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Soil cover: vertical and lateral morphology and its functioning

Soil cover

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

• It is formed by mineral and organic constituents and includes solid, liquid and gaseous phases.

• The constituents are organized in structures, specific for the pedological medium. These structures form the morphological aspect of the soil cover, equivalent to the anatomy of a living being. They result from the history of the soil cover and from its actual dynamics and properties. Study of the soil cover structures facilitates perception of the physical, chemical and biological properties, it permits understanding of the past and present of the soil, and predicting its future.

• The soil cover is in constant evolution, thus giving the soil its fourth dimension, time.

Structure of the soil

The morphological organization of the soil cover exists at different scales of observation: from the basic assemblages of particles which can be observed through a microscope, to the arrangement of pedological systems at the scale of a landscape. Four types of structures, corresponding with four levels of organization and observation of the soil cover, are particularly important to be described, measured and understood:

1. Elementary organizations: structures made up of the constituents. They are visible with the naked eye, or through a microscope. The main types of elementary organizations are aggregates (peds), voids (porosity), cutans, nodules, features of biological activity; colour of the soil material helps to recognize and understand the elementary organization.

2. Assemblages: pedological volumes determined by the presence of various elementary organizations. Examples of assemblages are andic, calcic, ferralic, vertic, etc.; each of these assemblages can be recognized in terms of specific associations of colour, peas, voids, cutans, nodules, etc.

3. Horizons: pedological volumes more or less parallel to the surface of the earth. A horizon is described by the presence of one or more types of assemblages and by the relationship between these assemblages. It is also described by its thickness, by its lateral extension, and by its vertical and lateral morphological limits. At the scale of a landscape, horizons are never infinite; laterally they disappear or merge into another horizon.

4. Pedological systems: spatial distributions and relationships of horizons at the scale of the landscape (Ruellan and Dosso, 1993). The structure of a pedological system can be described by the arrangement of the horizons: elementary organizations and assemblages of the horizons, vertical superposition and lateral successions of horizons, kinds of limits which separate the horizons.

The pedological studies have, until now, mainly considered the characterization and the genetic significance of the elementary organizations, of the assemblages, of the horizon itself and of the vertical sequences of horizons. Relatively few detailed studies have been made with regard to the three-dimensional, spatial organization of the soil cover and with respect to the historical and actual dynamics of the three-dimensional organization. Such studies are needed to understand the dynamic soil entities or soil units, at the scale of the landscape and ecosystems, and to disclose the relationships between the pedosphere and the other components of the earth: lithosphere, hydrosphere, atmosphere, biospere.

A morphogenetic soil reference system such as WRB is based on elementary organizations, assemblages, horizons, and the vertical superpositions of horizons. However, a comprehensive reference system for lateral distributions cannot yet be constructed with enough precision. Recognizing the existence of these lateral distributions, they are acknowledged, wherever possible, in the definition of the components of the WRB, thus opening the possibility to start a World Reference Base concerning pedological systems (lithosequences, toposequences, chronosequences, biosequences, climasequences, etc.)

Problems encountered

A basic philosophy in the WRB is that the final soil groups must show coherence in geographical distribution and pedogenetic character, and that soils preferably must be characterized by their morphological expression rather than by analytical data. In a number of proposed major soil groups this has led to divisions and in others to amalgamation. For example, FAO's Leptosols comprise "soils limited in depth by.... a continuous cemented layer within 30 cm of the surface", i.e. petrocalcic, petrogypsic, petroferric layers or duripan. These layers are considered in the WRB as pedogenetic horizons and soils having such layers should therefore be classed with their appropriate groups. Consequently, shallow soils over petrocalcic, petroduric, petrogypsic or petroplinthic horizons are grouped in the World Reference Base with the Calcisols, Durisols, Gypsisols and Plinthosols, respectively. It must be emphasized, however, that shallow soils over a petroplinthic horizon may occupy distinctly different positions in the landscape than those containing a plinthic horizon. The latter usually occur in depressions and on extensive plains, while the former are frequently encountered in elevated positions, e.g. as 'cuirasses' in western Africa, forming caps of table lands.

Leptosols also comprise soils with a very high content in coarse fragments. This combination left the Leptosols an odd group of either shallow soils or relatively deep soils, with as a common characteristic a low amount of available moisture. There was a proposal to group the deeper Leptosols with the Regosols. This would have purified the Leptosols but 'polluted' the Regosols. Therefore, coarse fragmental soils are retained in the Leptosols.

