Agriculture Reference
In-Depth Information
Soil classiÝcation may spur or deter scientists with an interest in soils. If a classiÝcation system
proves to be relevant and user-friendly, it stimulates and encourages further work because it is
recognized for its inherent capacity to create order and enhance the useful understanding of soils.
For example, The Factual Key for the Recognition of Australian Soils by Northcote (1979) provided
a generation of Australian scientists with valuable conceptual understanding of soils in terms of
textural-differentiation of proÝles, the relative development of A2 (E) horizons, soil reaction trends,
subsoil color and mottle differences. Many of the concepts in his classiÝcation also provided
effective pedotransfer functionality (Bouma, 1989), particularly in terms of soil-water attributes
(e.g., McKenzie et al., 2000).
If a classiÝcation is not useful, it hinders research. The Handbook of Australian Soils, by Stace
et al. (1968), described 43 Great Soil Groups as central concepts, supported by representative
proÝles. Because of the lack of distinct separation between classes, many soils were inconsistently
classiÝed and distinguished, leading to conceptual confusion and pointless argument of subtle
differences. While useful in many ways in a pedological context, it was signiÝcantly biased toward
the population of agricultural soils, the subject of study for most of AustraliaÔs soil scientists. Many
soils found in forests, rangelands, and nonagricultural contexts could not be allocated, as they did
not match the central concepts.
The Australian Soil Classification System
The most recent national classiÝcation, the Australian Soil ClassiÝcation (Isbell, 1996), is
broad in its application, and its hierarchical structure generally allows for unambiguous allo-
cation of unknowns to particular classes. It draws on concepts of the two previous systems
mentioned and borrows from overseas classiÝcation schemes as well. A simpliÝed outline is
provided in Figure 9.1.
There are 14 Soil Orders at the highest level that reÞect important features of the soil
continuum in Australia. For example, as an arid and strongly weathered continent with an
absence of glaciation, it has soil orders that reÞect this. For instance, the Sodosol order is
deÝned by a high subsoil exchangeable sodium percentage (ESP). It includes diverse soils that
have been affected by salt during formation, often the consequence of past semi-arid or arid
conditions. Other examples are the Kandosol and Ferrosol orders that usually have a history
of signiÝcant weathering and leaching. The Australian Soil ClassiÝcation not only deals with
agricultural soils, but also all soils of the rangelands, the tidal zone, the arid zone, and human-
made soils, which were less understood. Its strengths are derived from the large national and
state databases used to develop the classiÝcation, its general-purpose orientation, and a set of
clearly stated principles.
Many management-related properties of the scheme are revealed at the lowest levelÐthe
Family level. The Family level classiÝes soils based on soil depth, thickness, texture, and
gravel content of A horizon and the maximum texture of the B horizon. For water-related
devised functions (Bouma, 1989), the family criteria could well be the most technically useful
part of the classiÝcation.
Application and Future Improvements
In the six years since its release, the Australian Soil ClassiÝcation has been found to be very
applicable to agricultural soils and adequate for many engineering purposes. However, it has
attracted criticism regarding wet soils in general (Hydrosols soil order), tidal soils in particular,
saline soils, arid zone soils, sands, and soils rich in ferruginous gravel. Its formulation has been
an advance in that it focused attention on the limits of existing datasets and the inadequate
knowledge of many soils, thus providing challenges for soil scientists and the direction of further
useful research.
