_Absorptive Power Varies._—So far as the mere physical absorption is concerned, it will readily be understood that a coarse sandy soil exercises less retentive influence upon dissolved substances than clay or humous soils. In the humid region, where sand is substantially nothing but granular silica (see above, chap. 6, page 86), the same may be measurably true as regards the chemical absorption also. In the arid region, on the contrary, a great many sandy or silt soils, very poor in clay, exert fully as much chemical absorption as clay soils, and are no more liable to the washing-out of soluble fertilizers introduced than are the latter. For the chemical absorption lies chiefly in the zeolitic portion of the soil (see above chap. 3, p. 37), which in the humid region accumulates in the clay, while in the arid it remains encrusting the sand and silt grains. _Generalities regarding Chemical Absorption and Exchange._—In regard to the leaching-out and absorption or retention of substances important to agriculture, the following general statement may be made: The substances most likely to be leached out of soils are, of bases: soda, magnesia and lime; of acidic constituents: chlorine, sulfuric acid and nitric acid. Lime sometimes passes off with either of the above acidic ingredients, and also in the form of carbonate. Substances rather tenaciously retained in soils are: potash and ammonia among the bases, and phosphoric acid among the acids. Thus (as stated above) when a weak (one or two per cent) solution of potassic chlorid or sulfate is poured upon a column of good soil several inches thick, it will be found that the first portions passing through are free from potash, but contain the chlorids or sulfates of magnesium and calcium. If potassic nitrate be used, lime and magnesia will pass off as nitrates; while in the case of potassic phosphate, both ingredients will be retained. A solution of gypsum (calcic sulfate) will usually cause the passing-off of some of the magnesia, soda and potash contained in the soil, in the form of sulfates; but the amount of potash thus dissolved soon diminishes to a mere trace. Solutions of potassic or amnionic phosphates will be absorbed and retained by the soil to a very considerable extent, before the soil becomes saturated. While it is true that the degree to which the soil retains the several ingredients may serve in a very general way to indicate their richness or poverty in the same, the attempt to make such experiments serve to determine the agricultural needs of soils has met with but little practical acceptance.