Soil Acidity

Soil Acidity affects more than 70% of soils across the Southwest of Western Australia and is a natural consequence of agriculture caused by the removal of plant material (pasture and crop), application of nitrogen fertilisers and leaching of nitrates. Acid soils reduce root growth, increase subsurface aluminium toxicity to plants, reduce major nutrient uptake and inhibit nodulation in legumes.

Visual Symptoms

• Plant growth is poor and usually worse on sandy surface soils.
• In dry seasons or dry finishes, acid soils are more likely than other soils to have poor yields (due to stunted roots systems and inability to access moisture).
• Plant roots look stunted and end at a distinct layer without a visible cause (e.g. compaction, waterlogging, poor soil structure).
• Nodules on legumes roots are reduced and not very active (not bright red).
• Plants have finished growing but there is still plenty of moisture below the roots.

Measuring Acidity

• The acidity of soils can only be measured through pH testing.
• An acid soil is when the top 10 cm has a pHCaCl2 of less than 5.5 and/or the subsoil (10-30 cm) has a pHCaCl2 of less than 4.8;
• Paddocks should be soil tested to depth before lime is applied to determine the liming rate.
• Limed paddocks should be re-tested for pH to depth every 5-7 years to determine if more lime is needed.

Solutions

Applying lime or dolomite to the surface with or without incorporation is the main option available to WA landowners at present. Other strategies include reducing/replacing the use of ammonium based fertilizers and improving nitrogen use efficiency. Liming is practical, profitable and effective on most soils. Applying lime to the surface also treats subsoils although this is a slow process (5-10 years).
Mechanical incorporation can help get surface applied lime into the subsurface but is more expensive and risks moving acidic subsoils up to the surface, therefore subsoil testing is highly encouraged before undertaking incorporation.

Further Reading

Apal Agricultural Laboratory. Undated. Soil Test Interpretation Guide. http://www.apal.com.au/images/uploads/resources/Soil_Test_Interpretation_Guide_1.pdf

Department of Primary Industry and Regional Development (DPIRD). Accessed 26/04/2018. ‘Soil Acidity.’ https://www.agric.wa.gov.au/climate-land-water/soils/managing-soils/soil-acidity

DPIRD. 2017. ‘Soil acidity in Western Australia.’ https://www.agric.wa.gov.au/soil-acidity/soil-acidity-western-australia

Wheatbelt NRM. Accessed 26/04/2018. ‘Soil Acidity.’ http://www.nrmstrategy.com.au/soil-acidity