1  Background and basics

Modified

October 26, 2024

1.1 History

1.1.1 From its beginnings to the third edition 2014/15

The World Reference Base for Soil Resources (WRB) is based on the Legend (FAO-UNESCO 1974) and the Revised Legend (FAO-UNESCO-ISRIC 1988) of the Soil Map of the World (FAO-UNESCO 1971–1981). In 1980, the International Society of Soil Science (ISSS, since 2002 the International Union of Soil Sciences, IUSS) formed a Working Group ‘International Reference Base for Soil Classification’ for further elaboration of a science-based international soil classification system. This Working Group was renamed ‘World Reference Base for Soil Resources’ in 1992. The Working Group presented the first edition of the WRB in 1998 (ISSS-ISRIC_FAO 1998), the second edition in 2006 (IUSS Working Group WRB 2006) and the third edition in 2014/15 (IUSS Working Group WRB 2015). In 1998, the ISSS Council endorsed the WRB as its officially recommended terminology to name and classify soils.

A detailed description of the older WRB history is given in the second edition (IUSS Working Group WRB 2006) and the third edition of the WRB (IUSS Working Group WRB 2015).

1.1.2 From the third edition 2014 (Update 2015) to the fourth edition 2022

The third edition of the WRB was presented at the 20th World Congress of Soil Science 2014 in Jeju, Korea. In 2015, an Update was published online, which is the valid WRB from 2015 to 2022: https://www.fao.org/3/i3794en/I3794en.pdf.

The second edition was translated into several languages: Czech, French, Georgian, Polish, Russian, Slovene, and Spanish.

Since 2014, several WRB field workshops were organized to test the third edition:

2014: Ireland
2017: Latvia and Estonia
2018: Romania
2019: Mongolia
2022: Iceland

The field tours associated with the meetings of the IUSS Commission on Soil Classification in South Africa (2016) and Mexico (2022) were additional tests of the third edition and also the tours offered with the 21st World Congress of Soil Science 2018 in Brazil.

Now, after 8 years, a fourth edition has been prepared.

1.2 Major changes in WRB 2022

The major changes are:

  • The contents of the book were rearranged:
    • The former Annex 1 (Descriptions) was deleted. The descriptions were not fully up to date.
    • Annex 2 (Laboratory methods) was maintained.
    • The former Annex 3 (Codes) is now Chapter 6. This reflects that the codes, if used, are not only recommended but mandatory.
    • The former Annex 4 is integrated in the new Annex 1.
    • The new Annex 1 is a Field Guide. It replaces the FAO Guidelines (2006). Compared to the FAO Guidelines, the Annex 1 is more comprehensive for WRB, more precise and more didactical using many illustrations. It gives many definitions of field characteristics that up till now have been nowhere defined in WRB, neither in the WRB itself, nor in the FAO Guidelines. Many of these definitions were taken from the USDA Soil Survey Manual (2017) and the NRCS Fieldbook (2012), which brings WRB and Soil Taxonomy closer together.
    • The new Annex 3 provides brief definitions of layer symbols further developing the definitions of the FAO Guidelines.
    • The new Annex 4 explains a soil description sheet that is provided online.
    • The new Annex 5 gives a guidance on database set-up. The details are provided online.
    • The new Annex 6 gives recommendations for colour symbols for Reference Soil Group maps.
  • In Chapter 2.1, General rules and definitions, several definitions have been added for WRB: fine earth, whole soil, litter layer, soil surface, mineral soil surface, soil layer, soil horizon. Some new general rules have been added to make the definitions easier.
  • All Reference Soil Groups (RSGs) are maintained. There are some changes in the Key: Planosols and Stagnosols are now before Nitisols and Ferralsols. Fluvisols are before Arenosols.
  • The following diagnostics were deleted:
    • fulvic horizon, melanic horizon: belonged to an outdated concept of soil organic matter;
    • aridic properties: had a non-systematic combination of various characteristics (the wind deposition is now characterized by the aeolic material, see below);
    • geric properties: can be better expressed as qualifier;
    • sulfidic material: not needed after introducing the hypersulfidic and the hyposulfidic material in 2014.
  • The following diagnostics were introduced:
    • albic horizon: In the first and the second edition of WRB, the albic horizon was defined. However, it was only defined by colour, and results of soil-forming processes were not required. Therefore, it was changed to albic material in 2014. But this made the definition of the Albic qualifier difficult. Now, the albic horizon was reintroduced, explicitly requiring results of soil-forming processes. The albic material was maintained (just defined by colour) and renamed claric material (see below)
    • cohesic horizon: Dense subsurface horizon dominated by kaolinite. It is found in tropical regions with seasonal climate and was not considered so far in WRB.
    • limonic horizon: Accumulation of Fe by capillary rise in groundwater soils. The accumulation is so strong that Fe oxides cause a cementation. It is traditionally referred to as bog iron.
    • panpaic horizon: Buried A horizon.
    • tsitelic horizon: Accumulation of Fe by subsurface flow, usually from Planosols and Stagnosols further up in the landscape.
    • protogypsic properties: Accumulation of secondary gypsum, not sufficient for a gypsic or petrogypsic horizon.
    • aeolic material: Deposited by wind.
    • mulmic material: Mineral material with a high content of soil organic carbon, derived from organic material. Drainage of organic material causes accelerated decomposition, and eventually the content of soil organic carbon sinks below 20%, which transforms the organic material into mineral material.
    • organotechnic material: Contains large amounts of organic artefacts and relatively small contents of soil organic carbon in the fine earth.
  • The following diagnostic materials received new names:
    • claric material instead of albic material: After reintroducing the albic horizon, it had to be avoided that a diagnostic material and a diagnostic horizon have the same name. The albic material was therefore renamed in claric material.
    • solimovic material instead of colluvic material: The word colluvium has very different meanings in different countries. To avoid confusion, the new name solimovic material was coined. It explains that at least parts of the accumulated material underwent soil formation before having been transported.
  • Many criteria in the diagnostics, the key and in the definitions of the qualifiers were sharpened and refined. Special effort was undertaken to make sure that the same features are worded in the same way throughout the text, including the annexes.
  • Some new qualifiers were defined, some existing ones were deleted, and many definitions have been refined.

