Southern region growers who are battling acidic soils have been given the latest insights into remediation techniques and technology at a workshop in South Australia.

The workshop presented trial results and research findings from the ‘Acid Soils Southern Region’ project that is funded by GRDC and led by the South Australian Research and Development Institute (SARDI).

The main points shared were about the development of a new method to measure lime in soil, and how this approach can be used to inform growers about what is happening to lime once it has been applied – and to provide insights about when to re-lime.

The new method to measure lime was developed using mid-infrared spectroscopy, which provides a fast, effective and relatively cheap way of measuring many soil properties, according to Ruby Hume, a University of Adelaide PhD candidate whose research into using infrared spectroscopy to monitor soil acidification forms part of the project.

Ms Hume developed a way of measuring concentrations of lime that are relevant in an agricultural context (given that only small amounts of lime should be remaining if it is working to neutralise acidity). “Results showed that it was indeed possible to measure very low concentrations of various lime sources that were present in acidic soil,” she says.

“Following from this, the next component of this research was to use the new method to understand if and how lime moves down the soil profile to treat acidity in subsurface soil layers. Two key pieces of information were measured at high spatial resolution down the soil profile using mid-infrared spectroscopy.

“The first was soil pH, to understand how alkalinity from dissolved lime moves through the soil, and the second was the concentration of undissolved lime (or calcium carbonate) remaining in the soil profile, to understand if there was physical movement of the lime, and also to determine how much of the undissolved lime remained in the soil system to continue to remediate acidity into the future.”

By determining how much undissolved lime remains, in combination with how the soil pH has changed, it is possible to also calculate how much more lime is required in the system (if re-liming is needed and when).

“The information that can be provided using this new approach can provide additional information to growers about where and how quickly lime is working to treat acidity in the surface and subsurface layers, and can also provide insights into when re-applications are required, and which areas of the soil profile should be targeted,” Ms Hume says.

“The new technologies that provide this information continue to become more accessible and affordable as research develops and it is hoped that this extra information will be available to growers and advisers soon to improve the way that acidic soils are limed.”

Acid Soils Southern Region project

The ‘Acid Soils Southern Region’ project, formerly entitled 'Acid Soils SA' and ‘New Knowledge and Practices to Address Topsoil and Subsurface Acidity Under Minimum Tillage Cropping Systems of South Australia’, incorporated a wide range of research into acidification rates, lime movement and effectiveness in modern cropping systems.

The collaborative effort included project partners from SARDI, GRDC, the University of Adelaide, the SA Department for Environment and Water, AgCommunicators, Trengove Consulting and Penrice Quarry and Mineral.

Acid soils have a negative effect on crop development and yield, especially in sensitive crops such as lentils and faba beans, where acidity can inhibit rhizobial survival and nodulation. Low pH can also cause aluminium or manganese toxicity, which are highly damaging to roots and, hence, inhibit the uptake of water and nutrients.

Lime is mildly soluble and moves through the soil very slowly. As a result, surface-applied lime can take many years to move through the first five centimetres of topsoil. In many cases, this has led to stratified soil where the top five centimetres of topsoil is neutral or alkaline and below this the soil is still acidic.

As well as providing resources and recommendations to help growers test their soil pH to depth, a major focus of ‘Acid Soils Southern Region’ has been measuring lime movement and effectiveness in different soil types and exploring novel ways to manage soil acidity, especially in minimum-tillage systems.

The project has demonstrated that growers can use a range of strategies to address soil acidification. Higher lime rates, good-quality lime products and physical incorporation (as opposed to simply topdressing) all helped to increase soil pH relatively quickly, and generally delivered an increase in plant dry matter and yield. The effects were more notable in acid-sensitive crops such as lentils, faba beans and vetch.

The project has engaged more than 1600 farmers, advisers and agribusiness personnel through 42 extension events. An ‘Acid Soils SA’ website was developed to share grower resources, soil pH mapping, fact sheets, liming calculators, lime sources and results from the project.

Precision agriculture shows benefits

One early adopter of precision agriculture (PA) for liming, Wimmera grower David Kuchel, says he is seeing excellent results from variable-rate liming.

David and his family had extremely variable pH on their properties straddling the South Australia-Victoria border, and acidity was a major factor limiting production. Across their cropping areas, pH ranged from 4.5 to 7.0 in some areas.

In recent years, paddock mapping of soil acidity levels and variable-rate application of lime have made a big difference. David now grows crops in paddocks where he previously could not, and enjoys increased yields due to much-reduced soil pH variability.

“We had identified pH as being the main instigator of poor crop growth, particularly in areas where legumes would not grow well. But in the early days, we were just walking around and using manual probes to identify areas that needed lime,” he says.

University of Adelaide PhD candidate Ruby Hume is researching the use of infrared spectroscopy to monitor soil acidification. Photo: Courtesy Ruby Hume

He started by spreading lime at two tonnes per hectare in areas that had been identified as yielding poorly – and quickly saw benefits. “We did that for a couple of years, but there wasn’t much science to it. Then our agronomist told us about precision agriculture (PA) and grid mapping. Five years ago we started doing that in just one paddock, and now we are doing 200 to 300ha a year.”

After soil testing in grid maps of all paddocks (10 soil probes in each 2ha grid), he now applies lime at rates of between zero and 6t/ha, with a target of achieving pH of 6.0 across the property. “We knew that the pH was low, but we didn’t know the rates of lime that we needed to correct it,” he says.

David’s son Benjamin says the ability to combine lime spreading rate information and yield maps has made a huge difference on-farm. “The monitors on the tractor tell us where we have spread the lime, and we are able to overlay the spread maps with yield maps at harvest, which gives us a really good idea of exactly what is going on in each paddock.”

David says he is happy with the amount of “bang for your buck” he gets from variable-rate liming.

“I regard it as one of the easiest PA things to do. All you need is a telephone. We ring the agronomist and ask them to come and do the testing; we ring the cartage contractor to pick up whatever is needed to be picked up; and then you ring the spreading contractor to come and spread it.

“In the early days, when we first started, we didn’t need any expertise or any plant and equipment. Now we have progressed to doing nearly everything ourselves.”

David says the next big-ticket item for soil amelioration on the property is sodicity, and the soil testing he has done so far will still be useful.

“Once we have dealt with soil acidity, we will start working on sodicity with gypsum in the future. So the soil tests will still be of benefit for a long time to come. We have also started playing around with using dolomite to address magnesium deficiency, and have seen huge gains from doing that.”

David grows a three-year rotation of wheat (Vixen and Calibre ), canola (Clearfield varieties) and faba beans (Samira ) in roughly equal proportions. “We used to cut hay for ryegrass control, but now that our chemicals are working better, we no longer do that. It may change in the future, but for the time being we are happy because we don’t love cutting hay.”

David and Benjamin are keen to stay involved with PA technologies, with green-on-green weed seekers the next possible area for investment. “Moisture conservation is high on the agenda, as well as staying across carbon laws going forward, particularly for electricity usage in the irrigation side of the business. We are also looking at putting in some solar energy equipment in 2023.”

More information: David Kuchel, [email protected]; Nathan Tink, [email protected]; Ruby Hume, [email protected]

Read also: Project raises the pH outlook for southern soils.