World Reference Base for Soil Resources (WRB)

WRB structure

WRB has two levels for classifying soils:

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:
  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.

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:

  1. 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.
  2. 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.

  3. 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).