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Summer rainfall triggers nitrogen windfall

This year’s wet summer has provided a nitrogen windfall for many broad acre farmers in the Wheatbelt.

That’s one of the findings from a research project investigating the impact mineralisation has on soils.

The research, funded by natural resource management group Wheatbelt NRM, began as a simple trial in December 2016, testing nitrogen levels across 126 sites in the Wheatbelt.

But the massive rainfall events of late January and February, where falls of more than 100 millimetres were recorded, has triggered a change of direction.

The Nutrient Use Efficiency project has now morphed into a comprehensive study on the impact a massive summer rainfall event can have on soil nutrients, with a focus on nitrogen.

Funded by the Australian Government’s National Landcare Programme and the Department of Biodiversity, Conservation and Attractions – Rivers and Estuaries Division, the program aimed to deal with the efficiency of fertiliser use on farms.

Initial soil testing was done in December on 126 sites by York agronomist David Stead, using a corer giving access to a depth of 50 centimetres.

Retesting took place in May after the large summer rainfall events of January and February, where between 50 and 150 millimetres of rainfall was recorded.

“The hot topic amongst farmers was what impact these massive summer rainfall events had on nitrogen levels in the soil,” David Stead said.

“The question was being asked, had the rain leached the nitrogen out of the soil, or had it added nitrogen via mineralisation?

“Research has indicated a two-inch rainfall event can leach soluble nutrients out of the immediate root zone.

“Recently, a review was undertaken by Dr Louise Barton and her colleagues at UWA, which suggested between season nitrogen mineralisation rates of between 0.1 and 0.2 kilograms per hectare per day.

(Anderson, Murphy et al 1998 and presented at the GIWA Research Update presentation titled “Where does the Nitrogen go”).

 “Given the magnitude of summer events seen this year, there was a great opportunity to investigate this further.”

Mineralisation occurs when soil bacteria and micro organisms convert nitrogen oxides and other nitrogen sources into nitrate and ammonium forms, which are plant available.

David Stead said what was found was the significant amount of summer rainfall across all soil types and the cooler temperatures immediately following, allowed the top of the soil to stay at optimum field water holding capacity for four to six weeks.

“This prompted a massive mineralisation event that would have been reasonably unprecedented,” he said.

Wheatbelt NRM’s Chris Kennedy said this meant farmers needed to test nitrogen levels as close to seeding as possible.

“The aim of the project was to give farmers more information about their soils before they started planning their fertiliser program,” Chris Kennedy said.

“The biggest finding this research project has generated is that a major rainfall event generates nitrogen in the soil, which means farmers need to soil test close to seeding, so they can accurately budget their fertiliser requirements.”

David Stead agreed that soil testing closer to the break in the season was needed.

“We’re not investigating our soils very well,” David Stead said.

“We need to be combining soil testing using coring units, moisture probes, yield mapping and tissue testing to accurately plan our fertiliser program closer to and during the cropping season.

“This gives us an accurate picture for nitrogen, as this is the primary diver and most controllable fertiliser input when the crop is growing.

“In many cases, farmers need to adopt maintenance rates, effectively replacing what is taken out during the cropping season.”

Wheatbelt NRM will now continue the project by working with growers and testing at the same sites for nutrients in tissue tests and grain testing prior to harvest.