Chapter 3 Classification of vertisols

63 Chapter 3

CLASSIFICATION OF VERTISOLS W.A. BLOKHUIS

3.1. INTRODUCTION

In the major international soil classification systems Vertisols or a similar group of soils under a different name, appear at the highest categoric level. This is not surprising: Vertisols or "dark cracking clays" have a specific morphology that is strongly related to a combination of a fine-textured soil material, smectite clay mineralogy, poor drainage conditions and the alternation of wet and dry seasons. The morphology is characterized by surface cracks, a well-developed soil structure with wedge-shaped peds and slickensides, and weak horizonation. In various classification systems the criteria for defining Vertisols at the highest level emphasize either the morphology, or the soil material, or both, in combination with environmental factors such as climate and drainage. At the lower levels, differentiating criteria vary widely between classification systems and between successive editions within one and the same system. Changes in a system as more information becomes available and knowledge increases, are clearly shown in the various amendments and revisions of Soil Taxonomy (Soil Survey Staff, 1975), the soil classification of the US Department of Agriculture. In this chapter, emphasis is placed on Soil Taxonomy because of its multicategoric nature and its widespread international use. Soil Taxonomy was developed in the U.S.A. in collaboration with many soil scientists from abroad. It aims at accommodating all soils of the world. Other systems to be discussed are the FAO/Unesco classification for the Soil Map of the World project (FAO/Unesco, 1974; FAO, 1988), the French classification (CPCS, 1967) and the Austrahan classification (Northcote 1979). Recently international frameworks for soil classification have developed: the World Reference Base for Soil Resources and the Referentiel Pedologique Frangais. 3.2. SOIL TAXONOMY

Soil Taxonomy was developed to replace the system published in the Agricultural Yearbook of 1938, "Soils and Men" (Baldwin et al., 1938), with later amendments (Thorp and Smith, 1949). The new system was first published in a prehminary form as "Soil Classification, a comprehensive system; 7th Approximation" (Soil Survey Staff, 1960). The official puWication followed in 1975 as "Soil

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Taxonomy, a basic system of soil classification for making and interpreting soil surveys" (Soil Survey Staff, 1975). Soil Taxonomy is in line with the tradition that soil classification in the USA should serve soil mapping for farmers, engineers and foresters (Bartelli, 1982). Since 1975 several revisions have been pubUshed as "Keys to Soil Taxonomy", in 1983, 1985, 1987, 1990, 1992 and 1994. Revisions of specific aspects of the classification were prepared by international committees, the ICOM's. The works of two ICOM's have special importance for Vertisols: ICOMERT, for Vertisols, and ICOMAQ, for the aquic soil moisture regime. As from 1960, the USDA soil classification system has six categories. Classes on the four highest levels (order, suborder, great group and subgroup) are defined and named, whereas rules are given for the definition and nomenclature of the lower levels (soil family and soil series). Classes are defined according to quantified differentiating features: diagnostic horizons and other diagnostic properties. The definition of the order of Vertisols has not changed fundamentally since 1960, but in the lower defined categories of the suborder, great group and subgroup the modifications have been substantial. In the 1960 edition, Vertisols were defined as soils having more than 35% clay, a CEC of more than 30meq./100g soil, cracks 1-25 cm wide, and one or more of: gilgai, intersecting slickensides, wedge-shaped or parallelepiped structural aggregates. There were two suborders, Aquerts and Usterts. Aquerts had low chroma (less than 1.5) and/or mottling, Usterts had no mottling, and chroma was 1.5 or above. Mottles as well as low chroma were associated with periodic wetness: "The Aquerts are saturated with water at some season, and are gleyed, though they have so few noncapillary pores when wet that it is difficult to make specific statements about the presence of groundwater" (Soil Survey Staff, 1960, p.124). Usterts were (moderately) well-drained and not subject to flooding for long periods. In each suborder two great groups were recognized: the Grum- had a surface mulch of loose soil aggregates, the Maz- had a massive surface crust. The thin crust was considered to be a minor albic horizon. Each great group had an Orthic as well as an Entic subgroup, with colour values of 3.5 or less, and higher than 3.5, respectively, and intergrade subgroups with MoUisols, Alfisols, Ultisols and Aridisols, some with natric characteristics (exchangeable sodium percentage of 15 or more) and subgroups for low pH. Vertic subgroups in other orders had 40% or more expanding lattice clay. These were potentially shrink-swell soils. In the 1975 version. Soil Taxonomy, the definition of the order underwent some changes: clay percentage of 30 or more, CEC no longer diagnostic, and a limitation to mesic, isomesic or warmer soil temperature regimes. The other criteria (gilgai, slickensides, wedge-shaped peds, cracks) remained unchanged. In most orders, the definition of suborders was based on the newly introduced concept of the "soil moisture regime". Suborders of Vertisols were Torrerts, Xererts, Usterts and Uderts. A suborder Aquerts was not defined because an "aquic soil moisture regime"—based on the depth towards a water level in an unlined borehole—could not be defined in a Vertisol. Except for the Torrerts, each suborder had a Pellic and a Chromic great group, with chroma less than 1.5, or 1.5 and more, respectively. Low chroma was not associated with hydromorphic

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conditions per se, but Pelluderts, Pellusterts and Pelloxererts were supposed to occur in level or depressed positions. Aquic and aquentic subgroups, based on, inter alia, the presence of "distinct or prominent mottles" were recognized in Chromuderts and Chromoxererts. There was no longer a provision for natric subgroups. Soil surface characteristics were no longer diagnostic on any of the defined categoric levels. Vertic subgroups in other orders were defined as the combination of two properties: cracks (1-25 cm wide) and the shrink-swell potential of the soil material: a coefficient of Hnear extensibihty (COLE) of 0.09 or more, or a potential Unear extensibility of 6 cm or more. The revisions of 1983, 1985, 1987 and 1990 hardly affected the Vertisol order. In the 1992 revision (Soil Survey Staff, 1992), the definition of the order was based on intersecting slickensides or wedge-shaped peds, a clay percentage of 30 or more, and cracks that open and close periodically; there is no longer a limitation on soil temperature regime, and a gilgai microrelief is no longer diagnostic. In the key to soil orders, Vertisols come fifth, after Histosols, Spodosols, Andisols and Oxisols, respectively. Two suborders were added, the Aquerts and the Cryerts. The Cryerts, Vertisols of cold regions, could be included now that soil temperature regimes were no longer diagnostic. Aquerts had a come-back after their first and temporary introduction in 1960. Creation of this suborder was consistent with the principles of Soil Taxonomy, and differentiated a separate kind of soil and soil conditions (Eswaran et al., 1989). The Aquerts as defined in 1992, are supposed to have "aquic conditions" and, in addition either low chromas (two or less)^ with redox concentrations or very low chroma (one or less) without redox concentrations, or enough active ferrous iron to give a positive reaction to a a,a'-dipyridyl test. "Aquic conditions" refers to soils "which currently experience continuous or periodic saturation and reduction. The presence of these conditions is indicated by redoximorphic features and can be verified (. . .) by measuring saturation and reduction" (Soil Survey Staff, 1992, p.25). Saturation should be measured by piezometers or tensiometers, and reduction by direct measurement of redox potentials, or by the a,a'-dipyridyl test. The number of great groups was greatly enlarged, with provisions for salinity, sodicity, the presence of a calcic/petrocalcic or a gypsic horizon, and other soil properties (Table 3.1). However, there is on the great group level no provision for differentiating between low-chroma and high-chroma Vertisols. Subgroup differentiation in the 1992 Keys is based on subordinate reference to soil moisture regimes (Aquic, Xeric, Ustic, Aridic), chemical characteristics (Sulfaqueptic, Alic, Halic, Sodic), temporary water saturation without implications for soil colour (Oxyaquic), soil depth (Lithic, Leptic), coarser soil texture (Entic), higher chroma (Aerie, only in Aquerts). The last two subgroups to key out in any great group are Chromic and Typic. The Chromic subgroup can have either or

^All colour criteria are now in whole Munsell units.

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TABLE 3.1 Great group differentiae in suborders of Vertisols according to the Keys to Soil Taxonomy, 1994 edition Suborders Great group differentiae

Aquerts

Salinity Duripan Natric horizon Gypsic horizon Calcic horizon Dystric (low pH) Epizaturation Endosaturation High content organic matter Haplo

X X X

Cryerts

Torrerts

Uderts

Usterts

Xererts

X

X X

X X X X X X

X

both of a relatively high colour value (4 or above) and a relatively high chroma (3 or above).^ Vertic subgroups in other orders are defined solely on the shrink-swell capacity of the soil material: a linear extensibiUty of 6.0 cm or more. For Vertisols the 1994 Keys (Soil Survey Staff, 1994) are not different from the 1992 revision. 3.2.1.

