WITH global phosphorus (P) supply chain issues continuing and prices for fertiliser expected to stay high in 2022, decisions about P fertiliser application rates are expected to have a greater influence on the budgets of grain-producing businesses than usual next season.
Higher prices for P fertilisers have prompted growers to question whether they should reduce rates to maximise economic returns from applying it, however that answer depends on the level of yield response to P per hectare that is expected.
A current Grains Research and Development Corporation (GRDC) investment, led by Department of Primary Industries and Regional Development (DPIRD) research officer Craig Scanlan, is assessing how soil properties and climate can influence yield response to P fertiliser in wheat.
The research is still in progress, however completed field trials provide an opportunity to examine some economic case studies.
As part of this study, researchers are investigating what precision in P rates are required to maximise economic returns from P fertiliser, with the calculations showing the range of rates that maximise net return, referred to as the optimal range.
The impact of an increase in price for P fertiliser was assessed using three price scenarios for mono-ammonium phosphate (MAP) - $700 per tonne (close to the five-year average), $1050 per tonne (1.5 x average) and $1400 per tonne (2 x average).
The economic analysis was done for five sites that represent the range of responses observed, from very low to very high yield response to P, while the price for wheat (allowing for delivery and levies) was set at its five-year average of $273 per tonne.
Dr Scanlan said the resulted show that the level of yield response to P has a greater influence on the optimal range than fertiliser price.
"For example, doubling the MAP price from $700/t to $1400/t shifted the optimal range from 20-55 kilograms of P per hectare, to 11-56kg P/ha at Boyup Brook where a yield response of 2.27t/ha occurred," Dr Scanlan said.
"At Wandering, where a yield response to P of 0.65 t/ha occurred, the same increase in price shifted the optimal rate from 13-23kg P/ha to 6-22kg P/ha.
"The greatest shift in optimal range occurred at the Northam site, where a response to P of 0.38t/ha occurred, it decreased from 7-27kg P/ha to 2-11kg P/ha.
"At Tammin, where there was no significant yield response to P, the optimum rate was 0kg P/ha under all price scenarios."
The optimal range is quite wide in these case studies as the rate-return relationship is flat across a wide range.
In fact, there is overlap in the optimal range for the three price scenarios.
For example, at the Wandering site a P rate between 13 and 22 would fall within the optimal range for each price scenario.
Dr Scanlan said the wide range of rates within the optimal range demonstrates that growers have flexibility to reduce financial risk without reducing profit.
"The optimal range at Boyup Brook and Brookton are above rates typically used in those areas - we'll know more about how current practice compares to economic optimum when this research is complete," he said.
"The optimal range at Tammin is unlikely to be used by growers so it's more likely that growers will apply P at replacement rates in this scenario to maintain soil P fertility."
The economic impact of applying P rates below the optimum range is dependent on the yield response to P.
Dr Scanlan said the potential for lost income was greatest at the Boyup Brook site, followed by Brookton, Wandering and Northam.
"At Boyup Brook, the average reduction in net return between 10 and 19kg P/ha is $19 per hectare per kg of P," he said.
"At Brookton, the average reduction between 10 and 21kg P/ha is $5/ha per kg of P.
"At Wandering, the average reduction between 0 and 12kg P/ha is approximately $8/ha per kg of P and lastly, at Northam, the average reduction between 0 and 7kg P/ha is approximately $4/ha per kg of P."
While these case studies provide an assessment of the sensitivity of the optimal range of rates to increases in MAP prices, they are not intended to provide recommendations.
Soil testing for nutrients, in combination with assessing different price and yield scenarios, are the best approach for determining the rate required for individual paddocks.
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