soil, surface layer of the earth, composed of fine rock material disintegrated by geological processes; and humus humus (hy
..... Click the link for more information. , the organic remains of decomposed vegetation. In agriculture agriculture, science and practice of producing crops and livestock from the natural resources of the earth. The primary aim of agriculture is to cause the land to produce more abundantly and at the same time to protect it from deterioration and misuse.
..... Click the link for more information. , soil is the medium that supports crop plants, both physically and biologically. Soil may be from a few inches to several feet thick.

Components and Structure

The inorganic fraction of soil may include various sizes and shapes of rocks and minerals; in order of increasing size these are termed clay clay, common name for a number of fine-grained, earthy materials that become plastic when wet. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium, calcium, magnesium, or iron, in small amounts.
..... Click the link for more information. , silt silt, predominantly quartz mineral particles that are between sand size and clay size, i.e., between 1-16 and 1-256 mm ( 1-406 – 1-6502 in.) in diameter. Silt, like clay and sand, is a product of the weathering and decomposition of preexisting rock.
..... Click the link for more information. , sand sand, rock material occurring in the form of loose, rounded or angular grains, varying in size from .06 mm to 2 mm in diameter, the particles being smaller than those of gravel and larger than those of silt or clay .
..... Click the link for more information. , gravel gravel, particles of rock, i.e., stones and pebbles, usually round in form and intermediate in size between sand grains and boulders. Gravel is composed of various kinds of rock, the most common constituent being the mineral quartz.
..... Click the link for more information. , and stone. Coarser soils have lower capacity to retain organic plant nutrients, gases, and water, which are essential for plants. Soils with higher clay content, which tend to retain these substances, are therefore usually better suited for agriculture. In most soils, clay and organic particles aggregate into plates, blocks, prisms, or granules. The arrangement of particles, known as soil structure, largely determines the soil's pore space and density, which translates into its capacity to hold air and water. Organic matter consists of decomposed plant and animal material and living plant roots. Microorganisms, living in the organic portion of soil, perform the essential function of decomposing plant and animal matter, releasing nutrients to be used by growing plants.

Besides organic matter, soil is largely composed of elements and compounds of silicon, aluminum, iron, oxygen, and, in smaller quantities, calcium, magnesium, sodium, and potassium. Factors determining the nature of soil are vegetation type, climate, and parent rock material; geographic relief and the geological age of the developing soil are also factors. Acidic soils occur in humid regions because alkaline minerals are leached downward: alkaline soils occur in dry regions because alkaline salts remain concentrated near the surface. Geologically young soils resemble their parent material more than older soils, which have been altered over time by climate and vegetation. For advice and information on soils, consult state agricultural experiment stations and their publications.

Undisturbed soils tend to form layers, called horizons, roughly parallel to the surface. The Russian system of soil classification, from which most others derive, is based on the distinctive horizons of the soil profile. The A horizon, the surface layer, contains most of the humus. The B horizon contains inorganic compounds formed by decomposition of organic material, a process known as mineralization; the material is brought to the B layer by the downward leaching action of water. The lowest soil layer, the C horizon, represents the weathered mineral parent substance.

Soil Fertility and Conservation

Soil fertility—the ability to support plant growth—depends on various factors, including the soil's structure or texture; its chemical composition, esp. its content of plant nutrients; its supply of water; and its temperature. Agriculture necessarily lowers soil fertility by removing soil nutrients incorporated in the harvested crops. Cultivation, especially with heavy machinery, can degrade soil structure. Agricultural soils are also vulnerable to mismanagement. Exposure of soils to wind and rain during cultivation encourages erosion of the fertile surface. Excessive cropping or grazing can depress soil-nutrient levels and degrade soil structure.

Soil conservation techniques have been developed to address the range of soil management issues. Various methods of cultivation conserve soil fertility (see cover crop cover crop, green temporary crop grown to prevent or reduce erosion and to improve the soil by building up its organic content. Green-manure crops are usually classed as cover crops.
..... Click the link for more information. ; rotation of crops rotation of crops, agricultural practice of varying the crops on a piece of land in a planned series, to save or increase the mineral or organic content of the soil, to increase crop yields, and to eradicate weeds, insects, and plant diseases.
..... Click the link for more information. ). Minimum-tillage systems, often entailing herbicide herbicide (hr`bəsīd'), chemical compound that kills plants or inhibits their normal growth.
..... Click the link for more information. use, avoid erosion and maintain soil structure. Soil fertility and agricultural productivity can also be improved, restored, and maintained by the correct use of fertilizer fertilizer, organic or inorganic material containing one or more of the nutrients—mainly nitrogen, phosphorus, and potassium, and other essential elements required for plant growth.
..... Click the link for more information. , either organic, such as manure manure, term used in the United States to refer to excreta of animals, with or without added bedding; also called barnyard manure. In other countries the term often refers to any material used to fertilize the soil.
..... Click the link for more information. , or inorganic, and other soil amendments. Organic matter can be added to improve soil structure. Soil acidity can be decreased by addition of calcium carbonate or increased by addition of sulfuric acid.

Bibliography

See F. R. Steiner, Soil Conservation in the United States (1990); M. Alexander, Introduction to Soil Microbiology (2d ed. 1991); E. J. Plaster, Soil Science and Management (2d ed. 1991); publications of the U.S. Soil Conservation Service.

soil

The biologically active, porous medium that has developed in the uppermost layer of the Earth's crust. Soil serves as a natural reservoir of water and nutrients, as a medium for the filtration and breakdown of injurious wastes, and as a participant in the cycling of carbon and other elements through the global ecosystem. It has evolved through the weathering of solid materials such as consolidated rocks, sediments, glacial tills, volcanic ash, and organic matter. The bulk of soil consists of mineral particles composed of silicate ions combined with various metal ions. Organic soil content consists of undecomposed or partially decomposed biomass as well as humus, an array of organic compounds derived from broken down biomass.