Strategic soil testing and liming in summer and early autumn is needed to address the emerging problem of surface pH stratification and subsurface acidification in previously unaffected areas of South Australia.

More than four million hectares of land in SA is now considered prone to acidification. This is where pH has declined to critical levels, causing patchy plant growth and reduced grain yields - particularly in sensitive crops, such as beans, lentils and barley.

Detecting and remediating soil acidity prior to sowing winter crops gives growers the chance to address emerging acidity concerns.

Making a plan

Primary Industries and Regions SA (PIRSA) Principal Consultant for Rural Solutions, Brian Hughes, says growers and advisers need to rethink how they soil test and manage lime applications to maximise returns.

"Soil pH stratification and sub-surface acidification under no-till farming systems needs different approaches to soil testing and treatment," he says.

"Where acidification develops deeper in the soil profile, it is more difficult and costly to correct."

Mr Hughes, who is managing a collaborative project with investment from the GRDC - which is aiming to improve the treatment of surface and sub-surface soil acidity - says consultation of yield maps and soil testing to depths of up to 20-30 centimetres is sometimes needed to identify acid sub-surface areas.

Type of system

Mr Hughes says in a no-till system, there is a need to better understand how to get top-dressed lime down deeper in the soil profile - because lime has a low solubility and moves slowly in undisturbed soils.

"This type of system is commonly associated with the formation of an acid layer - or 'acid throttle' - in the soils, which restricts root growth and crop yields - particularly in sensitive crops such as pulses," he says.

"When soil testing after liming in a no-till situation, it is useful to examine pH at 5 cm intervals (e.g. 0-5cm, 5-10cm, 10-15cm) to determine where the lime has moved and the presence or not of an acid throttle."

Mr Hughes says mapping soil pH in paddocks will assist in the management of acidity, as lime can be applied in areas where it is most needed. He says variable rate spreaders are particularly effective in this situation.

Strategic tillage can be used to help get the lime further down the soil profile where applicable.

"Rainfall, soil texture, organic matter and lime quality all affect the movement of lime down the soil profile," Mr Hughes says.

What the future holds

The Acid Soils SA project is working towards developing new data and information regarding liming rates and mechanisms for moving lime down the profile to give growers greater confidence in making management decisions.

Trial sites are being established at 11 locations across SA where soil acidity is an emerging issue.

There is also work being done to identify, develop and validate novel acidity management practices, such as lime forms, placement and incorporation methods - including spading or topsoil slotting.

Complementary amendments will be examined at some sites.

The project is being conducted with input from:

• Primary Industries and Regions SA,
• the South Australian Department for Environment and Water,
• the University of Adelaide,
• Trengove Consulting,
• Penrice
• AgCommunicators.

More Information: Bridget Penna, AgCommunicators, 0429 676 413.