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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.

Loess field in Germany
Loess field in Germany

Soils are vital to all life on Earth because they support the growth of plants, which supply food and oxygen and absorb carbon dioxide and nitrogen.

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.

Natural soil development

An example of soil development from bare rock occurs on recent lava flows in warm regions under heavy and very frequent rainfall. In such climates plants become established very quickly on basaltic lava, even though there is very little organic material. The plants are supported by the porous rock becoming filled with nutrient bearing water, for example carrying dissolved bird droppings or guano. The developing plant roots themselves gradually breaks up the porous lava and organic matter soon accumulates but, even before it does, the predominantly porous broken lava in which the plant roots grow can be considered a soil.

Chemical processes in soils

Weathering releases ions such as Potassium (K+) and Magnesium (Mg2+) into the soil solution. Some of these elements (as ions) are taken up by bacteria, fungi and plants. The remaining portion can form secondary minerals, be chelated into organic complexes or be adsorbed into ion exchange complexes. Anion exchange complexes affect negatively charged ions ( phosphate) and compounds. Anion exchange surfaces occur most typically in humus. Cation exchange complexes affect positively charged ions. Cation exchange surfaces are typically clay minerals such as montmorillonite and organic materials such as humus. When the level of ions is relatively low in the soil solution, equilibrium processes convey ions into solution, where they satisfy demand for nutrients by plants, bacteria and fungi.

The pH level in soils affects the activity and availability of ionic nutrients (examples are Ca2+, Mg2+, K+, Na+) and non-nutrients (H+, Al3+). Nutrient uptake is highest in a neutral pH range of 5.5 to 8.2. At pH levels below 5.0, increased aluminum activity can have a toxic affect, exacerbating reduced nutrient availability. Additionally, Ca2+, Mg2+, K+, Na+ can be displaced by H+ and Al3+. Subsequent leaching can result in lower soil fertility and productivity. At elevated soil pH levels nutrient availability is limited, especially for zinc and phosphorus. Additionally, differential removal of cations can result in elevated Na+ relative to Ca2+ and Mg2+ with a deleterious affect on soil structure, permeability and tilth. Contributors to soil acidification include "acidic" parent material (granite), plant root exudates, decomposition of certain types of organic residue (pine needles), chemical changes that occur when perennially wet sediments are dried, acidifying fertilizers (anhydrous ammonia, ammonium sulfate), and natural rain as well as acid rain phenomena. Sources of alkalinity include "basic" parent material (serpantine, limestone) and airborne soil particulates from alkaline areas. To raise a soil's pH, farmers can apply alkaline materials such as lime. To lower a soil's pH, farmers can apply acid-forming materials such as elemental sulfur. To increase calcium content in an alkaline soil, farmers can apply gypsum.

Although the elements nitrogen, potassium and phosphorus, which are necessary for plant growth, may be abundant in soils, only a fraction of these elements may be in a chemical form which plants can use.

Processes such as the nitrogen cycle and carbon cycle continually exchange nitrogen and carbon nutrients between soils and the atmosphere. The raw products are initially present as gases in the atmosphere. In nitrogen fixation, atmospheric nitrogen is converted to plant available forms. In nitrogen mineralization, proteins and other organic forms are converted into mineral, plant available forms: NH4+ and NO3-. In nitrification, NH4+ is converted into the more usable NO3-. While NH4+ is especially important to young plants and early in the growing season, NO3- is the dominant form of nitrogen taken up by plants. NO3- moves to plants by mass transport and needs transpiration to drive uptake.

The organic component of soils originate in plant debris (such as fallen leaves), animal excreta, and other decomposing organic materials. These materials, when broken down, form humus, a dark, nutrient-rich material. Chemically, humus is composed of very large molecules including esters of carboxylic acid, phenolic compounds, and derivatives of benzene. Organic materials in soils provide nutrients necessary for plant growth. Organic material also contributes to water retention, drainage ability, and oxygenation of soils.

If oxygen enters a wet soil, because of lowered ground water table, organic matter in the soil will be broken down further by oxidation, which can lead to subsidence. An example of this can be seen in soils in the Everglades region of Florida, which have been drained by canals for agriculture, primarily sugar production. Originally very high in organic content, oxygenation and compaction have led to breakdown of the soil structure and nutrient content, and degradation of the soil's ability to support continued high crop yields.

Biological processes in soil

Wetland soil processes

The diffusion of dissolved oxygen in saturated soils is slower than in unsaturated soils. Wetland (also referred to as hydric) soils form due to soil microbial cellular respiration in excess of soil oxygen supply, resulting in oxygen depletion. Anaerobic soil chemistry results, which creates a reducing environment. This eliminates plants and creatures not adapted for life in saturated soil conditions.

Biological soil crusts

Biological soil crusts are formed by living organisms and their by-products, creating a surface crust of soil particles bound together by organic materials.

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