Soil Orders

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Alfisols

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Alfisols are moderately leached soils that have relatively high native fertility. These soils have mainly formed under forest and have a subsurface horizon in which clays have accumulated. Alfisols are primarily found in temperate humid and subhumid regions of the world. The combination of generally favorable climate and high native fertility allows Alfisols to be very productive soils for both agricultural and silvicultural use. They are divided into five suborders: Aqualfs, Cryalfs, Udalfs, Ustalfs and Xeralfs.

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Andisols

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Andisols (from Japanese ando, "black soil") are soils that have formed in volcanic ash or other volcanic ejecta. They differ from those of other orders in that they typically are dominated by glass and short-range-order colloidal weathering products such as allophane, imogolite and ferrihydrite. As a result, Andisols have andic properties — unique chemical and physical properties that include high water-holding capacity and the ability to "fix" (and make unavailable to plants) large quantities of phosphorus. They are divided into eight suborders: Aquands, Gelands, Cryands, Torrands, Xerands, Vitrands, Ustands and Udands.

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Aridisols

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Aridisols (from Latin aridus, "dry") are CaCO3-containing soils of arid regions that exhibit subsurface horizon development. They are characterized by being dry most of the year and limited leaching. Aridisols contain subsurface horizons in which clays, calcium carbonate, silica, salts and/or gypsum have accumulated. Materials such as soluble salts, gypsum and CaCO3 tend to be leached from soils of moister climates. They are divided into seven suborders: Cryids, Salids, Durids, Gypsids, Argids, Calcids and Cambids.

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Entisols

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Entisols are soils of recent origin. The central concept is soils developed in unconsolidated parent material with usually no genetic horizons except an A horizon. All soils that do not fit into one of the other 11 orders are Entisols. Thus, they are characterized by great diversity, both in environmental setting and land use. Many Entisols are found in steep, rocky settings. However, Entisols of large river valleys and associated shore deposits provide cropland and habitat for millions of people worldwide. Entisols are divided into five suborders: Wassents, Aquents, Psamments, Fluvents and Orthents.

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Gelisols

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Gelisols (from Latin gelare, "to freeze") are soils of very cold climates that contain permafrost within two meters of the surface. These soils are limited geographically to the high-latitude polar regions and localized areas at high mountain elevations. Because of the extreme environment in which they are found, Gelisols support only approximately 0.4 percent of the world's population — the lowest percentage of any of the soil orders. The frozen condition of Gelisol landscapes makes them sensitive to human activities. They are divided into three suborders: Histels, Turbels and Orthels.

 

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Histosols

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Histosols (from Greek histos, "tissue") are soils that are composed mainly of organic materials. They contain at least 20-30 percent organic matter by weight and are more than 40 cm thick. Bulk densities are quite low, often less than 0.3 g cm3. They are often referred to as peats and mucks and have physical properties that restrict their use for engineering purposes. These include low weight-bearing capacity and subsidence when drained. Histosols are divided into five suborders: Folists, Wassists, Fibrists, Saprists and Hemists.

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Inceptisols

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Inceptisols (from Latin inceptum, "beginning") are soils that exhibit minimal horizon development. They are more developed than Entisols, but still lack the features that are characteristic of other soil orders. Although not found under aridic climate regimes, Inceptisols nevertheless are widely distributed and occur across a wide range of ecological settings. They are often found on fairly steep slopes, young geomorphic surfaces and on resistant parent materials. Land use varies considerably with Inceptisols. A sizable percentage of Inceptisols are found in mountainous areas and are used for forestry, recreation and watershed. Inceptisols are divided into six suborders: Aquepts, Gelepts, Cryepts, Ustepts, Xerepts and Udepts.

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Mollisols

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Mollisols (from Latin mollis, "soft") are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon. This fertile surface horizon, known as a mollic epipedon, results from the long-term addition of organic materials derived from plant roots. Mollisols are among some of the most important and productive agricultural soils in the world and are extensively used for this purpose. They are divided into eight suborders: Albolls, Aquolls, Rendolls, Gelolls, Cryolls, Xerolls, Ustolls and Udolls.

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Oxisols

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Oxisols (from French oxide, "oxide") are very highly weathered soils that are found primarily in the intertropical regions of the world. These soils contain few weatherable minerals and are often rich in Fe and Al oxide minerals. Most of these soils are characterized by extremely low native fertility, resulting from very low nutrient reserves, high phosphorus retention by oxide minerals and low cation exchange capacity (CEC). Most nutrients in Oxisol ecosystems are contained in the standing vegetation and decomposing plant material. Despite low fertility, Oxisols can be quite productive with inputs of lime and fertilizers. The Oxisols are divided into five suborders: Aquox, Torrox, Ustox, Perox and Udox.

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Spodosols

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Spodosols (from Greek spodos, "wood ash") are acid soils characterized by a subsurface accumulation of humus that is complexed with Al and Fe. These photogenic soils typically form in coarse-textured parent material and have a light-colored E horizon overlying a reddish-brown spodic horizon. The process that forms these horizons is known as podzolization. Many Spodosols support forest. Because they are naturally infertile, they require additions of lime in order to be productive agriculturally. They are divided into five suborders: Aquods, Gelods, Cryods, Humods and Orthods.

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Ultisols

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Ultisols (from Latin ultimus, "last") are strongly leached, acid forest soils with relatively low native fertility. They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes. Intense weathering of primary minerals has occurred, and much Ca, Mg and K has been leached from these soils. Ultisols have a subsurface horizon in which clays have accumulated, often with strong yellowish or reddish colors resulting from the presence of Fe oxides. The "red clay" soils of the southeastern United States are examples of Ultisols. They are divided into five suborders: Aquults, Humults, Udults, Ustults and Xerults.

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Vertisols

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Vertisols (from Latin verto, "turn") are clay-rich soils that shrink and swell with changes in moisture content. During dry periods, the soil volume shrinks and deep wide cracks form. The soil volume then expands as it wets up. This shrink/swell action creates serious engineering problems and generally prevents formation of distinct, well-developed horizons in these soils. They are divided into six suborders: Aquerts, Cryerts, Xererts, Torrerts, Usterts and Uderts.

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