Phosphorus demands dictated by soil's ability to respond

Trengove Consulting's Sam Trengove, Bute said improved mapping techniques could help direct the use of P, beyond simple replacement to get the best crop outcomes.

"P is one of the biggest ticket items and anything we can do to make its use more efficient, should improve the bottom line," he said.

Mr Trengove said for about a decade on from 2011, he had been collecting tissue samples while out with clients, pairing them between the high performing and poor performing areas in the paddock.

In that time he collected 104 paired samples, which showed that in 62 per cent of cases, the poor performing zone had low P.

"P deficiency was the main nutrient popping up on a consistent basis," he said.

After investing in a Veris soil pH mapping machine in 2015, Mr Trengove said he had increasingly been looking at the soil pH data, and began to notice some trends.

"It was a lightbulb moment for me when you see the increase in soil pH connected with decreased crop vigour," he said.

He said high pH soil was quite often associated with reduced availability of P due to P fixation, or tie up.

"As we move into soils above pH 7, the general trend is that phosphorus becomes harder to access for the crop," he said.

Using funding from the SA Grains Industry Trust, he was able to host field trials to check this in 2019-20.

After initially aiming for four paddocks, each with four trials - or 16 plots - they ended up with five paddocks and 21 plots across the upper Yorke Peninsula and Mid North.

Each trial site assessed the crop response to P, with rates ranging from zero up to 50kg P/ha.

Mr Trengove said the trial sites had a good cross-section of high, neutral and lower pH areas within the paddock as well as data from the normalised difference vegetation index showing high and low performing zones.

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He said the trial sites at Bute had performed as expected in 2019, with a high response to additional P applications in some zones.

"We were able to increase yields in responsive zones by over 1t/ha tonne a hectare compared with the untreated, and by over 0.5t/ha compared with district practice (10-20kg P/ha) by using much higher rates than district practice (30-50kg P/ha)," he said.

"But in the same paddock we had sites that didn't respond to P application at all."

At Koolunga in 2019, the P response trials followed a similar trendy in areas of low-neutral pH (5-7 pH CaCl2) and good historical crop growth indicated from satellite imagery, there was little difference in the yields produced between zero and 50kg P/ha.

"The best economic response is to turn fertiliser off in those instances," Mr Trengove said.

He said there were similar trends in three trial paddocks in thee 2020 season, with an emphasis on the economics of different rates of P.

"In 2020 at sites predicted to be responsive, the economic optimum P rate ranged from 16-26kg/ha, which was anywhere from two to four times the replacement P rate calculated from grain yield," he said.

"However at non responsive sites replacement rates were more than sufficient to satisfy crop requirement."

"That indicates there is a huge upside when in responsive soils by treating accordingly rather than just replacing P."

He said they also went back to the 2019 trial sites in 2020 and the impact of the extra P in P responsive soils was also evident on the next year's crop.

"So the economic response goes beyond just one year," he said.

But he said it was a different case in non-responsive soils where there was little economic benefit between replacement and increased applications of P.

Mr Trengove said there were early signs that it was possible to combine data layers to create an index to measure potential P responsiveness.

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He said there were methods that could be employed to combine the use of soil pH and satellite imagery to identify areas of high P response, while still using P replacement calculated from yield maps to set a baseline P rate.

These can be formulated into a single fertiliser prescription map for variable rate application of P at seeding time.

"Going forward there can be a strategy of using grain yields to calculate a minimum replacement value so we're not mining P from the less responsive soils, but maintaining them at adequate levels," he said.

Mr Trengove said they had also received further funding from SAGIT to continue work for seasons 2021-23 to validate these results and test them in a broader range of environments.

"We also hope to look at the best long-term approach for higher P responsive sites," he said.

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