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Chapter 1: Background

Elements of the world reference base for soil resources

It has been a matter of great concern that after a hundred years of modern soil science a generally accepted system of soil classification has not yet been universally adopted (Dudal, 1990). This situation arises partly from the fact that soils constitute a continuum which, unlike easily identifiable plants and animals, needs to be divided into classes by convention. To remedy this situation work in soil systematics during the past 20 years has concentrated upon the development of a World Reference Base for Soil Resources.


The World Reference Base for Soil Resources (WRB) is the successor to the International Reference Base for Soil Classification (IRB), an initiative of FAO, supported by the United Nations Environment Programme (UNEP) and the International Society of Soil Science, dating back to 1980. The intention of the IRB project was to work toward the establishment of a framework through which existing soil classification systems could be correlated and through which ongoing soil classification work could be harmonized. The final objective was to reach an international agreement on the major soil groupings to be recognized at a global scale as well as on the criteria and methodology to be applied for defining and identifying them. Such an agreement was meant to facilitate the exchange of information and experience, to provide a common scientific language, to strengthen the applications of soil science, and to enhance communication with other disciplines.

After preliminary discussions in 1978 in Canada, three meetings were convened in Sofia, Bulgaria, in 1981 and 1982, to initiate an international programme toward a common soil classification. Draft definitions of 16 major soil groups were formulated, weakly developed soils, swelling/shrinking soils, groundwater influenced soils, saline/alkali soils, calcic/gypsic soils, mollic soils, umbric and shallow soils, sialic soils, fersialic soils, ferralic soils, andic soils, surface water influenced soils, podzolized soils, histic soils, pergelic soils and anthropogenic soils.

The project to create an International Reference Base for Soil Classification was initiated in 1982 as one of the proposed programmes to implement a World Soils Policy through UNEP. It was envisaged that the International Reference Base for Soil Classification was to be used as a basis to revise the Legend of the Soil Map of the World (FAO-UNESCO, 1974).

In 1982, in New Delhi, India, the 12th Congress of the International Society of Soil Science (ISSS) endorsed this programme and entrusted it to a Working Group within Commission V (Soil Genesis, Classification and Cartography).

In 1986, at the 13th Congress of the ISSS in Hamburg, Germany, the IRB programme was placed in Commission V under the responsibility of its chairman, assisted by a core group. Selected contributors were asked to work out in greater detail the definitions of the major soil groupings and relevant diagnostic attributes, to make proposals for further subdivision at a second/third level, and to establish correlation with existing soil units of major soil classification systems.

The further development of the IRB was discussed at consultations held in Rome, Italy, in 1987 and in Almaty (Alma-Ata), Kazakstan, in 1988. Progress was reported in 1990 at a symposium devoted to the International Reference Base for Soil Classification during the 14th Congress of the International Society of Soil Science in Kyoto, Japan. At this stage 20 major soil groupings were identified on the basis of their representativeness of the world's soil cover, viz. organic, anthric, vertic, andic, gleyic, stagnic, ferralic, podzic, luvic, nitic, lixic, fluvic, gypsic, calcic, salic, sodic, chernic, modic, cambic and primic soils. The attributes used to define these soil groupings were selected to reflect major soil forming processes.

In the meantime FAO had issued the Revised Legend of the Soil Map of the World (FAO, 1988). The number of major soil groupings in this legend was increased from 26 to 28 and that of the soil units from 106 to 153. Some of the main changes included the amalgamation of Lithosols, Rendzinas and Rankers into Leptosols, the split of Luvisols into Luvisols and Lixisols and, similarly, the separation of Acrisols into Acrisols and Alisols, the deletion of Xerosols and Yermosols, and the introduction of Anthrosols, Plinthosols, Calcisols and Gypsisols. Some diagnostic criteria were adapted, others were newly defined (e.g. argic and ferralic B horizons, and andic, fluvic, gleyic, stagnic, nitic, salic and sodic properties).