Discussion

(1) Introduction Notwithstanding the elaborate organization and well-planned international meetings and excursions, the classification of Vertisols in Soil Taxonomy still leaves open some important questions and there is a lack of agreement on a number of definitions and taxa. The most drastic changes since the inception of the system in 1960 are the introduction of soil moisture regimes as the only criterion on suborder level, changes in the use of soil colour aspects as differentiae, and the dropping of surface characteristics as a diagnostic property. (2) Soil moisture regimes Soil moisture regimes as defined in Soil Taxonomy could not be applied to Vertisols because of the irregular and incomplete moistening and drying: moistening by rain is often restricted to the surface soil and the areas around cracks, whereas the subsurface soil between the cracks remains dry. Therefore, suborders of Vertisols were not defined—as in other orders—on the soil moisture regimes as such but on the periods that cracks are open or closed. The use of such soil ^In Aquerts, Chromis subgroups are defined on value criteria alone.

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criteria for the definition of moisture regimes should, in the rationale of Soil Taxonomy, even have prevalence over the use of soil moisture criteria (Guthrie, 1985). However, discrepancies occurred between moisture regimes as estimated from climatic data, and as estimated from cracking behaviour (Dudal and Eswaran, 1988; Blokhuis, 1993), probably due to factors other than climatic conditions. This was confirmed by Isbell (1991) for the wide range of Vertisols in the Australian arid zone. Isbell (1991) considered the use of cracking patterns instead of soil moisture regimes a major weakness in Soil Taxonomy. He suggested that better methods of defining and estimating soil moisture regimes must be developed if the concept of the moisture regimes is to be used at the suborder level in Soil Taxonomy. Van Baren and Sombroek (1985) criticized the suborder criteria as being management-oriented and unsuitable once irrigation is practiced. (3) Aquic soil moisture regime and aquic conditions The definition of the aquic soil moisture regime was particularly difficult in Vertisols: the recommended borehole method to identify an aquic regime could not be used, whereas reducing conditions in a soil that is periodically dry, and perhaps only partially water-saturated when wet, are difficult to ascertain. Hydromorphic conditions in Vertisols, defined as "aquic conditions" or as "aquic soil moisture regime"^ was discussed by Comerma (1984) who found that saturation and reduction in a Vertisol normally occur due to wetting from the surface. Water saturation from groundwater sources should, however, in principle not be excluded (Comerma et al. 1988). Blokhuis (1985) stated that groundwater tables were alien to Vertisols, but transient perched water tables were feasible at the sites of cracks. If complete water saturation at 40-50 cm depth (as required for Aquerts) in a Vertisol, even temporary, is already open to doubt, even more unlikely are reducing conditions. ICOMAQ (1991) has recommended that terms like episaturation and endosaturation be used for saturation without reduction. This is particularly relevant for Vertisols which often have neutral or higher pH; in such soils extremely low E^ levels must be reached before iron is reduced. There is certainly a need for more observations on wet Vertisols to ascertain the presence or absence of aquic conditions as specified in the Keys to Soil Taxonomy (Soil Survey Staff, 1992). One monitoring study should be mentioned here. Griffin et al. (1992) studied watertable movements, soil saturation (by piezometer) and reduction (by a,a'-dipyridyl) in three wetland soil profiles in Texas during a 17-month period. One of these soils, an Entic Pelludert, had a seasonal movement of the watertable between 10 and 130 cm from the surface, temporary saturation and reduction, and redoximorphic features. The type of saturation was endosaturation, and the soil met the requirements for endoaquic conditions. A second Vertisol, a Typic Pelludert, had a perched water table for brief periods, temporary saturation, but no reduction. The type of saturation was ^In the Keys of 1994, the term "aqutic soil moisture regime" is no longer used as a criterion; suborders of hydromorphic soils are defined on "aquic conditions" and other properties.

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episaturation. The absence of ferrous iron was thought to be due to the very short period of saturation, and/or the influxes of oxygenated water from rainfall, and/or high pH. Unfortunately, macromorphological observations on the soil profile are not given in the paper; the Entic Pelludert had very large slickensides, the Typic Pelludert had a gilgai microreUef. We find it difficult to think of a soil with large, not necessarily intersecting, slickensides and a freely moving watertable, as a Vertisol. (4) Soil colour as a diagnostic property The most problematic issue at this moment is perhaps the use of colour aspects as diagnostic properties for water saturation and reduction in Vertisols. The criteria have changed since 1960 (*'7th Approximation") and so have the presumed genetic implications. Low chroma Vertisols have explicitly (Aquerts in the 1992 Keys) or implicitly (PeUic great groups)"^ been related to wetness. They often occupy sites subject to long periods of flooding, and occur most frequently in the higher-rainfall areas. In Vertisols of alluvial and coUuvio-alluvial clay plains of the Sudan, low chroma in combination with flooding of some duration appeared to be a reUable indicator of poor drainage (Blokhuis, 1993). Van Baren and Sombroek (1985) suggested that Aquerts could perhaps be defined on a quantified minimum duration of flooding or submergence, in addition to colour criteria. Low-chroma Vertisols that do not have aquic conditions key out on soil colour in the 1992 revision as a typic subgroup, or earlier in the key on other characteristics that are diagnostic on the great group or subgroup level. Indeed, low chroma and flooding—whether or not meeting the requirements on saturation and reduction as specified under "aquic conditions"—is not a necessary combination. Discussions in ICOMERT (Circular Letters 1 (February, 1981) and 2 (October, 1982)) showed that it was possible to define boundaries for a chromic/pellic differentiation that in a practical manner separated flooded from non-flooded Vertisols, but that such boundaries are different in various regions of the world. Besides, although chromic and pelHc separate two distinct morphological groups, little can be said of their performance-related characteristics (Dudal and Eswaran, 1988). Comerma et al. (1988), however, although acknowledging the fact that "pelHc" and "chromic" did not make the desired separation according to drainage condition, are in favour of their maintenance at the subgroup level as the colour often is a reflection of parent material, which influences physical and chemical soil properties. Whatever the relation between parent material and chroma of Vertisols is, the following examples show that many of the Vertisols developed from basalt are Pellusterts. Most of these occur on sloping terrain and are well-drained externally.

'^In the discussion, "pellic" refers to low-chroma Vertisols, "chromic" to high-chroma Vertisols, without quantification of the chromas.

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Under these conditions, reduction is unlikely to occur. Some of the Vertisols in Sudan that were developed in situ on basalt or other types of basic igneous rock were Pellusterts (Blokhuis, 1993). Similar observations were made by Quantin et al. (1977), Tejedor Salguero et al. (1978) and Fernandez Caldas et al. (1981), who studied Pellusterts on basalt in the Canary Islands. Vertisols on pedisediments, foothills and mountains of basaltic origin in Lesotho were Typic Pellusterts (Lekholoane, 1985). Most Vertisols in Zimbabwe formed from ultramafic and mafic rocks, are Pellic Vertisols (according to the FAO/Unesco classification) (Kanyanda, 1985). Two Vertisol profiles in Uruguay, one a "Vertisol lithomorphe", the other a "Vertisol topomorphe" in the French classification (CPCS, 1967), both derived from basalt weathering, had differences in leaching and in drainage characteristics. Both were classified as Pelluderts according to Soil Taxonomy (Rossignol, 1983). This could imply that the low-chroma colour of both soils is due to a relation with the parent material. Blokhuis (1993) showed for peUic Vertisols in the central clay plain of the Sudan that the low chroma of the soil could not be due to reduction because the colour did not change on exposure to air. On both field and laboratory evidence he ascribed the low chroma to stability of smectite and, consequently, to the lack of free ferric iron. Pellic Vertisols thus seem related to areas subject to flooding, as smectite stabiUty is promoted by poor drainage, and, in addition, by high pH levels and high levels of basic cations and silica. Smectite stability in pellic Vertisols developed in situ on basic rocks and occurring on slopes with sufficient run-off to prevent flooding, could be due to a very slow internal drainage or to some relation with the parent rock (Blokhuis, 1985). The higher chroma of Chromic Vertisols, generally occurring on (moderately) well-drained sites was ascribed to a partial transformation of smectite into kaolinite, with a simultaneous liberation of ferrous ions from the smectite lattice and oxidation of these to ferric oxides. The genetic implication of different values of the soil colour is even less clear than that of different chromas. Colour value was used as a differentiating property at the subgroup level in the 7th Approximation: Entic for moist values of four or more, Orthic for values of 3.5 or less. The low-value (dark) Vertisols were considered to be the typical ones. High-value Vertisols occurred in, for example, the Sudan in such contrasting environments as semi-arid level plains with Chromusterts, and higher-rainfall flooded depressions with Pellusterts. In 1975 the definition of Entic and Typic (Orthic in 1960) subgroups remained the same, apart from a 0.5 value unit shift: Entic for moist values of 3.5 or more. The combination of colour value and colour chroma to characterize Chromic and Typic subgroups in all suborders except Aquerts in the 1992 Keys, is most surprising. Entic subgroups had no longer any relation with soil colour, they had some layer with a low percentage of clay within 100 cm depth. Chromic was re-defined for subgroups with both a high chroma and a high value. There is no longer a Pellic subgroup; the subgroup to key out after Chromic, is Typic, suggesting that the typical Vertisol has both a low chroma and a low value. Worldwide this may be true, but the fact remains that the new definitions ignore the genetic implications of the terms chroma and value in their original meaning.

