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Chapter 4: Classifying subdivisions of the reference soil groups

General principles for distinguishing lower level units
Definitions of formative elements for lower level units

Since the inception of the Legend of the Soil Map of the World (FAO, 1974), the number of lower level units used in the Legend or soil classification has continued to grow: from 106 in 1974 to 152 in the Revised Legend of the Soil Map of the World (FAO, 1988) to 209 in the first draft of the World Reference Base for Soil Resources (ISSS-ISRIC-FAO, 1994). At the same time a serious effort was undertaken to expand this second level further with the introduction of third level units (Nachtergaele et al., 1994). Further proliferation of soil units and subunits in the World Reference Base might easily lead to a situation where it will become extremely difficult to recall and use all definitions within the main reference soil groups.

A further complication is that many soil unit names, and modifiers in the draft WRB, were inherited from the original FAO Legend and were defined depending on the grouping in which they occurred. For example, a "Dystric" soil unit may mean: "... having a base saturation of less than 75%" (in Dystric Vertisols), or "... having a base saturation of less than 50%", in different control sections (e.g. note the difference in control sections of Dystric Planosols and Dystric Cambisols).

Another limitation inherent to the close link with the Legend of the Soil Map of the World is that, although often used as a soil classification system, the original purpose of the FAO system was to serve as a Legend for a specific map, which made certain simplifications necessary. For example, Calcic Gleysols included soil with a gypsic horizon. Similarly, Umbric Fluvisols grouped alluvial soils with an umbric horizon together with Fluvisols with a desaturated histic horizon. This resulted in loss of information due to the generalization required for the Legend.

Last but not least, it is thought that a clear-cut separation must be made between the double objectives of the World Reference Base, which on one hand should be able to serve as a soil reference system for geographers, agronomists and other users who are mainly interested in the highest level of generalization explained in non-technical terms, while on the other hand WRB must be a sophisticated tool for soil correlation able to accommodate a wide range of national soil classification systems.

In order to remedy the soil classification constraints as discussed above, it is decided to aim for standardized definitions for each subdivision and to design a flexible soil classification system which allows for a maximum transfer of soil profile information. Therefore a limited number of names are defined for the subdivisions of the World Reference Base soil groups which may be used in a certain ranking order to qualify each group at lower levels. Moreover, to simplify its use as far as possible, a unique definition for each qualifier, as well as the use of standard depths and thicknesses, is promoted. In doing so, it is unavoidable that the certain link which now existed until 1994 with the FAO soil units will be partly lost. However, this loss is compensated by the gain of clarity and ease of use of the present approach.

A further advantage of standardized subdivisions is that it will facilitate and enhance soil correlation and technology transfer among countries and regions. It should, in addition, serve useful purposes (for example land evaluation and land use planning), and should not be considered as an end in itself but rather contribute to the better understanding of the soil resource.

At this stage it is not possible to provide a comprehensive list of lower level names for the World Reference Base. Reviewing the uses which have been made of at second level by FAO (1988), Soil Survey Staff (1996) and WRB (ISSS-ISRIC-FAO, 1994), and at third level in soil classifications of Botswana (Remmelzwaal and Verbeek, 1990), northeastern Africa (FAO, 1998), Bangladesh (Brammer et al, 1988) and the European Union (CEC, 1985), and by reclassifying a large number of typifying pedons of all reference soil groups, a provisional list of names and definitions has been established.

General principles for distinguishing lower level units

In order to keep the system simple and easy to use, criteria to differentiate soil subunits are selected that closely relate to the diagnostic criteria defined at the first level.

The newly introduced criteria relate to additional soil properties which are thought to be relevant at lower levels. The use of phases as differentiating criteria of the lower classification levels should, in principle, be kept at minimum. A few of them, however, have been included in the provisional list of names.

General rules

The general rules to be followed when differentiating lower level units are:

1. The diagnostic criteria applied at lower level are derived from the already established reference group diagnostic horizons, properties and other defined characteristics. They may, in addition, include new elements as well as criteria used for phase definitions at higher levels.

