Soils - Overview - WA

Western Australian Land Resource Assessment

Introduction

Land Resource mapping conducted by the Western Australian Department of Agriculture has a different focus for the intensive agricultural and pastoral regions.

Soil-landscape mapping predominates in the intensive agricultural regions. These are supplemented by minor soil association mapping in areas such as the Ord irrigation area. Most soil-landscape or land system mapping is at reconnaissance scale, ie 1:250,000 to 1:100,000. The map below shows the scale of soil landscape mapping in the south west of WA.

Land systems mapping covers most of the pastoral region. For the arid interior, the only comparable mapping is the Atlas of Australian Soils and Beard's vegetation maps. More information including contacts is available on the Western Australian Department of Agriculture web site.

Soil-landscape Mapping

A six level hierarchical rank structure for soil-landscape mapping has been adopted and is being implemented in the intensive agricultural portion of WA, essentially the south-west corner. The structure implies that map units adopt general properties of "parent" map units. These ranks approximate different scales of mapping with most mapping so far produced being of subsystems.

This mapping essentially began with the Decade of Landcare under the National Soil Conservation Program.

However, there is complete coverage of at least draft mapping from which a system level map has been produced. This rank approximates land systems. Such mapping has been used in several themes of the National Land and Water Resources Audit and for other "big picture" interpretations.

A major advantage of the system adopted is the enforcement of seamless boundaries between individual surveys, at least at the system level. This minimises the problems of matching adjacent surveys with different map scales.

Traditional methods, ie aerial photo interpretation and field point observations, have been used in identifying the map units. Similar polygons are identified by a common label against which attribution is recorded in a database. This which contrasts with some States which adopt a UMA (unique map area) approach.

Attribution

Map units (ie the common label), including "parent" units, are described in landscape and soil terms plus limited geology and vegetation. It is recognised that most map units are heterogeneous, in terms of soil and/or landscapes. Much of this variation could be recognised at larger scales of mapping. Also, the properties of the map units important for land management (land qualities) are heterogeneous making it difficult to record values of these directly to map units.

Therefore, map units represented by polygons have proportional attribution of unmapped components. What has been recognised as the unmapped components has varied over time from soil series, land units and WA Soil Groups. However, a process of standardisation of units based on a combination of WA Soil Groups and landscape position is well advanced.

Land quality values are assessed and recorded for each unmapped component. Therefore, the recognition of the unmapped components needs to take into account that each should be homogeneous in terms of their land quality values.

Land System Mapping

Land system mapping at a scale of 1:250,000 is nearly complete for the pastoral areas of Western Australia. This mapping also follows traditional methods. Vegetation is an important part of the recognition. Field observation focuses on typification of vegetation or pastoral types and documentation of pastoral condition.

Each land system is described in terms of geology, landscape, soil and vegetation types. However, documentation focuses on important vegetation units and plant species. However, the scale of mapping is too small to identify vegetation units separately. Thus, land units are used as the primary unmapped units.

Land units, proportionally allocated to each land system, are an important part of the land system description. Vegetation units are assigned to land units. Through this and the "pastoral potential" of vegetation units being assessed, the "pastoral potential" of the land systems can be derived. Land qualities in the terms used in the intensive agricultural areas are not assessed.

Western Australian Soil Groups

Communicating information based on soil units needs careful thought. In Western Australia, very few soil series have wide use. There is no uniform use of a technical classification system with Australian Soil Classification rarely used. Like with most vernacular soil names, those commonly used by the farming community and agricultural professionals alike suffer from inconsistent naming, usage and no have definition.

This lead to the development of the Western Australian Soil Groups. While it was not meant to replace such as the Australian Soil Classification, these soil groups are based on a clear key classification by which they are defined. The key requires only limited technical experience relying principally on texture, colour and pH. A feature of the system is that it only explicitly caters for the main soils of Western Australia. However, minor soils are catered for at a generic level within the classification.

The soils in soil-landscape map units are described in terms of WA soil groups. As attribution becomes complete, communication of soil names is increasingly in terms of these soil groups.

Land Resource Interpretation

The suitability of areas for different land uses and the distribution of serious degradation risks are products being increasingly asked of the mapping and its attribution. For the soil-landscape mapping, such relies on attribution of land quality values. This attribution uses standard definitions (Land Evaluation Standards for Land Resource Mapping, van Gool and Moore 1999) and is applied to the unmapped components. This is the major aspect of the soil-landscape mapping being completed.

By the use of tables being developed, the land quality values are converted to a 5-class scale of suitability for different land uses. "Most limiting factor" determines the suitability of each unmapped component for each land use. These are aggregated to provide a proportional occurrence of the 5 classes for each map unit.

Databases

Two basic classes of objects are incorporated in an integrated relational database; points and map units.

Points

Over 60,000 points have been incorporated in the Soil Profiles Database that is based on the standardised PC Sites model. Most points are from individual land resource projects, the code for which is a key field in the tables. These points are principally soil profile descriptions and observations but also include the vegetation descriptions collected during the pastoral Land System mapping. The results of laboratory analysis from a proportion of the sites are also incorporated.

Map Units

All soil-landscape, land system and soil map units are described and attributed in the Map Units database. So too are the unmapped components and their land quality values. Their proportional allocation to the map units is recorded. There are also tables for recording land suitability from land quality values.

Products

The designed outputs of the land resource surveys are printed reports and maps. All completed surveys are freely available for purchase. With the mapping being captured and maintained in digital formats, digital map products are also available for a standard fee. Licensing agreements apply to the provision of digital data.

Draft mapping and data may be available on application. The intended use of the data is considered in these cases and co-operative research is encouraged. Fees are also chargeable for this.

Interpretation mapping can be generated on a commissioned case by case basis.

Consideration is also given to the provision of point data in either in a database or document formats.

Research

Insights obtained during the conduct of a soil-landscape survey have resulted in the development of a new theory for the development of lateritic gravels that are a major feature of south western Australian soils. This collaboration between soil science and botany is soon to be published in the Australian Journal of Botany. The implication of this theory for soil and landscape evolution is still being evaluated.

Research programs are being developed to increase the quality of the existing soil-landscape mapping using digital elevation model derivatives and limited airborne geophysics. These offer great potential to provide reliable mapping at smaller scales than currently available in WA.

Beard, J.S. (1974) Vegetation survey of Western Australia - Sheet 3. Great Victoria Desert, University of Western Australia Press, Nedlands, Western Australia.

van Gool, D. and Moore, G.A. (1999) Land Evaluation Standards for Land Resource Mapping , Agriculture WA Resource Management Technical Report 181, 85pp.

Schoknecht, N.S. (1999) Soil Groups of Western Australia, a Guide to the Main Soils of Western Australia. Agriculture WA Resource Management Technical Report 193, 98pp.

Contact Agriculture WA