Agriculture Reference
In-Depth Information
of a hierarchy of classes which allows an orderly overview of the diversity of the objects concerned.
The success of taxonomic classiÝcations of organisms has prompted its application to soils. How-
ever, unlike plants and animals, which can easily be identiÝed, soils constitute a continuum that
needs to be broken into classes by convention. In the early days of soil science, ÑgenesisÒ was
considered to be the basis for classifying soils. The concept, borrowed from the biosciences, was
actually not appropriate for soils that do not have phylogenetic relationships and that cannot be
grouped by descent from common ancestors.
The successive systems of soil classiÝcation that were developed reÞected different concepts
of soil formation, as well as the state of knowledge at the time. Various hypotheses included much
conjecture. Soil formation spans long time intervals during which soil-forming factors vary con-
siderably. Hence links of soil formation to present climate were not proven to be applicable in
different parts of the world. Soil classiÝcation schemes based mainly on ÑgenesisÒ have retarded
rather than enhanced the development of a comprehensive soil taxonomy. Recent systems of soil
classiÝcation reÞect considerable progress in this respect. The USDA Soil ClassiÝcation (Soil
Survey Staff, 1960) was a breakthrough toward classifying soil in terms of its own properties and
toward deÝning taxa on the basis of quantitative differentiae that could be observed or measured.
The selection of differentiating properties took soil formation into account, but soil-forming pro-
cesses as such were no longer criteria for separating classes. SigniÝcance to plant growth also had
a bearing on the selection of differentiae. As the number of soil taxa far exceeds the amount of
information that the mind can comprehend and remember, a hierarchy of six categories was
established, from orders down to series, somewhat analogous to the hierarchy from phylum to
species in plant science. The branching sequence of organisms has evolution as an objective basis.
For soils, however, the hierarchy is a matter of opinion (Swanson, 1993).
Although soil-forming processes are no longer taxonomic differentiae as such, it appears that
concepts of soil formation still inÞuence the ranking of the hierarchical categories. The genetic
signiÝcance and the importance to use ascribed to the argillic horizon have led to the distinction
of two orders in the USDA Soil Taxonomy (Soil Survey Staff, 1999): the AlÝsols and the Ultisols.
The overriding weight given to the argillic horizon has been questioned because of the constraints
presented by strong weathering in soils of the tropicsÐferralic attributesÐthat are more pro-
nounced than the apparent advantage of clay increase with depth (Eswaran, 1990). In the South
African Soil Taxonomy (Soil ClassiÝcation Working Group, 1991), textural differentiation is a
major differentiating characteristic, however, with much more strongly expressed textural differ-
ences than those of the argillic horizon. While AlÝsols and Ultisols are separated at order level
by a difference in base saturation in the subsurface, base saturation in South Africa is taken into
account only at family level. The wide geographic extension of steppes in North America and
Europe justiÝed the distinction of Mollisols or HumusÏAccumulative Soils at order level in Soil
Taxonomy, and in the Russian Soil ClassiÝcation (Shishov et al., 2001), respectively. An equivalent
higher category is absent in the Australian (Isbell, 1996) and South African soil classiÝcation
systems. Hydromorphic soils are distinguished at order level in Australia and Russia, in contrast
to the suborder ranking to which they are assigned in the USDA Soil Taxonomy. In the Russian
soil classiÝcation, Halomorphic soils are recognized at order level, while in South Africa, no
distinct provision has been made for saline soils. Both the Australian and the Russian classiÝcation
systems acknowledge strong human inÞuence on soil formation at order level, as Anthroposols
and Agrozems.
There are 12 orders in Soil Taxonomy, 14 in Australia, 23 in Russia, and 73 Ñsoil formsÒ in
South Africa, reÞecting different opinions as to the weighting of soil characteristics as well as to
the impact of the geographic setting in which the classiÝcations were developed. The rationale for
the distinction and number of classes at different levels of generalization is not always apparent,
and neither is the justiÝcation for the very high number of classes in the higher categories, which
is in the range of thousands for the subgroups. Considering that their interpretation value is limited,
a clariÝcation of the rationale for this elaborate hierarchy is called for.