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In this way taxa are created that can hardly be understood from a genetic point of view. This is most confusing. It also remains unclear what the purpose of using the present colour criteria is. Do the subgroups, thus defined, have other characteristics in common? (5) Surface characteristics The mazic/grumic differentiation was dropped in Soil Taxonomy (Soil Survey Staff, 1975) because of the suspected non-permanency of the feature; it was found to be dependent on the length of the period of cultivation and on soil management practices, and it varied between years probably because of different weather conditions. In the 7th Approximation (Soil Survey Staff, 1960), the crust was considered a minor albic horizon, characterized by the presence of bleached sand and silt grains. Crusty Vertisols, showing a slight clay increase below this albic horizon have been regarded as intergrades towards Planosols (Dudal, 1973). Probert et al. (1987) criticized the loss of the mazic/grumic differentiation because of its importance in land suitability assessment. According to Dudal and Eswaran (1988), crusting of Vertisols seems to be more frequent in humid areas, where it is of importance to the water regime: less water intake, more hazards of waterlogging, difficult tillage, and poor seed bed conditions. In the Sudan, Vertisols with a surface crust had more silt and sand and less clay than grumic Vertisols and had a more compact structure (Blokhuis 1993). Such mazic Vertisols often had lower clay percentages throughout because of the uniform texture of most Vertisols. The differentiation of surface characteristics thus created taxa that also have other characteristics in common, notably clay content and a relatively poor structural development, and that form easily mappable polypedons. Van Baren and Sombroek (1985) suggested that mazic and grumic Vertisols be re-introduced, for instance at the third categoric level. A crusty surface may also occur in combination with sodic conditions as these promote a loss of structural stability. Clay illuviation will then form an incipient natric horizon. Field evidence of this type of a mazic surface is weaker than for the sandy/silty crust, but is not lacking (MacGarity, 1985; Blokhuis, 1993). The mazic/grumic issue was raised again in ICOMERT's 2nd Circular Letter (October 1982) because of its agronomic importance. In the 3rd Circular Letter (October 1983) mazic subgroups were proposed in all suborders except Aquerts. In the 4th and last Circular Letter (July 1984) it was reported that correspondents mainly connected with cultivated soils, were not in favour of grumic or mazic subgroups, whereas correspondents working in areas with less intensive agriculture or with contrasting parent materials were in favour. In the proposed key to the order of Vertisols, attached to the 4th Circular Letter, no mazic or grumic subgroups are to be found and apparently the issue was dropped. This remains unsatisfactory. (6) Sodic subgroups In the 1992 Keys sodic subgroups were defined on the basis of a minimum sodium saturation percentage of 15. This is the same figure as used for defining

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sodic subgroups of Aridisols. Ahmad (1983) suggested a higher percentage for Vertisols because exchangeable sodium would not have the same implications for aggregation, permeability and bulk density as in other soils. Purnell et al. (1976) considered an exchangeable sodium percentage of 15 too low to differentiate between sodic and non-sodic Vertisols of the Sudan clay plains. They suggested limits of 25 for sHghtly sodic, and 35 for strongly sodic, based on the observation that excellent crops are grown on Vertisols with ESP well above 15. In Australia, however, Vertisols with ESP in the range 8-14 are considered sodic (MacGarity, 1985); they have a crusty surface and a massive soil structure. 3.3. THE FAQ/UNESCO SYSTEM

The FAO/Unesco soil classification is perhaps as widely known as the US Soil Taxonomy, and in several countries both systems are used. Between the two are marked differences, but their common ground is that differentiating criteria between taxa are defined in terms of observable and measurable soil properties. Soil genesis is at the basis of these diagnostic features, but genetic processes as such are not diagnostic. The system was devised as a framework for the legend of the Soil Map of the World at a scale of 1:5,000,000. It was initiated at the 6th International Congress of Soil Science at Paris, in 1956. The project aimed at developing a classification and correlation of the soils of great regions of the world. It soon became clear that without a common denominator on methods, systems and nomenclature a world soils project could not be realized. A comprehensive soil mapping legend was required, and such a legend had to be built on a new nomenclature and a new system of classifying soils. Through world-wide cooperation in various working meetings a broad international panel arrived in 1968 at a "Definition of soil units for the soil map of the world" (FAO/Unesco, 1968). At this stage the structure of the classification system, and the methods to use the taxonomy in devising soil mapping units were well-established, and only relatively small changes and amendments were necessary to publish in 1974 Volume I, Legend, of the FAO/Unesco Soil Map of the World project (FAO/Unesco, 1974). Volumes II-X are regional monographs with 1:5,000,000 map sheets. The classification system presented in 1974 had two categories: major soil groupings and soil units. The definition of taxa was in line with that of the US ''7th Approximation" (Soil Survey Staff, 1960), "in the belief that the introduction of measurable differentiating characteristics would result in improved identification, description and correlation of soils" (FAO/Unesco, 1968). The Legend has 26 major soil groupings and 106 soil units. Major soil groupings are more or less on the order or suborder level of Soil Taxonomy, whereas soil units correspond roughly to the great group level. The soil mapping units are usually associations of two or more soil units. For mapping purposes, soil textural and soil slope classes and phases are used in addition to soil associations. Vertisols were a major soil grouping as from the 2nd draft (FAO/Unesco, 1964). In Volume I, Legend (FAO/Unesco, 1974), Vertisols were defined in the same way as in Soil Taxonomy (Soil Survey Staff, 1975), but, as soil temperature regimes

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are not diagnostic in the FAO/Unesco classification, there were no cUmatic Umitations. Vertisols were third in the Key of major soil groupings, after Lithosols and Histosols. Two soil units were distinguished: PeUic Vertisols (with moist chroma less than 1.5) and Chromic Vertisols (with chroma 1.5 or more). Vertic soil units were defined in Luvisols and Cambisols; these have cracks at least in the B horizon, but lack other diagnostic characteristics of Vertisols. Naturally, the 1974-version was not the last word. A Revised Legend appeared in 1988 (FAO, 1988); in this revision the International Soil Reference and Information Centre (ISRIC) participated with FAO and Unesco. There were relatively minor changes in diagnostic horizons, diagnostic properties, phases, major soil groupings and soil units. The general structure and rationale on which the orginal legend was constructed, were maintained. The Revised Legend is going to be used for updating the maps and for the preparation of new maps at scales of 1:5,000,000 or larger. There are now 28 major soil groupings, subdivided into 153 soil units. The definition of Vertisols has slightly changed: clay percentages must be at least 35%, and the gilgai microrelief is no longer required. The former subdivision of Vertisols according to colour chroma has been replaced by a subdivision based on presence/absence of a gypsic or a calcic horizon and on base saturation. No justification for this far-reaching change is given, and as it will take some time before new 1:5,000,000 maps are prepared, it will remain difficult to judge how this change will work out geographically. The soil units key out as follows: Gypsic Vertisols, with a gypsic horizon; Calcic Vertisols, with a calcic horizon or soft powdery lime; Dystric Vertisols, with base saturation (by NH4OAc)<50%; Eutric Vertisols. Vertic subgroups occur in Luvisols and Cambisols. These have "vertic properties", a term that is used in connection with "clayey soils which at some period in most years show one or more of the following: cracks, slickensides, wedge-shaped or parallelepiped structural aggregates, that are not in a combination, or are not sufficiently expressed, for the soils to qualify as Vertisols". This seems a suitable definition for soils that are not Vertisols, but have some of their characteristics. A third categoric level was developed in the Revised Legend to provide (1) a more detailed basis for the classification of individual soil profiles, and (2) a framework for the construction of soil mapping legends at scales more detailed than the original 1:5,000,000 scale of the World Soil Map. The third level is considered to suit soil mapping at scales of 1:1,000,000 to 1:250,000, on a global, regional or national basis (Nachtergaele et al., 1994). Experience with the use of third-level units was obtained in soil surveys in, for example, Botswana, Kenya, Zambia, Bangladesh, Mozambique and in the European Community. In the Revised Legend some examples of third-level modifiers are given, and at the 15th World Congress of Soil Science in Mexico, a provisional Ust of names and

