Aridisols are a soil order in USDA soil taxonomy. Aridisols form in an arid or semi-arid climate. Aridisols dominate the deserts and xeric shrublands, which occupy about one third of the Earth's land surface. Aridisols have a very low concentration of organic matter, reflecting the paucity of vegetative production on these dry soils. Water deficiency is the major defining characteristic of Aridisols. Also required is sufficient age to exhibit subsoil weathering and development. Limited leaching in aridisols often results in one or more subsurface soil horizons in which suspended or dissolved minerals have been deposited: silicate clays, sodium, calcium carbonate, gypsum or soluble salts. These subsoil horizons can also be cemented by carbonates, gypsum or silica. Accumulation of salts on the surface can result in salinization.
In both the World Reference Base for Soil Resources (WRB) and the USDA soil taxonomy, a Histosol is a soil consisting primarily of organic materials. They are defined as having 40 centimetres (16 in) or more of organic soil material starting within 40 cm from the soil surface. In Soil Taxonomy, Gelisols key out before Histosols, and in WRB, Histosols key out before Cryosols. Therefore, organic permafrost soils belong to the Histosols in WRB and to the Gelisols (Histels) in Soil Taxonomy.
In USDA soil taxonomy, a Psamment is defined as an Entisol which consists basically of unconsolidated sand deposits, often found in shifting sand dunes but also in areas of very coarse-textured parent material subject to millions of years of weathering. This latter case is characteristic of the Guiana Highlands of northern South America. A Psamment has no distinct soil horizons, and must consist entirely of material of loamy sand or coarser in texture. In the World Reference Base for Soil Resources (WRB), most Psamments belong to the Arenosols. However, Psamments of fluviatile, lacustrine or marine origin belong to the Fluvisols.
The World Reference Base for Soil Resources (WRB) is an international soil classification system for naming soils and creating legends for soil maps. The currently valid version is the fourth edition 2022. It is edited by a working group of the International Union of Soil Sciences (IUSS).
In soil classification, an Umbrisol is a soil with a dark topsoil and in which organic matter has accumulated within the mineral surface soil—in most cases with low base saturation—to the extent that it significantly affects the behaviour and utilization of the soil. Umbrisols are the counterpart of comparable soils with a high base saturation.
Gypsisols in the World Reference Base for Soil Resources (WRB) are soils with substantial secondary accumulation of gypsum (CaSO4.2H2O). They are found in the driest parts of the arid climate zone. In the USDA soil taxonomy they are classified as Gypsids (USDA Soil Taxonomy), in the Russian soil classification they are called Desert soils (USSR).
An Acrisol is a Reference Soil Group of the World Reference Base for Soil Resources (WRB). It has a clay-rich subsoil and is associated with humid, tropical climates, such as those found in Brazil, and often supports forested areas. In the USDA soil taxonomy, Acrisols correspond to the Humult, Udult and Ustult suborders of the Ultisols and also to Oxisols with a kandic horizon and to some Alfisols. The Acrisols low fertility and toxic amounts of aluminium pose limitations to its agricultural use, favouring in many places its use for silviculture, low intensity pasture and protected areas. Crops that can be successfully cultivated, if climate allows, include tea, rubber tree, oil palm, coffee and sugar cane.
Solonchak is a Reference Soil Group of the World Reference Base for Soil Resources (WRB). It is a pale or grey soil type found in arid to subhumid poorly-drained conditions. The word is Russian for "salt marsh", which is in turn from the Russian sol (соль) "salt". The Ukrainian folk word "солончак", which in turn from Ukrainian "salty" (солоний) + "чак"—suffix; designation of an object that has the property. A village with the name Solonchaky exists in Ukraine.
Lixisols are a Reference Soil Group of the World Reference Base for Soil Resources (WRB). They are soils with subsurface accumulation of low activity clays and high base saturation. They develop under intensive tropical weathering conditions and subhumid to semi-arid climate.
A Cambisol in the World Reference Base for Soil Resources (WRB) is a soil in the beginning of soil formation. The horizon differentiation is weak. This is evident from weak, mostly brownish discolouration and/or structure formation in the soil profile.
A Durisol is a Reference Soil Group under the World Reference Base for Soil Resources (WRB) referring to free-draining soils in arid and semi-arid environments that contain grains cemented together by secondary silica (SiO2) in the upper metre of soil, occurring either as concretions (durinodes – duric horizon) or as a continuously cemented layer (duripan – hardpan (Australia) – dorbank (South Africa) – petroduric horizon). The name is derived from Latin durus for hard.
A Regosol in the World Reference Base for Soil Resources (WRB) is very weakly developed mineral soil in unconsolidated materials. Regosols are extensive in eroding lands, in particular in arid and semi-arid areas and in mountain regions. Internationally, Regosols correlate with soil taxa that are marked by incipient soil formation such as Entisols in the USDA soil taxonomy or Rudosols and possibly some Tenosols in the Australian Soil Classification.
A Technosol in the World Reference Base for Soil Resources is a Reference Soil Group that combines soils whose properties and pedogenesis are dominated by their technical origin. They contain either a significant amount of artefacts, some sort of geotechnical liner, or are sealed by technic hard material. They include soils from wastes, pavements with their underlying unconsolidated materials, soils with geomembranes and constructed soils in human-made materials. Transported natural soil material does not qualify as Technosol and is described with the Transportic qualifier in WRB. Technosols are often referred to as urban or mine soils. They are recognized in the new Russian soil classification system as Technogenic Superficial Formations.
An Alisol is a Reference Soil Group of the World Reference Base for Soil Resources (WRB).
A fluvisol in the World Reference Base for Soil Resources (WRB) is a genetically young soil in alluvial deposits. Apart from river sediments, they also occur in lacustrine and marine deposits. Fluvisols correlate with fluvents and fluvaquents of the USDA soil taxonomy. The good natural fertility of most fluvisols and their attractive dwelling sites on river levees and higher parts in marine landscapes were recognized in prehistoric times.
Kastanozem is one of the 32 Reference Soil Groups of the World Reference Base for Soil Resources (WRB). These soils are brighter than Chernozems, and are related to the Mollisols in the USDA soil taxonomy. They are rich in humus, and originally covered with early maturing native grasslands vegetation, which produces a characteristic brown surface layer in the first meter in depth. They have a relative high level of exchangeable calcium ions and secondary carbonates starting within 70 cm of the mineral soil surface.
Luvisols are a group of soils, comprising one of the 32 Reference Soil Groups in the international system of soil classification, the World Reference Base for Soil Resources (WRB). They are widespread, especially in temperate climates, and are generally fertile. Luvisols are widely used for agriculture.
Nitisol, in the World Reference Base for Soil Resources (WRB), is a deep, red, well-drained soil with a clay content of at least 30% and a polyhedral structure or a blocky structure, breaking into a polyhedral or a flat-edged structure. The soil aggregates show pressure faces. Nitisols correlate with the kandic alfisols, ultisols and inceptisols of the USDA soil taxonomy.
A Phaeozem in the World Reference Base for Soil Resources (WRB) is a dark soil with a high base status, but without a secondary carbonates within one metre of the soil surface. Most Phaeozems correlate with the Udolls (Mollisols) of the USDA soil taxonomy.
A Retisol is a Reference Soil Group of the World Reference Base for Soil Resources (WRB). Retisols are characterized by clay migration and an additional specific feature: The clay-poorer and lighter coloured eluvial horizon intercalates netlike into the clay-richer more intensely coloured illuvial horizon. The illuvial horizon is the diagnostic argic horizon, and the intercalation is called retic properties.
