Hydrous or “zeolitic” silicates, from which dilute acids can take up the bases potash, soda, lime and magnesia. These silicates may be in either the gelatinous or powdery form; in the former case they may also occlude water-soluble substances.
Carbonates of lime and magnesia, which are readily dissolved by carbonated water as well as by the vegetable acids.
Phosphates of lime and magnesia, not very readily soluble in carbonated water, but more readily attacked by the acids of the soil and of plant roots; thus supplying phosphoric acid to plants. The more finely divided they are the more readily they are dissolved; some soils containing only crystalline needles of apatite ([see chap. 5, p. 63]) only are nevertheless poor in available phosphoric acid.
The natural phosphates of iron and alumina are practically insoluble in all solvents at the disposal of vegetation and though present in considerable amounts in some soils, ([see chapter 19, page 355]), may be considered as being permanently inert, and therefore not to be counted among the soil resources for plant nutrition. As yet no artificial process by which their phosphoric acid can be made available within the soil, has been discovered.
Water-soluble Ingredients.—As regards these it has already been explained that they are largely retained in the condition of purely physical adsorption, as in the case of charcoal or quartz sand, through which sea water filters and is thereby partially deprived of its salts. But these can be gradually withdrawn by washing with pure water alone, and still more easily when stronger solvents are used. Since the soil-water is always more or less charged with carbonic acid, and the roots themselves secrete carbonic as well as stronger acids in their absorption of mineral plant-food, there is no difficulty about explaining the manner in which such physically condensed ingredients are taken up.[108]
Recognition of the Prominent Chemical Character of Soils. In a former chapter the soils formed from the several minerals and rocks have been discussed in a general manner. We can as a rule obtain some insight into the nature of any soil which we can trace to its parent rock or rocks, if we are acquainted with the composition of the latter.
Similarly, but in a much more direct manner, we can obtain a strong presumption as to the nature of any soil by determining the undecomposed minerals present in it. In all ordinary cases the presumption must be that the decomposed portion of the soil has been derived from the minerals still found in it. Of course it may happen in the case of lands derived from widely distinct and distant regions that no such characteristic minerals can be found; this is very commonly true of the soils forming the deltas of large rivers, in which sometimes the only remaining recognizable mineral is quartz in its several forms, with occasional grains of such hardy minerals as tourmaline, garnet, etc. Apart from such cases, the hand lens or the microscope permits us to recognize in most soils the minerals that have mainly contributed to their formation, thus also gaining a clew to their prominent chemical nature.
Such recognition sometimes involves, of course, a somewhat intimate knowledge of mineralogy; yet a little practice will enable almost any one to identify the more important soil-forming minerals, under the lens or microscope, according to the degree of abrasion or decomposition they may have undergone. The details of such researches lie outside of the limits of this treatise, but some general directions on the subject are given farther on.[109]
Acidity, Neutrality, Alkalinity.—A test never to be omitted is that of the reaction of the soil on litmus or other test paper, to ascertain its acid, neutral or alkaline reaction. Should the latter occur quickly (by the prompt blueing of red litmus paper), “black alkali” would be indicated; but a blueing after 20 to 30 minutes means merely that a sufficiency of lime carbonate is present. An acid reaction (the reddening of blue litmus paper) of course indicates a “sour” soil ([see chap. 8, page 122]).
Chemical Analysis of Soils.—When the observations mentioned above give no very decisive results or inferences as to the soil’s chemical character, the more elaborate processes of qualitative and quantitative chemical analysis may be called in. It would seem at first sight that these ought to yield very definite results to guide the cultivator; yet such is by no means always the case. Both the previous history of the land, and the method of analysis, influence materially the practical utility of the results of chemical soil analysis.