Grains Research and Development

Date: 31.08.2015

Dig deep to understand soil acidity

Author: Rebecca Barr

Image of Lisa Miller

Southern Farming Systems project coordinator Lisa Miller says that growers need to sample below 10 centimetres to better understand soil acidity.

PHOTO: Bianca Hudson

An extensive soil survey in Victoria has shown that typical soil-sampling methods may not be providing growers with sufficient information to manage soil acidity. Southern Farming Systems is examining soil acidity and lime response as part of a GRDC project, ‘Soil acidity is limiting grain yield’, in south-west Victoria, New South Wales, South Australia and Western Australia.

Project coordinator Lisa Miller says a survey of Victorian soils in the first year of the study showed that a significant percentage of soils were acidic below 10 centimetres.

Acidity sampling and targets

  • Sample approximately every three years.
  • Sample at 0 to 10 centimetres, 10 to 20cm and 20 to 30cm.
  • If subsurface sampling shows the soil pH is greater than 4.8, then the target pH in the 0 to 10cm soil layer is greater than 5.
  • If subsurface sampling shows that the soil pH is less than 4.8 then the target pH in the 0 to 10cm soil layer is greater than 5.5.
  • Do not sample burnt windrows as burning can increase pH by 0.3 to 0.8 units, which can give an incorrect picture.
“Growers have generally sampled soil pH in the top 10cm. We looked at soil samples to depths of 0 to 10cm, 10 to 20cm and 20 to 30cm in the south-west Victorian Corangamite region to get a more detailed picture,” Ms Miller says.

“We found that 59 per cent of soils had a topsoil pH (CaCl2) that was less than five, and 26 per cent had a subsurface pH that was less than 4.8 at a soil depth of 10 to 20cm.

“Going deeper, 17 per cent of samples were still below 4.8 at 20 to 30cm and 28 per cent of soils had acidic subsoil or B horizon (generally 30 to 90cm).

“This highlighted that sampling just 0 to 10cm doesn’t tell the full story. Growers who want to manage acidity should also sample the subsurface soil layers. Sampling 0 to 10cm, 10 to 20cm and 20 to 30cm provides a good representation of soil acidity and can pick up the formation of any acidic layers. An ideal time to soil sample is when growers are also taking samples to assess deep nitrogen.”

In a soil with pH less than 4.8, the type of aluminium available is in a toxic form, which stunts roots and reduces crop performance. The topsoil pH might be above 4.8 and aluminium levels may be negligible, especially if lime has been applied, but the soil layer from 10 to 20cm could have a toxic layer, which roots have to grow through before they reach soil with lower acidity.

Research from WA has shown that where subsurface soil layers are acidic, a pH of at least 5.5 is required in the topsoil before lime can move below 10cm in the soil. While the target pH to combat soil acidity is greater than five in topsoils, if there is also subsurface acidity the target must be lifted to at least 5.5.

The project is also assessing the response to liming over four years. “We’ve recently gone back and looked at some old trial sites, where lime was applied three years ago,” Ms Miller says. “The results were an eye-opener for us – after three years, the lime had only neutralised the acidity and changed pH within the top 10cm. I’ve since looked at other studies and found this is not unusual.

“Either all the lime has been used up neutralising acid in the top 10cm of the soil because the surface pH was less than 5.5 or it takes more than three years for lime to reach beyond 10cm under minimum tillage, and it takes even longer for the lime to reach the acidic subsurface layers.

“As a consequence, we recommend growers start adjusting soil pH well before it is limiting production, as there may be a four-to-seven year delay from when it is applied to when the subsurface soil layers respond.”

Rather than running with a set program, such as the traditional approach that involves applying lime at 2.5 tonnes per hectare every 10 years for Victorian soils, growers are advised to test soil pH every three years so lime can be applied early and rates adjusted if needed; for instance, when either the topsoil or the subsoil are close to the minimum pH target.

Over the next three years, the project aims to provide more information on crop responses to lime applications, including assessing whether there are benefits from small and frequent additional lime applications and identifying any negative consequences of applying lime at rates that are too high.

More information:

Lisa Miller, Southern Farming Systems,
0488 600 226,
lmiller@sfs.org.au

See the soil constraints Ground Cover Supplement with this issue.

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