World Reference Base

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The World Reference Base for Soil Resources (abbreviation WRB ) is an international soil classification system for naming soils and creating legends for soil maps. It can be used worldwide and is used to exchange information about soils and their properties across regional and national borders.

Origin and background

Since the end of the 19th century, systems have emerged in many countries to describe soils and to divide them into classes. In the second half of the 20th century, however, it became increasingly clear that this development had led to several problems:

  • The independent development of the national classification systems had created an international problem of understanding. Individual terms appeared in different classifications, but each had different meanings. So which is podsolisation in Germany for the relocation of aluminum, iron and humus . In Russia, however, it also includes the relocation of clay . In addition, different names existed for one and the same soil.
  • The national systems are adapted to the circumstances of the respective country. So it happened that they are highly specific for their areas of origin, but hardly treat soils in other zones. Most classification systems even lack entire subject areas. The internationally important soil types of frost or salt soils , which do not even appear in the German soil system, can be named here as an example . At that time, no system was able to treat all soils in the world satisfactorily.
  • Although every system has weaknesses and cannot be used worldwide, all of them have great, non-repellant strengths. Since they are optimally adapted to the customary floors, they contain an enormous amount of attention to detail. If they had simply been abolished, this would have meant a loss of data and description options. Soils are so complex that a user-friendly classification for worldwide use can hardly achieve the accuracy of the national systems.
  • In addition, there were national interests, since no state was prepared to simply give up its classification, which had grown over decades and was taught in all educational institutions.

The United States began to radically reshape its classification to cover all soils around the world. The draft of their new system was presented in 1960 and the first edition published in 1975 under the name Soil Taxonomy . Despite strong efforts on the part of the US, it was not officially adopted internationally until 2014.

From 1971 to 1981, the FAO, together with Unesco, published the world soil map on a scale of 1: 5 million in 10 sheets. The legend for this map was published in 1974 and became known as the FAO Soil Classification . Under the leadership of Rudi Dudal, soil scientists from numerous countries contributed to their development, including Germany. Many ideas from existing systems were incorporated, e.g. B. the principle of diagnostic horizons from the draft of the Soil Taxonomy . In 1988 a Revised Legend was published.

In 1982 the International Soil Science Society (ISSS; now: International Union of Soil Sciences, IUSS) set up the International Reference Base for Soil Classification (IRB) working group. Ernst Schlichting became chairman . The working group was supposed to design an international soil classification system that was more genetically oriented than the FAO soil classification. She presented two designs, one in 1982 and one in 1990.

In 1992 the IRB working group decided to develop a new system called the World Reference Base for Soil Resources (WRB). It should be a further development of the Revised Legend of the FAO Soil Classification, taking into account soil genetic aspects from the IRB. Otto Spaargaren ( International Soil Reference and Information Center ) and Freddy Nachtergaele ( FAO ) were commissioned to develop a design. This was presented in 1994 at the 15th World Soil Congress in Acapulco. At the same congress, the WRB was set up instead of the IRB as a working group of the ISSS. At the 16th World Soil Science Congress in Montpellier in 1998, the first edition of the WRB was finally published. At the same congress, the ISSS established the WRB as the international reference system for soil classification. (In 2014 the US Soil Taxonomy also received the status of a reference system.) In 2006 a new, heavily revised version of the WRB was published, which incorporated suggestions for improvement from eight years of practical experience, and a corrected version was published in 2007. For the first time, there is an official German translation of the version from 2006. The 2014 version was published for the 20th World Soil Science Congress. In 2015 there was an update. In contrast to the 2006 version, it is not only suitable for naming individual soil profiles, but also for creating soil map legends.

A special feature of the WRB is that it should not replace the national classifications. It represents an international language to clearly publish findings from soil science worldwide and to exchange them more easily. This is also evident from the name, because WRB stands for World Reference Base or in German: worldwide reference base.

The WRB is also recognized in Germany, even if the German Soil Systematics ( KA5 ) is primarily used and taught at universities. In German-language specialist literature, the WRB designations are sometimes given parallel to those of the German soil systematics. There is no clear translation of the German soil designations into those of the WRB. The Federal Institute for Geosciences and Natural Resources (BGR) enables program-supported derivation of the WRB name from terrain data according to KA5 and laboratory values with the free derivation key . The authors of this tool are Einar Eberhardt (BGR), Peter Schad ( Technical University of Munich ) and Carsten Lehmann and Roland Harhoff (BGR).

