SOIL FORMATION - Soil Ecology - page 197

of landscape relationships and verified using limited profile sampling at locations
considered representative for the unit under study. Under Soil Taxonomy, the individual
soil is considered to be a polypedon, an association of similar and closely-associated
pedons. Practically, the profile remains the individual study unit with a very limited
lateral dimension related to sampling. Other decisions to be made include the extent of
variation acceptable within any one taxonomic unit, at all levels.
Other units have been advanced as the appropriate basic unit of classification systems.
Fitzpatrick (1967) considered that the soil horizon was the appropriate unit in this regard
and, more recently, the Référentiel Pédologique Français (discussed below) takes
a similar approach. Holmgren (1988) presented a scheme for the point recognition
of soil properties which avoids some of the conceptual ambiguities of the current
pedon definition.
A particular problem in soil classification is that of deciding the relative importances
of different characteristics. This becomes especially subjective in trying to interpret
imperfectly-known characteristics related to soil genesis. Of similar importance are
the large-scale, systematic geographical differences that occurs in the distribution of
soils across the face of the earth. Because of the varied environments, surficial
materials, landscape ages and biotas that occur world wide, most countries possess
only a small portion of the spectrum of world soils; this has lead to differing conceptions
and emphases in studies of soil genesis and relationships. In consequence, no agreed,
world-wide system of soil classification exists and this continues to pose real problems
of communication between soil scientists and those concerned with information transfer
associated with soil management.

4.6.3 TYPES OF SOIL CLASSIFICATIONS

Two pedological systems of soil classification dominate the world soil science literature,
namely, 'Soil Taxonomy', the American system of classification (Soil Survey Staff,
1999) and the 'Revised legend to the Soil Map of the World' published by FAO/
UNESCO (1990). More recently the 'World Reference Base for Soil Resources' (WRB)
has been established (ISSS Working Group RB, 1998a) to build on and extend the
FAO/UNESCO sytem through a merger with one of the reference based systems, IRB.
Other systems exist but have not received widespread recognition.
A range of national systems of soil classification has been developed. Some examples
include those of Australia (Isbell, 1996), Brazil (Camargo et al., 1986), Canada (Canada
Soil Survey Committee, 1978), China (Chinese Soil Taxonomic Classification Research
Group, 1990), France ( Association Française pour l'Etude du Sol, 1998), Germany
(Muckenhausen, 1965), New Zealand (Hewitt, 1992), South Africa (Soil Classification
Working Group, 1991) and the United Kingdom (Avery, 1980). However, most have
little currency beyond areas within which they were produced. In many countries
a succession of systems has occurred, new ones replacing the previous when their
utility is perceived to have diminished. In addition to the national classifications,
lower-level, local systems have also proved necessary (see, e.g., Isbell, 1990)
Taxonomic groupings may not necessarily be correlated between the different soil
classification systems because different criteria are used in defining the various categories.