A three-year GRDC investment aimed at raising awareness of soil acidification issues in South Australian cropping systems has delivered significant knowledge about acid soil management and treatment.

‘Acid Soils SA’, formally entitled ‘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 the South Australian Department of Primary Industries and Regions (PIRSA), its research division, the South Australian Research and Development Institute (SARDI), GRDC, the University of Adelaide, the South Australian Department for Environment and Water, AgCommunicators, Trengove Consulting and Penrice Quarry and Mineral.

Background

In 2018, the South Australian Department of Environment, Water and Natural Resources estimated that about two million hectares of agricultural land in SA had a soil pH (measured in calcium chloride) of 5.5 or below in the top 10-centimetre layer, with a further two million hectares also at risk of becoming acidic over the next four decades.

Acid soils have a negative impact 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.

Soil acidification is a natural consequence of productive farming systems where alkaline products are exported, and the acidity left behind needs to be managed with regular applications of lime. The Department for Environment and Water has predicted that at least 200,000 tonnes of lime are needed each year to neutralise recent rates of acidification across SA.

However, between 2015 and 2018 only about 100,000t per annum was applied. It has been estimated that about three million tonnes of lime are required to raise the state’s cropping topsoil pH to at least 5.5 so that acidity is no longer a problem for most crops and pastures.

Lentils are acid sensitive and generally demonstrate a positive response to lime treatments (right). Photo: Brian Hughes

PIRSA soil scientist Brian Hughes, who has been the project leader on Acid Soils SA, says product removal through grain or hay and fertiliser inputs contribute to soil acidification.

“Removing harvest products takes alkaline products such as calcium and magnesium out of the paddock,” Mr Hughes says. “Meanwhile, adding nitrogen fertilisers, especially those containing ammonia and – to a lesser extent – urea, increases acidity, especially if that nitrogen is not used by the crop and lost through leaching.”

As a rough rule of thumb, Mr Hughes says, each hectare of acid-prone soil should be treated with 10 kilograms of lime per tonne of wheat removed, while each kilogram of monoammonium phosphate (MAP), diammonium phosphate (DAP) or sulfate of ammonia fertiliser should be matched with a kilogram of lime.

“Farmers should plan to apply between 1.5 and 3t of lime per hectare every 10 years on high-yielding paddocks to neutralise acidity from the cropping system and they might need to also catch up with significant lime application from previous decades’ practices,” he says.

Testing lime types and incorporation methods

Lime is only mildly soluble and moves very slowly through the soil. As a result, surface-applied lime can take several years to move through the first five centimetres of topsoil. In many cases, this has led to stratified soil where the top 5cm of topsoil is neutral or alkaline and below this the soil is still acidic.

Along with providing recommendations and resources to help growers test their soil pH to depth, a major focus of Acid Soils SA 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 Acid Soils SA team has helped establish lime trials on different soil types across SA, allowing the project team to study liming techniques on a range of crops, systems, soil types and environments.

The trials compared applying lime from several sources and at several different rates, including double the recommended rate per hectare to establish whether heavier applications moved into the subsoil more quickly. Numerous trial sites tested the effectiveness of physically incorporating lime to better understand the benefits of soil amelioration or strategic tillage strategies for correcting soil acidification.

Complementary product applications were tested to see whether applying lime with additives such as manure, biochar or clay could support its rate of movement into the soil.

The project also monitored older trials dating back to 2015, which provided useful insights into the long-term movement and persistence of lime in the soil.

The trial plots were planted with a range of crops, including legumes (faba beans, vetch and lentils), barley, oats and wheat to compare the effects of soil pH and liming strategies on grain yield.

Project findings and extension

The project has indicated that growers can use a range of strategies to address soil acidification on their farms.

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.

Application rate

In minimum-tillage systems where the topsoil pH was about 4.5 or less, application rates of 6t/ha or more led to better movement into the soil profile than lower rates of the same product.

The Acid Soils SA project has improved knowledge of lime movement under high-rainfall conditions. Trials in drier environments and on sandy soils will need more time to deliver a similar level of understanding.

Complementary additives

At a small number of sites, the researchers also trialled applying clay, biochar clay, gypsum or manure in combination with lime, or as standalone products, to determine whether these additives could enhance the rate of pH change in minimum-tillage systems.

At a few sites, applying gypsum with lime improved pH to a greater depth than applying lime alone. However, high rates of gypsum alone were also linked to a greater decline in molybdenum content in plant tissues in lentils. This contradictory impact of gypsum required further work to determine which impacts continue over time.

