Soil

the loose surface of the earth in which vegetation takes root. Soil is derived chiefly from rocks, but in part from the water and air. The hardest rock wears off when exposed to the weather. Changes from hot weather to cold and back again crack the surface of rocks. Water enters the slightest crack, and, expanding with frost, pries off fragments, just as it bursts a waterpipe or bulges the bottom of a pail. Lichens send tiny roots into crevices and enlarge them. Flying dust strikes against the face of a rock and chisels off more dust. Sandy water scours out furrows, each sharp grain of sand in suspension acting like a chisel to scrape off tiny stone chips. If water or air or the acids of plant roots are able to dissolve any portion of a rock, the rest is all the more likely to crumble into soil. Soil is chiefly crumbled rock. Some kinds of rocks wear and crumble into sand; others are worn and ground into clay; a third sort of soil is vegetable soil or humus. Humus is formed by the decay of plants. Some soil is almost pure sand. Some is pure clay; and some, like peat and the muck of swamps, is almost purely vegetable. A mixture of sand, clay, and vegetable soil is called a loam. If sand predominates, it is a sandy loam; if clay, a clay loam; if humus, a vegetable or frequently a black loam. Clays and sands are of any color-red, gray, yellow, vermilion, or blue,-according to the color of the rocks from which they come. Soil may be found near the rock from which it is derived, as soil at the foot of a cliff lying next the surface of a stratum of disintegrating limestone; or it may be carried thousands of miles away by streams, dust storms, and ice. The soil at the mouth of a large river has come almost invariably from rocks in the interior of the continent. Mountain chains are worn down, not infrequently a mile or two, and the soil thus made is carried down to build up the lowlands. Glaciers make and transplant enormous quantities of soil. See Sand; Loess; Lava