1.3 The object classified in the WRB

Like many common words, ‘soil’ has several meanings. In its traditional meaning, soil is the natural medium for the growth of plants, whether or not it has discernible soil horizons (Soil Survey Staff 1999).

In the 1998 WRB, soil was defined as:

“… a continuous natural body which has three spatial and one temporal dimension. The three main features governing soil are:

  • It is formed by mineral and organic constituents and includes solid, liquid and gaseous phases.
  • The constituents are organized in structures, specific for the pedological medium. These structures form the morphological aspect of the soil cover, equivalent to the anatomy of a living being. They result from the history of the soil cover and from its actual dynamics and properties. Study of the structures of the soil cover facilitates perception of the physical, chemical and biological properties; it permits understanding the past and present of the soil and predicting its future.
  • The soil is in constant evolution, thus giving the soil its fourth dimension, time.”

Although there are good arguments to limit soil survey and mapping to identifiable stable soil areas with a certain thickness, the WRB has taken the more comprehensive approach to name any object forming part of the epiderm of the earth (Sokolov 1997; Nachtergaele 2005). This approach has a number of advantages; notably that it allows for addressing environmental problems in a systematic and holistic way and avoids sterile discussion on a universally agreed definition of soil and its required thickness and stability. Therefore, the object classified in the WRB is: any material within 2 m of the Earth’s surface that is in contact with the atmosphere, excluding living organisms, areas with continuous ice not covered by other material, and water bodies deeper than 2 m. If explicitly stated, the object classified in the WRB includes layers deeper than 2 m. In tidal areas, the depth of 2 m is to be applied at mean low water springs.

The definition includes continuous rock, paved urban soils, soils of industrial areas, soils on buildings and other (permanent/stable) constructions, cave soils as well as subaqueous soils. Soils under continuous rock, except those that occur in caves, are generally not considered for classification, but in special cases, the WRB may be even used to classify soils under rock, for example for palaeopedological reconstruction of the environment. The use of WRB for paleosols is still in an experimental stage.