As a follow-up to the 1990 Congress, a consultation was convened at Montpellier, France, in 1992, in order to take stock of the current status of the IRB in the light of the discussions held at the Kyoto Symposium. It had appeared that some of the 20 major soil groupings proposed were so broad that it proved difficult to prepare consistent definitions. These major soil groupings needed to be split in order to obtain a more significant subdivision. When comparing the Kyoto list of 20 IRB units and the 28 FAO major soil groupings of the Revised Legend, the question arose as to whether it was justified to develop two systems side by side. If a further split of some IRB units took place, one would end up with almost identical lists of units. Furthermore, as both IRB and the Soil Map of the World were co-sponsored by the ISSS, it was felt inappropriate to pursue two separate programmes which essentially had the same goal, namely to arrive at a rational inventory of global soil resources. An early motive for doing so had been that the 1974 FAO-UNESCO Legend was meant only to serve the sole purpose of the 1:5000000 Soil Map of the World. Since then, the Legend has progressively been developed to encompass the major soils of the world at three levels of generalization and is presently used widely for surveys both in developing and developed countries. Moreover, the terminology is well known and generally accepted.

Therefore it was decided that the IRB should adopt FAO's Revised Legend as a framework for its future activities. It would be IRB's task to apply its principles of definitions and soil relationships to the existing FAO units, providing greater depth and background. The merger of the two efforts was launched under the name: 'World Reference Base for Soil Resources', an ISSS/FAO/ISRIC undertaking.

When the Revised Legend was published in 1988, FAO called for continents and possible amendments. WRB has endeavoured to identify possible gaps and suggest adjustments accordingly. Proposed adjustments were presented in draft form during the 16th World Congress of Soil Science at Acapulco, Mexico (ISSS-ISRIC-FAO, 1994) and have been tested during meetings and field trips in Germany (1995), Russia (1996), South Africa (1996), Argentina (1997) and Austria (1997).


The main objective of the World Reference Base for Soil Resources is to provide scientific depth and background to the 1988 FAO Revised Legend, incorporating the latest knowledge relating to global soil resources and their interrelationships. To include some of the most recent pedological studies and to expand use of the system from an agricultural base to a broader environmental one. it was recognized that a limited number of important changes to the 1988 Legend were becoming necessary.

More specifically, the objectives are:

• to develop an internationally acceptable system for delineating soil resources to which national classifications can be attached and related, using FAO's Revised Legend as a framework;

• to provide this framework with a sound scientific basis so that it can also serve different applications in related fields such as agriculture, geology, hydrology and ecology;

• to recognize within the framework important spatial relationships of soils and soil horizons as characterized by topo- and chronosequences; and

• to emphasize the morphological characterization of soils rather than to follow a purely laboratory-based analytical approach.

WRB is designed as an easy means of communication amongst scientists to identify, characterize and name major types of soils. It is not meant to replace national soil classification systems, but be a tool for better correlation between national systems. It aims to act as a common denominator through which national systems can be compared. WRB also serves as a common ground among people with an interest in land and natural resources.

WRB is also a tool for identifying pedological structures and their significance. It serves as a basic language in soil science to facilitate:

• scientific communication;

• implementation of soil inventories and transfer of pedological data, elaboration of different systems of classification having a common base, interpretation of maps;

• acknowledgement of relationships between soils and soil horizon distribution as characterized by topo- and chronosequences;

• international use of pedological data, not only by soil scientists but also by other users of soil and land, such as geologists, botanists, agronomists, hydrologists, ecologists, farmers, foresters, civil engineers and architects, with as a particular objective to improve upon:

- the use of soil data for the benefit of other sciences;

- the evaluation of soil resources and the potential use of different types of soil cover;

- the monitoring of soils, particularly soil development which is dependent on the way soils are used by the human community;

- the validation of experimental methods of soil use for sustainable development, which maintain and, if possible, improve the soil's potential;

- transfer of soil use technologies from one region to another.


