Getting the most from your liming program
On-farm soil acidity and nutrient management (Watering WA – Clean Waterways)
Proving the effectiveness of agricultural lime
Results from a recently completed three-year project have shown the effectiveness of using agricultural lime to increase soil pH and highlighted the positive efforts made by growers in the Avon River Basin to address soil acidity.
The project, led by Department of Primary Industries and Regional Development research scientist Alice Butler, also helped growers assess the best method to apply lime for management of soil acidity into the future, with a focus on mixing or deeper incorporation of lime to accelerate the remediation of acidity (low pH) in subsurface soil.
The issue with soil acidity
Soil acidity has a significant impact on agricultural productivity due to its effect on plant root growth and nutrient-use efficiency.
It affects the availability of key nutrients in the topsoil (0-10 centimetres) and restricts crop root growth and access to moisture and nutrients in the subsurface layer (below 10cm).
To address this issue, agricultural lime is commonly used to increase soil pH and manage soil acidity, however the current lime usage in WA is only 60 per cent of what is estimated to be required annually to combat soil acidification.
Taking a trip down soil acidity lane
Between 2005 and 2012, DPIRD collaborated with Precision SoilTech and Wheatbelt NRM to collect and analyse a soil pHCa dataset for the WA grain growing region which offered insights into the state of soil acidity during that period.
That report found about 70 per cent of topsoils (0-10 cm) had a pH level (pHCa) below the recommended minimum target of 5.5, and about 50 per cent of subsoils (10-20cm and 20-30cm) had a pHCa below the recommended minimum target of 4.8, indicating higher acidity.
The 2013 report served as a benchmark for the recent work conducted on soil acidity.
Extensive sampling and positive improvements
Over the 2020-21, 2021-22 and 2022-23 soil sampling seasons, 182 farms were sampled, located from Dalwallinu in the north to Lake Grace in the south, and in total 40,470 soil samples were taken as part of the project from 13,359 individual sample sites.
The target pH for the topsoil (0-10cm) was above 5.5, with a critical pH above 5, while in the subsoil depths (10-50cm), the target pH was above 4.8, with a critical pH above 4.5.
Upon the completion of sampling, it was found that 44 per cent of topsoil samples were below the target pH level (pHCa), which represented an improvement compared to the 2005-2012 study where 70 per cent of topsoils were more acidic than the recommended pH level.
Similarly, there was improvement in subsoil pH levels, with 28 per cent of samples at 10-20cm and 22 per cent of samples at 20-30cm falling below the target pH level, compared to the previous study which found 50 per cent of subsoils did not meet the target pH of 4.8.
These results indicated the positive efforts made by growers in the Avon River Basin to address soil acidity.
Putting soil amelioration in the mix
While statistics of lime use in WA indicate growers have progressively adopted the application of lime to remediate the effects of soil acidification, until recently most of the lime has been applied to the soil surface and is below the level required to reach minimum targets for surface and subsurface soil pH.
Since the movement of lime from surface application is slow, researchers and growers have investigated mixing or deeper incorporation of lime to accelerate the remediation of acidification in subsurface soil.
This project worked collaboratively with four growers in the Avon River Basin to analyse their historic and future soil amelioration strategies using DPIRD’s iLime app.
Does it stack up economically?
Overall, surface soil pH improved when growers were applying lime, benefitting plant root growth, leading to improved access to water and nutrients and ultimately leading to increased returns for farmers. However, the subsurface soil pH increased at a much slower rate.
Incorporating lime into the subsurface soil via mechanical amelioration increased pH levels to the depth of incorporation and facilitated the movement of alkali deeper into the soil profile.
By modelling the soil pH change using the growers historical soil pH results and lime applications (date and amount applied) along with rotation, rainfall and yield within iLime, lime scenarios could be assessed economically.
All liming scenarios showed economic benefits compared to not liming.
When incorporation to 20cm was added into the analysis it did not provide additional economic gains over surface application, however it also did not show a reduction in cumulative net profit.
This indicated that the cost of incorporating lime to 20cm was no more expensive overall to the grower than surface spreading, but with more long-term benefits, especially within the subsoil.
In saying that, the study's comparisons were based on paddocks with previous lime applications and relatively higher subsurface pH levels.
Looking ahead, managing acidity will require a focus on higher lime application rates and strategic tillage in areas where it is economically feasible to effectively address subsurface acidity.
There are considerations to be made regarding the potential costs associated with recovering soil pH levels in more acidic subsurface soil in scenarios without or with only surface lime application.
Growers are encouraged to soil test to depth to ensure they understand soil acidity issues throughout the profile, allowing for the most economical method of management to be implemented.
Digging deep into the detail from 182 farms
In the topsoil, 56.5 per cent of samples were above the target pH, 26.9 per cent were between the target and critical pH, and 16.6 per cent were below the critical level.
For subsoil depths of 10-20cm, 72.2 per cent of samples were above target, 16.1 per cent were between target and critical, and 11.7 per cent were below critical.
In the 20-30cm samples, 77.9 per cent were above target, 11.2 per cent were between target and critical, and 10.9 per cent were below critical.
In the deeper subsoil depth of 30-40cm, 76 per cent of samples were above target, 6.7 per cent were between target and critical, and 17.2 per cent were below critical.
Lastly, for the 40-50 cm samples, 83.8 per cent were above target, 5.9 per cent were between target and critical, and 10.3 per cent were below critical.
DPIRD and the Department of Water and Environmental Regulation (DWER).
- Grains Convo podcast: ‘How does liming change soil acidity over 20 years?’ on Apple Podcasts and Spotify.
- Soil Acidity Report Card (2013)
- iLime app