Research into correcting soil acidification in cropping systems is showing that using different lime application techniques can suit differing soil types and farming practices.

After several years, the study has revealed economically significant third-year responses in pulse crops and indicated that lime might not always need physical incorporation to deliver the greatest benefit.

Supported by GRDC and the Department of Primary Industries and Regions South Australia (PIRSA), the ‘Acid Soils SA’ project started in 2019 and now includes trial sites across South Australia. Results from the 2020, 2021 and 2022 growing seasons have revealed economically significant third-year responses to lime in lentils and vetch, which are generally more sensitive to soil acidity, when compared to the control treatments.

Project leader Brian Hughes, who is the soils team leader at the South Australian Research and Development Institute (SARDI), says applying lime with physical incorporation was often linked to a positive pH response within 12 to 24 months following application. However, this response appeared to be in decline by year three at some locations.

“While the trials showed incorporating lime through strategic tillage can help to remediate subsurface soil acidity relatively quickly, allowing lime to move into the soil from surface application naturally has had similar responses at some sites in years three and four,” he says.

Soil type, incorporation method and pH profile all appear to influence how lime affects acidity and yields over time.

At Mallala, in the Lower Mid North of SA, a trial site was established in 2020 with treatments including lime, dolomite, biochar and gypsum, with and without incorporation. The soil at the trial location comprised coarse sandy loam 10 to 15 centimetres in depth over a reddish medium clay.

Lentils (PBA Hallmark XT ) were sown in year one (2020) and assessed for visual growth responses in July and October. Compared to the controls, these assessments showed dry matter increases of up to 0.7 tonnes per hectare for the manure, fertiliser matched to manure, and cultivated lime treatments. At harvest, the yield results followed a similar pattern with gains of up to 0.4t/ha over the controls.

A wheat rotation (Sheriff CL Plus ) was grown in year two (2021). This crop was given a nitrogen and phosphorus starter fertiliser but no follow-up nitrogen due to dry spring conditions. In-season assessments of biomass indicated some positive response to the incorporated lime treatments, but these were not observed at harvest, where yields were not different to controls.

For year three (2022), the site was again sown to lentils, with consistent lime response patterns carrying through from in-season dry matter and visual assessments to the final harvest results.

“Assessments through 2022 showed the control, biochar and sulfur (to demonstrate continuing soil acidification) treatments had consistently low normalised difference vegetation index (NDVI), visual, dry matter and yield scores,” Mr Hughes says.

“However, the medium to high-rate lime treatments (3t/ha and 5t/ha), both surface-applied and cultivated, delivered a yield response of up to 0.85t/ha above the control, for an economic benefit of about $600/ha based on 2022 grain prices.”

The results were similar for another trial at Sandilands, on the Yorke Peninsula, SA. This site was established in 2019 and treated with surface-applied and incorporated lime at a range of application rates.

The soil at this site is a coarse, sandy loam up to 25cm deep with some ironstone and quartz gravel content, over a reddish medium clay. The zero to 5cm and 5 to 10cm layers exhibited very low pH and aluminium in the plant toxic range when the trial treatments were applied.

In year one (2019), the site was sown to wheat (Scepter ), which is moderately tolerant of soil acidity. In-season biomass and post-harvest yield assessments revealed no significant difference between any of the surface lime treatments, while cultivated lime delivered notably less benefit.

In year two (2020), the site was sown to lentils (PBA Hurricane XT ) with the seed being inoculated on the day of sowing. Plant analysis of the lime treatments showed near-toxic levels of manganese while levels of zinc, sulfur and aluminium had been reduced. Nodulation appeared better where lime had been incorporated.

Yield response

Following a dry winter and early spring, rain in September and October supported yield responses of up to 30 per cent or 0.3t/ha better than the control across the lime treatments. In contrast, ripping without added lime only performed marginally better than the control.

The site was sown to wheat (Yitpi , which is considered more sensitive to acidity than Scepter ) for year three (2021). Most of the lime treatments showed a significant growth response compared to the control, although the difference between the surface-applied and cultivated lime treatments was less than had been observed with lentils the previous year.

At harvest, yield gains of up to 17 per cent (about 0.5t/ha) above the control were observed across the various lime treatments. Generally, yields from cultivated treatments were slightly lower than yields from the surface-applied treatments.

For year four (2022), the site was again sown to lentils (PBA Hurricane XT ). Dry matter analysis in October showed a benefit from all lime treatments, with only the low-pH sulfur treatment performing below the control.

“At harvest, all the lime treatments performed better than the control, while unlimed tillage and ripping performed worse,” Mr Hughes says.

“The control yielded just under 1.1t/ha, while the lime treatments yielded up to 0.3t/ha or about 27 per cent better.”

The soil profile at the Mallala, South Australia, site. Photo: SARDI

As at Mallala, physical incorporation provided no clear benefit after several years had elapsed. In fact, high rates (6t/ha) of surface-applied lime performed marginally better than the same treatment with tillage.

In contrast, the low-pH sulfur treatment yielded up to 0.7t/ha worse than the lime treatments, clearly demonstrating the economic cost of allowing soil acidification to continue without correction.

“What is really interesting is that the physically incorporated lime treatments were not performing significantly better than surface-applied treatments after a few years at Mallala or Sandilands,” Mr Hughes says.

“On these soil types at least, it is looking like incorporation may only be worthwhile if soil pH is less than 5.5, particularly at depths below 10cm, and needs to be corrected quickly.

“This could be good news for farmers on a no-till cropping system, as long as they monitor their soil pH and apply lime regularly to maintain soil pH in a healthy 6.0 to 7.0 band.”

Mr Hughes is careful to point out that the results from Mallala and Sandilands might not be consistent across all regions and soil types, and could even vary with different types of incorporation and soil.

For example, trials on sandy soils with a deeper acid layer at Yumali found lime incorporated by rotary hoe was still giving a better response than surface-applied lime after three years. Planted to vetch for the 2022 season, lime treatment yields at this site were up to 50 per cent higher than the control. Claying delivered a similar result to lime, while the benefits from deep ripping had clearly declined.

Time is emerging as an important consideration in the management and correction of soil acidity. Allowing lime to move through the soil naturally appears to offer a lasting improvement in soil pH, while physical incorporation might be beneficial for severe acidification or subsurface acid banding.

“Soil type, incorporation method and pH profile all appear to influence how lime affects acidity and yields over time,” Mr Hughes says.

“Growers should consult with their agronomist for advice relating to their specific growing conditions and use the resources on our website to keep up with the growing body of knowledge in this area.

“As we continue to gather data, we expect the interactions between different factors to become more clear.”

The results from Mallala and Sandilands will be compared to data from other sites and treatment methods, and data from different growing environments, as the project continues.

The GRDC-funded ‘Acid Soils SA’ project has been extended until 2025 as the ‘Acid Soils Southern Region’ project, with additional trial sites in Victoria. These will provide insights into soil acidity and liming strategies for a wider range of soil types and growing conditions across the south, while the SA sites continue to deliver long-term results.

Updates, information and grower resources are available online at acidsoilssa.com.au