A major concern has become how to treat human-influenced soils which do not qualify as Anthrosols. Especially in the reference soil groups of Podzols, Umbrisols and Arenosols, soils occur in which the surface layer has been modified by fertilization and liming to such an extent that the original characteristic of low base saturation has disappeared. If left to nature, the initial low base saturation will return in time. Following the principle that short-term management effects should not influence soil grouping, these kind of soils are kept with the Podzols, Umbrisols and Arenosols, and an "anthric" modifier may be used to indicate human influence.

Interfaces between some soils in the Revised Legend are arbitrary. FAO's separation of Luvisols, Alisols, Acrisols and Lixisols may be very useful, but their identification is based largely on analytical data, whereas differences between the soils are difficult to detect in the field. At one stage it was proposed to group Luvisols together with Alisols and Acrisols together with Lixisols. The result would have been that two very different soils in Africa would have been classed together, and that the fertile Luvisols of the loess belt in western Europe would have been grouped together with the extremely acid and infertile Alisols of the foothills in the Andes or on Kalimantan, thus ignoring the WRB principle of relationships in soil geographical distribution. It was therefore decided to retain the separation made in the Revised Legend, to base the difference between Luvisols and Alisols mainly on "alic properties", and to continue the search for morphological and associated criteria which would enable the four 'luvic' soils to be distinguished better in the field. Such criteria could be based, for instance, on their structural development.

Similar problems exist between Andosols and Podzols, especially between the Andosols which are dominated by alumino-organic complexes, and Podzols lacking an albic horizon. No sound field differentiating criteria could be established, apart from circumstantial evidence derived from the geography of the area, and so a clear distinction between the two soils still needs the support of analytical tests.

The limit between Ferralsols and Nitisols in the Revised Legend is unsatisfactory. Many Nitisol-like soils have nitic properties and also a ferralic B horizon; consequently they key out as Ferralsols. The general concept of Ferralsols, however, is that of soils which have weakly developed structures and are low in active Si, Al and Fe. In contrast, nitic properties apply to soil materials which are strongly structured and have a high content of active iron (FAO, 1988). This apparent contradiction has been removed in WRB by excluding the presence of a nitic horizon in Ferralsols and, at the same time, providing the possibility of an intergrade in Nitisols to Ferralsols by the qualifier "Ferralic".

Consensus has been reached on the problem concerning priority between ferralic horizons and argic horizons containing low activity clays. Horizons with a CEC of 16 cmolc kg-1 clay or less qualify for ferralic, provided all other requirements are met. If, however, the ferralic horizon is overlain by a horizon which qualifies for an argic horizon and which contains in its upper 30 cm more than 10 percent water-dispersible clay, the argic horizon takes precedence over the underlying ferralic horizon for classification purposes.

Standard depths of 10, 20, 25, 30, 40, 50, 75, 100, 150 and 200 cm have been used, unless there is an overriding argument not to do so. Such an argument can be the use of another depth value in the national classification system from which a description has been taken. This ensures compliance with one of the main WRB objectives, namely to serve as an internationally acceptable framework for delineating soil resources, to which national classifications can be attached and related.

Correlation with existing classification systems

The WRB, having taken the framework of the Revised Legend as a guide, obviously bears many similarities to it. The nomenclature has been adopted and, where necessary, adapted using the set rules. Its concepts of diagnostic horizons and properties, supplemented with diagnostic soil materials, have been incorporated.

The original FAO Legend (FAO-UNESCO, 1974) was built on knowledge and experience of many soil scientists from all over the world and reflects a consensus derived from a number of classification systems. For example, the Greyzems, Chernozems and Kastanozems stem from tile older Russian classification of Grey Forest soils, Chernozems and Chestnut soils. Similarly, Cambisols coincide largely with the German 'Braunerde' and the French 'Sols bruns', while Ferralsols closely follow the concept of the American Oxisols and Brazilian Latosols.

The World Reference Base for Soil Resources continues to build on existing classification systems. The Anthrosols contain many elements from the Chinese Soil Taxonomic Classification System (CSTC Research Group, 1995), the description and definition of Andosols correlates closely with the Andisols of the Référentiel pédologique (AFES, 1995), as is the case, to a lesser extent, with the Podzols and the French Podzosols. On the other hand, some of the proposals for Solonchaks, Gleysols, Plinthosols, Ferralsols, Durisols, Alisols, Umbrisols and Regosols result from original ideas not yet reflected in current classification systems.

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