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For the heavy metal band see Soil (band) Soil is the layer of minerals and organic matter, in thickness from centimetres to a metre or more, on the land surface. Its main components are rock and mineral matter, organic matter, water, and air. Soils differ in the ratio of these components. Air, trapped in spaces between the various particles, and water, trapped in spaces and on the surface of particles, can comprise up to half of the soil by volume. The rock and mineral content is categorized according to particle size as sand (coarsest), silt or clay (finest); the ratio of these particles to a great degree determines the soil classification and characteristics.

More precise definitions of soil vary by scientific discipline, theoretical model, and classification system. Generally speaking, though, soil is the uppermost layer of the Earth that can support plant life. It can be subdivided into two general layers or strata; topsoil (the upper few centimetres, where most roots and microorganisms are located) and subsoil which is more dense and less rich in organic matter.


Modern soil concepts

Soil is "the collection of natural bodies in the earth's [sic] surface, in places modified or even made by man of earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors. Its upper limit is air or shallow water. At its margins it grades to deep water or to barren areas of rock or ice. Its lower limit to the not-soil beneath is perhaps the most difficult to define. Soil includes the horizons near the surface that differ from the underlying rock material as a result of interactions, through time, of climate, living organisms, parent materials, and relief. In the few places where it contains thin cemented horizons that are impermeable to roots, soil is as deep as the deepest horizon. More commonly soil grades at its lower margin to hard rock or to earthy materials virtually devoid of roots, animals, or marks of other biologic activity. The lower limit of soil, therefore, is normally the lower limit of biologic activity, which generally coincides with the common rooting depth of native perennial plants" (Soil Survey Staff, 1975).

The "natural bodies" of this definition include all genetically related parts of the soil. A given part, such as a cemented layer, may not contain living matter or be capable of supporting plants. It is, however, still a part of the soil if it is genetically related to the other parts and if the body as a unit contains living matter and is capable of supporting plants.

The definition includes as soil all natural bodies that contain living matter and are capable of supporting plants even though they do not have genetically differentiated parts. A fresh deposit of alluvium or earthy constructed fill is soil if it can support plants. To be soil, a natural body must contain living matter. This excludes former soils now buried below the effects of organisms. This is not to say that buried soils may not be characterized by reference to taxonomic classes. It merely means that they are not now members of the collection of natural bodies called soil; they are buried paleosols.

Not everything "capable of supporting plants out-of-doors" is soil. Bodies of water that support floating plants, such as algae, are not soil, but the sediment below shallow water is soil if it can support bottom-rooting plants such as cattails or reeds. The above-ground parts of plants are also not soil, although they may support parasitic plants. Rock that mainly supports lichens on the surface or plants only in widely spaced cracks is also excluded.

The time transition from not-soil to soil can be illustrated by recent lava flows in warm regions under heavy and very frequent rainfall. Plants become established very quickly in such climates on the basaltic lava, even through there is very little earthy material. The plants are supported by the porous rock filled with water containing plant nutrients. Organic matter soon accumulates; but, before it does, the dominantly porous broken lava in which plant roots grow is soil. (Soil Survey Staff, 1993)

Soil components and processes

Minerals in soil are obtained from a variety of sources, but the process which delivers the bulk is weathering of rocks. Weathering is the actions of wind, rain, ice, sunlight, and biological processes on rocks that break them down into smaller particles.

Weathering also releases ions such as K+ and Mg2+ into the soil solution. Some of these ions are taken up by plants, but the majority not left in solution are absorbed through ion exchange by clays such as montmorillonite. When the level of ions is low in the soil an equilibrium process forces ions back into solution, where they can be used by plants.

However if acid is introduced into soil, hydrogen ions bind in preference to clays, forcing ions out where they can be washed away during rain. Acidity also encourages the weathering of clays, releasing toxic aluminium ions (of which clays are composed) into the solution. To stop this occurring, farmers may apply alkaline materials such as slaked lime.

Although there exist plenty of elements such as nitrogen, potassium and phosphorus necessary for plant growth in soil, very little of this is in a form which plants can use. In processes such as nitrification and mineralisation , bacteria and other organisms convert unusable forms (such as NH4+) in to usable forms (such as NO3-). The raw products are initially present as gases in the atmosphere. Processes such as the nitrogen cycle and carbon cycle continually exchange nutrients between the soil and atmosphere.

The organic store in soil is made up of plant debris, animal excreta and other decomposing materials. A lot of the carbon compounds react to form humus, which is composed of very large molecules including esters of carboxylic acid, phenolic compounds and derivatives of benzene. Organic material in soil provides nutrients necessary for plant growth.

If oxygen enters the soil, e.g. due to lowering the ground water table, organic matter in the soil will be oxidised and this may lead to subsidence, also due to increased ground pressure.


  • Soil Survey Staff. (1975) Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agric. Handb. 436. U.S. Gov. Print. Office. Washington, DC.
  • Soil Survey Division Staff. (1993) Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
  • Logan, W. B., Dirt: The ecstatic skin of the earth. 1995 ISBN 1-57322-004-3
  • Faulkner, William. Plowman's Folly. New York, Grosset & Dunlap. 1943. ISBN 0-93328-051-3
  • Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941 ASIN B0006APBY4

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