AGRICULTURE in the South West has a long history of phosphorus fertiliser application, but much of the knowledge base used to support its use is derived from cropping systems that differ considerably to those of today.
The long-term adoption of no-tillage alongside big changes in land use, such as the rise of cropping paddocks and a decrease in legumes, make it timely to revisit what is driving the response to phosphorus in our modern farming systems.
In a recent study, Department of Primary Industries and Regional Development (DPIRD) senior research scientist Craig Scanlan and the project team found the phosphorus buffering index (PBI) determined which properties affected the wheat response to phosphorus fertiliser.
In WA, lighter or sandy soils typically have a lower PBI, meaning there is more phosphorus available for the crop to access and as a result a lower level of soil phosphorus is required to meet crop demand.
Rainfall was a big factor in crop performance, specifically the amount of rainfall in the period of five to two weeks prior to seeding.
If less than eight millimetres of rain hit the paddock in this period, a larger crop response to phosphorus fertiliser was seen at harvest.
If rainfall was greater than 8mm in the five days to two weeks before seeding, the crop response was weaker and soil pH became an important factor.
"When there was a wetter start to the season before the crop went in, we found a relationship with rainfall and soil acidity," Dr Scanlan said.
"If soil pH was less than 5.1, the yield response was greater than if soil pH was higher than 5.1."
Dr Scanlan said one of the key benefits of this research is the inclusion of economic analysis, given the recent price spikes in phosphorus fertiliser, which means the optimal application rate was on the mind of a lot of WA farming businesses.
"By developing this research we can much better understand managing phosphorus for profitability," he said.
"We're seeing the biggest influence on optimal rate is the level of yield response to phosphorus fertiliser.
"At the higher end of yield responses, in the realm of greater than one tonne per hectare response, the optimal rate is relatively insensitive to the price of fertiliser.
"However, at lower response levels of less than 250 kilograms per hectare, the optimal rate is much more price sensitive."
A PBI value of 50 marked a key threshold at which different factors were driving the response to phosphorus fertiliser at different sides of this threshold.
On soil with a PBI greater than 50, the wheat yield response was most closely related to soil chemical property DGT-P.
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Where the PBI values was less than 50, the crop response to phosphorus fertiliser was driven by climate and soil acidity, measured by pH.
Despite discovering that other climate and soil properties such as DGT-P were closely related to the response to phosphorus fertiliser, there were still some scenarios where Colwell P remained an important component of the toolkit growers and advisers can use when making fertiliser decisions.
"On soils with a PBI of less than 20, which is that very low range, we found good calibrations of yield response and soil Colwell P value when sampling 0-30cm of soil," Dr Scanlan said.
He said he hoped more research on phosphorus would be able to build on the new knowledge generated from this four-year trial.
"Future research on phosphorus needs to continue to take climate into account so we can better quantify risk from a grower perspective," he said.
"This is the first time we've been able to create an in-depth dataset that brings together both soil properties and climate factors to dig deeper into what is driving the response wheat has to phosphorus fertiliser and it means advisers and growers can have more confidence in their decision making and profitability.
"These findings were a result of 40 field experiments set up and monitored over four years as part of a major GRDC investment in crop nutrition through the SoilsWest alliance, which brought together DPIRD, Murdoch University, The University of WA, Curtin University, Summit Fertilizers and CSBP, alongside growers who participated in the trials."
Dr Scanlan said the partnerships made the work's scale and speed possible and the research a reality.
"This kind of collaboration allowed us to achieve coverage of the whole Wheatbelt, including the north-south and east-west gradients of soil type, rainfall, and yield potential," he said.
"It's thanks to this level of investment and support from growers with industry that we can be really confident in the results we're seeing."
Each trial site had the same design and sampling protocol.
Crucially, the inclusion of climate measurements was a valuable new dimension to the analysis in comparison to a previous research.
Dr Scanlan and others on the team applied a suite of analysis tactics to untangle which soil or climate factors were influencing the response of wheat to phosphorus and how these factors might interact with each other.
"Because we wanted to investigate the role of climate we applied a novel analysis technique called the 'sliding window approach' that was initially developed for ecology research, which gave us much more insight into what was going on," he said.
"We were able to look at the influence of soil properties on their own, climate properties on their own and then finally how soil and climate interact to drive different levels of crop response."
More information and data analysis from this research is in the GRDC Update paper and a peer-reviewed scientific article will be published soon.
Jasmine Peart
Jasmine is a earnest listener and a passionate advocate for justice. She feels privileged to be trusted by farmers across Western Australia to tell their truth.
Jasmine is a earnest listener and a passionate advocate for justice. She feels privileged to be trusted by farmers across Western Australia to tell their truth.
