- Nitrogen application is usually more profitable and less risky on sandy dunes than heavy swales
- In recent long-term trials at Karoonda, SA, high nitrogen upfront gave the highest profitability for dunes, while low fertiliser addition provided the best result for swales
- Applying no nitrogen on the heaviest swale soils has been the most profitable approach for the past five years, but is not likely to be sustainable
- Zoning of paddocks by soil type allows for variable-rate nitrogen application to provide the best returns and risk management
Trials at Karoonda in the Mallee have shown big differences in profit and risk from nitrogen application on the dunes (foreground) compared with the swales (background).
PHOTO: Bill Davoren
GRDC-funded research into soil-specific nitrogen strategies in the Mallee has found it is not always the highest-yielding areas that provide the highest profitability from additional nitrogen application.
A recent CSIRO and Mallee Sustainable Farming trial near Karoonda, South Australia, investigated options to zone fertiliser addition by soil type.
CSIRO farming systems scientist Rick Llewellyn says the Mallee was ideal to investigate fertiliser zoning.
CSIRO farming systems scientist Dr Rick Llewellyn.
PHOTO: Brad Collis
“Our trials reflect the way growers are increasingly managing their paddocks. The Mallee currently has the highest rate of variable-rate fertiliser application in the country, due to the large range of different soil types experienced in single paddocks and properties. Using a combination of field trials and simulation modelling, we tested the theory that it would be profitable and better for risk management to zone according to responsiveness to nitrogen,” Dr Llewellyn says.
The trial is in its sixth year and compares the district practice of a constant fertiliser rate, applied at sowing, with higher and lower rates of nitrogen application, across a range of soil types.
Widely different results were observed across the soil types.
“We found that on the heavy flats the lowest possible rates of nitrogen have been most profitable over the past five years. In these soil types, there is opportunity to reduce nitrogen applications compared to district practice rates,” Dr Llewellyn says.
“But we also saw really good returns on nitrogen fertiliser investment on a lot of the sandier soils. This was particularly interesting because the sandy soils provided lower yield and profitability than the swales in recent years, but the nitrogen benefit was higher. This is different to some traditional thinking that the focus of nitrogen application should be the most profitable areas.
Nitrogen management resource for growers
A new booklet released as part of the GRDC’s Stubble Initiative provides growers with advice for managing nitrogen in stubble-retained systems. With the continuing increases in no-till farming, more focus is required on nitrogen levels, as the soil bacteria and fungi that break down stubble compete with crop plants for nitrogen, which reduces the nitrogen available to the plant.
Growers can estimate that for every tonne of cereal or canola crop harvested, five kilograms of nitrogen will be tied up by the stubble. This tie-up of nitrogen needs to be accounted for in the farming system and compensated for to ensure yields in the following year are not compromised.
The new booklet, Nitrogen management in stubble retained systems, addresses this required compensation by providing a simple reference guide to nitrogen management in four stages: soil testing, results assessment, application of nitrogen at sowing and top dressing.
The booklet is available from Farmlink Research
“We did note that while the yields were good on the swales, the protein levels in the ‘nil fertiliser’ treatment were starting to drop, which indicates persistent lack of nitrogen fertiliser would not be sustainable, even on the heaviest swales and regular monitoring of starting nitrogen levels is needed.”
Based on these findings, Dr Llewellyn says there is a very strong case for variable-rate fertiliser in the Mallee for both profit and risk management.
He recommends growers in the Mallee zone their paddocks, including soil testing to characterise their soils, and then – based on their total nitrogen budget – allocate nitrogen, with priority given to the sandy zones where return on investment is likely to be highest and most reliable.
Dr Rick Llewellyn,
08 8303 8502,
02 6980 1333,
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