[108] Whitney (Bull. 22, U. S. Bureau of Soils) claims on the basis of a large number of (three-minute) extractions of soils made with distilled water, that these solutions are essentially of the same composition in all soils; that all soils contain enough plant-food to produce crops indefinitely; and that the differences in production are due wholly to differences in the moisture supply, which he claims is, aside from climate, the only governing factor in plant growth. The tables of analytical results given in Bull. 22 fail to sustain the first contention; the second is pointedly contradicted both by practical experience, and by thousands of cumulative culture experiments made by scientific observers; the third fails with the second, except of course in so far as an adequate supply of moisture is known to be an absolute condition both of plant growth, and the utilization of plant-food. It is moreover well known that it is not water alone, but water impregnated more or less with humic and carbonic acids, that is the active solvent surrounding the plant root. _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] [109] See Appendix B.