Agriculture Reference
In-Depth Information
However, partial correlations may sometimes be made for certain groupings, particularly
at the higher levels of classification. The New Zealand system, for example, attempts to
link the national system with 'Soil Taxonomy' by making explicit correlations at certain
hierarchical levels (Hewitt, 1992).
Duchaufour (1997) divides the systems of soil classification into hierarchical and
reference-based and, within both of these categories, into those based on combinations
of soil characteristics and those with a genetic base. The hierarchical systems are those
in which soil individuals are divided into a ranked series of categories within which each
member is grouped with others of similar properties into a common member at a higher
level. Information is most specific at the lower levels and become increasingly
generalised with progress up the hierarchy.
Reference-based systems, for example, the '
Référentiel Pédologique Français'
(
Associa-
tion Française pour l'Etude du Sol,
1998), take a more flexible approach, reducing or
eliminating the hierarchical structure of taxonomic units and defining soil individuals in
relationship to a series of well-described soils that serve as bases for comparison.
Soil Taxonomy, the American system of classification (Soil Survey Staff, 1999), is
an hierarchical system with six levels. The highest level of classification is the order of
which there are currently twelve. The soil orders are defined by their degree of develop-
ment, by the possession of specific diagnostic horizons (either at the surface or forming
part of the B horizon), environmental factors, the degree of cracking, the possession
of highly organic surface horizons or, for the andisols, by their largely pyroclastic
parent materials. Recent keys to the orders and subordinate categories of this evolving
system are presented in Soil Survey Staff (1999). Table II.7 lists the orders and
summarises some major properties (Fanning and Fanning, 1989); Wilding
et al.
(1983)
provide detailed discussions of the ordinal concepts of Soil Taxonomy.
In contrast, the WRB as a potential successor to the FAO/UNESCO (1990)
classification, has two levels of classification with 30 highest level groups and a likeli-
hood that 200-250 second level groupings will eventually be defined (ISSS Working
group RB6, 1998). This system concentrates on creating major soil groupings at a glob-
al scale. At its upper level of classification, soils are divided on the basis of major
pedogenic processes with some groups being differentiated on the basis of specific
parent materials. Its emphasis is on morphological rather than analytical properties and
it has borrowed extensively from a number of national systems.
