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Loess field in Germany

Surface-water-gley developed in glacial till, Northern Ireland

For the American hard rock band, see SOiL.

For the System of a Down song, see Soil (song).

Soil is a naturally occurring, unconsolidated or loose material on the surface of the earth, capable of supporting life.^[1] In simple terms, soil has three components: solid, liquid, and gas. The solid phase is a mixture of mineral and organic matter. Soil particles pack loosely, forming a soil structure filled with voids.^[2] The solid phase occupies about half of the soil volume. The remaining void space contains water (liquid) and air (gas).^[3]

Darkened topsoil and reddish subsoil layers are typical in some regions.

Soil color is the first impression one has when viewing soil. Striking colors and contrasting patterns are especially memorable. The Red River in Mississippi carries sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma. The Yellow River in China carries yellow sediment from eroding loessal soils. Mollisols in the Great Plains are darkened and enriched by organic matter. Podsols in boreal forests have highly contrasting layers due to acidity and leaching.

Soil color results from chemical weathering. As the primary minerals in parent material weather, the elements combine into new and colorful compounds. Iron forms secondary minerals with a yellow or red color; organic matter decomposes into black and brown compounds; and manganese forms black mineral deposits. ^[4]

Soil structure is the arrangement of soil particles into aggregates. These may have various shapes, sizes and degrees of development or expression.^[5]

Soil texture refers to sand, silt and clay composition. Sand and silt are the product of physical weathering while clay is the product of chemical weathering. Clay content is particularly influential on soil behavior due to a high retention capacity for nutrients and water.^[6]

Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological, and anthropogenic processes on soil parent material resulting in the formation of soil horizons. Soil is always changing. The long periods over which change occurs and the multiple influences of change mean that simple soils are rare. While soil can achieve relative stability in properties for extended periods of time, the soil life cycle ultimately ends in soil conditions that leave it vulnerable to erosion. Little of the soil continuum of the earth is older than Tertiary and most no older than Pleistocene.^[7] Despite the inevitability of soils retrogression and degradation, most soil cycles are long and productive. How the soil "life" cycle proceeds is influenced by at least five classic soil forming factors: regional climate, biotic potential, topography, parent material, and the passage of time.

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.

Geologists have a particular interest in the patterns of soil on the surface of the earth. Soil texture, color and chemistry often reflect the underlying geologic parent material and soil types often change at geologic unit boundaries. Buried paleosols mark previous land surfaces and record climatic conditions from previous eras. Geologists use this paleopedological record to understand the ecological relationships in past ecosystems. According to the theory of biorhexistasy, prolonged conditions conducive to forming deep, weathered soils result in increasing ocean salinity and the formation of limestone.

Geologists use soil profile features to establish the duration of surface stability in the context of geologic faults or slope stability. An offset subsoil horizon indicates rupture during soil formation and the degree of subsequent subsoil formation is relied upon to establish time since rupture.

Soil examined in shovel test pits is used by archaeologists for relative dating based on stratigraphy (as opposed to absolute dating). What is considered most typical is to use soil profile features to determine the maximum reasonable pit depth than needs to be examined for archaeological evidence in the interest of cultural resources management.

Soils altered or formed by man (anthropic and anthropogenic soils) are also of interest to archaeologists. An example is Terra preta do Indio.

A homeowner tests soil to apply only the nutrients needed. Farmers practice the same testing procedure.

Due to their thermal mass, rammed earth walls fit in with environmental sustainability aspirations.

A homeowner sifts soil made from his compost bin in background. Composting is an excellent way to recycle household and yard wastes.

Gardening and landscaping provide common and popular experience with soils. Homeowners and farmers alike test soils to determine how they can be maintained and improved. Plant nutrients such as nitrogen, phosphorus, and potassium are tested . If specific soil is deficient in these substances, fertilizers may provide them. Extensive academic research is performed in an effort to expand the understanding of agricultural soil science.