The French Référentiel pédologique and the Soil Classification of Switzerland (KlaBS) have a similar approach to a reference framework . The Soil Taxonomy, on the other hand, has a hierarchical structure and also uses properties that are not soil properties in the narrower sense (e.g. the soil moisture regime).

The editors of the WRB

The WRB is published by the working group of the same name of the International Union of Soil Sciences (IUSS). The current chairman is Peter Schad ( Technical University of Munich , since 2010). Stephan Mantel ( International Soil Reference and Information Center , Netherlands, since 2018) is the deputy chairman .

Chairman at the time of the first edition in 1998 (and thus first publisher) was Seppe Deckers (Belgium), at the time of the second edition in 2006 Erika Michéli (Hungary) and at the time of the third edition in 2014 Peter Schad (Germany).

The WRB working group has its own homepage, which is currently located at the Chair of Soil Science at the Technical University of Munich . There you will find:

  • the currently valid documents of the WRB 2015 for download, i.e. the English original as well as the translations (so far: French, Spanish, Russian, Georgian, Slovenian and Czech - the Polish translation is only available in book form),
  • an explanation of the system,
  • Photos of profiles of all reference soil groups, which can be downloaded and used if the author of the photo is named (further photos can be found on the World of Soils page of the IUSS, see web links),
  • a presentation of the history of the WRB,
  • References to past and upcoming workshops,
  • Links to other institutions relevant to the WRB.

The WRB 2015

The structure of the WRB

Basis of the classification are diagnostic Horizonte ( Diagnostic Horizons ), diagnostic properties ( diagnostic properties ) and diagnostic materials ( diagnostic materials ). Diagnostic materials are materials that significantly influence soil-forming processes. They are either inherited from the parent rock or created through soil-forming processes. Diagnostic properties are typical results of soil-forming processes or special soil formation conditions. Diagnostic horizons are typical results of soil-forming processes that occur horizontally over a certain minimum thickness. The diagnostic horizons, properties and materials are combined as diagnostics ( diagnostics called).

The diagnostics have names (e.g. argic horizon, stagnic properties, fluvic material). The WRB does not use any horizon symbols (i.e. no Ah horizons, Bt horizons, etc.). Horizons that are not diagnostic horizons thus remain without names. The horizon symbols of the FAO Guidelines for Soil Description (2006) are recommended for naming horizon sequences in international publications.

The classification is divided into two levels:

On the first level reference soil groups ( reference soil groups - RSG) distinction, of which there are 32 (1998: 30).

On the second level there are qualifiers that further differentiate the reference floor assembly . The WRB 2015 has a total of 185 different qualifiers. There are two types of qualifiers:

  1. Principal qualifiers , which identify the typical characteristics of an RSG. They are arranged hierarchically in the order of their importance and are placed in front of the name of the RSG.
  2. Supplementary qualifiers that name further properties. They are not structured hierarchically and are listed in alphabetical order. They appear after the name of the RSG in brackets and separated by commas.

The names of the RSGs and the qualifiers are capitalized. They are not translated into other languages ​​in order to guarantee uniform soil names worldwide.

The names used for the soil types are predominantly artificial names that have their roots in Latin, the Greek language and, among the modern languages, especially Russian.

Due to the practically non-hierarchical division, it is possible to address several thousand different floor sub-units.

If the current WRB is compared with the legend of the old FAO world soil map, it is noticeable that the reference soil groups are more genetically defined and statements about usage potentials and usage restrictions have been shifted to the qualifier level in some cases. Most of the earlier soil phases were included directly in the classification. So were z. B. Floors with duripan phase to Durisols, and floors with stony phase have the Skeletic Qualifier.

Soil naming with the WRB

The classification of a soil in the respective RSG is based on a key, in which the presence and depth of diagnostics is asked in a clearly defined order. In addition, individual characteristics are asked for, such as certain clay contents or base saturation. The classification in an RSG is based on the first fully met set of criteria.

For each RSG there is a list of available qualifiers. The shortest list (Nitisols, Gypsisols) contains 35, the longest (Cambisols) 68 qualifiers. From the available qualifiers, all that apply must be added to the soil name. The order of the principal qualifiers is from right to left. The higher up in the list there is a Principal Qualifier, the further to the right (the closer to the name of the RSG) it is. Following the same principle, the supplementary qualifiers are given, from left to right. The further to the beginning of the alphabet a Supplementary Qualifier is, the further to the left (the closer to the name of the RSG) it is. If no other Principal Qualifier on the list applies, the floor receives the Principal Qualifier Haplic ("normal"). Some qualifiers are separated from each other with a /. In this case only one is used. Either they are mutually exclusive (e.g. Eutric / Dystric for high and low bass saturation) or the preceding one includes the following (e.g. Dolomitic / Calcaric for dolomitic and calcareous soil material). In general, redundant qualifiers are also not used. So has a floor z. B. the Calcaric Qualifier (primary carbonate from 20 to 100 cm), the Eutric Qualifier (high base saturation) is not used.

