An overview of the WRB reference soil groups with special attention to West Africa
The World Reference Base for Soil Resources (WRB) has been issued in 1998 (FAO, 1998), after a preliminary version was completed in 1994 (FAO, 1994) and tested in several parts of the world (Argentine, Germany, Russia and South Africa). The FAO Revised Legend of the Soil Map of the World (FAO, 1988) has formed the framework for WRB, according to the mandate of the Working Group on the World Reference Base for Soil Resources of the International Union of Soil Sciences (IUSS): "... to provide scientific depth and background to the 1988 FAO Revised Legend".
WRB comprises two tiers of categorical detail:
As compared to the 1988 Revised Legend, which has 28 Major Soil Groups, three reference groups have been added, while one of the 1988 major soil groups has been deleted. Newly defined reference groups are the Cryosols (soils from the arctic regions with evidence of cryoturbation), Durisols (soils in (semi-) arid regions with accumulation of secondary silica), and Umbrisols (soils with accumulation of organic matter and low base saturation). The major soil group of Greyzems in the Revised Legend has been merged with the Phaeozems, and the Podzoluvisols have been renamed Albeluvisols. An overview of the WRB reference soil groups is given in Table 2, page 19, of the present report.
The World Reference Base for Soil Resources uses diagnostic horizons, diagnostic properties and diagnostic soil materials to identify a soil. Thirty-four diagnostic horizons are recognized, as compared to 16 used in the Revised Legend. Many of the diagnostic properties in the Revised Legend have been given a thickness requirement, thus transforming them into diagnostic horizons. An example is the ferric properties, in the Revised Legend defined as having many coarse red mottles or discrete nodules, which in the WRB need to be present over a thickness of at least 15 cm. In addition, new diagnostic horizons have been formulated such as the melanic and fulvic horizons frequently occurring in volcanic soils.
As a result of this, WRB recognizes less diagnostic properties than the FAO Revised Legend of 1988, 12 versus 26. It should, however, be mentioned that the Revised Legend's diagnostic properties also include materials which are distinguished separately in the WRB.
For classifying subdivisions of the Reference Soil Groups, a comprehensive set of "qualifiers" has been developed. These qualifiers all have unique definitions, in contrast to some of the subunits defined in the Revised Legend, which may have more than one connotation. This is due to the fact that the subunits were designed as part of the Revised Legend, whereby certain soil properties were put together under one heading for the sake of simplification.
A total of 121 qualifiers are now distinguished in WRB. For each Reference Soil Group a list of applicable qualifiers has been established in a priority sequence. Both the number of qualifiers and the sequencing may be subject to change in the future. Already some proposals have been received to add qualifiers, as well as to structure the sequencing differently.
For the present sequencing the principle has been used that first intergrades to other Reference Soil Groups are listed, followed by extragrades and special characteristics. This did not prove to be effective in all soil groups, therefore sequencing was done differently in, for example, the Andosols whereby the properties of the unique soil material have been given priority.
In addition to the qualifiers, the expression of intensity of certain soil characteristics or properties, or the depth of occurrence, can be indicated using one of the ten prefixes defined. These can be added to the qualifier name, thus giving even more information about specific soil conditions.
The Reference Soil Groups, together with the qualifiers, of which up to four may be used to name the soil, and the prefixes, now permit very precise classification of individual soil profiles, and are therefore an excellent tool for exchange of information and experience, not only amongst soil scientists, but also towards other disciplines.
West Africa covers the humid tropical, seasonally dry tropical and (semi-) arid regions, and, consequently, soils typical for all these three environments can be found. In addition, soils conditioned by specific parent material, and soils conditioned by relief (including wetland soils) occur as well. The various soils of these groups, and their correlation with the 1988 Revised Legend will be discussed.
This grouping comprises six Reference Soil Groups, viz. Lixisols, Acrisols, Alisols, Nitisols, Ferralsols, and Plinthosols. Apart from the Alisols, all these soils frequently occur in West Africa, in particular Lixisols and Acrisols, and to a lesser extent Nitisols, Ferralsols and Plinthosols.
Lixisols and Acrisols are quite similar soils, separated from each other by their base status. They are characterized by subsoil clay accumulation ("argic horizon"), a low cation exchange capacity (less than 24 cmolc kg-1 clay) due to the predominance of kaolinitic clays, and, in case of Acrisols a low base saturation (less than 50%; Lixisols in contrast have a base saturation of 50% or more). The two Reference Soil Groups are quite similar to the ones from the 1988 Revised Legend, with one exception: included in the WRB Lixisols and Acrisols are soils with a deep (up to 200 cm thick) horizon of loamy sand or coarser texture overlying the argic horizon. In the Revised Legend these latter soils classified as Arenosols.
