The general process of soil formation constitutes one of the most important phases of katamorphism—the destructive side of the metamorphic cycle, described in Chapter II. Processes of katamorphism or weathering, usually accompanied by the formation of soils, affect the surface rocks over practically all the continental areas.
The weathering of a highly acid igneous rock with much quartz produces a residual soil with much quartz. The weathering of a basic igneous rock without quartz produces a clay soil without quartz, which may be high in iron. Where disintegration has been important the soil contains an abundance of the original silicates of the rock, and less of the altered minerals.
The production of soil from sedimentary rocks involves the same processes as alter igneous rocks; but, starting from rocks of different composition, the result is of course different in some respects. Sandstones by weathering yield only a sandy soil. Limestones lose their calcium carbonate by solution, leaving only clay with fragments of quartz or chert as impurities. A foot of soil may represent the weathering of a hundred feet of limestone. Shales may weather into products more nearly like those of the weathering of igneous rocks. Silicates in the shales are broken down to form clay, which is mixed with the iron oxide and quartz.
In some localities the soil may accumulate to a considerable depth, allowing the processes of weathering to go to an extreme; in others the processes may be interrupted by erosion, which sweeps off the weathered products at intermediate stages of decomposition and may leave a very thin and little decomposed soil.
Soils formed by weathering may remain in place as residual soils, or they may be transported, sorted, and redeposited, either on land or under water. It is estimated by the United States Bureau of Soils[14] that upward of 90 per cent of the soils of the United States which have been thus far mapped owe their occurrence and distribution to transportation by moving water, air, and ice (glaciers), and that less than 10 per cent have remained in place above their parent rock. Glaciers may move the weathered rock products, or they may grind the fresh rocks into a powder called rock flour, and thus form soils having more nearly the chemical composition of the unaltered rocks. Glacial soils are ordinarily rather poorly sorted, while wind and water-borne soils are more likely to show a high degree of sorting.
The character of a transported soil is less closely related to the parent rock than is that of a residual soil, because the processes of sorting and mixture of materials from different sources intervene to develop deposits of a nature quite different from residual soils; but even transported soil may sometimes be traced to a known rock parentage.
Where deposited under water, soil materials may be brought above the water by physiographic changes, and exposed at the surface in condition for immediate use. Or, they may become buried by other sediments and not be exposed again until after they have been pretty well hardened and cemented,—in which case they must again undergo the softening processes of weathering before they become available for use. Where soils become buried under other rocks and become hardened, they are classed as sedimentary rocks and form a part of the geologic record. Many residual and transported soils are to be recognized in the geologic column; in fact a large number of the sedimentary rocks ordinarily dealt with in stratigraphic geology are really transported soils.
The development of soils by weathering should not be regarded as a special process of rock alteration, unrelated to processes producing other mineral products. Exactly the same processes that produce soils may yield important deposits of iron ore, bauxite, and clay, and they cause also secondary enrichment of many metallic mineral deposits. For instance the weathering of a syenite rock containing no quartz, under certain conditions, as in Arkansas, results in great bauxite deposits which are truly soils and are useful as such,—but which happen to be more valuable because of their content of bauxite. The weathering of a basic igneous rock, as in Cuba, may produce important residual iron ore deposits, which are also used as soils. Weathering of ferruginous limestone may produce residual iron and manganese ores in clay soils.
Composition of Soils and Plant Growth
The mineral ingredients in soils which are essential for plant growth include water, potash, lime, magnesia, nitrates, sulphur, and phosphoric acid—all of which are subordinate in amount to the common products of weathering (pp. 20-22, 23-24). Of these constituents magnesia is almost invariably present in sufficient quantity; while potash, nitrates, lime, sulphur, and phosphoric acid, although often sufficiently abundant in virgin soil, when extracted from the soils by plant growth are liable to exhaustion under ordinary methods of cultivation, and may need to be replenished by fertilizers (Chapter VII). Some soils may be so excessively high in silica, iron, or other constituents, that the remaining constituents are in too small amounts for successful plant growth.