World Reference Base for Soil Resources


The World Reference Base for Soil Resources is an international soil classification system for naming soils and creating legends for soil maps. The currently valid version is the fourth edition 2022. It is edited by a working group of the International Union of Soil Sciences.
file:WRB,_4th_edition,_Cover.jpg|thumb|WRB, 4th edition

Background

History

Since the 19th century, several countries developed national soil classification systems. During the 20th century, the need for an international soil classification system became more and more obvious.
From 1971 to 1981, the Food and Agriculture Organization. The Legend for this map, published in 1974 under the leadership of Rudi Dudal, became the FAO soil classification. Many ideas from national soil classification systems were brought together in this worldwide-applicable system, among them the idea of diagnostic horizons as established in the '7th approximation to the USDA soil taxonomy' from 1960. The next step was the Revised Legend of the Soil Map of the World, published in 1988.
In 1982, the International Soil Science Society established a working group named International Reference Base for Soil Classification. Chair of this working group was. Its mandate was to develop an international soil classification system that should better consider soil-forming processes than the FAO soil classification. Drafts were presented in 1982 and 1990.
In 1992, the IRB working group decided to develop a new system named World Reference Base for Soil Resources that should further develop the Revised Legend of the FAO soil classification and include some ideas of the more systematic IRB approach. Otto Spaargaren and Freddy Nachtergaele were nominated to prepare a draft. This draft was presented at the 15th World Congress of Soil Science in Acapulco in 1994. At the same congress, the WRB was established as an ISSS working group replacing the IRB. At the 16th World Congress of Soil Science in Montpellier in 1998, the first edition of the WRB was published. At the same congress, the ISSS endorsed the WRB as its correlation system for soil classification. At the 18th World Congress of Soil Science in Philadelphia in 2006, the second edition of the WRB was presented, and at the 20th World Congress of Soil Science in Jeju in 2014, the third edition. An update of the third edition was issued in 2015. Whereas the second edition was only suitable for naming soils, the third and the following edition can additionally be used for creating map legends. At the 22nd World Congress of Soil Science in Glasgow in 2022, the fourth edition was published. The 4th edition is an open access document under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
The WRB has two hierarchical levels and has in that sense a similar approach as the French référencial pédologique. Contrary to that, the USDA soil taxonomy is strongly hierarchical and has six levels. The classification in WRB is based mainly on soil morphology as an expression of pedogenesis. Another difference with USDA soil taxonomy is that soil climate is regarded only as a soil-forming factor and not as a soil characteristic. The WRB is not meant to replace national soil classification systems, which, for their area, may be more detailed than the WRB.

WRB Working Group

The WRB is edited by a working group of the International Union of Soil Sciences. The current chair of the working group is Cezary Kabala. The current vice-chair is Stephan Mantel.
Chairs of the WRB working group and responsible first authors of the WRB editions are: Seppe Deckers, Erika Michéli and Peter Schad.
The WRB working group has a homepage that is currently hosted by the ISRIC. It provides the following:
  • the currently valid fourth edition of the WRB for download,
  • the third edition with the English original and the translations into Czech, French, Georgian, Polish, Russian, Slovene, and Spanish,
  • an explanation of the system,
  • soil profile photos of all RSGs, which may be downloaded and used if the author is accredited,
  • the history of the WRB,
  • the WRB leadership,
  • information about past and upcoming workshops,
  • teaching material,
  • invitations for publications,
  • links to other institutions important for the WRB.

    The WRB 2022

Architecture

The classification is based on diagnostic horizons, diagnostic properties and diagnostic materials, altogether called diagnostics:
  • Diagnostic materials are materials that significantly influence soil-forming processes or are indicative of them. They may be inherited from the parent material or be the result of soil-forming processes.
  • Diagnostic properties are typical results of soil-forming processes or reflect specific conditions of soil formation.
  • Diagnostic horizons are typical results of soil-forming processes showing a minimum thickness and therefore a horizontal appearance,thus forming a recognizable layer in the soil. The diagnostics have names.
For example, diagnostic materials related to the concentration of organic carbon:
  • Mineral material < 20% soil organic carbon
  • Organic material ≥ 20% soil organic carbon
  • Soil organic carbon organic carbon that does not meet the diagnostic criteria of artefacts.
The classification comprises two levels:
The first level has 32 Reference Soil Groups.
At the second level, for further differentiation a set of qualifiers is added to the name of the RSG. There are 202 qualifiers in total. For every RSG, there is a list of available qualifiers, which are subdivided into two types:
  • Principal qualifiers are ranked and given in an order of importance. The ranking of the principal qualifiers reflects major subdivisions of the respective RSG or properties strongly influencing the soil’s functionality.
  • Supplementary qualifiers describe additional characteristics and are not ranked.
Qualifiers may be principal for some RSGs and supplementary for others. The names of the RSGs and the qualifiers start with capital letters. They must be given in English and must not be translated into any other language in order to guarantee that a certain soil has the same name all over the world.

