Pastures on acid soils
Almost half of NSW agricultural land has subsurface acidity. By fixing soil acidity we improve productivity and the capacity to grow more diverse pastures. Our research team is working to:
- Demonstrate the benefits of 5cm sampling to identify soil acidity and inform its control.
- Promote feasible and profitable ways to fix acidic soils, including higher pH targets and lime incorporation.
- Evaluate the persistence of productive pastures and changes in forage quality after liming.
- Recommend productive pasture mixes and practices that capture the benefits of amended soils.
Soil acidification
Most plants and micro-organisms have defined ranges of pH for optimal growth. The optimal range of pH in CaCl2 (pHCa) for plants is between 5.5 - 8, whereas most soil organisms function best between pHCa 6 - 7.
In acidic soils:
- aluminium (Al) and/or manganese (Mn) may become toxic,
- microbial activity decreases,
- nodulation and nitrogen fixation are decreased in legumes,
- soil fertility decreases, particularly phosphorus, nitrogen and sulfur, or micro-nutrients, such as molybdenum, and
- drought resilience of pastures is decreased, due to reduced root growth constraining access to soil water.
Causes of acidification
Nitrogen cycling, from legumes or ammonium producing fertilisers, is a major cause of acidification in grazing systems. Efficient use of nitrogen helps to minimise the acidification from added nitrogen, but acidity will still result, often as stratified acidic subsurface layers in the topsoil.
Soil acidifies (soil pH gradually declines) as alkaline compounds are removed from the paddock by either grain, fodder or livestock production. Hence, acidification rates are higher in production systems that remove more plant and/or animal product, for example hay cutting compared with grazing; prime lambs compared with wool sheep; or grain and graze systems compared with grain-only production.
Superphosphate does not have a direct acidifying effect but leads to acidification by encouraging more vigorous legume growth, resulting in greater nitrogen inputs and ultimately, more alkali removal via increased agricultural production. Production gains from the use of superphosphate will acidify soil.
All agricultural systems acidify the soil if external alkali (e.g. lime) is not applied to balance the acidity produced by agriculture.
Acid soil management
Applying lime is the most effective method of correcting soil acidification. Liming acid soils can result in increased pasture production, especially when legumes are present, allowing greater carrying capacity and livestock production per hectare.
The first step in any acid soil management program is to sample soil to identify the extent of acidity. Ensure you create a soil sampling program to monitor pH and fertility status of soils over time and to check the impact of your soil management practices. It is recommended that soil be sampled in 5 cm increments to identify subsurface acid soil layers (0-5, 5-10, 10-15, 15-20 cm).
From soil test results, liming rates can be determined to meet prescribed pH targets. Research indicates that whilst liming to raise soil pHCa to 5.2 can mitigate aluminium toxicity, a target of pHCa 6.0 is required in the soil above acidic layers to enable the liming effect to move deeper.
Where possible (i.e. low risk of erosion), incorporation by tillage can hasten the lime response and ameliorate soil acidity more quickly by mixing the lime with the soil volume. On soils or topography where tillage is not practical, surface applied lime will still ameliorate soil acidity but over a longer timeframe.
Resources
The extent, significance and amelioration of subsurface acidity in southern New South Wales,
- Condon J, Burns H, Li G
- Australia. Soil Research 59, 1-11 (2020). https://doi.org/10.1071/SR20079
Changes in pasture and soil properties with liming and superphosphate application on five soils in the Central Tablelands of New South Wales over 12 years.
- Dowling PM, Vimpany IA, Conyers MK, Millar GD, Helyar KR, Michalk DL, Nicol HI, Bradley J, Milham PJ, Hayes RC.
- Crop and Pasture Science, 76(5) (2025). https://doi.org/10.1071/CP24336
A financial analysis of lime application in a long-term agronomic experiment on the south-western slopes of New South Wales.
- Li GD, Singh RP, Brennan JP, & Helyar KR.
- Crop and Pasture Science 61, 12-23 (2009). https://doi.org/10.1071/CP09103
Long-term surface application of lime ameliorates subsurface soil acidity in the mixed farming zone of south-eastern Australia.
- Li GD, Conyers MK, Helyar KR, Lisle CJ, Poile GJ, & Cullis BR.
- Geoderma 338, 236-246 (2019). https://doi.org/10.1016/j.geoderma.2018.12.003