The qualifiers can be refined in their statement with the help of specifiers , for example if a qualifier only refers to part of the soil profile. The specifier is added as a prefix (s) in front of the corresponding qualifier. The depth-related specifiers are particularly important, but their use is optional:

  • Epi-: only within ≥0 to ≤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,
  • Panto-: from 0 to ≥100 cm.

Creation of map legends with the WRB

While the naming of an individual soil (a soil profile) requires the listing of all applicable qualifiers, the level of detail of soil maps is determined by the scale. The WRB distinguishes between four levels of measurement:

  • first level of scale: only the RSG,
  • second benchmark level: the RSG plus the first applicable principal qualifier,
  • Third benchmark level: the RSG plus the first two applicable principal qualifiers,
  • fourth level of measurement: the RSG plus the first three applicable principal qualifiers.

The assignment of the scale levels to real scales (e.g. fourth scale level for scales from 1: 250,000 to 1: 1,000,000) is only possible to a limited extent, as the choice of the scale level depends heavily on the homogeneity or heterogeneity of the landscape.

The principal qualifiers are placed in front of the name of the RSG, following the same rules as for naming soil profiles. In addition, it is permitted at all scales to optionally add further qualifiers after the name of the RSG, separated from each other in brackets and with commas. These can be principal qualifiers that are further down the list and are actually not yet taken into account at the relevant benchmark level. But they can also be supplementary qualifiers. This enables the creators of the cards to communicate features that they consider important to the readership. Since different characteristics are seen as particularly important in different national soil classification systems, this enables the preservation of national traditions without violating the uniform structure of the legends.

The WRB expressly encourages not only showing a single soil on a mapping unit, but always an association of soils. The following terms apply to this:

  • dominant: the soil takes up> 50% of the area,
  • codominant: the soil takes up 25 - 50% of the area,
  • associated: the soil takes up 5 - 25% of the area.

When naming codominant and associated soils in the mapping unit, it is permitted to use fewer principal qualifiers than corresponds to the scale level. The use of specifiers is usually not possible because of the generalizations required. In map legends, the RSGs are in the plural, otherwise always in the singular.

The structure of the WRB document

The WRB document consists of five chapters and four annexes.

Chapter 1 gives a brief description of the background and the basics. This also includes tables of the diagnostic horizons and the RSGs. The latter is shown below. Chapter 2 names the rules for soil classification and for creating map legends. It is strongly recommended that you read this short chapter before using the WRB. Chapter 3 shows the diagnostic horizons, properties and materials with a general description, the diagnostic criteria and further information. For the decision as to whether a certain diagnostic agent is available, however, only the diagnostic criteria are decisive. Chapter 4 contains the identification key for the RSGs as well as the list of available principal and supplementary qualifiers for each RSG. The qualifiers are defined in Chapter 5. Next comes the bibliography.

This is followed by four appendices. Appendix 1 briefly describes the 32 RSGs. Appendix 2 names the laboratory methods required for the WRB. This is only a naming of the methods, not laboratory instructions. Appendix 3 lists the codes for the RSGs, the qualifiers and the specifiers as well as the rules for the sequence of codes for naming soils and for creating map legends. Appendix 4 contains a grain size triangle in which the areas of the soil type qualifiers are marked with different shades of gray.

List of reference soil groups of the WRB 2015

The list of reference soil groups includes 32 soils, which are shown here in the order of the WRB code. Such a list can also be found in Chapter 1 of the WRB document. The codes according to Annex 3 of the WRB document are also listed:

Organic soils

  • HS Histosol (with mighty horizons made of organic material)

Anthropogenic soils

  • AT Anthrosol (modified by humans to improve fertility and used for a long time)
  • TC Technosol (contain high proportions of artificial or man-made substrates)

Soils with limited root penetration

  • CR Cryosol ( shaped by permafrost )
  • LP Leptosol (shallow above rock or very skeletal)
  • SN Solonetz (clay enrichment in the subsoil and high levels of exchangeable sodium)
  • VR Vertisol (high content of swelling and shrinking clay minerals, which triggers a mixing of the soil when the climate is changeable)
  • SC Solonchak (characterized by easily soluble salts)