The general principles on which the WRB is based were laid down during the early Sofia meetings in 1981 and 1982, and further elaborated upon by the Working Groups entrusted with its development. These general principles can be summarized as follows:

• the classification of soils is based on soil properties defined in terms of diagnostic horizons and characteristics, which to the greatest extent possible should be measurable and observable in the field;

• the selection of diagnostic horizons and characteristics takes into account their relationship with soil forming processes. It is recognized that an understanding of soil forming processes contributes to a better characterization of soils but that they should not, as such, be used as differentiating criteria;

• to the extent possible at a high level of generalization it is attempted to select diagnostic features which are of significance for management purposes;

• climatic parameters are not applied in the classification of soils. It is fully realized that they should be used for interpretation purposes, in dynamic combination with soil properties, but they should not be part of soil definitions;

• WRB is meant to be a comprehensive classification system which enables people to accommodate their own national classification system. It comprises two tiers of categorical detail:

1. the "Reference base" which is limited to the first level only, having 30 reference soil groups; and

2. the "WRB Classification System" consisting of combinations of a set of prefixes as unique qualifiers (or modifiers) added to the reference soil groups, allowing very precise characterization and classification of individual soil profiles;

• the reference soil units in WRB should be representative of major soil regions so as to provide a comprehensive overview of the world's soil cover;

• the reference base is not meant to substitute for national soil classification systems but rather to serve as a common denominator for communication at an international level.
This implies that lower-level categories, possibly a third category of the WRB, could accommodate local diversity at country level. Concurrently the lower levels could emphasize soil features which are important for land use and soil management;

• the Revised Legend of FAO/UNESCO Soil Map of the World has been used as a basis for the development of the WRB in order to take advantage of the international soil correlation work which has already been conducted through this project;

• definitions and descriptions of soil units are to reflect variations in soil characteristics both vertically and laterally so as to account for spatial linkages within the landscape;

• the term 'Reference Base' is connotative of the common denominator function which the WRB will assume. Its units should have sufficient width to stimulate harmonization and correlation of existing national systems;

• in addition to serving as a link between existing classification systems the WRB may also serve as a consistent 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 will retain terms which have been traditionally used or which can easily be introduced in current language. These terms are precisely defined in order to avoid the confusion which occurs when names are used with different connotations.

Although the basic framework of the FAO Legend, with its two categoric levels and guidelines for developing classes at a third level, was adopted, it has been decided to merge the lower levels. Each reference soil group of WRB is provided with a listing of possible qualifiers in a priority sequence, from which the user can construct the various lower-level units. The broad principles which govern the WRB class differentiation are:

• at the higher categoric level classes are differentiated mainly according to the primary pedogenetic process that has produced the characteristic soil features, except where 'special' soil parent materials are of overriding importance; and

• at the lower categoric levels classes are differentiated according to any predominant secondary soil forming process that has significantly affected the primary soil features. In certain cases, soil characteristics that have a significant effect on use may be taken into account.

It is recognized that a number of reference soil groups may occur under different climatic conditions. It was decided, however, not to introduce separations on account of climatic characteristics so that the classification of soils is not subordinated to the availability of climatic data.

Elements of the world reference base for soil resources

The WRB reference soil groups

After reviewing FAO's Revised Legend, 30 reference soil groups were identified to constitute the World Reference Base for Soil Resources. Three new reference soil groups are included: the Cryosols, Durisols, and Umbrisols. The Greyzems have been merged with the Phaeozems, and the Podzoluvisols are renamed Albeluvisols.

The 30 major soil groups of the WRB are Acrisols, Albeluvisols, Alisols, Andosols, Anthrosols, Arenosols, Calcisols, Cambisols, Chernozems, Cryosols, Durisols, Ferralsols, Fluvisols, Gleysols, Gypsisols, Histosols, Kastanozems, Leptosols, Lixisols, Luvisols, Nitisols, Phaeozems, Planosols, Plinthosols, Podzols, Regosols, Solonchaks, Solonetz, Umbrisols, and Vertisols.

Cryosols are introduced at the highest level to identify a group of soils which occur under the unique environmental conditions of alternating thawing and freezing. These soils have permafrost within 100 cm of the soil surface and are saturated with water during the period of thaw. In addition, they show evidence of cryoturbation. Durisols comprise the soils in semi-arid environments which have an accumulation of secondary silica, either in the form of nodules, or as a massive, indurated layer. Umbrisols cover the soils which have either an umbric horizon, or have a mollic horizon and a base saturation of less than 50 percent in some parts within the upper 125 cm of the soil surface. They are a logical counterpart of the Chernozems, Kastanozems and Phaeozems.

