World Reference Base for Soil Resources (WRB)

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:

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:

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).

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.