1.4 Basic principles

General principles

  • The classification of soils is based on soil properties defined in terms of diagnostic horizons, diagnostic properties and diagnostic materials (together called the diagnostics), which to the greatest extent possible should be measurable and observable in the field. Table 1.1 provides an overview of the diagnostics used in the WRB.
  • The selection of diagnostic characteristics takes into account their relationship with soil-forming processes. An understanding of soil-forming processes contributes to a better characterization of soils but these processes should not, as such, be used as differentiating criteria.
  • To the extent possible at a high level of generalization, diagnostic features that are of significance for soil management are selected.
  • Climate parameters are not applied in the classification of soils. It is understood that they should be used for interpretation purposes, in combination with soil properties, but they should not form part of soil definitions. The classification of soils is therefore not subordinated to the availability of climate data. The name of a certain soil will not become obsolete due to global or local climate change.
  • The WRB is a comprehensive classification system that enables accommodation of national soil classification systems.
  • The WRB is not intended to be a substitute for national soil classification systems, but rather to serve as a common denominator for communication at the international level.
  • The WRB comprises two levels of categorical detail:
    • the First Level having 32 Reference Soil Groups (RSGs);
    • the Second Level, consisting of the name of the RSG combined with a set of principal and supplementary qualifiers.
  • Many RSGs in the WRB are representative of major soil regions so as to provide a comprehensive overview of the world’s soil cover.
  • Definitions and descriptions reflect variations in soil characteristics that occur both vertically and laterally in the landscape.
  • The term Reference Base is connotative of the common denominator function of the WRB: its units (RSGs) have sufficient width to facilitate harmonization and correlation with national systems.
  • In addition to serving as a correlation between existing classification systems, the WRB also serves as a communication tool for compiling global soil databases and for the inventory and monitoring of the world’s soil resources.
  • The nomenclature used to distinguish soil groups retains terms that have been used traditionally or that can be introduced easily into common language. They are defined precisely, in order to avoid the confusion that occurs where names are used with different connotations.
Table 1.1: The diagnostic horizons, properties and materials of the WRB. This table does not provide definitions. For diagnostic criteria, please refer to Chapter 3.
Simplified Description
1. Anthropogenic diagnostic horizons (all are mineral)
anthraquic horizon in paddy soils: the layer comprising the puddled layer and the plough pan, both showing a reduced matrix and oxidized root channels
hortic horizon dark, high content of organic matter and P, high animal activity, high base saturation; resulting from long-term cultivation, fertilization and application of organic residues
hydragric horizon in paddy soils: the layer below the anthraquic horizon showing redoximorphic features and/or an accumulation of Fe and/or Mn
irragric horizon uniformly textured, at least moderate content of organic matter, high animal activity; gradually built up by sediment-rich irrigation water
plaggic horizon dark, at least moderate content of organic matter, sandy or loamy; resulting from application of sods and excrements
pretic horizon dark, at least moderate content of organic matter and P, high contents of exchangeable Ca and Mg, with black carbon; including Amazonian Dark Earths
terric horizon evidence of addition of substantially different material, at least moderate content of organic matter, high base saturation; resulting from adding mineral material (with or without organic residues) and cultivation)
2. Diagnostic horizons that may be organic or mineral
calcic horizon accumulation of secondary carbonates, not continuously cemented
cryic horizon perennially frozen (visible ice or, if not enough water, < 0 °C)
salic horizon high amounts of readily soluble salts
thionic horizon with sulfuric acid and a very low pH value
3. Organic diagnostic horizons
folic horizon organic layer, not water-saturated and not drained
histic horizon organic layer, water-saturated or drained
4. Surface mineral diagnostic horizons
chernic horizon thick, very dark-coloured, high base saturation, moderate to high content of organic matter, well developed soil structure or structural elements created by agricultural practices, high animal activity (special case of the mollic
mollic horizon thick, dark-coloured, high base saturation, moderate to high content of organic matter, at least some soil structure or structural elements created by agricultural practices
umbric horizon thick, dark-coloured, low base saturation, moderate to high content of organic matter, at least some soil structure or structural elements created by agricultural practices
argic horizon subsurface layer with distinctly higher clay content than the overlying layer without a lithic discontinuity and/or presence of illuvial clay minerals (with or without a lithic discontinuity)
duric horizon concretions or nodules, cemented by secondary silica, and/or remnants of a broken-up petroduric horizon
ferric horizon ≥ 5% reddish to blackish concretions and/or nodules and/or ≥ 15% reddish to blackish coarse masses, with accumulation of Fe (and Mn) oxides
gypsic horizon accumulation of secondary gypsum, not continuously cemented
limonic horizon accumulation of Fe and/or Mn oxides in a layer that has or had gleyic properties; at least partially cemented
natric horizon subsurface layer with distinctly higher clay content than the overlying layer without a lithic discontinuity and/or presence of illuvial clay minerals (with or without a lithic discontinuity); high content of exchangeable Na
petrocalcic horizon accumulation of secondary carbonates, relatively continuously cemented