Nitisols are soils characterized by a "nitic horizon", in contrast to the revised Legend where Nitisols were defined as soils with a deeply stretched clay bulge and presence of "nitic properties". The nitic horizon has a number of specific properties, viz. 30% or more clay, a low water-dispersible clay/total clay ratio, a low silt/clay ratio, moderate to strong nutty structures with shiny pedfaces, and a relatively high percentage of "active" and "free" iron, the ratio of which should be 0.05 or more. Included in the Nitisols, in contrast to the Revised Legend, are soils which have a ferralic horizon overlain by a nitic horizon (see Ferralsols). Otherwise little differences exist between the 1988 Nitisols and the WRB Nitisols.
The definition of the "ferralic horizon" in the WRB has been slightly modified as compared to its definition in the Revised Legend; omitted has been the requirement of a silt / clay ratio of 0.2 or less (often subject to debate in the past), while the presence of less than 10 percent water-dispersible clay is not required anymore if the soil has a high organic matter content or "geric properties". Excluded from Ferralsols are soils which have a nitic horizon within 100 cm of the surface. Ferralsols normally have only weakly to moderately developed macrostructures. The strongly developed structure, typical for the nitic horizon, does not fit this concept, as does the relative high amount of "active" iron present in nitic horizons. Consequently, such soils are excluded from the Ferralsols, and admitted in the Nitisols where they form an intergrade to the Ferralsols ("Ferralic Nitisols").
Special attention has been paid in the WRB definition of Ferralsols to the apparent occurrence of an "agric horizon" on top of or in a ferralic horizon. In many instances, the top of the ferralic horizon is marked by a clay increase sufficient to fulfil the requirements for the argic horizon. Regularly disputes arose whether in such cases the argic or the ferralic horizon as diagnostic prevailed. By the position of the Ferralsols in the Key to the Major Soil Groups in the Revised Legend it could be argued that the ferralic B-horizon has precedence over the argic B-horizon. However, this was not always clear to the users of the Revised Legend. In the WRB it is now stated that "... if the upper 30 cm of the clay-increase horizon contains 10 percent or more water-dispersible clay, an argic horizon has preference over a ferralic horizon for classification purposes, unless the soil material has geric properties or more than 1.4 percent organic carbon ....". Such soils therefore classify as Lixisols or Acrisols.
WRB Plinthosols are soils that have an accumulation of iron oxides and clay, which hardens irreversibly upon repeatedly wetting and drying, or that has already such a hardened layer. This latter part is a major deviation from the 1988 Revised Legend, especially for those soils which have such a hardened layer at or near to the surface. In the Revised Legend such soils were considered Leptosols, in view of the limited rooting volume and the low water availability. Because WRB places more emphasis on genetical relations between soils it has been decided to merge these soils with a shallow petroferric contact with the Plinthosols. This change will have considerable impact on mapping soils in West Africa whereby all "cuirasses" need to be re-mapped and re-classified as Petric Plinthosols.
This grouping comprises five Reference Soil Groups, viz. Calcisols, Durisols, Gypsisols, Solonchaks, and Solonetz. Also found in these areas are specific man-made soils, the Irragric Anthrosols also known as "oasis soils". Due to their very limited extent in West Africa, these soils will not be considered in the present context.
The WRB Calcisols are quite similar to the Calcisols in the Revised Legend. Particular emphasis in WRB is paid to the type of calcic horizon present. Distinction is made between a calcic (15- 50 percent calcium carbonate equivalent) and a hypercalcic horizon (more than 50 percent calcium carbonate equivalent), whereby the accumulation of soft carbonates becomes such that all or most of the pedological and/or lithological structures disappear and continuous concentrations of calcium carbonate prevail. Included in the Calcisols are those soils which have a petrocalcic horizon at or near to the surface. These soils were included in the Leptosols of the 1988 Revised Legend.
Durisols are a new Reference Soil Group in the WRB with no direct equivalent in the Revised Legend. They comprise the soils that have accumulation of secondary silica ("durinodes" or "duripans"). On the Soil Map of the World (FAO-UNESCO, 1972-1982) they are shown as "duripan phase". They have not been recorded in West Africa, but do occur in southern Africa and west Australia, as well as locally under Mediterranean conditions.
