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The World Reference Base for Soil Resources

The WRB as a soil correlation system
Rules for identifying Soil Units
Ranking qualifiers in Soil Unit names
Polygenetic and buried soils


The WRB as a soil correlation system

The objectives of the World Reference Base are twofold. On the one hand the WRB is intended to be a reference system for users interested in a broad division of soils, at the highest level of generalisation and explained in non-technical terms. On the other hand, the WRB must facilitate soil correlation across a wide range of national soil classification systems.

To best reconcile such conflicting requirements, it was decided to design the WRB as a flexible system, with maximum use of `morphometric' (from Gr. morphos `shape' and L. metrum `size') soil profile information, but with rigidly standardised definitions. Using standardised diagnostic criteria and qualifiers facilitates soil correlation and technology transfer between countries and regions, which helps to better understand (relations between) soil resources and facilitates regional application of soil information, e.g. in land use planning.

Reference Soil Groups are distinguished by the presence (or absence) of specific diagnostic horizons, properties and/or materials. A limited number of `qualifiers', with unique definitions, describe individual Soil Units within Reference Soil Groups.

Annex 1 to this text presents the full key for identifying WRB Reference Soil Groups; Annex 2 defines the diagnostic horizons, properties and materials used to define the various Reference Soil Groups.

Note that the number of Reference Soil Groups in the WRB is fixed (30) but the number of Soil Units is not. Soil Units are distinguished on the basis of distinct `Rules for identifying Soil Units' (see hereafter); qualifiers used to identify Soil Units are presented in Annex 3.

Rules for identifying Soil Units

  1. Soil units are defined, and named, on the basis of WRB-approved `qualifiers'. See Annex 3.
  2. Qualifier names can be used in combination with indicators of depth, thickness or intensity. For instance, an Epi-Dystric Luvisol is a soil unit name in which `Epi-' signifies shallow depth whereas `Dystric' is a qualifier indicative of a low base status. If more than two qualifiers are needed, these are listed behind the Reference Soil Group name (between brackets), e.g. Acri-Geric Ferralsol (Abruptic and Xanthic).
  3. Names of soil units must not overlap or conflict with names of other soil units or with Reference Soil Group definitions. For example, a "Dystri-Petric Calcisol" is unacceptable because it contains a contradiction (`Dystri-' is incompatible with `Calcisol') and a "Eutri-Petric Calcisol" is rejected because the qualifier "Eutri-" overlaps with information inherent to the Reference Soil Group name "Calcisol".
  4. New units can only be established if documented by a soil profile description and supporting laboratory analyses.

Qualifiers are defined by unique sets of diagnostic criteria. Most diagnostic criteria in qualifier definitions are derived from already established Reference Soil Group criteria such as diagnostic horizons, properties and materials. Weak or incomplete occurrences of features are generally not considered to be differentiating. Attributes referring to climate, parent material, vegetation or to physiographic features such as slope, geomorphology or erosion, are not used to differentiate between soil units. Neither are soil-water related attributes such as depth of water table or drainage, substratum specifications, nor specifications of thickness and/or morphology of the solum or individual horizons.

Ranking qualifiers in Soil Unit names

It is widely felt that indiscriminate use of qualifiers would create confusion but the precise ranking of qualifiers in Soil Unit names is currently still under discussion. Annex 4 presents tentative ranking orders suggested for common qualifiers within each Reference Soil Group.

An example:

Within the Reference Soil Group of the Vertisols, the following qualifiers are considered to be `common' (See Annex 4):




intergrade to acid sulphate Gleysols, Fluvisols and Cambisols



intergrade to the Reference Soil Group of the Solonchaks



intergrade to the Reference Soil Group of the Solonetz



intergrade to the Reference Soil Group of the Gypsisols



intergrade to the Reference Soil Group of the Durisols



intergrade to the Reference Soil Group of the Calcisols



intergrade to the Reference Soil Group of the Alisols




containing gypsum



having a mulched surface horizon



having a very hard surface horizon; workability problems



having less than 75 percent base saturation



having an ESP of 6 to 15



having 50 percent or more base saturation



dark coloured, often poorly drained



reddish coloured



no specific characteristics

A reddish coloured Soil Unit within the Reference Soil Group of the Vertisols, having a calcic horizon, would be classified as a Calci-Chromic Vertisol because qualifiers 6 and 15 apply. If information on depth and intensity of the calcic horizon is available, e.g. occurring near the surface, one would classify the soil as an EpiCalci-Chromic Vertisol (indicating that the calcic horizon occurs within 50 cm from the surface).

If more than two qualifiers are needed, these are added behind the Reference Soil Group name. If, for instance, the Vertisol discussed would also feature a very hard surface horizon (qualifier 10), the soil would be named a Calci-Chromic Vertisol (Mazic).

Polygenetic and buried soils

Soils have vertical and horizontal dimensions that evolved over time. The vertical dimension is for practical purposes limited to a "control section" with a depth of 100 cm or, exceptionally, 200 cm below the surface. The qualifier bathic can be used to refer to horizons, properties or characteristics that occur below the control section.

Most soil profiles can be named without difficulty but some, more complex situations require additional classification guidelines. The WRB prefers to name soils as they occur, i.e. with present-day characteristics and functional behaviour, rather than emphasising their (supposed) genetic history. It is realised however that few soils have completely evolved in situ and that it may be useful in certain cases to indicate this. Some soils show signs of polygenetic development i.e. a different soil has evolved prior to the present one (often under different environmental conditions) and both soils can be classified. A qualifier thapto- indicates the presence of a buried soil or a buried horizon. This would be the case if a soil has a surface mantel of new material that is 50 cm thick or more. The surface mantel is named in the normal way (e.g. as a Regosol, Andosol or Arenosol) and the buried soil would be classified with a prefix qualifier `thapto-`. If the surface mantle is less than 50 cm thick, it is ignored in the soil name but the soil may be marked on the soil map by a phase indicator.

Note that it is not recommended to systematically include the qualifier thapto- if this adds no information that has practical implications for the user.

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