the biologically active, porous medium that has developed in the uppermost layer of the Earth's crust. Soil (pedology) is one of the principal substrata of life on Earth, serving as a reservoir of water and nutrients, as a medium for the filtration and breakdown of injurious wastes, and as a participant in the cycling of carbon and other elements through the global ecosystem. It has evolved through weathering processes driven by biological, climatic, geologic, and topographic influences.

This article covers the structure, composition, and classification of soils and how these factors affect soil's role in the global ecosystem. In addition, the two most important phenomena that degrade soils, erosion and pollution, are discussed. For a cartographic guide to the distribution of the world's major soils, featuring links to short descriptive entries on each soil type, see the interactive world map-->.

Soils differ widely in their properties because of geologic and climatic variation over distance and time. Even a simple property, such as the soil thickness, can range from a few centimetres to many metres, depending on the intensity and duration of weathering, episodes of soil deposition and erosion, and the patterns of landscape evolution. Nevertheless, in spite of this variability, soils have a unique structural characteristic that distinguishes them from mere earth materials and serves as a basis for their classification: a vertical sequence of layers produced by the combined actions of percolating waters and living organisms.

These layers are called horizons (horizon), and the full vertical sequence of horizons constitutes the soil profile (see the figure-->). Soil horizons are defined by features that reflect soil-forming processes. For instance, the uppermost soil layer (not including surface litter) is termed the A horizon. This is a weathered layer that contains an accumulation of humus (decomposed, dark-coloured, carbon-rich matter) and microbial biomass that is mixed with small-grained minerals to form aggregate structures.

Soil Horizon letter designationsThese simple letter designations are supplemented in two ways (see the table (Soil Horizon letter designations) of soil horizon letter designations). First, two additional horizons are defined. Litter and decomposed organic matter (for example, plant and animal remains) that typically lie exposed on the land surface above the A horizon are given the designation O horizon, whereas the layer immediately below an A horizon that has been extensively leached (leaching) (that is, slowly washed of certain contents by the action of percolating water) is given the separate designation E horizon, or zone of eluviation (from Latin ex, “out,” and lavere, “to wash”). The development of E horizons is favoured by high rainfall and sandy parent material, two factors that help to ensure extensive water percolation. The solid particles lost through leaching are deposited in the B horizon, which then can be regarded as a zone of illuviation (from Latin il, “in,” and lavere).

Soil Horizon letter designationsThe second enhancement to soil horizon nomenclature (also shown in the table (Soil Horizon letter designations)) is the use of lowercase suffixes to designate special features that are important to soil development. The most common of these suffixes are applied to B horizons: g to denote mottling caused by waterlogging, h to denote the illuvial accumulation of humus, k to denote carbonate mineral precipitates, o to denote residual metal oxides, s to denote the illuvial accumulation of metal oxides and humus, and t to denote the accumulation of clay.

Soils are natural elements of weathered landscapes whose properties may vary spatially. For scientific study, however, it is useful to think of soils as unions of modules known as pedons. A pedon is the smallest element of landscape that can be called soil. Its depth limit is the somewhat arbitrary boundary between soil and “not soil” (e.g., bedrock). Its lateral dimensions must be large enough to permit a study of any horizons present—in general, an area from 1 to 10 square metres (10 to 100 square feet), taking into account that a horizon may be variable in thickness or even discontinuous. Wherever horizons are cyclic and recur at intervals of 2 to 7 metres (7 to 23 feet), the pedon includes one-half the cycle. Thus, each pedon includes the range of horizon variability that occurs within small areas. Wherever the cycle is less than 2 metres, or wherever all horizons are continuous and of uniform thickness, the pedon has an area of 1 square metre.

The grain size of soil particles and the aggregate structures they form affect the ability of a soil to transport and retain water, air, and nutrients. Grain size is classified as clay if the particle diameter is less than 0.002 mm (0.0008 inch), as silt if it is between 0.002 mm (0.0008 inch) and 0.05 mm (0.002 inch), or as sand if it is between 0.05 mm (0.002 inch) and 2 mm (0.08 inch). Soil texture refers to the relative proportions of sand, silt, and clay particle sizes, irrespective of chemical or mineralogical (mineralogy) composition (see the figure-->). Sandy soils are called coarse-textured, and clay-rich soils are called fine-textured. Loam is a textural class representing about one-fifth clay, with sand and silt sharing the remainder equally.

