World Reference Base for Soil Resources (WRB)
The World Reference Base for Soil Resources is an international soil classification system for naming soils and creating legends for soil maps

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 ISRIC–World Soil Information and sponsored by the IUSS and the FAO and was designed to cater for any soil in the world.
WRB is a two-tier system of soil classification, with 32 Major Soil Groups (the “Reference Base”) and over 120 uniquely defined qualifiers for specific soil characteristics (the “WRB Classification System”).
The third edition, update 2015, is the currently valid WRB: Download document (pdf, 2.3 MB).
The definitions in the WRB are based on field and laboratory data:
- Field data collection is outlined in the FAO Guidelines for Soil Description (FAO 2006).
- Laboratory procedures are outlined in the Procedures for Soil Analysis (Van Reeuwijk 2002). This is published by ISRIC–World Soil Information.
- Guidelines for constructing small-scale map legends are included in the third edition.
WRB has two levels for classifying soils. The first level comprises 32 Reference Soil Groups (RSGs), identified by a key and the second level provides more detail and leads to constructed soil names.
First level
The first level comprises 32 Reference Soil Groups (RSGs), identified by a key
(Chapter 4 of the WRB document). The table below provides a summary of the key:
1. | Soils with thick organic layers: | Histosols |
2. | Soils with strong human influence - | |
With long and intensive agricultural use: | Anthrosols | |
Containing significant amounts of artefacts: | Technosols | |
3. | Soils with limitations to root growth - | |
Permafrost-affected: | Cryosols | |
Thin or with many coarse fragments: | Leptosols | |
With a high content of exchangeable Na: | Solonetz | |
Alternating wet-dry conditions, shrink-swell clays: | Vertisols | |
High concentration of soluble salts: | Solonchaks | |
4. | Soils distinguished by Fe/Al chemistry - | |
Groundwater-affected, underwater or in tidal areas: | Gleysols | |
Allophanes or Al-humus complexes: | Andosols | |
Subsoil accumulation of organic matter and/or oxides: | Podzols | |
Accumulation and redistribution of Fe: | Plinthosols | |
Low-activity clay, P fixation, many Fe oxides, strongly structured: | Nitisols | |
Dominance of kaolinite and oxides: | Ferralsols | |
Stagnant water, abrupt textural difference: | Planosols | |
Stagnant water, structural difference and/or moderate textural difference: | Stagnosols | |
5. | Pronounced accumulation of organic matter in the mineral topsoil - | |
Very dark topsoil, secondary carbonates: | Chernozems | |
Dark topsoil, secondary carbonates: | Kastanozems | |
Dark topsoil, no secondary carbonates (unless very deep), high base status: | Phaeozems | |
Dark topsoil, low base status: | Umbrisols | |
6. | Accumulation of moderately soluble salts or non-saline substances - | |
Accumulation of, and cementation by, secondary silica: | Durisols | |
Accumulation of secondary gypsum: | Gypsisols | |
Accumulation of secondary carbonates: | Calcisols | |
7. | Soils with clay-enriched subsoil - | |
Interfingering of coarser-textured, lighter coloured material into a finer-textured, stronger coloured layer: |
Retisols | |
Low-activity clays, low base status: | Acrisols | |
Low-activity clays, high base status: | Lixisols | |
High-activity clays, low base status: | Alisols | |
High-activity clays, high base status: | Luvisols | |
8. | Soils with little or no profile differentiation - | |
Moderately developed: | Cambisols | |
Sandy: | Arenosols | |
Stratified fluviatile, marine and lacustrine sediments: | Fluvisols | |
No significant profile development: | Regosols |
The full key (Chapter 4) relies on so-called diagnostic horizons, diagnostic properties and diagnostic materials (Chapter 3) which to the greatest extent possible should be measurable and observable in the field.
Second level
The second level provides more detail and leads to constructed soil names. Adjectives, called qualifiers, are added to the name of the RSG. The qualifiers, currently 185, are defined in Chapter 5 of the WRB document. They are divided into principal qualifiers - to go in front of the soil group name - and supplementary qualifiers, to be listed after the soil group name.
How then does the naming of a soil according to WRB work? There are three steps:
- The expression, thickness and depth of layers observed are checked against the requirements of WRB diagnostic horizons, properties and materials (17). This requires a certain level of soil field expertise, and lab data of soil characteristics (pH, CEC, total carbon, particle size) will be helpful to confirm those diagnostic features.
-
The described combination of diagnostic horizons, properties and materials is
compared with the systematic WRB Key (Chapter 4) in order to find the RSG.
The soil belongs to the first RSG for which it meets all specified requirements. -
Principal/supplementary qualifiers (Chapter 5) are used to further specify the soil. Principal qualifiers are ranked and given in order of importance; supplementary qualifiers are used in alphabetical order.
An example of a fully classified WRB soil is: Chromic Stagnic Leptic Luvisol (Cutanic, Differentic, Humic, Ruptic).
The WRB is not intended to be a substitute for national soil classification systems, but rather to serve as a common denominator for communication at the international level.
Applying the WRB to New Zealand soils is still in its infancy, and the USDA Soil Taxonomy is used in most cases for communicating New Zealand soil types internationally.
ISRIC–World Soil Information have used digital soil mapping techniques to produce a global 1km resolution map of WRB reference soil groups. The map is available from the SoilGrids web mapping portal, and is being updated regularly.