WRB has two levels, the first and the second level.
The first classification level comprises 32 Reference Soil Groups (RSGs), identified by a key
(Chapter 4 of the WRB document). Table 2 of the WRB document provides a summary:
|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 -|
|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 -|
|Stratified fluviatile, marine and lacustrine sediments:||Fluvisols|
|No significant profile development:||Regosols|
The second classification level provides 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. Some can be combined with many RSGs, others with only a few or even with just one. The qualifiers available for use with a particular RSG are listed in the Key (Chapter 4), along with the RSG. They are divided into principal and supplementary qualifiers. The principal qualifiers are ranked and given in an order of importance. The supplementary qualifiers are not ranked, but are, as a convention, used in alphabetical order. The number of available qualifiers ranges from 35 (Nitisols, Gypsisols) to 68 (Cambisols). Many of them are mutually exclusive.
Constructing the second level by adding qualifiers to the RSG has several advantages compared with a dichotomic key (Chapter 1 of the WRB document):
- For every soil, the RSG has the appropriate number of qualifiers.
- The soil name is informative and incorporates most of the soil’s properties.
- The system is robust. Missing data do not necessarily lead to a dramatic error in the classification of a soil. If one qualifier is erroneously added or erroneously omitted based on incomplete data, the rest of the soil name remains correct.
WRB uses diagnostic horizons, diagnostic properties and diagnostic materials, which are defined in Chapter 3 of the WRB document. Diagnostic materials are materials that significantly influence pedogenic processes or are indicative of them. They may stem from the parent material or be the result of soil-forming processes. Diagnostic properties are typical results of soil-forming processes or reflect special conditions of soil formation. Diagnostic horizons are typical results of soil-forming processes and have a minimum thickness and are therefore recognizable as horizontal layers. The definitions of many RSGs in the key (Chapter 4) and the definitions of many qualifiers (Chapter 5) rely on diagnostic horizons, diagnostic properties and diagnostic materials. In addition, many definitions ask for individual characteristics like sand content or base saturation.
Naming a soil consists of three steps (Chapter 2 of the WRB document). The first step is detecting diagnostic horizons, properties and materials considering the available field and laboratory data. The second step is identifying the RSG with the help of the key. In the third step, all applying principal and supplementary qualifiers are allocated. The principal qualifiers are added before the name of the RSG without brackets and without commas. The sequence is from right to left, i.e. the uppermost qualifier in the list is placed closest to the name of the RSG. The supplementary qualifiers are added in brackets after the name of the RSG and are separated from each other by commas. The sequence is from left to right, i.e. the first qualifier according to the alphabet is placed closest to the name of the RSG.
For naming a soil, all applying qualifiers must be listed in the soil name. For map legends, the number of qualifiers depends on the scale of the map. WRB distinguishes four scale levels (Chapter 2 of the WRB document):
a. For very small map scales (e.g. smaller than 1 : 10 000 000),
only the Reference Soil Group (RSG) is used.
b. For next larger map scales (e.g. from 1 : 5 000 000 to 1 : 10 000 000),
the RSG plus the first applicable principal qualifier are used.
c. For next larger map scales (e.g. from 1 : 1 000 000 to 1 : 5 000 000),
the RSG plus the first two applicable principal qualifiers are used.
d. For next larger map scales (e.g. from 1 : 250 000 to 1 : 1 000 000),
the RSG plus the first three applicable principal qualifiers are used.
The principal qualifiers are placed before the name of the RSG according to the rules for naming a soil. If there are fewer principal qualifiers applicable than described above, the lesser number is used.
Depending on the purpose of the map or according to national traditions, at any scale level, further qualifiers may be added optionally. They may be additional principal qualifiers from further down the list and not already used in the soil name, or they may be supplementary qualifiers. They are placed using the rules for supplementary qualifiers. If two or more optional qualifiers are used, principal qualifiers are placed first, followed by supplementary qualifiers.