Soil Orders

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AlfisolsArgillic, natric, or kandic horizon; high-to-medium base saturation.Alfisols are moderately weathered, and are found in cool to hot humid areas as well as in the semiarid tropics and Mediterranean climates. Often develop under native deciduous forests, although in some cases savanna is the natural vegetation. 

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Andisols From volcanic ejecta, dominated by allophane or Al-humic complexes.Andisols are usually formed on volcanic ash and cinders deposited in recent geological times. They are commonly found near the volcano source or in areas downwind from the volcano, where a sufficiently thick layer of ash has been deposited during eruptions. 

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Aridisols Dry soil, ochric epipedon, sometimes argillic or natric horizon.Aridisols occupy a larger area globally than any other soil order except Entisols. Water deficiency is a major characteristic of these soils. Usually alkaline soils. 

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Entisols Little profile development, ochric epipedon common.Weakly developed mineral soils without natural genetic B horizons belong to the Entisols order. This is an extremely diverse group of soils with little in common, other than the lack of evidence for all but the earliest stages of soil formation. 

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Gelisols Permafrost, often with cryoturbation (frost churning).Gelisols are young soils with little profile development. Cold temperatures and frozen conditions for much of the year slow the process of soil formation. Gelisols are also a large carbon sink.

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Histosols Peat or bog; >20% organic matter.Histosols consist of one or more thick layers of organic soil material. They have undergone little profile development because of the anaerobic environment in which they form. 

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Inceptisols Embryonic soils with few diagnostic features, ochric or umbric epipedon, cambic horizon.In Inceptisols, the beginning, or inception of a B horizon is evident, and some diagnostic features are present. However, the well-defined profile characteristics of soils thought to be more mature have not yet developed. 

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Mollisols Mollic epipedon, high base saturation, dark soils, some with argillic or natric horizons.The principal process int he formation of Mollisols is the accumulation of calcium-rich organic matter, largely from the dense root systems of prairie grasses. This humus-rich surface horizon may extend 60 to 80 cm deep. It's CEC is more than 50% saturated with base cations. 

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Oxisols Oxic horizon, no argillic horizon, highly weathered.The Oxisols are the most highly weathered soils in the classification system. They form in hot climates with nearly year-round moist conditions; hence the native vegetation is generally thought to be tropical rain forest. The clay content of Oxisols is generally high, but the clays are of the low-activity, non-sticky type.

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Spodosols Spodic horizon commonly with Fe, Al oxides and humus accumulation.Spodosols occur mostly on coarse-textured, acid parent materials subject to ready leaching. They occur only in moist to wet areas, commonly where it is cold or temperate and also in some tropical and subtropical areas. Spodosols form under forest vegetation, especially under conifers. 

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Ultisols Argillic or kandic horizon, low base saturation.The principal proccesses involved in forming Ultisols are clay mineral weathering, translocation of clays to accumulate in an argillic or kandic horizon, and leaching of non-acid cations from the profile. Most Ultisols have developed under most conditions in warm to tropical climates. 

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Vertisols High in swelling clays; deep cracks when soil is dry.The main soil-forming process affecting Vertisols is the shrinking and swelling of clay as these soils go through period of drying and wetting. The organic matter content of dark Vertisols typically ranges from as much as 5 or 6% to as little as 1%.