High rainfall research finds economic potential for higher P levels

But a five year GRDC-funded project (DAV00141) has aimed to quantify how much more fertiliser it is economically viable to apply.

It has found the critical recommended levels of phosphorus needed to reach the economic yield optimum for wheat and canola crops in the high rainfall zone could be more than double those recommended previously.

Agriculture Victoria soils research scientist Malcolm McCaskill says the historical recommendations had been derived from many trials in medium rainfall areas where yields were only 3 tonnes a hectare to 4t/ha, not the HRZ where croppers are chasing 8-10t/ha.

For example, of the 1225 previous trials conducted in Australia's cropping zone looking at Colwell P levels in wheat, just 28 were conducted in the HRZ, and most of those were from Tas in the late 1960s.

The project, led by Agriculture Victoria in collaboration with other organisations including the Mackillop Farm Management Group, has addressed this with 30 trials across Vic and Tas and the South East of SA, to fill the gaps.

Dr McCaskill says the project, which has just finished, looked at yield responses at different levels of phosphorus, potassium, sulphur and trace elements.

It found the soil Colwell P value needed to achieve 95 per cent of yield potential in wheat in the HRZ was 52 milligrams a kilogram, compared with the 24mg/kg result from previous trials on similar soil types that have been loaded into the Better Fertiliser Decisions for Cropping database.

Canola had an even higher phosphorus requirement with a soil Colwell P of 59mg/kg compared to 21mg/kg.

Dr McCaskill says it is not surprising higher inputs are needed in the HRZ, but it is not a simple calculation.

Many soils in the HRZ, especially black soils, have a much higher phosphorus buffering value, meaning the soils hold onto phosphorus more tightly and also lose nitrogen through waterlogging.

There was no clear result for potassium levels but Dr McCaskill suggested croppers follow the pasture guidelines of a critical level of 139mg/kg when growing canola and wheat rather than the 73mg/kg for cropping trials from Better Fertiliser Decisions.

Sulphur levels were found to be a limiting factor in some canola trials, and a critical value of 11mg/kg is recommended.

"Nutrients are the biggest variable cost of cropping but most of the decisions being made are based on rules of thumb," he said.

"Agronomists tend to want to get a good crop and prefer to put on more to avoid problems, whereas the farmers need evidence that the fertiliser they pay for will produce a good financial return."

A new economic tool to calculate the financial benefits of applying extra fertiliser to wheat and canola crops is now available to croppers in the high rainfall zone.

Some of the results from the GRDC DAV00141 project are still being added but the tool in GRDC Communities website aims to help them make better decisions.

"For example for a 2:1 benefit cost ratio in canola the model shows you that a grower can apply up to 88 kilograms of phosphorus per hectare at sowing and still get a financial response if there has been no prior application of phosphorus but if it is in fertile country you might only need 6kg P/ha," Ag Vic soils research scientist Malcolm McCaskill said.

He encouraged croppers to soil test their paddocks prior to sowing with their background research showing many paddocks were below the recommended values for phosphorus and sulphur even for lower rainfall areas.

From 150 soil tests taken in the SE of SA in 2015 and analysed by Nutrient Advantage, 21pc of the samples had phosphorus levels below the 22 milligrams a kilogram P-Colwell and nearly half were below 7.1mg/kg for sulphur.

Bool Lagoon cropper Bruce McLean, who hosted trials in 2015, 2016 and 2017, says the project has been an important step in his quest to consistently achieve higher yields.

"If we can eliminate 'x' and 'y' nutrients as the limiting factor we are a step closer to 10 t/ha crops."