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definitions, as well as rules for the priority sequence of third-level soil subunits were presented (Nachtergaele et al., 1994). The first rule is that the priority sequence should follow as much as possible the same sequence as used in the key for soil units; the second rule is that priority be given to properties in the topsoil rather than in the subsoil. The sequences are different for different major soil groupings. For Vertisols the priority listing is as follows: Calci- : having a calcic horizon or concentrations of soft powdery lime between 50 and 125 cm from the surface. Dystri- : having a base saturation of less than 50 percent (by NH4OAC) in at least some part between 50 and 125 cm from the surface. Gypsiri- : gypsiferous at least between 20 and 50 cm from the surface. Calcari- : calcareous at least between 20 and 50 cm from the surface. Sodi- : having sodic properties within 50 cm from the surface. Salt- : having salic properties within 30 cm from the surface. Pelli- : having a moist value of 3.5 or less and a chroma of 1.5 or less in the upper 30 cm of the matrix. Grumi- : indicating the presence of a surface layer with a thickness of 3 cm or more having a strong coarse, or finer, granular structure. Mazi- : indicating a massive structure and hard or very hard consistence in the upper 18 cm of soil. Orthi- : typical expression of (typical in the sense that there is no further or meaningful characterization at this level). Some examples with Vertisols are given in the Revised Legend, and these illustrate the different types of soil subunits. Grumi-Eutric Vertisols: Eutric Vertisols which, when dry, have a strong fine granular structure in the upper 18 cm of the soil. Pelli-Dystric Vertisols: Dystric Vertisols which have a moist value of 3 or less and a chroma of 2 or less in the soil matrix throughout the upper 30 cm of the soil. 3.3.1.

Discussion

The revision in 1988 of the FAO/Unesco legend has not been as widely discussed and documented as the changes in the USD A system, from 7th Approximation in 1960 till Soil Taxonomy in 1975, followed by six revisions until 1994. Interesting in comparison with the latest developments in Soil Taxonomy is the classification of "wet Vertisols", Vertisols that are temporarily water-saturated, and perhaps reduced, and that have low chroma matrix colours and redox concentrations. In the FAO/Unesco system, the effects of saturation by surface water are described as "stagnic properties": reduction in combination with low chroma with or without mottling. These properties are diagnostic for Stagnic soil units. On this subject the authors of the Revised Legend write: "Most Vertisols are subjected to surface water stagnation at some period of the year. However,

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no Stagnic Vertisol has been distinguished because of the lack of precise information as to the occurrence, duration and location of reduction in these soils. Furthermore, a relationship between reduction and visual criteria has not been established" (FAO, 1988, p.31). The third level accommodates former second level differentiating features (Pelhc/Chromic), as well as properties formerly apphed in the US system (Grumic/Mazic). It is confusing that Pelli- subunits are no longer defined on low chroma alone, but on low chroma in combination with low value; besides, one notices a difference in boundary values between the Revised Legend (FAO, 1988) and its presentation by Nachtergaele et al. (1994). Some field observations allow comparison between the 1974 Legend and the 1988 Revised Legend, including third-level soil subunits. Vertisols in the central clay plain of the Sudan belong to the Calcic and Eutric soil units in the lowerand higher-rainfall regions, respectively, in line with a rainfall gradient. The chromic/pelHc differentiation of the 1974 Legend also worked well on this clay plain, both in characterizing soil profiles and in soil mapping on a 1:2,000,000 scale (Blokhuis, 1993). On the Kenana clay plain between the rivers White Nile and Blue Nile, typical clay plain soils on level terrain not subject to flooding of any duration, had colours 10YR3/2 or 2.5Y3/2, whereas in large shallow depressions the colours were 10YR4/1 or 2.5Y4/1. These Vertisols conformed to Chromic and Pellic, respectively. The Chromic/Pellic differentiation had little meaning in Vertisols developed in situ on basic igneous rock; although occurring on sloping terrain, these were mostly Pellic. In the 1988 classification some of these Vertisols would key out as Calci-, Calcari- or Sodi- soil subunits. Most of the other Vertisols would be Grumi-, a few Mazi-. Pelli- subunits would not occur, as the particular combination of value and chroma required is not found on this clay plain. 3.4. THE WORLD REFERENCE BASE FOR SOIL RESOURCES (WRB)

The first step towards an international reference base for soils was taken in 1960 by FAO and Unesco, in cooperation with UNEP and the International Society of Soil Science. The project was then known as the International Reference Base for Soil Classification (IRB). Although at the time international soil classification systems which aimed at covering the world's soil continuum were operational (Soil Taxonomy, FAO/Unesco, the French system) a need was felt to build a framework in which ongoing soil classification work could be harmonized. There was sufficient international consensus as to the main soil bodies to be separated, but major differences persisted betweeen the classification schemes with regard to level of generalization, diagnostic criteria for the definition of classes, terminology and nomenclature. The final objective of IRB was to reach international agreement on the criteria and methodology to define and separate major soil groups of the world. From the beginning there was a close cooperation between IRB and the FAO-Unesco-ISRIC programme on the Revised Legend. In 1994 the two

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programmes merged; with 20 (IRB) or 28 (FAO) major soil groupings at the highest level, it was felt inappropriate to pursue a differentiation between them. Indeed, both projects had, essentially, the same goal, namely to arrive at a rational inventory of the global soil resources (Spaargaren 1994a). The merger of the two efforts was launched under the name "World Reference Base for Soil Resources", an ISSS/FAO/Unesco/ISRIC undertaking. The WRB was "to provide scientific depth and background to the 1990 FAO-Unesco-ISRIC Revised Legend, so that it incorporates the latest knowledge relating to global soil resources and interrelationships" (Spaargaren, 1994b). The WRB framework has a wider scientific basis than the FAO/Unesco Legend of 1974; it is to serve not only agriculture, but also geology, hydrology and ecology. At the 15th World Congress of Soil Science at Acapulco, Mexico, in 1994, a Symposium was devoted to WRB, and a first draft of the reference base was presented (Spaargaren, 1994b). It was emphasized that WRB is not a new international soil classification system, but a basis for a better correlation between national systems. The morphological characterization of soil is emphasized rather than a purely analytical (laboratory) approach. Lateral aspects of soil and soil horizon distribution, as characterized by topo- and chronosequences, will receive appropriate attention. WRB now consists of 30 major soil group(ing)s. The basic framework of the FAO Revised Legend, with its two categoric levels and guideUnes for developing classes at a third level, was adopted. WRB major soil groups are defined on the basis of diagnostic horizons, properties and materials. The FAO terminology of diagnostic characteristics is retained, with some modifications in definitions and names. New diagnostic horizons, properties and materials are added. Definitions will mainly be quahtative, at least at this stage. In the revised key order of WRB, Vertisols follow Histosols, Anthrosols, Leptosols and Cryosols. Vertisols are defined on the presence of a "vertic horizon", the presence of cracks, and a COLE above 0.06. The vertic horizon is defined as: "a subsurface horizon that as a result of shrinking and swelling has either slickensides, or wedge-shaped or parallelepiped structural aggregates whose longitudinal axis is tilted between 10° and 60° from the horizontal. It contains 30% or more clay throughout. To be diagnostic a vertic horizon must be 25 cm or more thick". In the WRB draft, eight soil units are proposed in the Vertisol major group. In key order: Thionic Vertisols: with a sulfidic or sulfuric horizon; Salic Vertisols: with a salic horizon; Sodic Vertisols: with 15% or more exchangeable sodium, or 50% or more exchangeable sodium plus magnesium; Gypsic Vertisols: with a (hyper-)gypsic horizon; Calcic Vertisols: with a (hyper-)calcic or cemented hypercalcic horizon, or soft powdery lime; Dystric Vertisols: with less than 75% base saturation;

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Chromic Vertisols: with a dominant Munsell hue of 7.SYR and a chroma, moist, of more than 4, or a hue redder than 7.SYR; Haplic Vertisols: other Vertisols. Depth requirements for the presence of the diagnostic horizons or properties are quantified in the full definitions. 3,4.1.

