Two step approach with soil testing saves dollars

Two step approach with soil testing saves dollars

BIG SAVINGS: North Star, NSW, grower Jack Pearlman saved himself $50,000 in gypsum costs when a soil test found it would not be economically beneficial.

BIG SAVINGS: North Star, NSW, grower Jack Pearlman saved himself $50,000 in gypsum costs when a soil test found it would not be economically beneficial.


A simple second soil test could save farmers thousands in assessing whether there is a benefit from applying soil conditions like gypsum.


A NEW two-step approach to soil testing could help farmers save money on soil ameliorants by better assessing the need for the treatments.

University of Southern Queensland's (USQ) professor John Bennett is leading a $3.3 million Grains Research and Development Corporation (GRDC) project looking into the best strategies to overcome two key soil constraints, dispersive and compacted soils.

Prof Bennett said a key to the project was understanding that dispersive and sodic soils were not interchangeable terms.

"Growers and farm advisers have tended to use the terms sodic and dispersive interchangeably," Prof Bennett said.

"However, importantly this new research has reinforced that not all sodic soils are dispersive," he said.

He said the project was particularly useful in providing information for growers looking to assess whether their constrained soils needed gypsum to improve soil structure.

At present he said an over-reliance on the exchangeable sodium percentage (ESP) test created costly and ineffective soil management strategies in as much as 38 per cent of cases.

Prof Bennett said growers should continue to do ESP tests but also conduct soil dispersion tests to check whether the sodic soils were also dispersive.

The findings from the five-year project, which also featured the Queensland Department of Agriculture and Fisheries (DAF) and the University of New England (UNE), have delivered new understandings on when gypsum is needed.

Using an ESP test, where the ESP has been greater than five per cent, gypsum application has been recommended to correct dispersed soils.

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Dispersive soils have structural instability which often result in surface crusting; a reduction in infiltration, soil water storage and availability; increased run-off; and associated poor crop emergence and yield.

There is much in common between dispersive sodic soils, which by definition refer to the presence of excess sodium.

However, while sodic soils are often dispersive they are not always and vice versa.

Prof Bennett said this was an important distinction.

He said it meant relying on an ESP test alone as the sole indicator as to whether soil would benefit from gypsum could lead to inaccurate diagnoses.

These incorrect analyses can mean farmers put out gypsum where it does not provide an economic benefit.

As part of their research, Prof Bennett and University of Southern Queensland researcher Stirling Roberton assessed 61 farms.

Of those, 75pc were considered sodic when ESP tests were used. However, only 48pc were found to have sodic soils that were also dispersive soils.

"This is a huge error rate, of all soils assessed as being dispersive using the ESP criterion, 38pc would be incorrectly diagnosed," Prof Bennet said.

"This has economic consequences for growers who apply gypsum where it is not needed and also erodes confidence in gypsum use."

As a result of this new data Professor Bennett and his team are pushing for a new approach to soil analysis.

"An ESP test will still be carried out - to calculate the amount of gypsum needed - but a gypsum recommendation will only be made where dispersion is observed," he said.

Prof Bennett said this two-step approach would result in better testing, diagnosis and management plans.

"Gypsum can cost more than $120 per hectare spread so we need to get it right.

"Comparatively, the cost of dispersion analysis can be as little as $4.5 per hectare, depending on the sampling density," he said.

Northern NSW grain grower Jack Pearlman has already benefited from the research findings thanks to a soil testing trial on his family property near North Star.

The grower, who farms 3300 hectares with his parents, Michael and Kylie, had planned to apply gypsum to about 185ha to address what he believed were sodicity issues at an estimated cost of $300/ha.

But before he applied the gypsum, he sent soil samples to Dr Roberton and Professor Bennett.

"In just 15 minutes, with an aggregate stability test, they saved us more than $50,000," Jack said.

Professor Bennett said the Pearlmans' experience highlights the importance of accurate soil constraint diagnosis before investing in amelioration.

His team is now working with Mr Pearlman and other growers involved in the GRDC on-farm trials to develop variable-rate gypsum plans.

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