petroduric horizon accumulation of secondary silica, relatively continuously cemented
petrogypsic horizon accumulation of secondary gypsum, relatively continuously cemented
petroplinthic horizon consists of oximorphic features inside (former) soil aggregates that are at least partially interconnected and have a yellowish, reddish and/or blackish colour; high contents of Fe oxides at least in the oximorphic features; relatively continuously cemented
pisoplinthic horizon ≥ 40% at least moderately cemented yellowish, reddish, and/or blackish concretions and/or nodules, with accumulation of Fe oxides, and/or remnants of a broken-up petroplinthic horizon
plinthic horizon has in ≥ 15% of its exposed area oximorphic features inside (former) soil aggregates that are black or have a redder hue and a higher chroma than the surrounding material; high contents of Fe oxides, at least in the oximorphic features; not continuously cemented
sombric horizon subsurface accumulation of organic matter other than in spodic or natric horizons; not a buried surface horizon
spodic horizon subsurface accumulation of Al with Fe and/or organic matter
tsitelic horizon lateral accumulation of Fe, usually derived from Planosols and Stagnosols further upslope
6. Other mineral diagnostic horizons
albic horizon light-coloured; loss of coloured substances (e.g. oxides, organic matter) due to soil-forming processes
cambic horizon evidence of soil-forming processes; not meeting the criteria of diagnostic horizons that indicate stronger alteration or accumulation processes
cohesic horizon massive or subangular blocky structure, root penetration restricted, drainage normally free, rich in kaolinite, poor in organic matter
ferralic horizon strongly weathered, dominated by kaolinites and oxides
fragic horizon with large soil aggregates, roots and percolating water penetrate the soil only in between these aggregates, not or only partially cemented
nitic horizon rich in clay minerals and Fe oxides, moderate to strong structure, shiny soil aggregate surfaces
panpaic horizon buried mineral surface horizon with a significant content of organic matter
protovertic horizon influenced by swelling and shrinking clay minerals
vertic horizon dominated by swelling and shrinking clay minerals
takyric properties fine-textured surface crust with a platy or massive structure; under arid conditions in periodically flooded soils
yermic properties combination of desert features: desert pavement, varnishing, ventifacts, vesicular pores, platy structure
8. Diagnostic properties defining the relationship between two layers
abrupt textural difference very sharp increase in clay content within a limited depth range
albeluvic glossae interfingering of coarser-textured and lighter coloured material into an argic horizon forming vertically continuous tongues (special case of retic properties)
lithic discontinuity differences in parent material
retic properties interfingering of coarser-textured and lighter coloured material into an argic or natric horizon
9. Other diagnostic properties
andic properties short-range-order minerals and/or organo-metallic complexes
anthric properties applying to soils with mollic or umbric horizons, if the mollic or umbric horizon is created or substantially transformed by humans
continuous rock consolidated material (excluding cemented pedogenetic horizons)
gleyic properties saturated with flowing or upwards moving groundwater (or upwards moving gases), permanently or at least long enough that reducing conditions occur
protocalcic properties carbonates derived from the soil solution and precipitated in the soil (secondary carbonates), less pronounced than in calcic or petrocalcic horizons
protogypsic properties gypsum derived from the soil solution and precipitated in the soil (secondary gypsum), less pronounced than in gypsic or petrogypsic horizons
reducing conditions low rH value and/or presence of sulfide, methane or reduced Fe
shrink-swell cracks open and close due to swelling and shrinking of clay minerals
sideralic properties relatively low CEC
stagnic properties saturated with surface water (or intruding liquids), at least temporarily, long enough that reducing conditions occur
vitric properties ≥ 5% (by grain count) of volcanic glasses and related materials, and containing a limited amount of short-range-order minerals and/or organo-metallic complexes
mineral material < 20% soil organic carbon and < 35% (by volume) organic artefacts
mulmic material developed from water-saturated organic material after drainage; 8 - 20% soil organic carbon
organic material ≥ 20% soil organic carbon
organotechnic material < 20% soil organic carbon and ≥ 35% (by volume) organic artefacts
soil organic carbon organic carbon that does not meet the diagnostic criteria of artefacts
claric material light-coloured fine earth, expressed by high Munsell value and low chroma
12. Technogenic diagnostic materials
artefacts created, substantially modified or brought to the surface by humans; no subsequent substantial change of chemical or mineralogical properties
technic hard material consolidated and relatively continuous material resulting from an industrial process
13. Other diagnostic materials
aeolic material sedimented by wind
calcaric material ≥ 2% calcium carbonate (CaCO3) equivalent, at least partially inherited from the parent material
dolomitic material ≥ 2% of a mineral that has a ratio CaCO3/MgCO3 < 1.5
fluvic material fluviatile, marine or lacustrine deposits with evident stratification
gypsiric material ≥ 5% gypsum (CaSO4), at least partially inherited from the parent material
hypersulfidic material containing sulfides and capable of severe acidification
hyposulfidic material containing sulfides and not capable of severe acidification
limnic material deposited in water by precipitation (possibly with sedimentation), or derived from algae, or derived from aquatic plants with subsequent transport or subsequent modification by aquatic animals or micro-organisms
ornithogenic material excrements or remnants of birds or bird activity
solimovic material heterogeneous mixture that has moved down a slope, suspended in water; dominated by material that underwent soil formation at its original place
tephric material ≥ 30% (by grain count) volcanic glass and related materials