The WRB Gypsisols are also quite similar to the Gypsisols in the Revised Legend. Again, particular emphasis is paid to the type of gypsic horizon present. Distinction is made between a gypsic (15-60 percent gypsum, CaSO4.2H2O) and a hypergypsic horizon (more than 60 percent gypsum). Included in the Gypsisols are those soils which have a petrogypsic horizon at or near to the surface. These soils were included in the Leptosols of the 1988 Revised Legend.
A special type of Gypsisol is recognized, namely the "Arzic Gypsisol", developing under hydromorphic conditions. In such Gypsisols, accumulation of secondary gypsum is evenly distributed, and a gypsic horizon as such (having more gypsum than overlying and underlying horizons) is not present.
Solonchaks in WRB are quite comparable to the Solonchaks in the 1988 Revised Legend. They are characterized by the presence of a salic horizon (horizon with high electrical conductivity). New is that in the subdivision of Solonchaks the possibility exists to indicate the type of salts that are dominantly present, e.g. "Chloridic Solonchaks".
The Solonetz, soils defined by the presence of a "natric horizon", of WRB are identical to the Solonetz defined in the 1988 Revised Legend.
Two out of the three soils comprising this group are important for West Africa, viz. the Arenosols and the Vertisols. The third one, the Andosols, is only important for western Cameroon.
The important change made in WRB for the Arenosols as compared to the 1988 Revised Legend is their place in the Key to the Reference Soil Groups. In the Revised Legend Arenosols keyed out as 9th Major Soil Group (after the Histosols, Anthrosols, Leptosols, Vertisols, Fluvisols, Solonchaks, Gleysols and Andosols), whereas in the WRB Key Arenosols come one but last. As a consequence such tropical soils as "Giant Podzols" and Acrisols with a thick (more than 100 cm) coarse textured eluvial horizon are classified as Podzols and Acrisols respectively, rather than Arenosols as is the case in the 1988 Revised Legend, provided the spodic or argic horizon occurs within a depth of 200 cm. Included now in the Arenosols are also soils in shifting sands, soils that were formerly excluded from Arenosols.
Vertisols have virtually been unchanged apart from the inclusion of soils that have a strongly contrasting layer (lithic or paralithic contact, petrocalcic, petroduric or petrogypsic horizons, sedimentary discontinuity) between 50 and 100 cm depth below the soil that fulfils the requirements for Vertisols. Vertisols in WRB are characterized by the presence of a vertic horizon, which replaces the lengthy description in the 1988 Revised Legend comprising the presence of cracks, intersecting slickensides, wedge-shaped or parallepiped aggregates. The options for subdivision of Vertisols include surface characteristics ("grumic" or "mazic") as well as the colour characteristics of the original 1974 Legend of the Soil Map of the World ("pellic" or "chromic").
The main change that has taken place in the Andosols in WRB with respect to the 1988 Legend is the division based on the type of andic material. Three main types are recognized: a) the "vitr-andic" type, still dominated by a large amount of volcanic glass, representing an intermediate stage of weathering of the volcanic material, b) the "sil-andic" type, characterized by the presence of large amount of newly formed siliceous minerals (mainly opal), and c) the "alu-andic" type, in which large amounts of aluminium are present. The latter two soils are distinctly different in terms of soil acidity, the sil-andic types being slightly acid to more or less neutral, whereas the alu-andic types are strongly acid.
An important addition has been made with respect to the volcanic soils in the sense that freshly deposited or only very slightly altered volcanic ashes have been given a separate place in the Regosols as "Tephric Regosols", being the forerunner of the various types of Andosols.
This grouping of soils comprises the Leptosols, Regosols, Fluvisols and Gleysols. They occur all over West Africa, in association with the soils discussed above.
Leptosols cover shallow soils over hard rock as well as highly gravely (more than 90 percent, by weight, gravel, stones or boulders) soils, and soils overlying highly calcareous materials. With respect to the 1988 Revised Legend soils have been taken out that have pedogenetically formed hard layers at or near to the surface (petrocalcic, petroduric, petrogypsic and petroplinthic horizons), which have been placed with the appropriate Reference Soil Group in WRB, viz. the Calcisols, Durisols, Gypsisols and Plinthosols.
