When Geoff Minchin wanted to explore and ameliorate soil constraints to lift profits, he enlisted the help of Eva Moffitt, a senior research officer at FarmLink Research.
In 2017, Precision Agriculture (PA) did an electromagnetic survey on 200 hectares of Mr Minchin’s 1000-hectare farm at Ardlethan, in southern NSW.
Mr Minchin says the 200ha block was representative of many of the soils in southern NSW and comprised a heavy gilgai clay with mounds and depressions, a hard-setting red clay and a duplex loam.
In 2018, an intensive round of ground-truth soil sampling was undertaken on a 49ha sub-area of the block in a joint project between FarmLink Research and PA.
The sampling included a 2ha zero-to-10cm grid survey, in addition to: 5cm segmented samples to 20cm; deep core samples to 100cm; and Emerson’s dispersion tests performed at every second grid point.
Mr Minchin and Ms Moffitt expected to see some sodicity with high exchangeable sodium percentages (ESP) because parts of the block had previously displayed dispersive characteristics.
Although ESP was elevated at depth, none of the topsoils showed values of more than six per cent, the standard threshold for sodicity.
Ms Moffitt says ESP values do not always provide the best representation of where dispersive symptoms exist in soils.
“I’ve had grid ESP maps come back and the grower has said they don’t match up with where they understand the dispersive soils to be,” she says.
“ESP works well for some soils however I don’t think it works in all cases because sodium is just one factor out of a number of factors that dictate whether a soil will disperse or hold together.”
In her experience, Ms Moffitt says many advisers only look at ESP when suggesting gypsum rates to their clients.
“Suggested application rates also tend to be general due to the lack of a scientifically sound gypsum calculator,” she says.
Meanwhile, Mr Minchin discovered research data that pointed to high magnesium and high potassium as potential causes of soil dispersion.
Another interesting element for Mr Minchin was finding a stratified acidity problem.
“The zero to 5cm band was more alkaline than the five to 10cm layer; and in the lighter soils, acidity was more of an issue than dispersion,” Ms Moffitt says.
“Low pH was also found at 10 to 15cm in the lighter soils, but the higher CEC (more buffered) soils were less acidic in the subsurface.”
In the 15 to 20cm layer across all soil types, soil pH increased.
“Considering most of the paddock had a pH of 5.0 in the zero to 10cm layer, we were surprised to see a pH band of between 4.4 and 4.9 in the five to 15cm layer across two-thirds of the area tested,” she says.
The zero to 5cm band was more alkaline than the five to 10cm layer; and in the lighter soils, acidity was more of an issue than dispersion.
Ms Moffitt suspects subsurface acidity is a far bigger problem across the Riverina region than many people think.
“The standard zero-to-10cm soil test does not provide a good representation of pH variability throughout the profile,” she says.
In her work over the years, Ms Moffitt has witnessed the full range of soil constraints, with some soils proving highly dispersive and others highly acidic.
“There is also a large area of soil that is slightly acidic but also dispersive,” she says.
“We are grid sampling to help growers find out where to put lime, but they also want to know where to put gypsum.”
After discussion with Mr Minchin, Ms Moffitt developed two maps that enabled lime and gypsum to be differentially applied (see Figure 2).
Lime was earmarked to be applied at rates between nil and 3 tonnes per hectare, targeting acidity amelioration to a depth of 15cm.
Gypsum was to be applied at rates from nil to 1.4t/ha with a focus on improving crop establishment success in areas demonstrating weakly dispersive characteristics.
“In the future, we hope researchers will develop a new test that we can build into a formula to create variable-rate gypsum maps based directly on the dispersiveness of the soil,” Ms Moffitt says.
“A new test would add more objectivity to the job of recommending gypsum rates to ameliorate soils.”
GRDC continues to make significant investments into soil amelioration research and strategies to address varying constraints in different soil types.
Originally published by GRDC Ground Cover.
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