2. Lower level units may be defined, and named, on the basis of the presence of diagnostic horizons. In general, weaker or incomplete occurrences of similar features are not considered as differentiae.

3. Differentiating criteria related to climate, parent material, vegetation or to physiographic features such as slope, geomorphology or erosion are not considered. The same applies to criteria derived from soil-water relationships such as depth of water table or drainage.
Substratum layers, thickness and morphology of solum or individual horizons, are not considered as diagnostic criteria for the differentiation of the lower level units.

4. There is one set of diagnostic criteria for the definition of the lower level soil units. This name contains in its definition the diagnostic criterion and functions at the same time as second and third level connotative. Each soil qualifier is given one unique meaning which should be applicable to all reference soil groups in which it occurs.

5. A single name should be used to define each lower level. However, these names can be used in combination with indicators of depth, thickness or intensity. If additional names are needed, these should be listed after the reference soil group names between brackets, e.g. Acri-Geric Ferralsol (Abruptic and Xanthic).

6. Definitions of the lower level units should not overlap or conflict with other soil subunits or with reference soil group definitions. For example, a Dystri-Petric Calcisol is a contradiction, whereas a Eutri-Petric Calcisol is an overlap in the sense that the name "eutric" does not give more information.

New units can only be established after being documented by soil profile descriptions and supporting laboratory analyses.

7. Priority rules for the use of lower level soil names are to be followed strictly to avoid confusion. Precise ranking orders for each qualifier in each reference soil group are given later in the text.


In Vertisols the following qualifiers have been recognized, in order of priority:



intergrade with acid sulphate Gleysols and Fluvisols



intergrade with the Solonchak reference soil group



intergrade with the Solonetz reference soil group



intergrade with the Gypsisol reference soil group



intergrade with the Durisol reference soil group



intergrade with the Calcisol reference soil group



intergrade with the Alisol reference soil group



containing gypsum



dark coloured, often poorly drained



mulched surface horizon



very hard surface horizon; workability problems



reddish coloured



having less than 75 percent base saturation (occurs in Venezuela)



having an ESP of 6 to 15



having 75 percent or more base saturation



no specific characteristics

To classify a reddish coloured Vertisol with a calcic horizon one would follow the priority list and note that qualifiers 6 and 12 apply. Therefore, the soil is classified as Chromi-Calcic Vertisol. If more information on depth and intensity of the calcic horizon is available, e.g. Occurring near to the surface, one may specify this by classifying the soil as Chromi-Epicalcic Vertisol, indicating the occurrence of the calcic horizon within 50 cm from the surface.

When more than two qualifiers are required, they can be added between brackets after the standard name. If, for instance, the Vertisol discussed also has a very hard surface horizon (qualifier 11), the soil would be named Mazi-Calcic Vertisol (Chromic).

Future expansion and applications

This system allows a maximum transfer of soil knowledge because all qualifiers have a unique meaning, are relatively few in number, and can easily be taught and memorized.

When used for mapping purposes at different levels of detail in combination with existing national soil classification systems, the system will need to be adapted to serve as a legend for soil maps. This may require a simplification and grouping several of the qualifiers together. For example, in the list of qualifiers the name "Thionic" already combines "Protothionic" and "Orthithionic"; another example would be to consider only intergrade names for a small-scale map.

It should also be noted that in the ongoing preparation of a World Soil and Terrain Database by UNEP, ISSS, ISRIC and FAO (Van Engelen & Wen, 1995; Nachtergaele, 1996), mapping units also contain soil profile information which can be classified using the present system.

It is also realized that for soil management purposes often more information is required, in particular on topsoil characteristics and on soil climate. It is proposed that guidelines on both issues should be further developed in line with existing proposals such as "The Characterization of Topsoils" (Fitzpatrick, 1988; Spaargaren, 1992; Purnell et al., 1994), and the climatic regimes of the Global Agro-ecological Zones Methodology (Fisher et al., 1996).

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