Discussion

Interesting in comparison with the FAO Revised Legend of 1988 are the extension of the number of soil units, the definition of a Dystric Vertisol having less than 7S% base saturation (less than S0% in the Revised Legend) and the consistent (?) waiver of the Eutric Vertisol. Colour criteria re-appear at this level, but now to single out distinct reddish-coloured Vertisols as Chromic. The World Reference Base for Soil Resources (WRB) has merged with the FAO/Unesco Soil Map of the World project, and differences in the classification aspects between the two projects are, at this moment, small. However, comparing the second categoric level of WRB with the third level of the FAO/Revised Legend, the definitions and use of the terms Chromic and Pellic are most confusing. On the second categoric level of WRB, one finds Chromic Vertisols with a colour definition equal to third-level Chromi- soil subunits of the FAO/Unesco Revised Legend. Chromi- soil subunits, however, are not meant to be used in Vertisols. The WRB recognizes Chromic Vertisols, but not Pellic Vertisols. The Revised Legend has PelU- soil subunits in Vertisols, and no Chromi-. There is, obviously, scope for an effort to synchronize both systems.

3.5. THE FRENCH CLASSIFICATION SYSTEM (CPCS, 1967) AND THE "REFERENTIEL PEDOLOGIQUE FRANCAIS"

3.5.1. CPCS 1967 The classification system of the "Commission de Pedologie et de Cartographic des sols" was published in 1967, and has since been used in France and in francophone countries, especially in West Africa. It was first presented at the 5th International Congress of Soil Science at Paris (Aubert and Duchaufour, 1956), and was further developed in the years 1964—1967. The French system was devised for world-wide us but did not, in 1967, pretend to cover the entire spectrum of world soils on the lowest categoric level of named taxa, that of the "sous-groupe". The system was devised as a framework ("systeme de reference"). There are seven categoric levels, the four highest ones ("classe", "sous classe", "groupe" and "sous-groupe") are named, whereas definition of the lower categories ("famille", "serie" and "type") is subject to certain conventions. Diagnostic criteria are defined as "central concepts" in quaUtative or semiquantitative terms. Soil genetic concepts, as well as morphological characteristics selected on the basis of such concepts, serve as differentiating characteristics. The "classes" are treated in succession from pedologically young ("sols

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mineraux bruts") till strongly weathered ("sols a sesquioxydes de fer et manganese"); in addition there are classes for hydromorphic and sodic soils. Vertisols are third in this pedogenetic key and are defined on the basis of morphological characteristics, clay content and base exchange capacity. The two subclasses are: Vertisols with Uttle or no external drainage, also referred to as ''Vertisols topomorphes", and occurring on flat or depressed terrain; Vertisols with possible external drainage, also referred to as "Vertisols lithomorphes", and occurring on sloping terrain. In each subclass there are two groups, one with a crumb to blocky surface soil to at least 15 cm depth, the other with a blocky structure from the surface, or with a thin surface horizon where a fine, wedge-shaped structure forms upon strong desiccation. In the fine-structured groups, four subgroups are defined: "modal" (the typic subgroup); vertic (with moderately developed vertic characteristics); hydromorphic (with mottles/concretions); halomorphic (with sahne characteristics); In the coarse-structured groups we find the same subgroups and in addition one with a self-mulching surface. 3.5.2. Referentiel Pedologique Frangais A recent development in French soil classification, and to some extent an updating of the system of 1967, is the "Referentiel Pedologique Frangais" (Baize and Girard, 1990), a first draft of which was published in 1987 by the INRA, the "Institut National de Recherches Agronomiques". A more complete third draft was presented at the 14th International Congress of Soil Science at Kyoto (Baize et al., 1990). The "Referentiel" is a flexible system, giving freedom to the pedologist in interpreting his data; it is also an open system in which it is possible to define new references or additional qualifiers. The new system remains in line with the French morphogenetic approach. However, two important aspects are new: 1. the system is not a hierarchical classification, but a pedological reference base; 2. "soil mantles" form the object of study. In the study of the "soil mantle" a distinction is made between "horizons" and "pedological systems". Pedological systems comprise both the vertical succession and the lateral extension of horizons, together forming a three-dimensional natural object. RPF has two categoric levels: "references" and "types". References are defined by a particular vertical sequence of reference horizons. Names of the references are often borrowed from the FAO terminology. References which have many common characteristics, showing, for example, the same "reference horizons" are often grouped into "grands ensembles de references" (GER). The GERs are

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named after one or more of the composing references. This grouping is not meant to be a hierarchical level, but serves practical purposes only, e.g. a group of references may have a specific soil forming process in common. At this moment 22 GERs have been defined and another three are in discussion. At a national or local level, more detailed information is necessary, and this is embodied in second-level "types". Types are defined by adding several "qualificatifs" to the reference, e.g. "CALCOSOL, fluvique, vertique, reductique, argileux". Vertisols form one of the 22 GERs of the RPF. They have in common one or more "horizons vertiques". Three vertic horizons have been defined: Vs: vertic surface horizon ("V de surface"); Vm: vertic subsurface horizon ("V median"); Vv: typical vertic horizon ("V typique"). Important characteristics common to Vs, Vm and Vv are: smectite clays; clay percent over 40; colours in hues 5Y, 7.5Y, lOYR, values 2-5, chromas 1 or 2; strong bonding between clay and organic matter; high CEC; exchange complex saturated by bases, mainly Ca and Mg; pH in the alkahne range. Vs, Vm and Vv differ in soil structure characteristics, resulting from desiccation intensity and weight of overlying soil. The vertic surface horizon, Vs, normally develops a surface mulch, in some cases a surface crust. The vertic middle horizon, Vm, is transitional in characteristics between Vs and Vv and is several decimeters thick. It has a blocky structure. The typical vertic horizon, Vv, has wedge-shaped structural aggregates and slickensides with glossy and striated surfaces. The GER Vertisols has four references: Topovertisols: situated in relatively low-lying and depressed areas, and subject to receiving additional water through on-flow. Lithovertisols: on slopes, formed in situ from basic rocks. Due to some leaching of the surface horizons, Lithovertisols may develop towards Planosols. Paravertisols have a more sandy surface horizon, a high amount of organic matter and a base saturation of less than 100%. They occur in regions with a long rainy season and over 1000 mm rainfall. Leptismectisols are shallow Vertisols on calcareous rock or on a petrocalcic horizon. They have a Vs horizon, sometimes a Vm, but never a Vv. In the 1967 classification they were classified as "groupe des sols calciques melanises" (dark-coloured calcic soils). Of the defined quahfiers, the following may be applied to any of the Vertisol references: gypsique: with a "gypsic horizon"; calcarique: with a "calcic horizon"; petro-calcarique: with a "petrocalcic horizon"; melanise: very dark colour in combination with a moderate amount of organic matter.

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The following are relevant to references in other GERs: vertique: some horizons have vertic characteristics, but the soil is not a Vertisol; vertisolique: the solum strongly resembles that of a typical Vertisol.

3.5.3.

Discussion

Like Soil Taxonomy and FAO/Unesco, the French classification system is based on soil genetic concepts, but there are two major differences in the way diagnostic criteria are selected and defined. First, not only morphological characteristics, but also soil genetic concepts serve as diagnostic criteria. Secondly, the definition of diagnostic properties may sometimes be in quantitative terms, more often they are semi-quantitative or quaUtative. In the French approach to soil classification and soil mapping there is a distinct relation between taxonomic units and soil mapping units, which in different ways also applies to Soil Taxonomy and FAO/Unesco. This relation is maintained in the RPF. The two subclasses of Vertisols in the classification of 1967 have some similarity with Pellic and Chromic Vertisols in earlier versions of Soil Taxonomy and FAO/Unesco. Although they are defined in a different way, there is a close similarity as regards the original genetic concept of this distinction. The group differentiation is reminiscent of the distinction between grumic and mazic; however, it refers to the surface soil, not merely to the surface mulch or crust. The "Referentiel" is undeniably a successor to CPCS (1967), which was also presented as a framework rather than a comprehensive soil classification system. The morphogenetic approach has been largely maintained, as well as the freedom to specify at the lowest level (''sous-groupe" in CPCS; "type" in RPF) by giving additional adjectives to the name of the higher categoric level. The aims of the "Referentiel" are similar to those of the WRB/FAO/Unesco/ISRIC project and so is its two-category structure. Whether or not the GERs are considered a major hierarchical level, it is undeniable that they are comparable and often equal to the 30 major soil groups of WRB and the 28 of FAO. In the French classification and the Referentiel Pedologique Frangais, colour criteria are not diagnostic. However, RPF gives the most frequent hues of Vertisols as 5Y, 7.5Y and lOYR, and the most common value/chroma combinations as 2/1, 4/2 and 6/4. Distinctly red-coloured soils, having horizons with slickensides and vertic structure, are intergrades towards Fersialsols. The distinction seems to be based solely on soil colour, which is, however, not a diagnostic property of Vertisols. Topovertisols and Lithovertisols of the "Referentiel" represent the "Vertisols topomorphes" and "Vertisols lithomorphes" of D'Hoore (1964). The grumic/mazic differentiation on the group level, and the criteria for subgroup definition have not been retained in the RPF, probably with a view on the scale of mapping equivalents.