Pore radii (space between soil particles) can range from millimetre-scale between sand grains to micrometre-scale between clay grains. Soil particles falling into the three principal size categories may have various mineralogical or chemical compositions, although sand particles often are composed of quartz and feldspars, silt particles often are micaceous, and clay particles often contain layer-type aluminosilicates (the so-called clay minerals (clay mineral)). Organic matter and amorphous mineral matter also are important constituents of soil clay particles.

Aggregates of soil particles whose formation has not been influenced by human intervention are called peds. The peds in the surface horizons of soils develop into clods under the effects of cultivation and the traffic of urbanization. Soils whose A horizon is dense and unstructured increase the fraction of precipitation that will become surface runoff and have a high potential for erosion and flooding. These soils include not only those whose peds have been degraded but also coarse-textured soils with low porosity, particularly those of arid regions.

Freshwater leaching of soils brings hydrogen ions (hydrogen ion) (H⁺) that increase mineral solubility, releasing Al³⁺ (aluminum), a toxic ion that can displace nutrients such as Ca²⁺. The gradual loss of nutrients and the accumulation of adsorbed H⁺ and Al³⁺ characterize the buildup of soil acidity, with its harmful effects on organisms. Soils display their acidity by a decrease in content of acid-soluble minerals (for example, feldspars or clay minerals) and an increase in insoluble minerals (iron and aluminum oxides). Soils weathered by freshwater leaching evolve from clay particles with a prevalence of metal ion-binding sites to highly weathered metal oxides that do not have sites that bind readily with metal ions.

The second major component of soils is organic matter produced by organisms. The total organic matter in soil, except for materials identifiable as undecomposed or partially decomposed biomass, is called humus. This solid, dark-coloured component of soil plays a significant role in the control of soil acidity, in the cycling of nutrients, and in the detoxification of hazardous compounds. Humus consists of biological molecules such as proteins and carbohydrates as well as the humic substances (polymeric compounds produced through microbial action that differ from metabolically active compounds).

The soil flora and fauna play an important role in soil development. Microbiological activity in the rooting zone of soils is important to soil acidity and to the cycling of nutrients. Aerobic and anaerobic (oxygen-depleted) microniches support microbes that determine the rate of the production of carbon dioxide (CO₂) from organic matter or of nitrate (NO₃⁻) from molecular nitrogen (N₂).

The carbon (carbon cycle) and nitrogen (nitrogen cycle) cycles are two important microbe-mediated cycles that are described in more detail in the section Soils in ecosystems (soil). In this section, however, it is worth pointing out how they illustrate the complex, integrated nature of a soil's physical, chemical, and biological behaviour: soil peds and pore spaces provide microniches for the action of carbon- and nitrogen-cycling organisms, soil humus provides the nutrient reservoirs, and soil biomass provides the chemical pathways for cycling. The carbon in dead biomass is converted to CO₂ by aerobic microorganisms and to organic acids or alcohols by anaerobic microorganisms. Under highly anaerobic conditions, methane (CH₄) is produced by bacteria. The CO₂ produced can be used by photosynthetic microorganisms or by higher plants to create new biomass and thus initiate the carbon cycle again.

The nitrogen (N) bound into proteins in dead biomass is consumed by microorganisms and converted into ammonium ions (NH₄⁺) that can be directly absorbed by plant roots (for example, lowland rice). The ammonium ions are usually converted to nitrite ions (NO₂⁻) by Nitrosomonas (nitrifying bacterium) bacteria, followed by a second conversion to nitrate (NO₃⁻) by Nitrobacter bacteria. This very mobile form of nitrogen is that most commonly absorbed by plant roots, as well as by microorganisms in soil. To close the nitrogen cycle, nitrogen gas in the atmosphere is converted to biomass nitrogen by Rhizobium bacteria living in the root tissues of legumes (e.g., alfalfa, peas, and beans) and leguminous trees (such as alder) and by cyanobacteria and Azotobacter bacteria.