Soils characterized by special iron and / or aluminum chemistry

  • GL Gleysol (with redox processes triggered by rising groundwater (or rising reduct gases), including underwater bottoms and tidal bottoms)
  • AN Andosol (with allophanes and / or Al-humus complexes, often formed from pyroclasts )
  • PZ Podzol (enrichment of iron and aluminum compounds and / or organic substances in the subsoil)
  • PT Plinthosol (concrete or network-like enrichment of iron oxides, in some cases hardened)
  • NT Nitisol (well-developed structure with shiny aggregate surfaces , high iron oxide content , mostly kaolinitic)
  • FR Ferralsol (dominance of kaolinite and oxides, poor in nutrients, but physically stable)
  • PL Planosol (seasonally low-pore subsoil horizon with significantly higher clay contents than the upper horizons, abrupt transition between horizons with less clay and more clayey horizons, resulting in water retention with redox processes)
  • ST Stagnosol (seasonal subsoil horizon with little coarse pores, thereby triggered water stagnation with redox processes, but without a significant difference in clay content)

Pronounced accumulation of humus in the mineral topsoil

  • CH Chernozem (dark to black, humic, well-structured, base-rich topsoil and, further down, secondary carbonate)
  • KS Kastanozem (dark, humic, base-rich topsoil and, further down, secondary carbonate)
  • PH Phaeozem (dark, humic, base-rich topsoil, secondary carbonate at most at a very great depth, consistently high base saturation)
  • UM Umbrisol (dark, humus topsoil, with low base saturation)

Soils with accumulation of salts or silicon compounds, typical for arid areas

Soils with clay enrichment in the subsoil

  • RT Retisol (network-like interlocking of clay depletion and clay enrichment horizon)
  • AC Acrisol (clay enrichment horizon with low cation exchange capacity, low base saturation in the subsoil)
  • LX Lixisol (clay enrichment horizon with low cation exchange capacity, high base saturation in the subsoil)
  • AL Alisol (clay enrichment horizon with high cation exchange capacity, low base saturation in the subsoil)
  • LV Luvisol (clay enrichment horizon with high cation exchange capacity, high base saturation in the subsoil)

Little differentiated soils

  • CM Cambisol (with a browned or tarnished or otherwise changed by soil formation)
  • AR Arenosol (very sandy)
  • FL Fluvisol (relatively little developed, still stratified, from young river, lake or sea sediment)
  • RG Regosol (little or no developed soils)

Procedure examples

Examples of soil naming and the creation of map legends are given below. Further examples can be found in the WRB document, Chapter 2.

Example of soil naming with the WRB

Our example floor has the following characteristics:

Description of the terrain: A soil developed from loess shows a marked increase in clay content at a depth of about 60 cm and clay cutane in the clay-rich horizon. Based on the history of the landscape, we assume that three-layer clay minerals dominate. In the field, a pH value of around 6 is measured in the subsoil. The lower area of ​​the less clayey topsoil is strongly lightened. In the clay enrichment horizon, rust and bleaching spots with a total area of ​​around 30% can be observed, the intense colors in the interior of the unit. Reducing conditions occur in spring. The soil is plowed annually. The humus content in the topsoil is relatively low.

Laboratory analyzes: The laboratory analyzes confirm the high cation exchange capacity per kg clay in the clay enrichment horizon and the high base saturation in the subsoil. In the topsoil we find 20% clay , 10% sand and 70% silt , in the subsoil 35% clay, 10% sand and 55% silt.

The naming takes place in four steps.

Question 1: Are there diagnostic horizons, properties or materials?

The following diagnostics are available:

  • albic material (in the brightened horizon)
  • argic horizon (tone enrichment horizon)
  • stagnic properties (in the clay enrichment horizon)
  • reducing conditions (in the tone enrichment horizon)

Question 2: Which RSG does the floor belong to?

You go through the key RSG for RSG. The present soil is not a Histosol, not an Anthrosol, not a Technosol, etc. Finally, you end up with the Luvisols . This is the first RSG in the key whose criteria the sample floor fully meets.

Question 3: Which qualifiers are available?

Of the principal qualifiers, Stagnic (stagnic properties and reducing conditions) and Albic (lightening) apply. Stagnic is higher up in the list, so the soil is now called Albic Stagnic Luvisol. Of the supplementary qualifiers, Siltic (silty from 0–60 cm), Loamic (loamy from 60 cm), Aric (plowed), Cutanic (Toncutane) and Ochric (low humus content) apply. If you put the latter in alphabetical order, the name of the floor is Albic Stagnic Luvisol (Aric, Cutanic, Loamic, Ochric, Siltic).