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3.6. THE AUSTRALIAN SOIL CLASSIFICATION

The Australian soil classification system deserves attention in this chapter because of the three countries with the largest area of Vertisols, India, Sudan and AustraUa, only Australia has a national soil classification system. Besides, some of the Australian Vertisols have characteristics that are different from Vertisols in other continents. The AustraUan classification, according to the most recent version of "A Factual Key for the Recognition of Austrahan soils" (Northcote, 1979), has five to seven categories: divisions, subdivisions, sections, (subsections), classes, (subclasses), and principal profile forms (PPF). Subsections and subclasses are not always distinguished. Profile form is central in this purely morphological and quantitative system. The profile form expresses "the overall material impression created by the soil properties as these are considered at different stages of the key" (Northcote, 1979, p. 22). At the divisional level, the Primary Profile Form which refers to soil texture, is diagnostic. Vertisols key out in division 2: uniform primary profile forms (code: U). They further belong to subdivision g (fine-textured, cracking). Clay mineralogy is not specified in the definition; many Vertisols in Australia are dominated by ilhte and kaolin, with some containing little or no smectite (Isbell, 1991). The subdivision Ug contains six sections, differentiated according to pedologic organization, "a broad term used to include all changes in soil material resulting from the effect of physical, chemical and biologic process, that is soil formation". Vertisols belong to section Ug5, showing "pedologic organization characterized by dominantly smooth-faced peds throughout the profile; A2 horizons are absent". Ug5 has two subsections: subsection (a) has "a high grade of pedality right to the surface (self-mulching, granular, blocky) or there is a thin surface crust 6 mm or less, over soil material with a high grade of pedahty, or the soil will reform one of these conditions if left uncultivated"; subsection (b) is "massive, with large coarse peds. . .". Subsections do not appear in the code. On a class level the value/chroma rating of the horizon immediately underlying the surface mulch or crust, is diagnostic. Two value/chroma ratings may serve as examples: VCl covers 3/0, 3/1, 3/2, 2/0, 2/1 and 2/2; VC2 covers 6/0-6/4, inclusive, 5/0-5/4, inclusive, 4/0, 4/1 and 4/2. Classes Ug5.1, 5.2 and 5.3 belong to subsection (a), classes 5.4, 5.5. and 5.6 to subsection (b). The value/chroma ratings of the classes and other diagnostic colour criteria are as follows for subsection (a): Ug5.1: value/chroma rating 1 (dark clay horizon, DCH); Ug5.2: value/chroma rating 2 or 3 (grey clay horizon, GCH): subclass (a): GCH has a hue redder than 2.5Y; subclass (b): GCH has a hue as yellow or yellower than 2.5Y, or has no hue (Munsell: N); Ug5.3: value/chroma rating 5 or 4: subclass (a): hue yellower than 5YR (brown clay horizon, BCH); subclass (b): hue is 5YR or redder (red clay horizon, RCH);

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Classes Ug5.4, 5.5 and 5.6 are subdivided in the same way, but 5.5. and 5.6 have no subclasses. Classes Ug5.1, 5.2 and 5.3 are further differentiated according to the Principal Profile Form (PPF), which "represents the complete concept and character of profile form and is the end point of the present key classification" (Northcote, 1979, p. 23). Strata or horizons underlying the DCH, GCH, etc., are diagnostic for PPF's, e.g. secondary limestone, weathered or unweathered country rock, brown or mottled brown clay, grey or mottled grey clay, or more than one of such underlying horizons or strata. In the classes Ug5.4, 5.5 and 5.6, no PPF's have been defined as the full range of soils in the class was not well known at the time. The classes were described by Northcote et al. (1975) who also give the more or less corresponding soil units of the FAO/Unesco classification of 1974: Ug5.1, Ug5.2, Ug5.3, Ug5.4, Ug5.5, Ug5.6,

Black self-mulching cracking clays; Pellic Vertisols; Grey self-mulching cracking clays; Chromic Vertisols. Brown and red self-mulching cracking clays; Chromic Vertisols. Black massive cracking clays; Pellic Vertisols. Gray massive cracking clays; Chromic Vertisols. Brown and red massive cracking clays; Chromic Vertisols.

Stace et al. (1968) give general descriptions of grey, brown and red clays, with profile descriptions and analytical data. Poor drainage is shown in the profile by rusty or ochreous spotting in the surface 30 cm and some mottling below. Northcote (1984) emphasized that the classes and profile forms of the Ug5 soils are realistic constructs, as is shown by their geographic distribution, their properties and land use, and their close relation with geological structure, land form and lithology. 3.6.1.

Discussion

The Australian classification shows similarities with Soil Taxonomy and FAO/Unesco, and to a lesser extent also with the French classification, in the choice of diagnostic properties. Contrary to these systems, however, Vertisols do not appear on the highest categoric level, but belong to the third category. Colour value, chroma and hue are important diagnostic criteria in the Australian classification. The motivation for the choice of particular value/chroma ratings and hues to define classes and subclasses is not given, but the relation with parent materials is important. The approach to soil colour differs from Soil Taxonomy and FAO/Unesco as relations with wetness, flooding, and possibly reduction are not indicated; poor drainage is shown in mottling, not in low chroma. On the subsectional level, surface characteristics, comparable with Grumic/Mazic are diagnostic.

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3.7. GENERAL DISCUSSION

3.7.1. On classification systems This is a difficult moment to judge international soil classification systems and frameworks: new projects (WRB, RPF) have just started and Httle experience has been gained, older systems have recently presented a revison (FAO/Unesco) or have reached the end of a period of revisions (Soil Taxonomy). Of the international systems, we see that one. Soil Taxonomy, develops towards increasing detail, refinement of definitions that sometimes require sophisticated laboratory and field determinations, whereas the other two, the joint project of the FAO/Unesco/ISRIC Revised Legend and WRB, and the RPF, deliberately refrain from detail. WRB and RPF are reference base systems, they aim at serving as a platform for discussion on concepts and definitions, and as a framework to accommodate more detailed national classification systems. Soil Taxonomy is faced with the problem that some of the diagnostic properties on the lower categoric level are valid in some regions, but do not work in other parts of the world. WRB and the FAO/Unesco Revised Legend, having introduced a third categoric level, are faced with the danger that they are going into much detail and move beyond an international framework. In this way there would be the temptation for some countries to use the WRB as a national classification system, for which it is not intended. RPF faces the same danger by adding an unlimited number of qualifiers in order to define second-level types; however, there is more freedom on this level which befits a framework. One could defend RPF's semi-quantitative approach to definitions, as this adds to the flexibility of the system. On the other hand, this will undoubtedly create confusion. Perhaps the best approach at this moment is that of WRB: give definitions in qualitative terms, "as it might not be realistic, at an initial stage, to put priority on strictly quantitative criteria which may be restrictive to reaching an international consensus" (Spaargaren, 1994b). The validity of frameworks like WRB and RPF is strongly linked to the recognition of the role of national soil classification systems. At the present stage, it is increasingly realized that a generally accepted soil classification would be realistic only at the highest levels of generalization (Dudal, 1990). This is, for example, shown in Australia, where the term Vertisols has wide currency, but subdivisions below the order level of Soil Taxonomy are seldom used. The causes of this are, according to Isbell (1991), first, that the soil moisture-based suborders are not meaningful in a land-use sense, and secondly, that the "pell" and "chrom" separation does not reflect any useful or consistent distinction. In the central clay plain of the Sudan, minor differences in chroma and value separated large areas with otherwise uniform Vertisols; the differences found had a genetic implication, but were only valid in that area (Blokhuis, 1993). The discussion in ICOMERT on how to define pellic and chromic in order to separate, on level clay plains, wet and temporarily flooded Vertisols from moderately well-drained ones, showed how regional differences prevented agreement on a common denominator. These