Parent material is the initial state of the solid matter making up a soil. It can consist of consolidated rocks, and it can also include unconsolidated deposits such as river alluvium, lake or marine sediments, glacial tills, loess (silt-sized, wind-deposited particles), volcanic ash, and organic matter (such as accumulations in swamps or bogs). Parent materials influence soil formation through their mineralogical composition, their texture, and their stratification (occurrence in layers). Dark-coloured ferromagnesian (iron- and magnesium-containing) rocks, for example, can produce soils with a high content of iron compounds and of clay minerals in the kaolin (clay mineral) or smectite (clay mineral) groups, whereas light-coloured siliceous (silica-containing) rocks tend to produce soils that are low in iron compounds and that contain clay minerals in the illite or vermiculite (clay mineral) groups. The coarse texture of granitic rocks leads to a coarse, loamy soil texture and promotes the development of E horizons (the leached lower regions of the topmost soil layer). The fine texture of basaltic rocks, on the other hand, yields soils with a loam or clay-loam texture and hinders the development of E horizons. Because water percolates to greater depths and drains more easily through soils with coarse texture, clearly defined E horizons tend to develop more fully on coarse parent material.

In theory, parent material is either freshly exposed solid matter (for example, volcanic ash immediately after ejection) or deep-lying geologic material that is isolated from atmospheric water and organisms. In practice, parent materials can be deposited continually by wind, water, or volcanoes and can be altered from their initial, isolated state, thereby making identification difficult. If a single parent material can be established for an entire soil profile, the soil is termed monogenetic; otherwise, it is polygenetic. An example of polygenetic soils are soils that form on sedimentary rocks or unconsolidated water- or wind-deposited materials. These so-called stratified parent materials can yield soils with intermixed geologic layering and soil horizons—as occurs in southeastern England, where soils forming atop chalk bedrock layers are themselves overlain by soil layers formed on both loess and clay materials that have been modified by dissolution of the chalk below.

Often the dominant effect of topography is on subsurface runoff (or drainage (drainage basin)). In humid temperate regions, well-drained soil profiles near a summit can have thick E horizons (the leached layers) overlying well-developed clay-rich Bt horizons, while poorly drained profiles near a toeslope can have thick A horizons overlying extensive Bg horizons (lower layers whose pale colour signals stagnation under water-saturated conditions). In humid tropical regions with dry seasons, these profile characteristics give way to less distinct horizons, with accumulation of silica, manganese, and iron near the toeslope, whereas in semiarid regions soils near the toeslope have accumulations of the soluble salts sodium chloride or calcium sulfate.

The term climate in pedology refers to the characteristics of weather as they evolve over time scales longer than those necessary for soil properties to develop. These characteristics include precipitation, temperature, and storm patterns—both their averages and their variation.

On a global scale, the integrated effects of climate can readily be seen along a transect from pole to Equator. As one proceeds from the pole to cool tundra or forested regions, polar desert soils give way to intensively leached soils such as the Podzols (Podzol) (Spodosols) that exhibit an eye-catching, ash-coloured E horizon indicative of humid, boreal climates. Farther into temperate zones, organic matter accumulates in soils as climates become warmer, and eventually lime (calcium carbonate) also begins to accumulate closer to the top of the soil profile as evapotranspiration increases. Arid subtropical climate then follows, with desert soils that are low in organic matter and enriched in soluble salts. As the climate again becomes humid close to the Equator, high temperature combines with high precipitation to create red and yellow tropical soils, whose colours reveal the prevalence of residual iron oxide minerals that are resistant to leaching losses because of their low solubility.

In principle, soil profile characteristics that are closely linked to climate can in turn be interpreted as climatic indicators. For example, a soil profile with two well-defined zones of lime accumulation, one shallow and one deep, may signal the existence of a past climate whose greater precipitation drove the lime layer deeper than the present climate is able to do.

Human beings are also part of the biological influx that influences soil formation. Human influence can be as severe as wholesale removal or burial (by urbanization) of an entire soil profile, or it can be as subtle as a gradual modification of organic matter by agriculture or of soil structure by irrigation. The chemical and physical properties of soils critical to the growth of crops often are affected significantly by cultural practices. Among the problems created for agriculture by cultural practices themselves are loss of arable land, erosion, the buildup of salinity, and the depletion of organic matter.

