Soil is a natural body consisting of layers (soil horizons) that are primarily composed of minerals which differ from their parent materials in their texture, structure, consistency, colour, chemical, biological and other characteristics. It is the unconsolidated or loose covering of fine rock particles that covers the surface of the earth. Soil is the end product of the influence of the climate (temperature, precipitation), relief (slope), organisms (flora and fauna), parent materials (original minerals), and time. In engineering, soil is referred to as regolith, or loose rock material: this is the 'drift deposit' lying on top of the 'solid geology'. However, in horticulture, the term 'soil' is defined as the humic layer of topsoil, or the depth of regolith containing organic material that influences and has been influenced by plant roots and may range in depth from centimetres to many metres. Expressions such as lunar soil or Martian soil are commonly used for extraterrestrial regolith, even though there is no known biological component.
Soil is composed of particles of broken rock (parent materials) which have been altered by chemical and mechanical processes that include weathering (disintegration) with associated erosion (movement). Soil is altered from its parent material by the interactions between the lithosphere, hydrosphere, atmosphere, and biosphere. It is a mixture of mineral and organic materials in the form of solids, gases and liquids. Soil is commonly referred to as "earth" or "dirt"; technically, the term "dirt" should be restricted to displaced soil.
Soil forms a structure filled with pore spaces and can be thought of as a mixture of solids, water, and air (gas). Accordingly, soils are often treated as a three-state system. Most soils have a density between 1 and 2 g/cm³. Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic, although fossilised soils are preserved from as far back as the Archean.
On a volume basis a good quality soil is one that is 45% minerals (sand, silt, clay), 25% water, 25% air, and 5% organic material, both live and dead. The mineral and organic components are considered a constant while the percentages of water and air are the only variable parameters where the increase in one is balanced by the reduction in the other.
Given time, the simple mixture of sand, silt, and clay will evolve into a soil profile which consists of two or more layers called horizons that differ in one or more properties such as texture, structure, colour, porosity, consistency, and reaction. The horizons differ greatly in thickness and generally lack sharp boundaries. Mature soil profiles in temperate regions may include three master horizons A, B and C. The A and B horizons are called the solum or “true soil” as most of the chemical and biological activity that has formed soil takes place in those two profiles.
The pore space of soil is shared by gases as well as water. The aeration of the soil influences the health of the soil's flora and fauna and the emission of greenhouse gases.
Of all the factors influencing the evolution of soil, water is the most powerful due to its involvement in the solution, erosion, transportation and deposition of the materials of which a soil is composed. The mixture of water and dissolved and suspended materials is called the soil solution. Water is central to the solution, precipitation and leaching of minerals from the soil profile. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil profile.
Soil colloidal particles (clay and humus) behave as a repository of nutrients and moisture, and buffer the variations of soil solution ions. Their contributions to soil nutrition are out of proportion to their part of the soil. Colloids act to store nutrients that might be leached and to release those ions in response to soil pH.
Soil pH, a measure of the hydrogen ion (acid-forming) soil reactivity, is a function of the soil materials, precipitation level, and plant root behavior. Soil pH strongly affects the availability of nutrients.
Most nutrients, with the exception of the lack of nitrogen in desert soils, are present in the soil but may not be available to plants due to extremes of pH. Most nutrients originate from minerals and are stored in organic material both live and dead and on colloidal particles as ions. The action of microbes on organic matter and minerals may free nutrients for use, sequester them, or cause their loss from the soil by their volatilisation to gasses or by leaching upon their conversion to soluble forms. Most of the nitrogen available in soils is the result of nitrogen fixation by bacteria.
The organic material of the soil has a powerful effect on its development, fertility, and available moisture. Following water, organic material is next in importance to soil's formation and fertility.
Read more about Soil: Soil Forming Factors, Physical Properties of Soils, Soil Water, Soil Atmosphere, Chemical and Colloidal Properties, Nutrients, Organic Matter, Soil Horizons, Classification, Uses, Degradation, Reclamation
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“A grimy fly can soil the entire wall and a small, dirty little act can ruin the entire proceedings.”
—Anton Pavlovich Chekhov (18601904)