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Soil is the layer of minerals and organic matter, in thickness from centimeters to a meter or more, on the land surface. Its main components are mineral matter, organic matter, moisture, and air.
Wikipedia: Soil is material capable of supporting plant life. Soil forms through a variety of soil formation processes, and includes weathered rock "parent material" combined with dead and living organic matter and air.
Soil components
Soils vary widely in composition and structure from place to place. Soils are formed through the weathering of rock and the breakdown of organic matter. Weathering is the action of wind, rain, ice, sunlight and biological processes on rocks, which breaks them down into small particles. The proportions of minerals and organic matter determine the structure and other characteristics of a particular soil.
Soils can be divided into two general layers (strata): topsoil, the topmost layer, where most plant roots, microorganisms, and other animal life are located, and subsoil, which is deeper and often more dense and contains less organic matter.
Water and air are also components of soils. Mineral and organic solids comprise about half of the soil by volume. Water occupies the spaces between soil particles and is held by surface tension on particle surfaces. Air occupies the remaining void space. Both water and air components of soils are important to plant growth and other life in the soil profile of a particular ecosystem.
The rock and mineral content of soils is categorized according to particle size, from sand (coarsest), to silt and clay (finest). The ratio of these particles to a great degree determines the soil classification and characteristics.
Soils serve as habitats for soil organisms varying in size from microorganisms to small animals. The character of soils is intricately tied to bioturbation and the biochemical functions performed by soil organisms.
Former soils which become buried below the effects of organisms are called paleosols.
Soils develops naturally over time through the action of plants, animals, and weathering. Soils are also affected by human habitation. People can alter soils to make them more suitable for plant growth through the addition of organic materials and natural or synthetic fertilizer, and by improving their drainage or water-retaining capacity. Human actions also can degrade soils through the depletion of nutrients, pollution, contamination, and compaction, and by increasing the rate of erosion, which is the relocation of soil through the movement of water or wind.
Source: wikipedia
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