Jackson-Sherman soil weathering stagesThe accumulation of clay and lime in soil profiles as a result of their translocation downward is also an indication of aging. For example, older soils that have formed on calcium-containing loess deposits have better-developed E and Bt horizons (as well as thinner A horizons) than younger soils forming on these deposits. Similarly, soils in a chronosequence developed on alluvium can exhibit a clayey hardpan after 100,000 years or so. Soils also tend to redden in colour as they age, irrespective of climatic conditions, reflecting the persistence of poorly crystalline or crystalline oxide minerals containing Fe³⁺. Indeed, the dominant mineralogy of the clay-size particles in soils is itself a reliable indicator of soil age. Any particular sequence of predominant clay mineralogy found in a soil is known collectively as the set of Jackson-Sherman weathering stages (see the table (Jackson-Sherman soil weathering stages)). Each downward increment through the table corresponds to increasing mineral residence time, both among and within the three principal stages (early, intermediate, and advanced).

The two principal systems of soil classification in use today are the soil order system of the U.S. Soil Taxonomy and the soil group system, published as the World Reference Base for Soil Resources, developed by the Food and Agriculture Organization (FAO) of the United Nations. Both of these systems are morphogenetic, in that they use structural properties as the basis of classification while also drawing on the five factors of soil formation described in the previous section in choosing which properties to emphasize.

Primary Diagnostic HorizonsCentral to both systems is the notion of diagnostic horizons, well-defined soil layers whose structure and origin may be correlated to soil-forming processes and can be used to distinguish among soil units at the highest level of classification (see the table (Primary Diagnostic Horizons) of primary diagnostic horizons). Diagnostic horizons may be found very near the land surface (epipedons) or deep in the soil profile (subsurface horizons); they need not correspond to the horizon letter designations.

U.S. Soil TaxonomyThe U.S. Soil Taxonomy classifies soils within a hierarchy of six categories. Only the highest-level category, order, is discussed here. Soil orders are named by adding the suffix -sol to a root word, as shown in the table (U.S. Soil Taxonomy) of the U.S. Soil Taxonomy. The resulting 12 soil order names thus represent a classification based either on parent material or on processes related to the five factors of soil formation as reflected in diagnostic horizons.

The soil orders associated with specific kinds of parent material ( Andisol, Histosol, and Vertisol) account for less than 5 percent of the Earth's continental areas covered by soil. Soils that show little development because they are too young ( Entisol) or lie in an adverse weathering environment ( Inceptisol) represent nearly 33 percent of the land area. Soils that are likely to exhibit natural toxicity to agricultural plants because of accumulations of salts ( Aridisol) or of acidity and aluminum ( Spodosol, Oxisol, and Ultisol) make up almost 40 percent of the total. This leaves essentially only the Alfisols (Alfisol) and Mollisols (Mollisol)—with about 15 percent of the total land area—as the inherently more fertile soils of the world. They occupy a strategic belt at middle latitudes in the Northern Hemisphere and in South America.

FAO Soil Classification SystemThe classification system of the FAO primarily involves a two-level nomenclature comprising the name of a soil group and a modifying adjective that serves to identify a soil unit within a group on the FAO Soil Map of the World. It is not meant to substitute for national soil classification systems such as the U.S. Soil Taxonomy but instead is designed to facilitate comparisons among these systems. Only the major soil groups are discussed here. Four of the soil groups are defined principally by their parent material (first cluster in the table (FAO Soil Classification System) of the classification system of the FAO), four are largely related to topographic factors in soil formation, and the remaining 22 groups are based on the three other soil-forming factors: climate, organisms, and time. Like the U.S. soil orders, the soil groups in the FAO system are based on extensive sets of field and laboratory observations and on technical criteria.

The FAO designates eight soil groups— Cambisol, Chernozem, Fluvisol, Gleysol, Kastanozem, Phaeozem, Umbrisol, and Vertisol—as having a high inherent soil fertility. They constitute 31 percent of the total land area. This figure would drop to 16 percent if the Cambisol and Vertisol groups were excluded, an estimate quite close to that made above for the more fertile U.S. soil orders. The soil groups that according to the FAO present toxicity hazards from salt accumulation ( Calcisol, Gypsisol, Solonchak, and Solonetz) or aridity and aluminum accumulation ( Acrisol, Alisol, and Ferralsol) cover about 25 percent of the land area. This figure would increase to about 33 percent if more tropical soil groups and the Podzol and Albeluvisol groups were included. Thus, both systems of soil classification conclude that the inherently more fertile soils are but a small portion of the total soil resources on Earth.

