SOIL, a term used to designate the super ficial portion of the earth's surface composed of broken and disintegrated rock mixed with vary ing proportions of decayed and decaying animal and vegetable matter (humus). For the entire mantle of unconsolidated material covering the earth's surface and including the soil Merrill proposes the expressive name regolith, from kyoc, a blanket, and a stone. The soil proper is the upper portion of the regolith. In humid regions as a rule it is easy to trace the gradation from soil proper at the surface through subsoil to the underlying undecom posed rock. The subsoil is distinguished from the surface soil mainly by a smaller percentage of organic matter and greater compactness, the latter being due to the accumulation of finer particles carried down by percolating water (clay subsoil) or to the formation of hardpan resulting from the compacting effect of con tinued cultivation at a uniform depth or to the cementing action of salts formed in the soil (calcerous and alkali hardpan). Hilgard has shown that on account of the absence of the leaching action of water, the soils of arid re gions are often uniform to a great depth, there being little or no distinction between soil and subsoil, although under irrigation and cultiva tion calcerous and alkali hardpan frequently makes its appearance.
Origin, Formation, and Soils.have fit the main been derived from the solid rock of the earth's crust through the dis integrating (weathering) and transporting ac tion of various agencies, among which are changes of temperature (heat and frost), mov ing water or ice (glacial action), chemical action of air and water and the influence of animal and vegetable life (including the action of micro-organisms) • and since some of these agencies are continually at work the properties of soils are constantly being more or less modi fied. Soils are grouped according to the method of their formation into two main classes: (1) Sedent soil formed by the weathering of roc c El sit residual deposits) or by the ex tensive accumulation of organic matter as in case of marsh or peat soils (comulose de posits) •, and (2) kg_nsported soils composed of materials transported from other localities than that in which the soil is found .by•water, ice (glaciers) or wind. Under residK1 soils Mer rill includes such as are composed of °those products of rock degeneration which are to-day found occupying the sites of the rock masses from which they were derived, and immedi ately overlying such portions as have as yet escaped destruction.° Such soils occur most ex
tensively in the United States east of the Mis sissippi River and south of the southern margin of the ice sheets of the Glacial Epoch and are typically exemplified in the usually highly col ored brown, red and yellow ferruginous clay soils ot jJs southern Appalachian region. The cumulo4i deposits are typically illustrated in the United States in the Dismal Swamp lands or the muck soil of Florida.
The more important examples of transported soils are alluvial soils, familiarly typified in the river bottom lands and deltas like those of the Nile and Mississippi; crolian soils composed of materials transported by the wind and typified by the sand dunes of seacoast regions and the characteristic loess of China and other coun tries; and glacial drift soils due to glacial action. Such drift soils cover a large portion of the northeastern and north central United States and are composed of the debris of disintegrated rocks of various kinds brought down from the north during the glacial period. Besides these main types of soils there are several others of more or less importance in the United States, including the soils composed of fine volcanic ash found in considerable areas in Kansas, Ne braska, Colorado, Montana and other Western States; adobe, a stiff clayey soil distributed in circumscribed areas over a large portion of the arid region of the United States; gumbo soil, a compact fine silty soil; and a so-called loess supposed to be of alluvial origin. In practice soils are, as a rule, described simply as gravelly, sandy, loamy, clayey, calcareous, humous or peaty, etc., according to the fineness of the soil particles and the proportions of sand, clay, lime and humus. They are also distinguished as light or heavy, but as so used these terms do not refer to the actual weight of the soil, but rather to the ease with which it is cultivated. Thus sandy soils, which are termed °light° in an agricultural sense, are actually heavier than clay soils, which are considered "heavy" from an agricultural standpoint.