Page:The American Cyclopædia (1879) Volume I.djvu/220

 196 AGRICULTURAL CHEMISTRY most soluble salts. If dilute solutions of am- monia, potash, soda, magnesia, &c., be agitated for a few moments with clay, or allowed to ni- 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 alu- mina 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 physi- cally 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 suffer 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 oc- curs 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, &c., but also from the fact that in its decay it is a continuous source of carbonic acid 1 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 j soil by the slow oxidation of humus acts also, according to the amount of its production, in the chemical disintegration of the insoluble 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 nu- triment 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 de- pends on its physical or chemical condition irrespective or 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 faith- ful analysis is considerable, it is impossible in the present state of science to distinguish from each other two soils, one of which is just fer- tile 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 Ibs., and since the total amount of matter withdrawn from the soil by the heaviest crops rarely exceeds 500 Ibs., l-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. Again, there are numerous instances of soils naturally sterile, which, after application of 400 Ibs. 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 -^s.^Y 15 ' ^ e on ' y me thod of chemical ex- amination that promises to be useful is the fol- lowing: A large quantity of soil, say 10 or even 100 Ibs., 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. Pe- ter, chemist to the Kentucky geological sur- vey, has recorded in his report some results obtained in this way, except that instead of exhausting 10 Ibs. 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 sim- pler 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 indi- cate with certainty the value of a new soil in this respect. In nearly all cases of unproduc- tive 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 sun- shine and parching winds. Some soils, how- ever, 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 man-