Salinity, soil carbon stocks, acidification and erosion affect soil condition and productivity in Australia.
Increases in dryland salinity appear to have been slowed by the millennium drought, although the return to wetter conditions is likely to increase the spread of salinity.
The management of soil carbon is central to maintaining soil health and ensuring global food security, as well as providing an important sink for atmospheric carbon. Australia has a lower baseline soil organic carbon stock than other parts of the world, and few regions have increasing soil carbon stores. The time since vegetation clearing is a key factor determining current trends. For example, large parts of Queensland are on a declining trend, because widespread clearing for agriculture was still occurring in the 1990s. Similarly, regions with intensifying systems of land use (e.g. northern Tasmania) and most regions with a projected drying climate have declining trends. The savanna landscapes of northern Australia have significant potential for increasing soil carbon stores, but this requires changes in grazing pressures and fire regimes.
Soil acidification is another challenge facing agriculture. Soil acidification restricts options for land management, because it limits the choice of crops and vegetation to acid-tolerant species and varieties. Soil acidity affects approximately 50 million hectares (or 50 per cent) of Australia’s agricultural land, and about 23 million hectares of subsoil layers, mostly in Western Australia and New South Wales (NLWRA 2001). Soil acidification is of greatest concern in situations where:
- the soil already has a low pH (i.e. is already acidic)
- agricultural practices increase soil acidity (e.g. use of high-ammonium nitrogen fertilisers, large rates of product removal)
- the soil has a low capacity to buffer the increase in acidity (e.g. infertile, light-textured soils).
It is relatively straightforward to reverse short-term surface soil acidification through the application of lime. However, it is much harder to reverse the problem if the acidification has advanced deeper into the soil profile, because incorporating lime at depth is prohibitively expensive. Although rates of lime application appear to be increasing, they still fall far short of what is needed to arrest the problem in some jurisdictions.
Current rates of soil erosion by water across large areas of Australia exceed soil formation rates, although progress has been made in reducing soil erosion through soil conservation measures.
A new generation of fine-grained soil mapping is being conducted that will inform national mapping and monitoring of carbon, biodiversity, agricultural impact and ecosystem functions in general. It is the result of national collaborative research, funded through the Terrestrial Ecosystem Research Network, involving CSIRO; the University of Sydney; Geoscience Australia; and national, state and territory government agencies.