World Reference Base for Soil Resources
Encyclopedia
The World Reference Base for Soil Resources (WRB) is the international standard taxonomic soil classification
Soil classification
Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.- Overview :...

 system endorsed by the International Union of Soil Sciences (IUSS). It was developed by an international collaboration coordinated by the International Soil Reference and Information Centre
International Soil Reference and Information Centre
World Soil Information, established in 1964 is an independent foundation with a global mandate, funded by the Netherlands Government, and with a strategic association with Wageningen University and Research Centre.Our aims:...

 (ISRIC) and sponsored by the IUSS and the FAO
Fão
Fão is a town in Esposende Municipality in Portugal....

 via its Land & Water Development division. It replaces the previous FAO soil classification
FAO soil classification
The Food and Agriculture Organization of the United Nations developed a supra-national classification, also called World Soil Classification, which offers useful generalizations about soils pedogenesis in relation to the interactions with the main soil-forming factors. It was first published in...

.

The WRB borrows heavily from modern soil classification concepts, including USDA soil taxonomy
USDA soil taxonomy
USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...

, the legend for the FAO Soil Map of the World 1988, the Référentiel Pédologique and Russian concepts. The classification is based mainly on soil morphology
Soil morphology
Soil morphology is the field observable attributes of the soil within the various soil horizons and the description of the kind and arrangement of the horizons. C.F...

 as an expression pedogenesis
Pedogenesis
Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...

. A major difference with USDA soil taxonomy
USDA soil taxonomy
USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...

 is that soil climate is not part of the system, except insofar as climate influences soil profile characteristics. As far as possible, diagnostic criteria match those of existing systems, so that correlation with national and previous international systems is as straightforward as possible.

The WRB is meant for correlation of national and local systems. The level of detail corresponds to USDA soil taxonomy subgroups, without the soil climate information. It is not detailed enough for mapping at scales larger than about 1:200k, although proposal have been made to couple WRB with substrate information to map at 1:50k in regional studies.

Key to the WRB reference soil groups (2006)

Identification key to the 32 reference soil groups:
1. Soils with thick organic layers: Histosols (HS)
2. Soils with strong human influence
Soils with long and intensive agricultural use: Anthrosols
Anthrosols
An Anthrosol in the FAO World Reference Base for Soil Resources is a type of soil that has been formed or heavily modified due to long-term human activity, such as from irrigation, addition of organic waste or wet-field cultivation used to create paddy fields....

 (AT)
Soils containing many artefacts: Technosols
Technosols
A Technosol in the FAO World Reference Base for Soil Resources is a new type of soil that combines soils whose properties and pedogenesis are dominated by their technical origin...

 (TC)
3. Soils with limited rooting due to shallow permafrost or stoniness
Ice-affected soils: Cryosols (CR)
Shallow or extremely gravelly soils: Leptosols
Leptosols
A Leptosol in the FAO World Reference Base for Soil Resources is a very shallow soil over hard rock or highly calcareous material or a deeper soil that is extremely gravelly and/or stony. Leptosols cover approximately 1.7 billion hectares of the Earth's surface. They are found from the tropics to...

 (LP)
4. Soils influenced by water
Alternating wet-dry conditions, rich in swelling clays: Vertisols (VR)
Floodplains, tidal marshes: Fluvisols
Fluvisols
A Fluvisol in the FAO World Reference Base for Soil Resources is a genetically young soil in alluvial deposits . Apart from river sediments, they also occur in lacustrine and marine deposits....

 (FL)
Alkaline soils: Solonetz
Solonetz
Solonetz is a type of soil in FAO soil classification. They have, within the upper 100 cm of the soil profile, a so-called "natric horizon" . There is a subsurface horizon , higher in clay content than the upper horizon, that has more than 15% exchangeable sodium...

 (SN)
Salt enrichment upon evaporation: Solonchaks (SC)
Groundwater affected soils: Gleysols
Gleysols
A Gleysol in the FAO World Reference Base for Soil Resources is a wetland soil that, unless drained, is saturated with groundwater for long enough periods to develop a characteristic gleyic colour pattern...