Water-induced erosion can take various forms depending on climate and topography. The force of rainfall (rain) striking a land surface unimpeded by vegetation or man-made structures is sufficient to raise 15 cm (6 inches) of material from an A horizon nearly 1 metre (39 inches) into the air. The impact of raindrops breaks the bonds holding soil aggregates together and catapults the particles into the flowing water from surface runoff. Wholesale removal of soil particles by the sheet flow of water (sheet erosion) or by flow in small channels (rill erosion) accounts for most of the water-induced soil loss from exposed land surfaces. More spectacular but less prevalent types of erosion are gully erosion, in which water concentrates in channels too deep to smooth over by tilling, and streambank erosion, in which the saturated sides of running streams tumble into the moving water below. The same forces at work in streambank erosion are seen in soils on hillslopes that become thoroughly saturated with water. Gravity, able to overcome the cohesive forces that hold soil particles together, can cause the entire soil profile to move downslope—a phenomenon called mass movement. This movement may be either slow (soil creep), rapid (debris flow or mudflow), or sometimes catastrophic (landslide).

The mechanisms involved in wind erosion depend on soil texture and the size of soil particles. Dry soil particles of silt or clay size can be transported over great distance by wind. Larger particles that are the size of fine sand, 0.05 mm (0.002 inch) to 0.5 mm (0.02 inch) in diameter, can be vaulted as high as 25 cm (10 inches) into the air, then drop to the ground after a short flight, only to rebound under the continual driving force of the wind. Coarser sand particles are not lifted, but they can tumble along the land surface. The major cause of wind erosion is the jumping motion of the smaller soil particles, a process called saltation. The texture of the windblown surfaces of these soils becomes coarser, making them less chemically reactive and less able to retain plant nutrients or trap pollutants. In arid regions, wind erosion often produces a gravelly land surface known as desert pavement.

When climate and topography are fixed and soil cover is varied, the rate of soil loss by water erosion has a predictable and dramatic dependence on vegetation. Irrespective of location, erosion losses are usually very small from forestland or permanent pastureland, moderate to high from land planted with grain crops, and very high from clean-tilled orchards, vineyards, and land planted with row crops, as shown in the figure-->.

The ability of soils to resist water and wind erosion depends on their texture and topographic characteristics. Clay-rich soils resist erosion well because of strong cohesive forces between particles and the gluelike characteristics of humus. Both loam and sandy soils are moderately resistant to erosion—the former because they have sufficient clay content to hold the particles together, the latter because their high permeability limits the amount of surface runoff that can wash soil particles away, while their larger particle size makes them too heavy to be easily entrained (transported) in flowing water. Silty soils, on the other hand, exhibit the least resistance to erosion because their permeability is low (resulting in more surface runoff), and their particle size is neither small enough to promote cohesion nor large enough to prevent entrainment. Soils on steep, long slopes are much more susceptible to erosion than those on shallow, short slopes because the steeper slopes accelerate the flow of surface runoff.

Nitrogen (N), one of the major nutrients, originates in the atmosphere. It is transformed and transported through the ecosystem by the water cycle and biological processes. This nutrient enters the biosphere primarily as wet deposition to the soil surface (throughfall), where plants, microbial decomposers, or nitrifiers (microbes that convert ammonium 【NH₄⁺】 to nitrate 【NO₃⁻】) compete for it. This competition plays a major role in determining the extent to which incoming nitrogen will be retained within an ecosystem.

Carbon (C) also enters the ecosystem from the atmosphere—in the form of carbon dioxide (CO₂)—and is taken up by plants and converted into biomass. Organic matter in the soil in the form of humus and other biomass contains about three times as much carbon as does land vegetation. Soils of arid and semiarid regions also store carbon in inorganic chemical forms, primarily as calcium carbonate (CaCO₃). These pools of carbon are important components of the global carbon cycle because of their location near the land surface, where they are subject to erosion and decomposition. Each year, soils release 4–5 percent of their carbon to the atmosphere by the transformation of organic matter into CO₂ gas, a process termed soil respiration. This amount of CO₂ is more than 10 times larger than that currently produced from the burning of fossil fuels (coal and petroleum), but it is returned to the soil as organic matter by the production of biomass.

