Agriculture Reference
In-Depth Information
Soil Supergroups and Groups of Western Australia
The Soil Groups of Western Australia (Schoknecht, 2001) were developed to satisfy a need for
a simple, standardized and easy-to-understand way to recognize the most common soils in Western
AustraliaÐa state covering about 250 million hectares.
The three main objectives of Soil Groups are to provide common nomenclature to the main
soils of Western Australia; provide a simple method of soil identiÝcation; and assist with the
communication of soil information at a general level.
The Soil Groups aim to create common names that are more consistent than current terms,
which are based on location, geology, and native vegetation. As a consequence, current common
names describe very different soils in different geographic areas. Soil Group names have provided
a standardized substitute for locally used common names.
Soil Groups can be allocated by non-technical people who usually have difÝculty using formal
soil classiÝcation systems such as the Australian Soil ClassiÝcation (Isbell, 1996). It aims to
categorize soils giving emphasis to soil characteristics that are relevant to land management and
easy to identify by people with little soil description experience. For example, many soils in the
southwest portion of the state are dominated by ironstone gravels (Figure 9.2). These gravels are
used locally to identify these soils and are known to have a major effect on their management.
Their presence is used as a primary division in the key for allocation to Soil Supergroups. The Soil
Supergroups and their deÝnitions are presented in Table 9.3.
Within the soil classiÝcation structure, two levels of generalization are identiÝed: Soil Super-
groups and Soil Groups. Thirteen Soil Supergroups for Western Australia are deÝned using three
primary criteria: texture or proÝle permeability; coarse fragments (presence and nature); and
water regime.
Soil Supergroups are further divided into 60 Soil Groups based on one or more of the
following secondary and tertiary criteria: calcareous layer (presence of carbonates); color;
depth of horizons/proÝle; pH; and structure. The deÝnitions of the criteria for determining
Soil Supergroups and Soil Groups are deÝned in Schoknecht (2001). An example of how one
of the Soil Supergroups (Deep sands) is subdivided into Soil Groups using a simple key is
presented in Table 9.4.
Table 9.3
The Soil Supergroups for Western Australia
Soil Supergroup
a
Definition
Wet or waterlogged soils
Soils seasonally wet within 80 cm of surface for a major part of the year.
Rocky or stony soils
Soils, generally shallow, with > 50% of coarse fragments >20 mm in size (coarse
gravels, cobbles, stones or boulders) throughout the profile. Includes areas of
rock outcrop (all lithologies).
Ironstone gravelly soils
Soils that have an ironstone gravel layer (> 20% and > 20 cm thick) or
duricrust/cemented gravels within the top 15 cm, and ironstone gravels a
dominant feature of the profile.
Sandy duplexes
Soils with a sandy surface and a texture contrast or a permeability contrast
(reticulite) at 3 to 80 cm.
Sandy earths
Soils with a sandy surface and grading to loam by 80 cm. May be clayey at depth.
Shallow sands
Sands
≤
80 cm over rock, hardpan, or other cemented layer.
Deep sands
Sands >80 cm deep.
Loamy duplexes
Soils with a loamy surface and a texture contrast at 3 to 80 cm.
Loamy earths
Soils with loamy surface and either loamy throughout or grading to clay by 80
cm.
Shallow loams
Loams
≤
80 cm over rock, hardpan, or other cemented layer.
Cracking clays
Soils with clayey surface at least 30 cm thick and crack strongly when dry.
Non-cracking clays
Soils with clayey surface at least 30 cm thick and do not crack strongly when dry.
Miscellaneous soils
Other soils.
a
Adapted from Schoknecht (2001)
