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At a glance 
The issue Soil structure describes the aggregation of soil particles (sand, silt, clay) and the pore spaces. Soil structure decline is a detrimental change in these soil characteristics as a result of certain land use practices. It is regarded as one of Tasmania's more serious forms of land degradation because it is comparatively widespread and has well-documented impacts on agricultural productivity. Soil structure decline results in reduced crop yields, greater surface run-off and the potential for soil erosion, which may reduce water quality. Soil compaction may result in less infiltration and drainage, thereby reducing amounts of water available for plant growth. Other impacts include a reduced ability of soils and plants to withstand stress imposed by diseases, climate and machinery effects (arising from lower organic matter levels) and reduced resistance to wind and water erosion. Structural decline is indicated if soils develop a more massive condition (i.e. lacking aggregates), denser overall structure (i.e. cloddiness and compaction) or suffer a partial or complete shattering or disaggregation of structure to a powdered soil. Problems associated with soil structure damage include reduced crop yields and increased run-off and soil erosion. Changes in land use patterns, particularly the change from dryland pasture to irrigated cropping, can increase the areas at risk of soil structure decline. Some newly cropped areas, where the existing soil structure is not strong or robust, are potentially sensitive to soil structure decline. Excessive cultivation, as well as stock and vehicular movement, may also increase the risk of structural decline on some soils. Severe structure decline may occur due to the presence of sodic soils, salinity or use of poor quality water. This 'At a glance' section provides an overview of the issue of soil structure decline. More detailed information and references are available in the Soil Structure Decline and Compaction Issue Report. Two indicators are included in this report: Potential for Soil Structure Decline and Percent of Cropped Land with Reduced Tillage Plus Stubble Retention. Recommendations are presented on Strategic Land Resource Management and Productivity of Soils. Favourable news - A project supported by the Natural Heritage Trust entitled 'Minimising Land Degradation and Salinity Risk using Resource Information' is investigating the degradation risks, including soil structure decline, of intensified land use on soil types in three pilot areas around the State. Each area receives less than 800 mm rainfall per annum and is currently experiencing, or will experience in the near future, a significant land use change away from traditional grazing enterprises to more intensive cropping activities such as irrigated cropping.
Unfavourable news - In 1988, the annual cost of soil structure decline to Tasmania was estimated to be approximately $6 million (Nothrop et al. 1988). No subsequent calculations have been made, so the cost may now be significantly higher. There is no direct evidence of an increase in the extent of soil structure decline and an increase in its economic cost. However, an overall increase in land use intensity and the expansion of cropping into more marginal soils suggests that this may have occurred. The Land Tenure, Land Use and Land Cover Issue Report describes some of these changes in land use intensity and cropping in Tasmania.
- Grice (1995) estimated that approximately 15% of private land in Tasmania is susceptible to moderate to severe soil structure decline. However, any change in land use from pasture-based agriculture to cropping could increase the area affected by soil structure decline.
Uncertain news - The only data currently available for potential soil structure decline (i.e. hazard) are from a survey of soil structure decline hazard conducted in 1992 (Grice 1995). This survey was confined to private land. No information is available on the extent of soil structure decline or hazard on Crown Land. The State of the Forests Report makes no specific reference to soil structure decline in State Forests, as there are no data to support such an assessment at this stage.
- There have been a number of research studies of soil compaction on forest soils at the Warra Long Term Ecological Research Site in the Southern Forests. Whether soil compaction and soil structural change is a general problem in forested areas is not known.
- There is little reliable information on the nature and distribution of Tasmania's dominant soils. It is therefore impossible to reliably determine the extent of soils at risk or even the degree of risk. The implementation of an appropriate land resource inventory would provide a baseline from which future trends in soil structure decline, and a suite of other land resource issues, could be determined.
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