Soil Acidity in WA

Truck in a paddock.

More than 70% of surface soils and almost half of subsurface soils across WA’s south-west are below appropriate pH levels for agricultural production.

Subsurface acidity is a major constraint to crop and pasture productivity right across WA’s wheatbelt, estimated to reduce crop yields by 9–12 per cent.

Some trace elements become more soluble at lower pH, as does aluminium (Al), which is toxic to plants when in solution. If soil pH is too low – below about 4.8 – the concentration of Al in the soil water increases rapidly and reduces root growth. The resultant smaller root systems have limited access to nutrients and water, leading to lower plant biomass and grain yields.

Adding lime to the soil raises the pH. Above pH 4.8 the Al becomes non-toxic in the soil, enabling the plants to develop effective root systems. Research shows that as well as improving crop yields and widening rotation options, liming has a long-term positive impact on the ecosystem by potentially boosting soil microbial activity, improving availability of major plant nutrients and helping to reduce weed seed banks.

DAFWA’s recommended minimum pH levels for WA’s agricultural soils to maintain or achieve yield potential (measured in pH calcium chloride– or pHCa) are:

Surface (0–10 cm) pHCa 5.5

Subsurface (10–20 cm and  20–30 cm) pHCa 4.8, and no less than pHCa 4.5 to avoid Al toxicity.

Wheat yields will not be constrained by acidity when these levels are met.

Lime use is most effective if it is precisely targeted to property areas based on subsurface soil testing during the summer months to a depth of at least 30 cm (in 10 cm increments), and to 40 or 50 cm on sandy soil. Scheduled liming applications are required to maintain the desired pH level in the soil, with priority given to the farm’s most productive cropping areas.

It is important that growers source the best value for money lime based on neutralising value (i.e. the carbonate content of lime determines the capacity of the lime to neutralise acidity) and particle size (i.e. the size of the lime particles determines how quickly the lime can neutralise acid). Lime with a high proportion of finer particles has a larger surface area to react with the acid in the soil.

Mechanically incorporating lime into the subsurface during amelioration with deep tillage, spading or mouldboard ploughing generally achieves more rapid improvements in subsurface pH levels. Variable rate technology can also be successfully applied to lime spreading.