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examples support Dudal's observation, that it has to be questioned whether quantitatively defined taxa on a subgroup level can have a global validity. 3.7.2. On diagnostic criteria and taxa (1) Aquic conditions, Aquerts and low-chroma soil colours In Soil Taxonomy (Soil Survey Staff, 1975), "aquic" suborders were defined in most orders on the basis of an "aquic soil moisture regime". In Vertisols it was technically impossible to perform the required measurements for ascertaining the presence of an "aquic moisture regime", and so a suborder Aquerts could not be defined. This conclusion implied not only a lack of adequate technology, it also indicated that Vertisols had a specific hydrology. In the 1992 revision of Soil Taxonomy, a suborder Aquerts was defined in a way that strongly resembled the definition of "aquic" suborders in other orders, and that insufficiently took into consideration the specific hydrology of Vertisols. The question remains whether in this respect Vertisols can be treated in the same way as other orders, which largely deal with permeable soils, horizontally moving waterfronts and a stable pore system. Water saturation and reduction are not the only possible causes of low-chroma and non-chroma soil colours: some Pellic Vertisols are moderately well-drained and never flooded. Blokhuis (1993) suggested that the main cause for the low-chroma colour was related to the stability of smectite under relatively wet conditions, whereby ferrous iron is not released from the smectite lattice. That the separation of poorly-drained from (moderately) well-drained Vertisols is not merely a Pellic/Chromic issue, is also demonstrated in the lack of consistency on where to put the boundary between Pellic and Chromic. This is clearly shown in the various revisions following "the 7th Approximation" of 1960. Suggestions have been made to define Aquerts on length of flooding in addition to low chroma (Van Baren and Sombroek, 1985; Blokhuis, 1993). Van Baren and Sombroek (1985) even pleaded for the re-introduction of the differentiation between "hthomorphic" and "topomorphic" Vertisols, a distinction that, in their opinion, was both straightforward and useful for soil mapping. If colour is not the right property to characterize wet vertisols, should they not better be defined by flooding alone? Or, alternatively, by measuring saturation, or saturation and reduction, but this would be rather unpractical in soil mapping. (2) Other colour criteria Soil colour is, in general, an attractive differentiating property, being easily observable and measurable. This is particularly true for Vertisols which have few distinct morphological features, but show differences in colour chroma, value and hue, often in a consistent manner—at least regionally. It has been emphasized in the discussion on Soil Taxonomy, that chroma differences in Vertisols had a genetic impact of their own, and so had differences in value. The combination of value and chroma to define diagnostic colours was criticized on pedogenetic grounds: the genetic meaning of subgroups thus created was not made clear. However, Ahmad (1983) criticized a distinction between

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Pellusterts and Chromusterts based entirely on chroma. He suggested that Pellusterts could be allowed with higher chromas as long as slopes are less than 1 percent. This suggestion, introducing a physiographic element, approaches the concept of the ''Vertisol topomorphe" of D'Hoore (1964). But it is not only low chroma that is critical. Blokhuis (1993) described hues of 2.5Y and 5Y as indicators of wetness if accompanied by reddish mottles. Bunting and Lea (1962) found that low chroma in combination with yellowish hue (2.5Y or even 5Y) indicated waterlogging in Vertisols on an alluvial clay plain on the Blue Nile westbank, with an annual rainfall of about 700 mm. In the 1974 edition of the FAO/Unesco classification system, the major soil grouping Vertisols had two soil units: PeUic Vertisols with chroma less than 1.5, and Chromic Vertisols with chroma 1.5 or above. The same differentiation was used in the 7th Approximation and in Soil Taxonomy. In the 1988 revision of the FAO classification, the chroma criterion for separating soil units was dropped. In the proposed third-level soil subunits, "pelHc" and ''chromic" returned, but with a completely different meaning: Pelli- for chroma of 1.5 or less (Nachtergaele et al., 1994), or 2 or less (FAO, 1988) and value of 3.5 (3) or less. This new Pellidefinition, with its mix of chroma and value criteria, is confusing and not in accordance with the original meaning of Pellic. One would expect the definition of Chromi- to be a mirror image of that of Pelli-, viz. chroma of 3 or more and value of 4 or more. However, the new definition of Chromi- is: hue of 7.5YR and chroma more than 4, or hue redder than 7.5YR. Although different from its earlier definition and not consistent with the definition of Pelli-, this concept of Chromi-, combining hue and chroma rather than value and chroma, makes sense; it singles out Vertisols with stronger and redder colours. It has to be added, however, that Chromi- is not on the priority list for third-level soil subunits (Nachtergaele et al., 1994). The WRB recognizes Chromic Vertisols, with the same definitions for colour as the Chromi-soil subunits in the FAO system. However, WRB does not have Pellic Vertisols. Soil colour aspects are prominent differentiating criteria in the Austrahan system, more so than in Soil Taxonomy and FAO/Unesco. Combinations of value and chroma, the value/chroma ratings, are diagnostic. Although we have earlier in this chapter strongly criticized such combinations, VC ratings may be appropriate in Australia: they have for many years been used in the same way, they are strongly related to parent material, and they are not meant to be used outside Australia. There is consistency in the use of colour aspects, and they may be better understood because they refer to one particular, albeit large, region of the world. In the French classification and the Referentiel Pedologique Frangais, colours are sometimes part of the general description of (higher-level) taxa, but they are not diagnostic, for reasons not made explicit. The way colour criteria have been used in Soil Taxonomy, the FAO/Unesco system and the WRB clearly indicates that the genetic impUcations of colour are not understood, or perhaps their importance is overrated. Even so, it remains questionable whether specific colour criteria will have the same meaning world-

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wide. World-wide systems should, perhaps, not go beyond two categoric levels, leaving the details to national soil classification systems. From the above discussion it should be clear that the use of soil colour aspects as diagnostic properties in Vertisols is dangerous and confusing. One wonders whether the concepts of value and chroma are understood well: in the Uterature, low-chroma Vertisols are often referred to as "dark-coloured", which is a connotation of "value". In fact, many of the former Pelluderts and Pellusterts are "light-coloured", i.e. they have a high value. (3) Qualitative or quantitative diagnostic criteria In both French systems which are not only historically but also conceptually closely connected, the approach to Vertisols is based on a sound understanding of their genesis, and the choice of differentiating criteria for lower categoric levels is firmly based on this understanding. On the other hand, the often qualitative way diagnostic features and taxa are defined, makes the use of these systems difficult for those who have not been working in the field with these systems— preferably under expert French guidance. (4) Surface characteristics As we have seen above, the mazic/grumic differentiation in the "7th Approximation" (Soil Survey Staff, 1960) was brought into discussion in ICOMERT, but has not been re-introduced in the 1992 version of the Keys. In the French and Austrahan classification system, surface characteristics are and have always been diagnostic at middle categoric levels. In the FAO/Unesco Revised Legend, Maziand Grumi- third-level soil subunits have been defined. Soil Taxonomy, a system which is also to serve detailed soil surveys, stands alone in not using the specific surface aspects of Vertisols on any of the defined levels of classification. REFERENCES Ahmad, N., 1983. Vertisols. In: L.P. Wilding, N.E. Smeck and G.F. Hall (Editors), Pedogenesis and Soil Taxonomy. II. The Soil Orders. Developments in Soil Science, No. IIB. Elsevier, Amsterdam, pp. 91-123. Aubert, G. and Duchaufour, P., 1956. Projet de classification des sols. In: Rapports, Vie Congres International de la Science du Sol, Paris, Vol. E, pp. 597-604. Baize, D. and Girard, M.C., 1990. Referentiel Pedologique Frangais. 3 eme proposition. INRA/AFES, 279 pp. Baize, D., Girard, M.C., Ruellan, A. and Boulaine, J., 1990. The new French Reference Base for Soils ("Referentiel Pedologique"). Trans. 14th Int. Congress of Soil Science, Kyoto, Japan, August 12-18, 1990, ISSS, Vol. V, pp. 10-16. Baldwin, M., Kellogg, Charles E. and Thorp, J., 1938. Soil classification. In: Soils and Men. USDA Yearbook Agr., U.S. Govt. Printing Office, Washington DC, pp. 979-1001. Bartelli, L.J., 1982. Soil Taxonomy: its Evolution, Status and Future. Soil Science Society of America, Spec. Publ. no. 14, pp. 7-13. Blokhuis, W.A., 1985. Relationships between morphological properties and drainage in Vertisols. Proc. 5th Int. Soil Classification Workshop: Taxonomy and Management of

