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Soil acidity

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Summary

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 1.4 million hectares (15%) of cleared land in SA has the potential to become acidic in the longer term future, i.e. 10 – 50 year timeframe to 2050, assuming that current farming production practices continue.

This page will help you understand:

soil acidity causes and problems
the extent of soil acidity in South Australia
current monitoring programs and trends.

Soil Acidity

Soil acidification is a natural process in the higher rainfall (>400 mm) areas of South Australia, as alkaline nutrients (e.g. calcium and magnesium) are leached down the soil profile. However, soil acidification can be significantly accelerated by agricultural practices such as:

removal of grain, hay and livestock products from the paddock (i.e. removal of alkaline nutrients from the system)
use of ammonium-containing or ammonium-forming fertilisers (i.e. release of hydrogen ions through the nitrification process)
incorrect use of nitrogen fertilisers (i.e. nitrate leaching promotes the loss of alkaline nutrients from the system)
use of legume plants (i.e. release of hydrogen ions into the soil through nitrogen fixation process).

Consequently, higher levels of production tend to lead to higher acidification rates.

Sandy textured soils are at the highest risk of acidification, due to their lower capacity for retaining alkaline cations (i.e. lower pH buffering capacity) and higher risk of nutrient leaching.

Soil acidity becomes a serious problem, affecting agricultural production and the environment, when the level of acidity reaches pH_CaCl of 5 or less. 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.

Extent

About 2.5 million hectares (28%) of cropping land in South Australia (SA) currently has surface soil acidity (0-10cm < pH 5.5) or is close to becoming acidic, and this area is expected to increase to 3.9 million hectares over the next 30 years if remedial action is not taken.

Monitoring programs

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 acidic over time)
the amount of lime needed to treat soils that are already acidic.

Some data on land managers’ awareness and understanding of soil acidity and its treatment is obtained from DEW’s land manager surveys.

Estimating annual acidification rates

Annual soil acidification rates in agricultural regions of SA 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. This is expressed as tonnes of lime per year needed to balance the acidification process).

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 (0-10 cm 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.

DEW is in the process of developing a spatial model to estimate annual rates of soil acidification in surface soils.

Soil acidity in the subsurface soil layer is also an issue of concern in agricultural areas, with some evidence of ‘subsurface’ acidic soil layers developing just below the surface (i.e. below 5 cm depth) under no till cropping systems. However, very little data is subsurface data available and this would be very costly to monitor extensively.

Monitoring trends

Between 2022 and 2025, the mean annual rate of lime use applied to acidic agricultural soils in SA was 262,000 tonnes. This was more than double the quantity used in 2016 (110,000 tonnes).
The annual lime requirement to balance soil acidification in surface soil layers is projected to increase from 200,000 tonnes in 2022 to 293,000 tonnes by 2050. This is due to increasing agricultural production, particularly more intensive cropping systems and higher nitrogen use.
Annual summary ‘acidity status reports’ have been produced for agricultural regions and the state.

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Page updated: April 2026