Manure was observed to increase soil pH by up to 0.6 units, but it is unclear whether the effect is long-lasting. More generally, the inclusion of chicken manure at Mallala and cow manure at Spalding did result in increased dry matter at both sites. However, plots treated with fertiliser alone had a similar result, which indicated a possible response to added nutrients (in particular, nitrogen) rather than improved soil pH.

Adding clay was particularly beneficial on sandy country, with soil pH, seed germination, dry matter production and yields all improving substantially.

Biochar was not observed to have any benefit for lime movement or soil pH.

Given the limited number of trials, further monitoring is needed to refine recommendations for these complementary additives to lime.

Physical incorporation

An important element of the Acid Soils SA liming trials has been the exploration of physically incorporating lime to reverse soil acidification – especially in relation to acidification in the subsurface layers (deeper than 10cm).

Physical incorporation using rotary hoes, ploughs, spading and deep ploughs have all indicated significant and accelerated benefits for soil pH when compared to the naturally slow movement of unincorporated lime.

This is an important result for the southern region’s many no-till growers. Ideally, lime application should commence early in no-till systems to avoid subsoil acidity. However, if soil pH is allowed to fall below 5.0, especially in subsurface layers, some form of strategic tillage might be the best approach to rectify the issue quickly.

Careful timing is important to prevent erosion and minimise the impact on soil moisture and soil health. Applying lime shortly before seeding will allow the seeder to provide some physical incorporation and take advantage of autumn rainfall to help move the lime into the soil profile.

Using spectroscopy to measure soil pH

The Acid Soils SA project was joined by University of Adelaide PhD candidate Ruby Hume, who conducted research into using infrared spectroscopy for monitoring soil acidification.

Spectroscopy can detect carbonates and other minerals in the soil and plant matter at very low concentrations (below 1 per cent) and in real time.

“Calcium carbonate has a very strong infrared signature, so spectroscopy is ideal for seeing how far lime has moved into the subsurface layers,” Ms Hume says.

“It is a relatively simple, low-impact alternative to intensive soil sampling by a laboratory, which can be expensive and is not always practical.”

Ms Hume’s research compared her spectroscopic readings with laboratory-verified measurements of organic carbon, pH, aluminium, pH buffering capacity and cation exchange capacity in both soil and plant samples, to ground-truth and calibrate the results.

“Infrared spectroscopy has the potential to measure a wide range of chemicals and nutrient profiles in both soil samples and leaf tissue,” she says. “With appropriate data models, farmers could also use it to assess soil properties such as organic carbon, carbonate and clay content, and pH buffering capacity.”

Grower engagement and lime usage

The project has engaged more than 1600 farmers, advisers and agribusiness personnel through 42 extension events. An Acid Soils SA website was also developed to share grower resources, soil pH mapping, fact sheets, liming calculators, lime sources and results from the project. It receives about 100 page views per month.

These results reflect strong interest in soil acidification and remediation, as well as growing awareness of the issue among SA growers.

More importantly, there has been a strong uptake in demand for lime from SA suppliers, indicating that more lime is being applied now than when the Department of Environment, Water and Natural Resources announced its findings in 2018 and since the Acid Soils SA investment was initiated in 2019.

Andrew Graetz from Penrice Quarry & Mineral says demand for the company’s Penlime Plus™ superfine product has increased strongly over recent years.

“Sales of Penlime Plus™ have nearly doubled over the last three years and our customers also seem more aware of how lime and the release of fertiliser nutrients can benefit from improved soil pH,” he says.

“We’re seeing greater engagement with agronomists and use of technologies like pH mapping and variable-rate application to maximise the value of the lime that is applied.”

Further areas of study

As a result of the Acid Soils SA project, traditional rule-of-thumb approximations between lime treatments and pH change are being validated and relationships between percentage relative yield and soil pH adjustment are being developed for SA soils and crops.

Although the term of initial investment is ending in 2022, ongoing research and extension will be supported in a new three-year GRDC investment that combines efforts in SA and Victoria.

While a handful of new trial sites will be instigated in 2022 and 2023, the new investment will continue monitoring established trials for crop responses and soil impacts in the medium to long term, leading to improved recommendations and decision support tools for acidity management.

While parts of SA have alkaline soils, acidity is emerging in new areas such as sandy zones within paddocks. As a result, ongoing extension and communication are considered necessary to continue raising awareness of acidity issues.