Earth sheltering is the architectural practice of using soil for external thermal mass against building walls. The principle is that earthen material undergoes slow temperature changes and thus presents a fairly constant surface temperature at the wall. In higher latitudes with low average annual air temperature, the potential for heat leaching requires floor and base wall insulation. Earth-based, wall-construction materials include adobe, chirpici, cob, mudbrick, rammed earth, and sod. An earthen wall facing the mid-day sun can be designed as a trombe wall. A trombe wall is glazed on the exterior to enhance heat gain. Heat is vented to the interior at night.

Organic soils, especially peat, serve as a significant fuel resource. Peat deposits are found in many places around the world. The majority of peatlands are found in high latitudes; approximately 60% of the world's wetlands are peat. Peatlands cover around 3% of the global land mass or 3,850,000 to 4,100,000 km². Peat is available in considerable quantities in Scandinavia: some estimates put the amount of peat in Finland alone to be twice the size of North Sea oil reserves.^[8] Peat is used to produce both heat and electricity, often mixed with wood. Peat accounts for 6.2% of Finland's yearly energy production, second only to Ireland.^[9] Peat is arguably a slowly renewable biofuel but is more commonly classified as a fossil fuel.^[9]

Waste management often has a soil component. Using compost and vermicompost are popular methods for diverting household waste to build soil fertility and tilth. The technique for creating Terra prêta do índio in the Amazon basin increasingly appears to have started from knowledge of soil first gained at a household level of waste management. Industrial waste management similarly relies on soil improvement to utilise waste treatment products. Compost and anaerobic digestate (also termed biosolids) are used to benefit the soils of land remediation projects, forestry, agriculture, and for landfill cover. These products increase soil organic content, provide nutrients, enhance microbial activity, improve soil ability to retain moisture, and have the potential to perform a role in carbon sequestration.

Compost and digestate are the finished products of treatment. Soil performs a more direct treatment role when it comes to septage effluent and in land application of industrial waste water.

Septic drain fields treat septic tank effluent using aerobic soil processes to degrade putrescible components. Pathogenic organisms vulnerable to predation in an aerobic soil environment are eliminated. Clay particles act like electrostatic filters to detain virus in the soil adding a further layer of protection. Soil is also relied on for chemically binding and retaining phosphorus. Where soil limitations preclude the use of a septic drain field, the soil treatment component is replaced by some combination of mechanical aeration, chemical oxidation, ultraviolet light disinfection, replaceable phosphorus retention media and/or filtration.

For industrial wastewater treatment, land application is a preferred treatment approach when oxygen demanding (putrescible) constituents and nutrients are the treatment targets. Aerobic soil processes degrade oxygen demanding components. Plant uptake and removal through grazing or harvest perform nutrient removal. Soil processes have limited treatment capacity for treating metal and salt components of waste.

Wikiquote has a collection of quotations related to:

Soil

Wikimedia Commons has media related to:

Soil

• Geohumus
• Geoponic
• Humus
• Manure
• Mud
• Soil functions
• Soil mechanics

• Soil Science Society of America
• Adams, J.A. 1986. Dirt. College Station, Texas : Texas A&M University Press ISBN 0890963010
• 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. (1999) 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 0-933280-51-3
• Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941
• Why Study Soils?
• Soil notes
• 97 Flood. USGS. Retrieved on 2006-05-31. Photographs of sand boils.
• Oregon State University's Soils (wiki)
• OpenAg.info's Soil Science Encyclopedia (wiki)
• European Soil Portal EUSOILS (wiki)
• Soil-Net.com A free schools-age educational site teaching about soil and its importance.
• LandIS Soils Data for England and Wales a pay source for GIS data on the soils of England and Wales and soils data source; they charge a handling fee to researchers.
• LandIS Free Soilscapes Viewer Free interactive viewer for the Soils of England and Wales
• Wossac the world soil survey archive and catalogue.
• Geo-technological Research Paper, IIT Kanpur, Dr P P Vitkar - Strip footing on weak clay stabilized with a granular pile http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_abst_e?cgj_t78-066_15_ns_nf_cgj4-78
• Percolation Test Learn about Soil, Percolation, Perc and Perk Tests.
• USDA-NRCS Web Soil Survey Inventory of the soil resource across the U.S.