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Soil is unconsolidated rock particles in combination with organic matter.

Soil is vital to all life on Earth because it supports the growth of plants, which supply food and oxygen and absorbs carbon dioxide and nitrogen.

Contents 1 Soil components 2 Natural soil development 3 Chemical processes in soils 4 Biological processes in soil 4.1 Wetland soil processes 4.2 Biological soil crusts 5 References 6 See also:

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 or 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 less rich in organic matter.

Water and air are also components of most soils. Air, trapped in spaces between soil particles, and water, trapped in spaces and on the surface of particles, comprises about half of the soil by volume. Both are important to plant growth and other life in the soil profile of a particular ecosystem.

The rock and mineral content of soil 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.

Soil serves as a habitat soil organisms varying in size from microorganisms to small animals. The character of soil 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.

Soil develops naturally over time through the action of plants, animals, and weathering. Soil is also affected by human habitation. People can alter soil to make it more suitable for plant growth through the addition of organic material and natural or synthetic fertilizer, and by improving its drainage or water-retaining capacity. Human actions also can degrade soil 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 break 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 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 soil 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 soil pH, farmers can apply alkaline materials such as lime. To lower soil 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 soil, 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 the soil and 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 material in soil provides nutrients necessary for plant growth. Organic material also contributes to water retention, drainage ability, and oxygenation of soil.

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.

References

• 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 1573220043
• Faulkner, William. Plowman's Folly. New York, Grosset & Dunlap. 1943. ISBN 0933280513
• Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941
• Why Study Soils?
• Soil notes
• Soil articles

See also:

• Alluvium
• Compost
• Denitrification
• Derelict soil
• FAO - Soil Unit Classification Scheme
• Humus
• Manure
• Nitrification
• Nitrogen cycle
• Nitrogen fixation
• Pedology
• Pedogenesis
• Soil degradation
• Soil moisture
• Soil pH
• Soil profile
• Soil salination
• Soil science
• Soil structure
• Soil survey (soil mapping)
• Soil test
• Soil types
• Topsoil
• USA soil taxonomy

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