1.4.1 Structure

Each RSG of the WRB is provided with a listing of possible principal and supplementary qualifiers, from which the user can construct the second level of the classification. The principal qualifiers are given in a priority sequence. The broad principles that govern the WRB class differentiation are:

  • At the First Level (RSGs), classes are differentiated mainly according to characteristic soil features produced by primary pedogenic process, except where special soil parent materials are of overriding importance.
  • At the Second Level (RSGs with qualifiers), soils are differentiated according to soil features resulting from any secondary soil-forming process that has significantly affected the primary characteristics. In many cases, soil characteristics that have a significant effect on land use are taken into account.

1.4.2 Evolution of the system

The Revised Legend of the FAO/UNESCO Soil Map of the World (FAO-UNESCO-ISRIC 1988) was used as a basis for the development of the WRB in order to take advantage of the international soil correlation that had already been conducted through this project and elsewhere. The first edition of the WRB, published in 1998, comprised 30 RSGs; the following editions have 32 RSGs.

1.5 Architecture

The WRB comprises two levels of categorical detail:

  1. the First Level having 32 Reference Soil Groups (RSGs);
  2. the Second Level, consisting of the name of the RSG combined with a set of principal and supplementary qualifiers.

1.5.1 First Level: The Reference Soil Groups

Table 1.2 provides an overview of the RSGs and the rationale for the sequence of the RSGs in the WRB Key. The RSGs are allocated to groups on the basis of dominant identifiers, i.e. the soil-forming factors or processes that most clearly condition the soil.

1.5.2 Second Level: The Reference Soil Groups with their qualifiers

In the WRB, a distinction is made between principal qualifiers and supplementary qualifiers. Principal qualifiers are regarded as being most significant for a further characterization of soils of the particular RSG. They are given in a ranked order. Supplementary qualifiers give some further details about the soil. They are not ranked but listed alphabetically (exception: the supplementary qualifiers related to the texture are given first). Chapter 2 gives the rules for the use of qualifiers for naming soils and for creating map legends. Constructing the second level by adding qualifiers to the RSG has several advantages compared with a dichotomous key:

  • Every soil receives the appropriate number of qualifiers. Soils with few characteristics have short names; soils with many characteristics (e.g. polygenetic soils) have longer names.
  • The WRB is capable of indicating most of the soil’s properties, which are incorporated into an informative soil name.
  • 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.
Table 1.2: Simplified guide to the WRB Reference Soil Groups (RSGs) with suggested codes. This table is not to be used as a key. For full definitions, please refer to Chapter 3 and the Key (Chapter 4).
RSG Code
1. Soils with thick organic layers: Histosols HS
2. Soils with strong human influence –

Strongly altered by long and intensive agricultural use: Anthrosols AT
Containing significant amounts of artefacts: Technosols TC
3. Soils with limitations to root growth –

Permafrost-affected: Cryosols CR
Thin or with many coarse fragments: Leptosols LP
With a high content of exchangeable Na: Solonetz SN
Alternating wet-dry conditions, shrink-swell clays: Vertisols VR
High concentration of soluble salts: Solonchaks SC
4. Soils distinguished by Fe/Al chemistry –

Groundwater-affected, underwater and 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 saturation: Phaeozems PH
Dark topsoil, low base saturation: 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 –

Moderately developed: Cambisols CM
Stratified fluviatile, marine or lacustrine sediments: Fluvisols FL
Sandy: Arenosols AR
No significant profile development: Regosols RG

1.6 Topsoils

Topsoil characteristics are prone to rapid change with time and are therefore used only in some cases in the WRB. Several suggestions for topsoil classification systems have been made (Broll et al. 2006; Fox, Tarnocai, and Broll 2010; Graefe et al. 2012; Jabiol et al. 2013; Zanella et al. 2018). They may be combined with the WRB.

1.7 Subsolum

A classification scheme for subsolum materials has been proposed by (Juilleret et al. 2016, 2018) that may be combined with the WRB. Subsolum material is any material occurring below the diagnostics of WRB.

1.8 Translation into other languages

Translations into other languages are most welcome. For copyright, please contact IUSS. However, all elements of the soil names (RSG, qualifiers, specifiers) must not be translated into any other language nor transliterated into another alphabet. Soil names must preserve their grammatical form. The rules for the sequence of qualifiers must be followed in any translation. Names of RSGs and qualifiers start with capital letters.