Soil properties: soil acidity
Indicator Status: For Advice
Department of the Environment and Heritage
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Why do we need to monitor soil acidity?
Soil acidification is a major soil degradation issue in many parts of Australia. It is often an insidious soil degradation process, developing slowly although indicators such as falling yields, leaf discolouration in susceptible plants and lack of response to fertilisers can indicate that soil pH is falling to critical levels. If it is not corrected, acidification can continue until irreparable damage takes place in the soil. Australia's ancient and highly weathered soils and current systems of agricultural land use are particularly vulnerable to this process. In addition, many surface or subsoils in agricultural areas are already acidic in their natural state.
The NLWRA (2001) estimates soil acidity impacts on 50 million hectares of surface layers and 23 million hectares of subsoil layers of Australia's agricultural zone. Large areas of acidic soils occur in New South Wales and Western Australia and, to a lesser extent, Victoria.
Impacts of soil acidity on agriculture and the wider community can include:
- increased nitrate pollution of groundwater and reduced water quality;
- reduced plant yields, farm income, land values and domestic/export earnings;
- reduced plant species options for agriculture;
- reduced vegetative cover, leading to accelerated run-off and erosion;
- irreversible degradation of the clay minerals of soil (i.e. hard setting) resulting in reduced fertility;
- increased infrastructure costs as a result of increased salinity, waterlogging, flooding and sediment on roads and in drains.
Reversal of the acidification process through the addition of lime is straightforward and effective, at least in the early stages. Over time, subsoils become acidified and remediation is difficult. A long term liming program that manages surface pH may also be effective in avoiding subsoil acidification. If subsoil becomes highly acid it is difficult and expensive to ameliorate in the short to medium term. The NLWRA (2001) estimates that:
- in the absence of remedial lime applications, the pH of 29-60 million hectares will fall to 4.8 or lower within ten years (i.e. below optimum for acid-sensitive agricultural plants), and a further 14-39 million hectares will decline to pH 5.5; and
- a very large increase in the quantity of applied lime is required to adjust pH to suitable levels. Regular applications are then needed, at rates that still exceed current-day rates, to maintain the new level.
The appendix of Dolling et al. (2001) contains information on maintenance lime requirements for each river basin identified by the Australia Water Resources Council (AWRC).
The findings of the National Land and Water Resources Audit represent an important step in assessing the magnitude and distribution of soil acidification in Australia. However, we still need to establish an accurate baseline for measuring future change. Such a baseline is essential to determine whether the problem is increasing, stabilising or improving through better land management.
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