The Plinthosols bring together the Plinthosols of the Revised Legend and the soils which have a petroplinthic layer at shallow depth. In the Revised Legend the latter soils belong to the Leptosols. For the World Reference Base it was decided to exclude from the Leptosols soils with pedogenetic horizons such as indurated calcic or gypsic horizons or hardened plinthite. This necessitated the definition of a reference soil group which included these soils. Although it is realized that soils with shallow petroplinthic layers and soils having plinthite normally occupy different positions in the landscape, it was felt appropriate to group them together as they are genetically related.

Podzoluvisols are renamed Albeluvisols. The name Podzoluvisols suggests that in these soils both the processes of cheluviation (leading to Podzols) and subsurface accumulation of clay (resulting in Luvisols) take place, while in fact the dominant process consists of removal of clay and iron/manganese along preferential zones (pea faces, cracks) in the argic horizon. The name Albeluvisols is therefore thought to be more appropriate, expressing the presence of a bleached eluvial horizon ("albic horizon"), a clay-enriched horizon ("argic horizon") and the occurrence of "albeluvic tonguing".

WRB diagnostic horizons, properties and materials

Earlier it was agreed that the soil groups should be defined in terms of a specific combination of soil horizons, called 'reference horizons' rather than 'diagnostic horizons'. Reference horizons were intended to reflect genetic horizons which are widely recognized as occurring in soils. Unfortunately, the distinction between reference and diagnostic horizons created confusion and it was agreed to retain the FAO terminology of diagnostic horizons as well as the diagnostic properties. Additionally it appeared necessary to define diagnostic soil materials. This together resulted in a comprehensive list of WRB diagnostic horizons, properties and materials, defined in terms of morphological characteristics and/or analytical criteria. In line with the WRB objectives, attributes are described as much as possible to help field identification.

Modifications to definitions of FAO's diagnostic horizons and properties

Of the 16 diagnostic horizons of the Revised Legend only the fimic A horizon has not been retained. It covers too wide a range of human-made surface layers and is replaced in the WRB by the hortic, plaggic and ferric horizons.

For the WRB, the definition of the histic horizon was broadened by reducing its minimum thickness to 10 cm and removing the maximum thickness. This is because of a second use of the definition. In the Revised Legend the histic H horizon is used to distinguish soils at second level to identify histic soil units; in the WRB it is used also at the highest level to define Histosols. It was agreed that Histosols over continuous hard rock should have a minimum thickness of 10 cm in order to avoid very thin organic layers over rock being classified as Histosols.

The P2O5 content requirement for FAO's mollic and umbric A horizons has been deleted from the WRB definition of mollic and umbric horizons. This requirement cannot be considered diagnostic since thick, dark coloured, human-made horizons in, for instance, China, also have low amounts of phosphate. Other criteria have to be found to separate mollic and umbric horizons from anthropedogenic horizons.

A chernic horizon is defined as a special kind of mollic horizon. The present definition of the mollic horizon was felt to be too broad to reflect properly the unique characteristics of the deep, blackish, porous surface horizons which are so typical for Chernozems.

The definition of the ochric horizon is similar to the ochric A horizon. The colour requirement for the albic horizon have been slightly changed compared to FAO's albic E horizon, to suit albic horizons which show a considerable shift in chrome upon moistening. Such conditions are frequently found in soils of the southern hemisphere.

The argic horizon definition differs from that of the argic B horizon of the Revised Legend in that the percentage clay skins on both horizontal and vertical ped faces and in pores has been increased from one to five percent. This is expected to provide a better correlation with the earlier requirement of at least one percent oriented clay in thin sections.

Guidelines to recognize a lithological discontinuity, if not clear from the field observation, were added to the description of the argic horizon. It can be identified by the percentage of coarse sand, fine sand and silt, calculated on a clay-free basis (international particle size distribution or using the additional groupings of the United States Department of Agriculture (USDA) system or other), or by changes in the content of gravel and coarser fractions. A relative change of at least 20 percent in any of the major particle size fractions is regarded as diagnostic for a lithological discontinuity. However, it should only be taken into account if it is located in the section of the solum where the clay increase occurs and if there is evidence that the overlying layer was coarser textured.

