Soil acidity is the second highest priority threat to the sustainable management of agricultural soils in South Australia. Approximately 1.9 million hectares of agricultural land (20%) are affected by soil acidity. Many soils in the higher rainfall areas of the state are naturally acidic.
Soil acidification can be significantly accelerated by agricultural practices including removal of grain, hay and livestock products from the paddock, use of ammonium-containing or ammonium-forming fertilisers, and leaching of nitrate nitrogen derived from legume plants or fertilisers. Higher levels of production also tend to lead to higher acidification rates. Sandy textured soils are at highest risk of acidification.
The consequences of untreated highly acid soils include:
- reduced growth and production of most agricultural plants
- increased soil salinity due to increased drainage of rainfall to groundwater
- increased leaching of iron, aluminium and some nutrients leading to contamination of surface and ground water
- structural breakdown of the soil.
Surface soil acidity can be readily treated by application of liming products, but subsurface acidity is more difficult and expensive to treat. If acidic topsoils are not adequately treated over time, there is an increased risk of subsurface acidification. Acidity can also be ameliorated by incorporation of calcareous or alkaline clay or by use of alkaline irrigation water. The use of deeper rooted perennial plants and effective management of soil nitrogen can reduce the rate of acidification.
Soil acidification will continue to increase unless the level of remedial action is significantly improved.
Lime use in the state rose through the late 1990s but then declined through the early 2000s, and has been relatively steady since 2007-08. The estimated amount of lime required to balance the annual acidification rate in the agricultural zone of South Australia is approximately 213,000 tonnes. The average amount of lime applied per year over this period was approximately 113,000 tonnes, only 53% of that required to balance acidification. There are still large areas of land where acidification continues to damaging levels.
Apart from the lime required to balance ongoing soil acidification, additional lime needs to be applied to raise the pH level of soils that are already acidic. An estimated 1.1 million tonnes of lime is required to treat topsoils that are already acidic.
Soil acidification is becoming an increasing issue in cropping districts due to high levels of production and increased use of nitrogenous fertilisers. Sub-surface (below 10 cm depth) acidity is more widespread than previously recognised, and is a significant issue in the Mt Lofty Ranges, Kangaroo Island and South East.
DEWNR is working in partnership with industry groups to develop and deliver programs and projects to:
- improve land manager understanding and awareness of soil acidity, its causes and treatment options
- re-test previous monitoring sites
- test additional sites to assess the extent of surface and subsurface acidity.
An improved understanding of the rate of topsoil and subsoil acidification under various land uses and land management systems and soil types is required. In addition, a better understanding of the impacts and treatment of soil acidity, especially in the sub-soil, are required as well as programs that increase the recognition and management of acidity by land managers.
See also soil acidity monitoring.