World Reference Base for Soil Resources

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The World Reference Base for Soil Resources (WRB) is the international standard taxonomic soil classification 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 (ISRIC) and sponsored by the IUSS and the FAO via its Land & Water Development division. It replaces the previous FAO soil classification.

The WRB borrows heavily from modern soil classification concepts, including USDA soil taxonomy, 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 as an expression of pedogenesis. A major difference with USDA soil taxonomy 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. The second edition was not detailed enough for mapping at scales larger than about 1:200 000, and a third edition has been published, improving the system for soil mapping.[1][2]


Identification key to the 32 reference soil groups:[3]

1. Soils with thick organic layers: Histosols (HS)
2. Soils with strong human influence
Soils with long and intensive agricultural use: Anthrosols (AT)
Soils containing many artefacts: Technosols (TC)
3. Soils with limited rooting due to shallow permafrost or stoniness
Ice-affected soils: Cryosols (CR)
Shallow or extremely gravelly soils: Leptosols (LP)
4. Soils influenced by water
Alternating wet-dry conditions, rich in swelling clays: Vertisols (VR)
Floodplains, tidal marshes: Fluvisols (FL)
Alkaline soils: Solonetz (SN)
Salt enrichment upon evaporation: Solonchaks (SC)
Groundwater affected soils: Gleysols (GL)
5. Soils set by Fe/Al chemistry
Allophanes or Al-humus complexes: Andosols (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 (GY)
Silica: Durisols (DU)
Calcium carbonate: Calcisols (CL)
9. Soils with a clay-enriched subsoil
Albeluvic tonguing: Albeluvisols (AB)
Low base status, high-activity clay: Alisols (AL)
Low base status, low-activity clay: Acrisols (AC)
High base status, high-activity clay: Luvisols (LV)
High base status, low-activity clay: Lixisols (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 (CM)
Soils with no significant profile development: Regosols (RG)

Soil groups[edit]

Code Soil type Brief description
AC Acrisol 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 an A horizon
CL Calcisol Soil with a substantial secondary accumulation of lime
CM Cambisol Transformation of soil matter (Fe particularly) in situ without moving in profile. Mostly brownish color.
CH Chernozem Fertile black-coloured soil containing a high percentage of humus, phosphoric acids, phosphorus and ammonia
CR Cryosol Soil in permafrost areas, exhibits cryoturbation and is usually rich in organic matter
DU Durisol 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 Soil developed above flood plain sediments, A horizon is commonly directly above C horizon
GL Gleysol
GY Gypsisol
HS Histosol Soil consisting primarily of organic materials, common in wetlands
KS Kastanozems
LP Leptosol Shallow soil over bedrock, calcareous material or a deeper soil that is gravelly or stony, common in mountains
LX Lixisol
LV Luvisol
NT Nitisol
PH Phaeozem Sod organic-accumulative
PL Planosol
PT Plinthosol
PZ Podzol Soil that presents significant podzolization, common in coniferous forests
RG Regosol
SC Solonchak
SN Solonetz
UM Umbrisol Soil with a dark topsoil and in which organic matter has accumulated significantly within the mineral surface soil
VR Vertisol Shows significant and recurrent swelling with water, high content of expansive clay


  1. ^ IUSS Working Group WRB (2014). World Reference Base for Soil Resources 2014. International soil classification system for naming soils and creating legends for soil maps (PDF) (3rd ed.). Rome: FAO. ISBN 978-92-5-108370-3. Retrieved 29 August 2014. 
  2. ^ Schad, P. (2014). "Presenting the 3rd edition of WRB" (PDF). Geophysical Research Abstracts 16. Retrieved 29 August 2014. 
  3. ^ IUSS Working Group WRB. (2006). World reference base for soil resources 2006. 2nd edition. World Soil Resources Reports No. 103. FAO, Rome. ISBN 92-5-105511-4,
  • Bridges, E. M. (1997). World soils (3rd ed.). Cambridge: Cambridge University Press.
  • Bridges, E. M., Batjes, N. H., & Nachtergaele, F. O. (Eds.). (1998). World Reference Base for soil resources: atlas. Leuven: ACCO.
  • Deckers, J. A., Nachtergaele, F. O., & Spaargaren, O. C. (Eds.). (1998). World Reference Base for soil resources: introduction. Leuven: ACCO.
  • Driessen, P., Deckers, J., Spaargaren, O., & Nachtergaele, F. (Eds.). (2001). Lecture notes on the major soils of the world. Rome: FAO.
  • FAO. (1998). World Reference Base for Soil Resources. Rome: Food and Agriculture Organization of the United Nations.
  • Jahn, R., Joisten, H., & Kabala, C. (2004). The “Reference Soil Series” Concept of the First European Joint Soil Map at a Scale of 1:50 000, Sheet Zittau – a Framework to Upgrade the Information Content of Lower Level WRB Units. Paper presented at the EUROSOIL 2004, Freiburg im Breisgau (D).

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