Soil, superficial covering of most of the earths land area; an aggregation of unconsolidated mineral and organic particles produced by the combined action of wind, water, and organic decay. Soils vary widely from place to place. The chemical composition and physical structure of the soil at any given location are determined by the kind of geologic material from which it originates, by the vegetation cover, by the length of time that the soil has been weathered, by the topography, and by artificial changes caused by human activities. In nature, changes in soil are gradual, except those resulting from natural disasters. The cultivation of land, however, deprives soil of its natural vegetational covering and of much of its protection against erosion by water and wind, permitting more rapid changes. Agriculturists have had to develop methods of preventing harmful alteration of soil resulting from cultivation and of rebuilding soil already detrimentally altered.

Although knowledge of the gross texture of soil is important to engineers who construct buildings, roads, and other structures on the earths surface, all the specific properties of soil are of great concern to agriculturists. For agriculturists, knowledge of the mineral and organic components of soils, aeration and water-holding capacity, as well as many other aspects of soil structure, is necessary for the production of successful crops. The soil requirements of different plants vary widely, and no generalizations can be made concerning an ideal soil for the growth of all plants. Many plants, such as sugarcane, require moist soils that would be deemed poorly drained by wheatgrowers. Characteristics suitable for successful growth of crops are not necessarily inherent in the soil itself; some of these characteristics are created by successful soil management.

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The primary components of soil are (1) undissolved inorganic or nonliving components produced by the weathering and breakdown of surface rocks; (2) soluble nutrients used by plants; (3) various forms of organic matter, both living and dead; and (4) gases and water required by plants and subterranean organisms.

The physical nature of the soil is determined by the proportions of particles of various sizes. Inorganic particles in soil range in size from fairly large pieces of stone and gravel to extremely small particles less than 1/40,000 cm (less than 1/100,000 in) in breadth. Large soil particles, such as sand and gravel, are mostly inactive chemically, but small inorganic particles, the chief components of fine clays, serve also as a reservoir from which nutrients are drawn by plant roots. The size and nature of these tiny inorganic particles also largely determine the ability of a given soil to store water, which is vital to all plant growth processes.

The organic fraction of soil is composed of undecayed plant and animal debris, together with variable amounts of an amorphous organic material called humus. The organic fraction makes up 2 to 5 percent of the surface soil for many soils in humid regions, but may be less than 0.5 percent in arid soils or more than 95 percent in peat soils.

The liquid component of soils, called by scientists the soil solution, is largely water containing a number of mineral substances in solution, as well as comparatively large amounts of dissolved oxygen and carbon dioxide. The soil solution is highly complex and is not well understood scientifically; it is primarily important as the medium through which nutrient materials reach and are absorbed by the roots of plants. When the soil solution is defective in one or more of the nutrient elements needed for plant growth, the soil is infertile.

The gases contained in soil are chiefly oxygen, nitrogen, and carbon dioxide. The first of these gases is important for plant metabolism because its presence is necessary for the growth of the various soil bacteria and other organisms causing the decomposition of organic materials. The presence of oxygen is also vital for plant growth, in that its absorption by the roots of plants is necessary to their metabolic processes.

Soils exhibit marked variation in appearance, fertility, and chemical characteristics depending on the mineral and plant materials from which they have been formed. Color is one of the simplest criteria for judging varieties of soil. The general but not invariable rule is that dark soils are more fertile than lighter-colored soils. Darkness in soil color usually results from the presence of a large amount of humus in the soil. Sometimes, however, soils that are dark brown or black owe their hue to mineral materials or to excessive dampness; in such cases, dark color is not an index of fertility. Soils having a red or reddish-brown hue usually contain a large proportion of iron oxides (derived from the parent rocks) that have not been subjected to excessive humidity. Red color in a soil, therefore, is generally an indication that the soil is well drained, not excessively humid, and fertile. This generalization is particularly true in the southeastern U.S. but is not always true in other parts of the wor