The adjustments made in the description of the argic horizon also apply to the natric horizon.

The definition of FAO's cambic B horizon has been slightly amended by deleting the requirement '....and has at least eight percent clay'. This requirement forces some soils, which have a well-developed structural-B horizon and silt loam or silt textures with a low clay content, as found, for instance, in fluvio-glacial deposits of the nordic countries, into the Regosols rather than in the Cambisols. Because there is also no need for this requirement to separate Cambisols from Arenosols (defined in the WRB as soils having a loamy sand or coarser texture) it has not been used in the definition proposed for the WRB cambic horizon.

Major alterations are made in the definition of the spodic horizon. It has been brought into line with the recent modifications in soil taxonomy (Soil Survey Staff, 1996) regarding the definition of spodic materials. Colour requirements were added, a limit of 0.5 or more in percentage oxalate extractable aluminium plus half that of iron is used, and a value for the optical density of oxalate extract (ODOE) of 0.25 or more is introduced. Moreover, the upper limit of spodic horizons has been set at 10 cm depth.

The silt-clay ratio of 0.2 or less has been deleted from the definition of the ferralic horizon. This criterion was felt to be too strict; the silt particle size fraction has been increased from 2-50 to 2-63 Ám (FAO, 1990). Other values have been proposed (silt-clay ratio of 0.7 or less; fine silt-clay ratio of 0.2 or less) but, as yet, no consensus has been reached.

Some alterations are made in the definitions of the calcic and gypsic horizons. For WRB purposes they are split into calcic/gypsic and hypercalcic/hypergypsic horizons. These latter horizons have a calcium carbonate equivalent and gypsum content of 50 and 60 percent, respectively, but are not cemented.

The definition for the sulfuric horizon remains the same as in the Revised Legend.

In addition to these diagnostic horizons, 19 new ones are proposed. Some are adopted from FAO's diagnostic properties, others are newly formulated. Together they bring the total of diagnostic horizons recognized in the WRB to 34. The newly defined diagnostic horizons are the andic, anthropedogenic, chernic, cryic, duric, ferric, folic, fragic, fulvic, glacic, melanic, nitic, petroduric, petroplinthic, plinthic, salic, takyric, vertic, vitric and yermic horizons. Definitions and descriptions are given in Chapter 3.

A combination of an anthraquic horizon at the surface with an underlying hydragric horizon, totalling together a thickness of at least 50 cm, defines certain Anthrosols which show evidence of alteration through wet-cultivation practices. It comprises a puddled layer, a plough pan and an illuvial subsurface horizon. This combination is characteristic for soils which have been used for long-term paddy rice cultivation.

Newly defined diagnostic properties and materials are albeluvic tonguing, alic and aridic properties, and anthropogeomorphic, calcaric, fluvic, gypsiric, organic, sulfidic and tephric soil material. Descriptions and definitions are also given in Chapter 3.

Gleyic and stagnic properties have been reformulated. Slight changes are made in FAO's definitions of abrupt textural change and geric properites, while the definitions of permafrost and soft powdery lime, renamed secondary carbonates, have been adopted without change.

In the description of the gleyic and stagnic properties the occurrence of 'gleyic' and 'stagnic colour patterns' is introduced. These terms apply to the specific distribution pattern of Fe/Mn (hydr)oxides caused by saturation with groundwater or stagnating surface water. A gleyic colour pattern has 'oximorphic' features on the outside of structural elements, along root channels and pores, or as a gradient upwards in the soil. A stagnic colour pattern on the other hand shows these features in the centre of peas or as a gradient downwards resulting from impedance of the water flow.

The slight changes in the descriptions of abrupt textural change and geric properties refer to a different depth in which tile change in texture must occur and another way of calculating the effective cation exchange capacity (ECEC)1, respectively.

1 ECEC: effective cation exchange capacity (sum of exchangeable bases plus extractable acidity).

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