Naming a soil

A key is used for allocating a soil to a certain RSG. In a defined sequence, the key asks for the presence or absence of certain diagnostics in a certain depth range. In addition, the key asks for single characteristics, e. g., a certain clay content or a certain base saturation. The soil belongs to the first RSG, for which it fulfils the set of criteria.
The qualifiers available for use with a particular RSG are listed in the key, along with the RSG. Their number is from 40 to 79. All applying qualifiers must be added to the soil name. Qualifiers:
  • The principal qualifiers are added before the name of the RSG. The sequence is from right to left, i. e., the uppermost qualifier in the list is placed closest to the name of the RSG. If no other principal qualifier applies, the Haplic qualifier is used.
  • 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. Supplementary qualifiers related to the texture, if applicable, are the first in the list. If several ones apply, they are placed in the sequence from the top to the bottom of the soil profile. All other supplementary qualifiers follow them and are used in alphabetical order.
If two or more qualifiers in the list are separated by a slash, only one of them can be used. The slash signifies that these qualifiers are either mutually exclusive or one of them is redundant with the redundant qualifier listed after the slash. In the soil name, supplementary qualifiers are always placed in the order of the alphabet, even if their position in the list differs from the alphabetical sequence due to the use of the slash. It is a general rule that qualifiers conveying redundant information are not used. Example: If a soil has the Calcaric qualifier the Eutric qualifier is not used.
Qualifiers may be combined with specifiers to form subqualifiers. The depth-related specifiers referring to layers are of special importance, although their use is optional:
  • Epi-: only between ≥0 and ≤50 cm,
  • Endo-: only below ≥50 cm,
  • Amphi-: starting between >0 and <50 and ending between >50 and <100 cm,
  • Ano-: starting at 0 and ending between >50 and <100 cm,
  • Kato-: starting between >0 and <50 and ending at ≥100 cm,
  • Poly-:
  • * diagnostic horizons: two or more diagnostic horizons are present at the depth required by the qualifier definition, interrupted by layers that do not fulfil the criteria of the respective diagnostic horizon;
  • * other layers: two or more layers within 100 cm fulfil the criteria of the qualifier, interrupted by layers that do not fulfil the criteria of the respective qualifier,
  • Panto-: from 0 to ≥100 cm.

    Creating map legends with the WRB

The number of qualifiers used in a map legend depends on the scale. The WRB distinguishes three map scale levels:
  • first map scale level: RSG only,
  • second map scale level: the RSG plus the first applying principal qualifier,
  • third map scale level: the RSG plus the first two applying principal qualifiers.
Correlating the map scale levels with concrete scales is difficult because selecting a map scale level depends very much from the homogeneity/heterogeneity of the landscape.
The principal qualifiers are added before the name of the RSG following the rules explained for naming a soil. 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 above-mentioned rules for supplementary qualifiers; principal qualifiers first, then supplementary qualifiers.
The WRB recommends that on a map unit not just one soil is indicated but an association of soils. For this purpose, WRB uses the following nomenclature:
  • dominant: the soil represents ≥ 50% of the soil cover,
  • codominant: the soil represents ≥ 25 to < 50% of the soil cover,
  • associated: the soil represents ≥ 5 to < 25% of the soil cover.
Soils representing smaller areas are ignored in the denomination of the map unit.
For codominant and associated soils, it is allowed to use less principal qualifiers than would correspondent to the used map scale level. The use of specifiers is not recommended due to the generalization that is required when making maps. In map legends, the names of the RSGs are given in plural; in all other cases they are given in singular.