soil, surface layer of the earth, composed of fine rock material disintegrated by geological processes; and humus, the organic remains of decomposed vegetation. In agriculture, 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, silt, sand, gravel, 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; rotation of crops). Minimum-tillage systems, often entailing herbicide 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, either organic, such as manure, 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.

prairie soil: see podzol.

calcareous soil, soil formed largely by the weathering of calcareous rocks and fossil shell beds. Different varieties usually contain chalk, marl, and limestone and frequently a large amount of phosphates. They are often very fertile, as in the case of the buckshot soils of the S United States. Sometimes calcareous soils are flinty, thin, and dry. They often form a large part of the soil of deserts, which may prove very fertile when sufficient moisture for crops is applied.

Free-Soil party, in U.S. history, political party that came into existence in 1847-48 chiefly because of rising opposition to the extension of slavery into any of the territories newly acquired from Mexico. The struggle in Congress over the Wilmot Proviso helped to consolidate the Free-Soil forces, which comprised those New York Democrats known as Barnburners, the antislavery Whigs, and members of the former Liberty party. These forces met in mass convention at Buffalo in Aug., 1848, where the party was formally organized and Martin Van Buren and Charles F. Adams (1807-86) were chosen as its candidates for president and vice president. The platform also declared for a homestead law, internal improvements, and a tariff for revenue only. The party polled nearly 300,000 votes and, by giving New York state to the Whigs, was a decisive factor in making Zachary Taylor president. The party elected one senator, Salmon P. Chase of Ohio, and 13 congressmen. The Compromise of 1850 supposedly settled the slavery issue, and the Barnburner element went back to its old allegiance. A few radical antislavery men kept the organization in existence and nominated John P. Hale for president in 1852; he received more than 150,000 votes. In 1854 the party was absorbed into the new Republican party.

A branch of soil science focusing on the formation, morphology, and classification of soils as bodies within the natural landscape. Pedology seeks to understand how the properties and distribution patterns of soils worldwide (collectively called the pedosphere) have developed along with broader landforms, biogeochemical environments, and habitats of living organisms. Through their understanding of the causes and magnitudes of soil variations, pedologists help to broaden knowledge of the surface of the Earth.

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Study of soils and their utilization, especially in planning foundations for structures and highways. How the soil of a given site will support the weight of structures or respond to movement in the course of construction depends on a number of properties (e.g., compressibility, elasticity, and permeability). Examination techniques include trench-digging, boring, and pumping samples to the surface with water. Seismic testing and measurement of electrical resistance also yield helpful information. In road construction, soil mechanics helps determine which type of pavement (rigid or flexible) will last longer. The study of soil characteristics is also used to choose the most suitable method for excavating underground tunnels. Seealso foundation, settling.

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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.

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