In 1998, the International Union of Soil Sciences (IUSS) adopted the World Reference Base for Soil Resources (WRB) as the official system for international correlation of soils. In addition, the WRB is the accepted common scheme of soil databases and maps of the European Union (see Figure C37).

Figure C37
figure 1_104

World Soil Resources global map.

Full size image

The structure, concepts and definitions of the WRB are strongly influenced by the revised legend of the FAO/UNESCO Soil Map of the World (FAO/UNESCO‐ISRIC, 1988), which in turn borrowed the diagnostic horizons and properties approach from USDA Soil Taxonomy.

The taxonomic units of the WRB are defined in terms of measurable and observable ‘diagnostic horizons’, the basic identifiers in soil classification. Diagnostic horizons are defined by (combinations of) characteristic ‘soil properties’ and/or ‘soil materials’.

WRB comprises two levels:

  1. 1.

    the “Reference Base”, which is limited to the first level only having 30 reference soil groups; and

  2. 2.

    the second level consists of combinations of qualifiers added to the reference soil groups providing more specific characterization and classification of soils.

Short summary of the reference groups

Organic soils, such as peat, are brought together in one reference soil group called Histosols while all man‐made soils, which vary widely in properties and appearance but have in common that their properties are strongly affected by human intervention are aggregated to the Anthrosols reference soil group. In some man altered environments non‐natural materials, called ‘technic materials’ dominate the unit, therefore Technosols have now been added as a new Reference Group of the WRB.

Mineral soil whose formation is conditioned by the particular properties of their parent material are sub‐divided into the Andosols of volcanic regions, the sandy Arenosols of desert areas, beach ridges, inland dunes or areas with highly weathered sandstone and the swelling / shrinking heavy clayey Vertisols of back‐swamps, river basins, lake bottoms and other areas with a high clay content.

Mineral soils whose formation was influenced by their topographic setting (for example, soils associated with recurrent floods or on steep terrain) range from the Fluvisols, which show stratification or other evidence of recent alluvial sedimentation, non‐stratified Gleysols in waterlogged areas and shallow Leptosols over hard rock or highly calcareous material, to the deeper Regosols, which occur in unconsolidated materials that have a minimal profile development because of low soil temperatures, prolonged dryness or erosion.

Soils that are only moderately developed on account of their young pedogenetic age or because of rejuvenation of the soil material are referred to as Cambisols.

The wet tropical and subtropical regions where high soil temperatures and ample moisture promotes rock weathering, rapid decay of soil organic matter, and a long history of dissolution and transport of weathering products have five types of deep and mature soil types. Plinthosols are marked by the presence of a mixture of clay and quartz (‘plinthite’) that hardens irreversibly upon exposure to the open air while deeply weathered Ferralsols have a very low cation exchange capacity and are virtually devoid of weatherable minerals. Alisols have high cation exchange capacity and much exchangeable aluminum, Nitisols have deep profiles in relatively rich parent material, Acrisols develop on acid parent rock with a clay accumulation horizon, low cation exchange capacity and low base saturation while Lixisols possess a low cation exchange capacity but high base saturation percentage.

Soil in arid and semi‐arid regions are differentiated to Solonchaks with a high content of soluble salts, Solonetz with a high percentage of adsorbed sodium ions, Gypsisols with a horizon of secondary gypsum enrichment, Durisols with a layer or nodules of soil material that is cemented by silica and Calcisols with secondary carbonate enrichment.

Soils that occur in the steppe zone between dry and humid temperate climates where vegetation consists of ephemeral grasses and dry forest classify to three reference soil groups. Chernozems with deep, very dark surface soil and carbonate enrichment in the subsoil, Kastanozems with less deep, brownish surface soils and carbonate and/or gypsum accumulation at some depth and the Phaeozems, the dusky red soils of prairie regions with high base saturation but no visible signs of secondary carbonate accumulation.

The brownish and greyish soils of humid temperate regions show evidence of clay or organic matter redistribution. Eluviation and illuviation of metal‐humus complexes produce the greyish (bleaching) and brown to black (coating) colors of soils. Five reference soil groups include the acid Podzols with a bleached eluviation horizon over an accumulation horizon of organic matter with aluminum and/or iron, Planosols with a bleached topsoil over dense, slowly permeable subsoil, base‐poor Albeluvisols with a bleached eluviation horizon tonguing into a clay‐enriched subsurface horizon, base‐rich Luvisols with a distinct clay accumulation horizon and Umbrisols with a thick, dark, acid surface horizon that is high in organic matter.

All soils of permafrost regions that show signs of ‘cryoturbation’ (i.e., disturbance by freeze‐thaw sequences and ice segregation) are assembled in one reference soil group, the Cryosols.

A simplified key to the 30 reference groups is given in Table C12. It is based on the presence, sequence or exclusion of diagnostic horizons and additional diagnostic features. Further characterization of soils is possible with the application of the qualifiers. Each qualifier has a unique meaning generally derived from the definitions of diagnostic features. The qualifiers are listed in table format in priority sequence for each Reference Groups. Two qualifiers may be used in soil unit names. If additional qualifiers are needed, they follow the reference soil group name between brackets. For example: Silti‐Calcic Chernozem (Vermic).

Table C12 Simplified key to the WRB Reference Groups (modified from Deckers et al., 2002)
Full size table

Cross‐references

Classification of Soils: FAO

Classification of Soils: World Reference Base (WRB) Soil Profiles

Classification of Soils: Soil Taxonomy

Horizon Designations in the WRB: See also individual articles on each of the Reference groups