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Managing soil acidity to stop its spread in South Australia

Brian Hughes, left, of PIRSA-Rural Solutions SA, and David Woodard, of PIRSA, testing soil acidity at a SA trial site.
Photo: Belinda Cay

Growers are set to benefit from a new collaborative project designed to improve management of surface and subsurface soil acidity.

This is an issue that currently affects more than two million hectares in South Australia and has the potential to affect five million hectares, if not managed.

The collaborative project, 'New knowledge and practices to address topsoil and subsurface acidity under minimum tillage cropping systems of South Australia' (or Acid Soils SA) is a GRDC investment which brings together project partners from:

  • Primary Industries and Regions South Australia (PIRSA);
  • The South Australian Department for Environment and Water;
  • The University of Adelaide;
  • Trengove Consulting;
  • Penrice; and
  • AgCommunicators.

They are researching soil acidification across a range of soil types and farming systems in SA.

GRDC manager of soils and nutrition, south, Dr Stephen Loss, says the project will generate new information regarding lime movement and effectiveness when applied to the surface of different soils and environments in modern farming systems.

"The project team is working to identify, develop and validate novel acidity management practices, such as lime forms, placement and incorporation methods...spading or topsoil slotting," Dr Loss says.

"Eleven new trial sites will be established across SA, including several sites where soil acidity is a newly-emerging issue.

"We are also collaborating with the University of Adelaide on a new PhD project.

"This will investigate complementary products and innovative practices, including: quick paddock methods of determining pH buffering capacity; alternative methods to pH mapping; the impact of seeding systems and fertiliser applications on the movement of lime through the soil profile; and the generation of prescription dyes to help identify acid soils in a paddock."

PIRSA-Rural Solutions SA principal consultant Brian Hughes, who is managing the project, says soil acidification is an unfortunate result of a productive farming system which includes high nitrogen fertiliser use - as well as increased cropping or hay intensity.

New areas affected

In general, the more production, the greater the acidification, but this hasn't had much impact on soils in SA, which tend to be alkaline. However, the issue is now emerging in previously unaffected areas.

"Historically, soil acidity was seen more on sandy soils in high-rainfall grazing and cropping areas, but it is starting to become more of an issue in low and medium-rainfall areas, such as the lower north of SA and the Yorke Peninsula, where soils are not well buffered against pH change," Mr Hughes says.

"Of particular concern is the fact that we are also seeing more issues in the subsurface (10 to 30 centimetres below the surface), even where the topsoil has been limed."

"Soil acidity can be corrected with lime, however total application rates in the state have been well below the rate of annual acidification.

"Between 2015 and 2018 about 100,000 tonnes of lime was applied in SA per annum, but this needs to increase to around 200,000 tonnes to correct current rates of acidification.

"More importantly, about three million tonnes of lime is required immediately to raise current topsoil pH to the critical value of 5.5 measured in calcium chloride."

Of particular concern is the fact that we are also seeing more issues in the subsurface (10 to 30 centimetres below the surface), even where the topsoil has been limed. - PIRSA-Rural Solutions SA principal consultant Brian Hughes

Mr Hughes says many growers in the southern region appear to be unaware of the emerging acidity issue, or are not convinced of the response to justify the cost, time and effort in implementing a liming program.

"If the soil pH is below 5.5, this directly impairs root growth and creates highly toxic forms of aluminium and manganese in soils," he says.

"In addition, the rhizobial symbiosis in most pulses is particularly sensitive to acidity. The impact on yield and productivity can be significant, so management is a must."

Building knowledge

Mr Hughes says the new investment will build on current knowledge and provide important information to growers about the sustainability of their farming systems.

"Over the next three years we will be encouraging growers and advisers to rethink how they test their soils and then manage lime applications to maximise returns," he says.

"Soil pH stratification under no-till and subsurface acidification need different approaches to soil testing and treatment.

"Soil pH should be monitored every five to six years to determine liming rates and application timing.

"It is also important to check the subsurface layer, as acidification occurs down the profile and, once affected, the subsoil is more difficult and costly to correct.

"In a no-till system, we need to work to get the lime down deeper because it has low solubility and moves slowly in undisturbed soils.

"Unfortunately, acidity cannot be easily ameliorated with top-dressed lime under low disturbance systems and strategic tillage may be necessary."

While current knowledge is useful, Mr Hughes says, new data and information which considers rates, the mechanisms for moving lime down the profile and even the quality of the lime will give growers greater confidence in making management decisions.

"Careful management of soil acidity can produce great cost savings for growers in the short and medium term," he says.

"For example, acidity often occurs in patchy areas and mapping acid zones using on-the-go pH mapping and then applying lime with variable-rate spreaders can represent cost savings in the order of 30 per cent.

"The highest cost savings with variable lime applications are on paddocks with a high degree of variability, particularly those with a proportion of neutral to alkaline soils."

GRDC Research Code DAS1905-011RTX

More information: Belinda Cay, 0423 295 576,

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