Soil acidity is the second highest priority threat (after soil erosion) to the sustainable management of agricultural soils in South Australia. It has been estimated that a further 2.2 million hectares (24%) of cleared land in SA has the potential to become acidic in the longer term future, ie 10 – 50 year timeframe to 2050, assuming that current farming production practices continue.
The consequences of untreated highly acid soils include:
- loss of production and financial returns, particularly for acid-sensitive plants
- progressive acidification of subsurface and subsoil layers, which are much more costly to ameliorate
- reduced uptake of soil water that can lead to rising water tables and increased soil salinity
- increased leaching of iron, aluminium and some other nutrients from the soil, potentially contaminating surface and ground water
Soil acidification is a natural process in the higher rainfall (>400mm) areas of SA, but 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 fertilisers or even legume plants. Higher levels of production also tend to lead to higher acidification rates. Sandy textured soils are at highest risk of acidification.
About 1.8 million hectares (20%) of cropping land in South Australia (SA) currently has surface soil acidity (0-10cm < pH 5.5) or is considered acid prone, and this area is expected to double over the next 40 years if remedial action is not taken.
Monitoring and Trends
Soil acidity becomes a serious problem, affecting agricultural production and the environment, when the level of acidity reaches pHCaCl of 5 or less. Direct monitoring of soil acidity would require soil samples from across the state to be collected and analysed on a regular basis. This is not feasible due to the enormous cost and effort that would be required.
Soil acidity is usually corrected with lime, therefore DEW monitors soil acidity in agricultural areas by comparing actual lime use with:
- The amount of lime needed to balance soil acidification (soils becoming acid over time)
- The amount of lime needed to treat soils that are already acidic.
Annual soil acidification rates in agricultural regions of SA (expressed as tonnes of lime per year needed to balance the acidification process) have been estimated on the basis of agricultural land use intensity and the area of soils that are already acidic or becoming prone to acidity.
DEW has previously assessed the extent of soils that are already acidic, from databases of commercial soil testing services that obtain soil samples for fertility analysis, throughout the state’s agricultural areas. With this information, the amount of lime needed to treat the topsoil layer (0-10cm depth) of the acidic soils has been determined.
Lime sales data (tonnes) has been collected annually from commercial lime product suppliers in South Australia since 1998 to estimate the annual amount of lime applied to land in the agricultural regions.
Soil acidity in the subsurface soil layer (below 10cm) is also an issue of concern in agricultural areas, but very little data is available and this would be very costly to monitor extensively. There is evidence of ‘subsurface’ acidic soil layers developing just below the surface (i.e. below 5cm depth) particularly under no till cropping systems.
Some data on land managers’ awareness and understanding of soil acidity and its treatment is obtained from DEW’s land manager surveys.
Annual summary ‘acidity status reports’ have been produced for agricultural regions and the state