every acre of highly cultivated ground. It is only needful, then, that this water should contain a few thousandths of ash ingredients in solution, in order to supply the mineral matters in an average crop, since even root crops, e. g. beets, remove but about 600 lbs. of these substances from the acre. In cultivated soils there is a constant removal of available ash ingredients, both by the harvests that are taken off", and by the rains which soak through or run over them. In a productive soil there is a constant renewal of available plant food, by the mechanical and chemical disintegration of the insoluble portion (the pulverization of the soil by the operations of tillage), by the alternate contractions and expansions of water (frost), and by the affinities of oxygen and carbonic acid.

In a few rare soils the disintegrating and solvent processes are so rapid (act on such finely divided or easily decomposable materials), that they always present a surplus of food to the plant. Such are certain soils of southern Russia (tchernozem or black earth), and of the Scioto valley, Ohio. They yield successive crops for many years without manure. In most cases, however, the removal of a few crops exhausts the store of available plant food. - Soils, when reduced in fertility, may be restored to productiveness by lying in fallow; mechanical and atmospheric agencies thus bring into solution enough of ash ingredients for a new crop. A soil consisting entirely of coarse sand is infertile, because it is too dry, and because there can occur in it no sufficient accumulations of available plant food. A soil consisting of fine sand may be highly productive, especially if it originates from easily decomposable rocks, because the amount of surface that the grains expose, and the close texture of the soil, maintain it in a proper degree of moisture (by capillarity), and allow a sufficient solution and accumulation of food for crops. Clay has a remarkable porosity and retentiveness for water, for ammonia, and for most soluble salts.

If dilute solutions of am- ! monia, potash, soda, magnesia, etc, be agitated for a few moments with clay, or allowed to fil-ter through it, a portion of these bodies is removed from solution, and absorbed by the clay. Putrid urine loses both odor and color by such treatment. The use of salts of alumina as mordants, and for the preparation of lakes, is another example of the same effect. Soils too rich in clay are heavy, and in wet climates intractable from their physical proper-ties; but in dry countries like Egypt, or when mixed with enough sand to render them physically adapted to the growth of plants, they usually possess a great and durable fertility, since they naturally abound in the aliment of vegetation, and are not liable to sutler loss of their soluble matters from the washing effects of rains or floods. . Organic matter (humus), when formed in wet places, constitutes muck and peat, which are not fertile; but as it occurs in arable soils, in quantities usually not exceeding 3 to 10 per cent., it is of great value, not only on account of its power of absorbing water, etc., but also from the fact that in its decay it is a continuous source of carbonic acid and ammonia, thus satisfying to some extent one condition of rapid growth, already insisted upon, viz.: supplies of atmospheric plant food by the soil.

The carbonic acid formed in the soil by the slow oxidation of humus acts also, ! according to the amount of its production, in the chemical disintegration of the insoluble i parts of the soil, and thus indirectly furnishes to the plant increased quantities of ash ingredi- ents. Until Liebig turned his attention to the applications of chemistry to agriculture, it was thought by the most eminent philosophers that humus in some of its forms was the chief nutriment of plants. Liebig denied.its immediate value as plant food, but recognized its use as an indirect supply of carbonic acid and ammonia. The best soils always contain soluble organic matter, and, although it has not been proved that cultivated plants are directly fed upon it, yet there is evidence that some of the lower orders of vegetation do assimilate it, and there is no reason to suppose that it may not be appropriated by agricultural plants, since it is sufficiently soluble to find its way into their circulation. - Analysis of Soils for Economical Purposes. When chemistry first indicated the relation between the composition of the soil and that of the plant, and showed that certain instances of barrenness and fertility in the former could be explained by the results of chemical analysis, the idea that. the farmer might profitably employ analysis in improving his soil took deep root.

A few considerations will suffice to show, however, that as a general rule even the most accurate analysis can be of no practical benefit. Saying nothing of the facts that the productiveness of a soil often depends on its physical or chemical condition irrespective of composition, that it is in most cases impracticable to get a specimen of soil that shall fairly represent a large field or farm, and that the expense of a thorough and faithful analysis is considerable, it is impossible in the present state of science to distinguish from each other two soils, one of which is just fertile and the other just barren; for the processes that have been usually employed in soil analysis are not nice enough to estimate quantitatively differences of l-10th per cent, with invariable accuracy. Now, since an acre of soil, taken to the depth of only 7 or 8 inches, weighs at least 2,000,000 lbs., and since the total amount of matter withdrawn from the soil by the heaviest crops rarely exceeds 500 lbs., 1-4000th of the whole, it is folly to expect that analysis can indicate any difference in the composition of a soil before and after one, two, or even three crops have been removed from it.