Question 4: Which specifiers can be used to form subqualifiers?

If the soil is from 0–60 cm Siltic and from 60 cm Loamic, the subqualifiers Anosiltic and Endoloamic can be formed using the depth-related specifiers Ano- and Endo-. The stagnic properties only occur in the sub-floor and the albic material only in the middle area, so you can use the subqualifiers Endostagnic and Amphialbic for this.

The soil name is now: Amphialbic Endostagnic Luvisol (Aric, Cutanic, Endoloamic, Ochric, Anosiltic).

The use of the codes from Appendix 3 of the WRB document allows us to use the following short form: LV-stn.abm-ai.ct.lon.oh.sia.

Example for the creation of map legends with the WRB

Our example floor Amphialbic Endostagnic Luvisol (Aric, Cutanic, Endoloamic, Ochric, Anosiltic) should take up 60% of the area of ​​the mapping unit. The other 40% were from a Eutric Endoluvic Amphialbic Stagnosol taken (Humic, Endoloamic, Anosiltic). So the name of the mapping unit is:

First scale level:

  • dominant: Luvisols
  • codominant: Stagnosols

Second level of scale:

  • dominant: Stagnic Luvisols
  • codominant: Albic Stagnosols

Third level of measurement:

  • dominant: Albic Stagnic Luvisols
  • codominant: Luvic Albic Stagnosols

Fourth level of scale:

  • dominant: Albic Stagnic Luvisols
  • codominant: Eutric Luvic Albic Stagnosols

The use of the depth-related specifier is not recommended for map legends because of the generalizations required.

Three principal qualifiers are planned for the fourth level. Our dominant soil only has two.

Optional qualifiers can be added at each scale level. Who z. B. wants to describe the carbon budget in particular, can already write on the first level:

  • dominant: Luvisols (Ochric)
  • codominant: Stagnosols (Humic)

If you want to refer to the genesis, you can also reproduce it more clearly on the first level:

  • dominant: Luvisols (Stagnic)
  • codominant: Stagnosols (Luvic)

The combination of both on the second level of scale would be:

  • dominant: Stagnic Luvisols (Ochric)
  • codominant: Albic Stagnosols (Luvic, Humic)


  • IUSS Working Group WRB: World Reference Base for Soil Resources 2014. (= World Soil Resources Reports. 106). Update 2015. FAO, Rome 2015, ISBN 978-92-5-108369-7 ( PDF ; 2.3 MB).
  • P. Schad, S. Dondeyne: World Reference Base for Soil Resources. In: R. Lal (Ed.): Encyclopedia of Soil Science. 3. Edition. CRC Press, New York 2017, pp. 2650-2653.
  • W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the world. 2nd Edition. Springer Spectrum, Heidelberg 2014, ISBN 978-3-642-36574-4 .
  • W. Amelung, H.-P. Blume , H. Fleige, R. Horn, E. Kandeler , I. Kögel-Knabner , R. Kretschmar, K. Stahr , B.-M. Wilke: Scheffer / Schachtschabel textbook of soil science. 17th edition. Heidelberg 2018, ISBN 978-3-662-55870-6 .
  • WEH Blum , P. Schad, S. Nortcliff: Essentials of Soil Science. Soil formation, functions, use and classification (World Reference Base, WRB). Borntraeger Science Publishers, Stuttgart 2018, ISBN 978-3-443-01090-4 .
  • IUSS Working Group WRB: World Reference Base for Soil Resources 2006. (= World Soil Resources Reports. 103). FAO, Rome 2006, ISBN 92-5-105511-4 .
  • World Reference Base for Soil Resources, by ISSS-ISRIC-FAO. (= World Soil Resources Reports. 84). FAO, Rome 1998, ISBN 92-5-104141-5 .
  • FAO – UNESCO: Soil map of the world. Volume 1, Legend. Paris 1974.
  • FAO: Guidelines for Soil Description. Prepared by R. Jahn, V. Asio, H.-P. Blume, O. Spaargaren and P. Schad. Rome 2006, ISBN 92-5-105521-1 ( PDF ).
  • H.-P. Blume, P. Schad: 90 Years of Soil Classification of the IUSS. In: IUSS Bulletin. 126, 2015, pp. 38-45 ( [1] ).

Web links

Individual evidence

  1. 90 years of soil classification of the IUSS. In: IUSS Bulletin. 126, pp. 38-45.
  2. Derivation of the WRB name (2015) Federal Institute for Geosciences and Raw Materials.
  3. ^ IUSS Working Groups
  4. WRB homepage
  5. WRB 2015