Major soil pollutantsThe presence of substances in soil that are not naturally produced by biological species is of great public concern. Many of these so-called xenobiotic (from Greek xenos, “stranger,” and bios, “life”) chemicals have been found to be carcinogens or may accumulate in the environment with toxic effects on ecosystems (see the table (Major soil pollutants) of major soil pollutants). Although human exposure to these substances is primarily through inhalation or drinking water, soils play an important role because they affect the mobility and biological impact of these toxins.

For instance, one species of bacteria might use the pollutant toluene, a solvent obtained from petroleum, as a carbon source, and naturally occurring Fe³⁺ might serve as a normal electron acceptor. Another species might use natural organic acids as a carbon source and selenium-containing pollutants as electron acceptors. Often, however, the ultimate decomposition of a contaminating xenobiotic compound requires a series of many chemical steps and several different species of microorganism. This is especially true for organic compounds that contain chlorine (Cl), such as chlorinated pesticides, chlorinated solvents, and polychlorinated biphenyls (PCBs; once used as lubricants and plasticizers). For example, the chlorinated herbicide atrazine is gradually degraded by aerobic microorganisms through a variety of pathways involving intermediate products. The complexity of the decomposition processes and the inherent toxicity of the pollutant compounds to the microorganisms themselves can lead to long residence times in soil, ranging from years to decades for toxic metals and chlorinated organic compounds.

Major soil pollutantsMost of the metals (metal) that are major soil pollutants (see table (Major soil pollutants)) can form strong complexes with soil humus that significantly decrease the solubility of the metal and its movement toward groundwater. Humus can serve as a detoxification pathway by assuming the role taken by biomolecules in the metal toxicity mechanisms discussed above. Just as strong complex formation leads to irreversible metal association with a biomolecule and to the disruption of biochemical functions, so, too, can it lead to effective immobilization of toxic metals by soil humus—in particular, the humic substances. The very property of toxic metals that makes them so hazardous to organisms also makes them detoxifiable by humus in soil.

Perhaps the finest book on soils for the layperson is Milo I. Harpstead, Thomas J. Sauer, and William F. Bennett, Soil Science Simplified, 3rd ed. (1997). An in-depth introductory treatment is given by Frederick R. Troeh and Louis M. Thompson, Soils and Soil Fertility, 5th ed. (1993). A holistic account of soils in their ecological setting is presented in Hans Jenny, The Soil Resource (1980, reprinted with corrections, 1983). Technical terms are defined in Glossary of Soil Science Terms (1997), published by the Soil Science Society of America.An excellent technical discussion of pedology in an ecological context is Stanley W. Buol et al., Soil Genesis and Classification, 4th ed. (1997). Guides to soil classification are Keys to Soil Taxonomy, 8th ed. (1998), published by the U.S. Department of Agriculture; and Soil Map of the World: Revised Legend (1990, reprinted with corrections, 1994), published by the Food and Agriculture Organization (FAO) and UNESCO.The modern classic on soil formation is Hans Jenny, Factors of Soil Formation (1941, reissued 1994). See also the commentary volume, Factors of Soil Formation: A Fiftieth Anniversary Retrospective (1994), published by the Soil Science Society of America. The geomorphic processes related to soil formation and erosion are discussed by Cliff Ollier, Weathering (1984). Erosion is considered in more technical detail in Frederick R. Troeh, James Arthur Hobbs, and Roy L. Donahue, Soil and Water Conservation, 3rd ed. (1999). The special problems of soil loss in the humid tropics are brought out in R. Lal and Pedro A. Sánchez (eds.), Myths and Science of Soils of the Tropics (1992); and, on a practical level, in Ted C. Sheng, Soil Conservation for Small Farmers in the Humid Tropics (1989).Soil pollution issues are given modern treatment in Garrison Sposito, The Chemistry of Soils (1989); Murray B. McBride, Environmental Chemistry of Soils (1994); Eldor A. Paul and Francis E. Clark (eds.) Soil Microbiology and Biochemistry, 2nd ed. (1996); and F.J. Stevenson and M.A. Cole, Cycles of Soil, 2nd ed. (1999). The last four texts also discuss biogeochemical cycling in soils at a technical level.Garrison Sposito