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 1.2 of the WRB document provides a summary:
|1.||Soils with thick organic layers:||Histosols||HS|
|2.||Soils with strong human influence -|
|With long and intensive agricultural use:||Anthrosols||AT|
|Containing significant amounts of artefacts:||Technosols||TC|
|3.||Soils with limitations to root growth -|
|Thin or with many coarse fragments:||Leptosols||LP|
|With a high content of exchangeable Na:||Solonetz||SN|
|Alternating wet-dry conditions, shrink-swell clay minerals:||Vertisols||VR|
|High concentration of soluble salts:||Solonchaks||SC|
|4.||Soils distinguished by Fe/Al chemistry -|
|Groundwater-affected, underwater or in tidal areas:||Gleysols||GL|
|Allophanes and/or Al-humus complexes:||Andosols||AN|
|Subsoil accumulation of humus and/or oxides:||Podzols||PZ|
|Accumulation and redistribution of Fe:||Plinthosols||PT|
|Stagnant water, abrupt textural difference:||Planosols||PL|
|Stagnant water, structural difference and/or moderate textural difference:||Stagnosols||ST|
|Low-activity clays, P fixation, many Fe oxides, strongly structured:||Nitisols||NT|
|Dominance of kaolinite and oxides:||Ferralsols||FR|
|5.||Pronounced accumulation of organic matter in the mineral topsoil -|
|Very dark topsoil, secondary carbonates:||Chernozems||CH|
|Dark topsoil, secondary carbonates:||Kastanozems||KS|
|Dark topsoil, no secondary carbonates (unless very deep), high base status:||Phaeozems||PH|
|Dark topsoil, low base status:||Umbrisols||UM|
|6.||Accumulation of moderately soluble salts or non-saline substances -|
|Accumulation of, and cementation by, secondary silica:||Durisols||DU|
|Accumulation of secondary gypsum:||Gypsisols||GY|
|Accumulation of secondary carbonates:||Calcisols||CL|
|7.||Soils with clay-enriched subsoil -|
|Interfingering of coarser-textured, lighter-coloured material into a finer-textured, stronger coloured layer:||Retisols||RT|
|Low-activity clays, low base status:||Acrisols||AC|
|Low-activity clays, high base status:||Lixisols||LX|
|High-activity clays, low base status:||Alisols||AL|
|High-activity clays, high base status:||Luvisols||LV|
|8.||Soils with little or no profile differentiation -|
|Stratified fluviatile, marine or lacustrine sediments:||Fluvisols||FL|
|No significant profile development:||Regosols||RG|
The second classification level provides constructed soil names. Adjectives, called qualifiers , are added to the name of the RSG. The qualifiers, currently 202, 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 start with the qualifiers related to texture (if present for the respective RSG) followed by all others in alphabetical order. The number of available qualifiers ranges from 40 (Nitisols) to 79 (Gleysols). 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):
- Every soil receives 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 like diagnostic properties, but with a minimum thickness and therefore recognizable as horizontal layer. 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. First, the qualifiers related to texture (if present for the respective RSG) are added, followed by all others in alphabetical order.
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 three scale levels (Chapter 2 of the WRB document):
- a. For very small map scales, only the Reference Soil Group (RSG) is used.
- b. For next larger map scales, the RSG plus the first applicable principal qualifier are used.
- c. For next larger map scales, the RSG plus the first two applicable principal qualifiers are used.
It is not possible to give general numbers for these scales, because this depends very much on the homogeneity or heterogeneity of the landscape. In landscapes of intermediate homogeneity, very small scales would be smaller than 1 : 10 000 000, the next larger scales smaller than 1 : 5 000 000 and the next larger scales smaller than 1 : 1 000 000.
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, elective qualifiers may be added. 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 elective qualifiers are used, principal qualifiers are placed first, followed by supplementary qualifiers.