Like with the Arenosols, the major change for the Regosols has been replacement in the WRB Key to the Reference Soil Groups. In the 1988 Revised Legend Regosols keyed out after the Arenosols on the 10th place. In WRB these soils key out last, thus forming the "basket" of soils that do not fit the criteria for the other 29 Reference Soil Groups. In the Revised Legend the Cambisols took up this function, a situation that always has been considered unsatisfactory.
Regosols comprise soils in unconsolidated deposits that hardly show signs of pedogenesis. Included in the Regosols are soils in man-made deposits that do not show significant evidence of soil formation ("Anthropic Regosols"). In the 1988 Revised Legend these soils qualified as Aric Anthrosols or Urbic Anthrosols. Anthropic Regosols can be subdivided according to the material they consist of (Aric for deep cultivation, Garbic for organic waste materials, Reductic for materials under anaerobic conditions due to gaseous emissions, Spolic for industrial waste material, and Urbic for earthy materials mixed with building rubble and artefacts).
Fluvisols in WRB have not changed significantly compared to their meaning in the 1988 Revised Legend. The main change has been the basis of their definition; where in 1988 "fluvic properties" diagnostic for the Fluvisols, WRB uses "fluvic soil materials" to define Fluvisols. An attempt has been made to more accurately describe the statement on "the recent past", something that always raised many questions when applying the Revised Legend. "Recent past" is now described as covering the period during which the soil has been protected from flooding, and during which time soil formation has not resulted in the development of any diagnostic subsurface horizon (other than a salic of sulfuric horizon).
The main change in the Gleysols as compared to the 1988 Revised Legend is the more restrictive conditions in the diagnostic criteria for "gleyic properties" with respect to the percentage of the soil mass affected by oximorphic and/or reductomorphic properties. WRB requires Gleysols to have these properties in either more than 50 percent of the soil mass, or in 100 percent of the soil mass below any surface horizon. Oximorphic properties reflect alternating reducing and oxidizing conditions, whereas reductomorphic properties reflect permanently wet conditions. The result of this change may be that soils, presently classified as Gleysols in the Revised Legend, may become a gleyic subdivision of one of the other Reference Soil Groups in WRB.
A few other soils may be important to discuss amongst the West African soils, viz. Planosols, Luvisols, and Cambisols. These belong to different soil groupings, mainly characteristic for temperate regions, but are known to occur in the region.
Apart from the maximum depth requirement for the lower boundary of the abrupt textural change (125 cm in the 1988 Revised Legend vs. 100 cm in the WRB), which moves 1988 Planosols with an abrupt textural change between 100 and 125 cm into Arenosols (if coarse textured) or Regosols, and the switch of places in the Key between Solonetz and Planosols, their essence remains unchanged: soils with poor physical characteristics due to the slowly permeable subsoil causing stagnic conditions in the surface or shallow subsurface horizons.
Due to the significant changes made in the WRB definition of the Alisols, restricting these soils to humid tropical and subtropical regions, a gap developed as compared to the 1988 Revised Legend with respect to those soils with an argic horizon, which have a low base saturation but do not fulfil the requirements for the WRB Alisols. Permitting Luvisols to have a base saturation of less than 50 percent, a major change with respect to the Revised Legend, has solved this problem. Such soils, now classifying as "Dystric Luvisols", occur in many forested areas of Europe and North America, and to a lesser extent in small areas in Latin America, Africa and Asia. Some "terres de barre" in Benin and Togo are an example of such soils.
Cambisols have become a Reference Soil Group of minor importance in West Africa. Most Cambisols according to the 1988 Revised Legend classified as "Ferralic Cambisols", mainly because not all criteria were met for the ferralic horizon, in particular the criterion on having a silt/clay ratio of 0.2. Parent materials in West Africa produce rather silt-rich weathering products, and as a consequence many of the strongly weathered soils do have a silt/clay ratio of 0.5 or more. In the definition of the ferralic horizon in WRB the silt/clay ratio criterion has been omitted, therefore most of the 1988 Ferralic Cambisols become Ferralsols under WRB. A few soils remain which do not fulfil the requirements for the WRB ferralic horizon; these soils remain in the Reference Soil Group of Cambisols.
Overviewing the changes made in the World Reference Base for Soil Resources compared to the 1988 Revised Legend of the Soil Map of the World, one may conclude that revisions may be necessary only on basis of a few details. Introduction and implementation of the World Reference Base in West Africa, with the knowledge about the 1988 Revised Legend already present should therefore not be a problem.