 (GL)
5. Soils set by Fe/Al chemistry
Allophanes or Al-humus complexes: Andosols
Andosols
Andosols are soils found in volcanic areas formed in volcanic tephra. In some cases can andosols also be found outside active volcanic areas. Andosols cover an estimate of 1-2% of earth's ice-free land surface...

 (AN)
Cheluviation and chilluviation: Podzols (PZ)
Accumulation of Fe under hydromorphic conditions: Plinthosols (PT)
Low-activity clay, P fixation, strongly structured: Nitisols (NT)
Dominance of kaolinite and sesquioxides: Ferralsols (FR)
6. Soils with stagnating water
Abrupt textural discontinuity: Planosols (PL)
Structural or moderate textural discontinuity: Stagnosols (ST)
7. Accumulation of organic matter, high base status
Typically mollic: Chernozems (CH)
Transition to drier climate: Kastanozems (KS)
Transition to more humid climate: Phaeozems (PH)
8. Accumulation of less soluble salts or non-saline substances
Gypsum: Gypsisols
Gypsisols
Gypsisols in the FAO World Reference Base for Soil Resources are soils with substantial secondary accumulation of gypsum . They are found in the driest parts of the arid climate zone.In the USDA soil taxonomy they are classified as Gypsids , in the Russian soil classification they are called...

 (GY)
Silica: Durisols
Durisols
A Durisol in the FAO World Reference Base for Soil Resources is a very shallow to moderately deep, free-draining soil of arid and semi-arid environments, that contain cemented secondary silica in the upper metre of soil. Durisols are internationally known as "hardpan soils" or "dorbank" or they...

 (DU)
Calcium carbonate: Calcisols
Calcisols
A Calcisol in the FAO World Reference Base for Soil Resources is a soil with a substantial secondary accumulation of lime. Calcisols are common in calcareous parent materials and widespread in arid and semi-arid environments...

 (CL)
9. Soils with a clay-enriched subsoil
Albeluvic tonguing: Albeluvisols
Albeluvisols
An albeluvisol in the FAO World Reference Base for Soil Resources is a soil with a thin, dark surface horizon on a bleached subsurface horizon that tongues into a clay illuviation horizon. The Bt horizon has an irregular or broken upper boundary resulting from the tonguing of bleached soil...

 (AB)
Low base status, high-activity clay: Alisols
Alisols
Alisols define a soil group within the World Reference Base for Soil ResourcesProperties include having an argic horizon, which has a specific cation exchange capacity; a predominantly alic properties zone between 250mm and 1000 mm from the soil surface; and no diagnostic horizons other than...

 (AL)
Low base status, low-activity clay: Acrisols (AC)
High base status, high-activity clay: Luvisols (LV)
High base status, low-activity clay: Lixisols
Lixisols
Lixisols are soils with subsurface accumulation of low activity clays and high base saturation. They develop under intensive tropical weathering conditions....

 (LX)
10. Relatively young soils or soils with little or no profile development
With an acidic dark topsoil: Umbrisols (UM)
Sandy soils: Arenosols (AR)
Moderately developed soils: Cambisols
Cambisols
A Cambisol in the FAO World Reference Base for Soil Resources is a soil with a beginning of soil formation. The horizon differentiation is weak...

 (CM)
Soils with no significant profile development: Regosols
Regosols
A Regosol in the FAO World Reference Base for Soil Resources 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...

 (RG)

WRB 98 soil groups

CodeSoil typeBrief description
AC Acrisol
Acrisol
An acrisol is a type of soil as classified by the Food and Agriculture Organization. It is clay-rich, and is associated with humid, tropical climates, such as those found in Brazil, and often supports forested areas. It is one of the 30 major soil groups of the World Reference Base for Soil Resources...

