_Drain Waters._—The table on p. 22, chapter 2, illustrates forcibly the working of the above principles, which are verified by the composition of drain-waters. In all, the chief nutritive ingredients of plants, except nitrogen, are present in traces only; chlorids, nitrates and sulfates of sodium and magnesium form the bulk of the permanently soluble matter, with usually a considerable proportion of calcic (and magnesic) carbonate, depending upon the amount of the earth-carbonates present in the soil, as well as upon that of oxidizable organic matter from which carbonic acid can be formed. That calcic carbonate filters readily through the soil has already been somewhat elaborately discussed (see chap. 3, p. 41); one of the results being that the surface soil is sometimes almost completely depleted of this important substance, while it accumulates at a greater or less depth in the subsoil, or in underdrains, as the case may be. Of the ingredients appearing in the above list, the one of greatest agricultural importance is nitric acid, since chlorine and sulfuric acid, as well as soda, are required only in very small quantities by most culture plants; so that they rarely need to be supplied in fertilizers. Nitric acid, however, is not only one of the most important fertilizers, but also the most expensive; hence the passing-off of nitrates in drainage-water is of such serious concern to the farmer, that the causes of its occurrence, and the means of preventing such loss, should be fully understood. This subject will, however, be more fully considered farther on. _The above Distinctions not Absolute._—It should, however, be also understood that while the above statements hold good in a general way, yet the line drawn is by no means an absolute one. For just as in the case of physical adsorption the long passing-through of distilled water will gradually abstract the substances condensed on the surface of the soil-grains, so an overwhelming amount of a solution of any one kind will have a tendency to substitute its own ingredients for those already present in the soil, removing the latter to a greater or less extent, even in the case of potash and phosphoric acid.