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Vertisols and Aridisols, Sudan, 2-11 November, 1982. Soil Survey Administration, Khartoum, pp. 231-242. Blokhuis, W.A., 1993. Vertisols in the central clay plain of the Sudan. Doctoral thesis. Agricultural University, Wageningen, xviH-418 pp. Bunting. A.H. and Lea, J.D., 1962.The soils and vegetation of the Fung, east-central Sudan. J. Ecol., 50: 529-558. Comerma, J.A., 1984. Hydromorphic Vertisols? In: Wetland Soils, Characterization, Classification and utilization. International Rice Research Institute, Manila, pp. 407-421. Comerma, J.A., Wilhams, D. and Newman, A., 1988. Conceptual changes in the classification of Vertisols. In: L.P. Wilding and R. Puentes (Editors), Vertisols: their Distribution, Properties, Classification and Management. Soil Management Support Services, Techn. monograph no. 18. Texas A&M University, pp. 41-54. CPCS, 1967. Classification des Sols. Commission de Pedologie et de Cartographic des Sols, Travaux CPCS, 96 pp. D'Hoore, J.L., 1964. La carte des sols d'Afrique au 1/5.000.000. Commission de Cooperation Technique en Afrique (CCTA), Projet Cojoint no. 11. Lagos, 209 pp., with map supplement. Dudal, R., 1973. Planosols. In: E. Schlichting and U. Schwertmann (Editors), Pseudogley und Gley, Verlag Chemie, Weinheim, pp. 275-285. Dudal, R., 1990. Recent developments in national soil classification systems. Trans. 14th Int. Congress of Soil Science, Kyoto, Japan, August 12-18, 1990, ISSS, Vol. V, p. 3. Dudal, R. and Eswaran, H., 1988. Distribution, properties and classification of Vertisols. In: L.P. Wilding and R. Puentes (Editors), Vertisols: their Distribution, Properties, Classification and Management. Soil Management Support Services, Techn. monograph no. 18. Texas A&M University, pp. 1-22. Eswaran, H., Kimble, J. and Cook. T., 1989. Properties, genesis and classification of Vertisols. Trans. Int. Workshop Swell-Shrink Soils: Classification, Management and Use Potential of Swell-Shrink Soils, October 24-28, 1988, NBSSLUP, Nagpur, India. Balkema, Rotterdam, pp. 1-22. FAO, 1988. FAO-Unesco Soil Map of the World, Revised Legend. World Soil Resources Report 60. FAO, Rome, 119 pp. FAO/Unesco, 1964. Preliminary definitions. Legend and Correlation Table for the Soil Map of the World. World Soil Resources Report 12, Rome, 16 pp. FAO/Unesco, 1968. Definitions of Soil Units for the Soil Map of the World. World Soil Resources Report 33, Rome, 72 pp. FAO/Unesco, 1974. Soil Map of the World 1: 5 000 000. Vol. I, Legend. Unesco, Paris, 59 pp. Fernandez Caldas, E., Quantin, P. and Tejedor Salguero, M.L., 1981. Sequences climatiques de sols derives de roches volcaniques aux lies Canaries. Geoderma, 26: 47-62. Griffin, R.W., Wilding, L.P. and Drees, L.R., 1992. Relating morphological properties to wetness conditions in the Gulf Coast Prairie of Texas. 8th Int. Soil Correlation Meeting (VIII ISCOM): J.M. Kimble (Editor), Characterization, Classification, and Utilization of wet soils, Louisiana and Texas, October 6-21, 1990. USDA, U.S. Soil Cons. Service, pp. 126-134. Guthrie, R.L., 1985. Refinement of taxa based on soil moisture regimes. Proc. 5th Int. Soil Clasification Workshop: Taxonomy and Management of Vertisols and Aridisols, Sudan, 2-11 November 1982. Soil Survey Administration, Khartoum, pp. 37-39.

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ICOMAO, 1991. Circular 11, March 15. ICOMERT Circular Letters: nr. 1, February, 1981; nr. 2, October, 1982; nr. 3, October, 1983; nr. 4, July, 1984. Isbell, R.F., 1991. Australian Vertisols. In: J.M. Kimble (Editor), Proc. 6th Int.Soil Correlation Meeting (VI ISCOM): Characterization, Classification, and Utilization of Cold Aridisols and Vertisols. Canada/U.S., August 6-18, 1989, pp. 73-80. Kanyanda, C.W., 1985. Properties, management and classification of Vertisols in Zimbabwe. 5th Meeting Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Wad Medani, Sudan, 5-10 December, 1983. World Soil Resources Report 56, FAO, pp. 94-109. Lekholoane, L., 1985. Properties, management and classification of Vertisols in Lesotho. 5th Meeting Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Wad Medani, Sudan, 5-10 December, 1983. World Soil Resources Report 56, FAO, pp. 84-93. MacGarity, J.W., 1985. Vertisols of AustraUa. Proc. 5th International Soil Classification Workshop: Taxonomy and Management of Vertisols and Aridisols. Soil Survey Administration, Khartoum, Sudan, pp. 173-290. Nachtergaele, F.O., Remmelzwaai, A., Hof, J., Van Wambeke, J., Souirji, A. and Brinkman, R., 1994. Guidelines for distinguishing soil subunits. Trans. 15th World Congress of Soil Science, Vol. 6a, pp. 818-834. Northcote, K.H., 1979. A Factual Key for the Recognition of Australian Soils. Rellim Techn. Publ., Adelaide, South Australia, 124 pp. Northcote, K.H., 1984. Considerations regarding the classification of Austrahan cracking clay soils. In: J.W.McGarity, E.H. Floult and H.B. Suo (Editors), The properties and Utilization of Cracking Clay Soils. Reviews in Rural Science 5, University of New England, pp. 14-19. Northcote, K.H., Hubble, G.D., Isbell, R.F., Thompson, C.H. and Bettenay, E., 1975. A Description of Austrahan Soils. CSIRO, AustraUa, 170 pp. Probert, M.E., Fergus, I.F., Bridge, B.J., McGarry, D., Thompson. C.FI. and Russell, J.S., 1987. The Properties and Management of Vertisols. CAB International, Wallingford, Oxon, 36 pp. Purnell, M.F., De Pauw, E.F. and Khodary, O., 1976. Soil resource regions of the Blue Nile, White Nile, Gezira and Khartoum Provinces of the Sudan. Soil Survey Report no. 80. Soil Survey Administration, Wad Medani, 149 pp. Quantin, P., Tejedor-Salguero, M.L. and Fernandez-Caldas, E., 1977. Climatosequence de la region meridionale de ITle de Tenerife (lies Canaries), lere partie: I'ecologie, morphologic, caracteristiques physico-chimiques. Cahiers ORSTOM, ser. Pedologie 15: 391-407. Rossignol, J.P., 1983. Les vertisols du nord de I'Urugay. Cahiers ORSTOM, ser. Pedologie, 20: 271-291. Soil Survey Staff, 1960. Soil Classification, a Comprehensive System. 7th Approximation. Soil Conservation Service, USD A, Washington D.C., 265 pp. Soil Survey Staff, 1975. Soil Taxonomy—a Basic System of Soil Classification for Making and Interpreting Soil Surveys. Agr. Handbook no. 436, Soil Conservation Service, USDA, Washington D.C., 754 pp. Soil Survey Staff, 1992. Keys to Soil Taxonomy, 5th ed. SMSS Techn. Monograph no. 19, Pocahontas Press Inc., Blacksburg, Virginia, 541 pp. Soil Survey Staff, 1994. Keys to Soil Taxonomy, 6th ed. USDA Soil Conservation Service, 306 pp.

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Spaargaren, O., 1994a. Introduction to the World Reference Base for Soil Resources. Trans. 15th World Congress of Soil Science, Vol. 6a, pp. 804^818. Spaargaren, O., (Editor) 1994b. World Reference Base for Soil Resources. Draft. ISSS/ISRIC/FAO, 161 pp. Stace, H.C.T., Hubble, G.D., Brewer, R., Northcote, K.H., Sleeman, J.R., Mulcahy, M.J. and Hallsworth, E.G., 1968. A Handbook of Australian Soils. Rellim Techn. Publ., Glenside, South AustraUa, 435 pp. Tejedor Salguero, M.L., Quantin, P. and Fernandez Caldas, E., 1978. Climatosequence de la region meridionale de I'lle de Tenerife (lies Canaries). 2eme partie: caracteristiques mineralogiques; interpretation et classification. Cahiers ORSTOM, Ser. Pedologie 16: 83-106. Thorp, J. and Smith, Guy D., 1949. Higher categories of soil classification. Soil Sci., 67: 117-126. Van Baren, H. and Sombroek, W.G., 1985. Vertisols in the collection of the International Soil Museum and some suggestions on classification. Proc. 5th Int. Soil Classification Workshop: Taxonomy and Management of Vertisols and Aridisols, Sudan, 2-11 November 1982. Soil Survey Administration, Khartoum, pp. 63-69.