Red, brown or yellow coloured soil, develops in areas of intense weathering, has a clay rich B horizon
AB Albeluvisol
AL Alisol
AN Andosol Soil developed from volcanic material, are young immature soils, characteristics depend on type of volcanic material
AT Anthrosol
AR Arenosol Sandy soil with no more profile development than a A horizon
CL Calcisol Soil with a substantial secondary accumulation of lime
CM Cambisol Transformation of soil matter (Fe particularly) in situ
In situ
In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...

 without moving in profile. Mostly brownish color.
CH Chernozem
Chernozem
Chernozem , also known as "black land" or "black earth", is a black-coloured soil containing a high percentage of humus 7% to 15%, and high percentages of phosphoric acids, phosphorus and ammonia...

Fertile black-coloured soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 containing a high percentage of humus, phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

s, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

CR Cryosol Soil in permafrost
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...

 areas, exhibits cryoturbation
Cryoturbation
In gelisols , cryoturbation refers to the mixing of materials from various horizons of the soil right down to the bedrock due to freezing and thawing.Cryoturbation occurs to varying degrees in most gelisols...

 and is usually rich in organic matter
DU Durisol
Durisol
Durisol may refer to:* a trade name for Cement-bonded wood fiber* Durisols, soil type...

Soil of some arid and semi-arid environments, contains cemented secondary silica
FR Ferralsol Red to yellow soil rich in iron and aluminium, common in temperate to tropical humid areas
FL Fluvisol
Fluvisols
A Fluvisol in the FAO World Reference Base for Soil Resources is a genetically young soil in alluvial deposits . Apart from river sediments, they also occur in lacustrine and marine deposits....

Soil developed above flood plain sediments, A horizon is commonly directly above C horizon
GL Gleysol
GY Gypsisol
HS Histosol
Histosol
In both the FAO soil classification and the USA soil taxonomy, a histosol is a soil consisting primarily of organic materials. They are defined as having or more of organic soil material in the upper . Organic soil material has an organic carbon content of 12 to 18 percent, or more, depending on...

Soil consisting primarily of organic materials, common in wetland
Wetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....

s
KS Kastanozem
LP Leptosol Shallow soil over bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

, calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 material or a deeper soil that is gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

ly or stony, common in mountains
LX Lixisol
LV Luvisol
NT Nitisol
PH Phaeozem Sod organic-accumulative
PL Planosol
Planosol
A planosol in the FAO World Reference Base for Soil Resources is a soil with a light-coloured, coarse-textured, surface horizon that shows signs of periodic water stagnation and abruptly overlies a dense, slowly permeable subsoil with significantly more clay than the surface horizon...

PT Plinthosol
PZ Podzol Soil that presents significant podzolization
Podzolization
Podzolization is the comprehensive name for the process of mobilization and precipitation of dissolved organic matter, together with aluminum Al and iron Fe as they are leaching down from the A and E horizons to the B horizon . Through this process the overlying eluvial horizons are getting bleached...

, common in coniferous forests
RG Regosol
SC Solonchak
Solonchak
Solonchak is pale or grey soil type found in arid to subhumid, poorly drained conditions. The word is Russian for "salt marsh" in turn from Russian sol , "salt"....

SN Solonetz
Solonetz
Solonetz is a type of soil in FAO soil classification. They have, within the upper 100 cm of the soil profile, a so-called "natric horizon" . There is a subsurface horizon , higher in clay content than the upper horizon, that has more than 15% exchangeable sodium...

UM Umbrisol
Umbrisol
An Umbrisol in the FAO World Reference Base for Soil Resourcesis a soil with a dark topsoil and in which organic matter has accumulated within the mineral surface soil to the extent that it significantly affects the behaviour and utilization of the soil...

Soil with a dark topsoil and in which organic matter
Soil organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

 has accumulated significantly within the mineral surface soil
VR Vertisol
Vertisol
In both the FAO and USA soil taxonomy, a vertisol is a soil in which there is a high content of expansive clay known as montmorillonite that forms deep cracks in drier seasons or years. Alternate shrinking and swelling causes self-mulching, where the soil material consistently mixes itself, causing...

Shows significant and recurrent swelling with water, high content of expansive clay
Expansive clay
Expansive clay is a clay that is prone to large volume changes that are directly related to changes in water content.The mineral make-up of this type of soil is responsible for the moisture retaining capabilities. Soils with smectite clay minerals, including montmorillonite, have the most dramatic...


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