World Reference Base for Soil Resources

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.

Key

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 (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

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

References

• 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).

External links

• World Reference Base for Soil Resources 2014 The latest WRB classification manual
• ISRIC Lecture notes on the Major Soils of the World. 2001.(with maps showing their distribution, soil profiles)
• WRB home page at the FAO
• WRB links at the Compendium of On-line Soil Survey Information