SOIL (OF., Fr. son, from Lat. so/um, ground, soil, foundation, sole). A term applied to the superficial unconsolidated portion of the earth's crust (regolith), which is composed of broken and disintegrated (weathered) rock mixed with varying proportions of decayed and decaying organic matter (humus). The processes by which soils are formed from the parent rocks are mechanical and chemical, and in some cases biolo gical. The fertility of a soil will, therefore, he determined to a considerable extent by the char acter of the parent rock and the stage of its. decomposition. Thus granite, being richer in the elements of plant food, yields a more fertile soil as a rule than the siliceous sandstones.
According to the method of their formation.
soils are classed as sedentiary or transported. When a soil is found resting on the parent rock from whose decay it has originated it is spoken of as sedentary soil. It may show a gradual transition from the fully formed soil at the sur face to the solid rock beneath. With this class may be grouped the humus or peaty soils due to accumulations of organic matters in bogs, swamps, and marshes. In many cases the residu al products have been removed from the place of their formation by the action of water, ice (glaciers), and wind and deposited elsewhere in the form of clayey, sandy, or loamy soils, often representing the mingling of material from sev eral different sources. This type is termed transported soil, and, though naturally very vari able in, character, includes some of the most productive soil in the world. The most important soils of this class are the alluvial soils, which often form a broad flo6d-plain (q.v.) bordering a river or a delta (q.v.) at the mouth, as in the ease of the Nile and the Mississippi rivers. In the northern half of the United States much of the soil is of the glacial drift type and repre sents the dOris of decayed rocks of various kinds brought down from the north during the glacial period (q.v.).
.Eo/ian soils are formed by wind action. They include: (1) Sand dunes, those shifting, sandy soils heaped up by wind action upon many ocean coasts and the shores of inland seas. (See DUNE; DUNE VEGETATION.) (2) Ash soils, the accumulations of ashes ejected by volcanoes. The deposits are often of considerable extent and are frequently very fertile. Much of the highly
productive region around Mount Vesuvius, in Italy, is of this kind. Such soils are found in Nebraska, Colorado, and Montana. Soils derived from disintegration of volcanic lava are of fre Tient occurrence, as, for instance, in the Hawaiian Islands, in Idaho and other Northwestern States. The loess soils of China and other countries are of Icolian origin, although the so-called loess soils of America are believed to be for the most part of alluvial origin. Soils containing an ex cess of soluble salts are found scattered through out regions of deficient or irregular rainfall and are known as alkali soils (q.v.), Ilumous, peaty, or moor soils are composed largely of organic matter. The purest types are represented by the accumulations of peat (q.v.) formed in ponds aml swamps; marine marshes, and muck soils represent a less pure variety. When properly drained and aerated and, in the case of marine marshes, freed from excess of solu ble salts, they often prove very productive.
In practice soils are classified as gravelly, sandy, loamy, calcareous. humus, or peaty, etc., distinctions based on the fineness of the soil par ticles and the relative proportions of sand, clay, lime, and humus, which they contain. Soils are also frequently classed as light and heavy, ac cording as they are easy or difficult to till. In this sense a sandy soil is termed 'light' (easy to till), although actually having greater weight than a clayey soil, which is termed 'heavy' (dif ficult to till). The productiveness of a soil de pends chiefly upon its chemical composition and its physical properties. Chemical and physical or mechanical analysis separates soil constituents into two general classes: (I) food constituents, and (2) physical constituents. The food constit vents necessary to plant growth are nitro gen, silicon, sulphur, phosphorus, chlorine, alu minum, calcium, magnesium, potassium, sodium, and iron in various forms of chemical combina tion. The mechanical constituents include clay, silt, sand, humus, etc., which act as a physical support to plants and have an indirect fertilizing value. They form as a rule the large proportion of the soil mass, usually 90.95 per cent.