Optimise nitrogen efficiency

Mr Straight said emissions of nitrous oxide are highly variable due to large variations is soil, climate and management practices.

However, a three-year trial has shown, by adopting certain practices, nitrogen loss can be minimised and at the same time yield potential is maintained.

Mr Straight noted the loss of a kilo of nitrogen in an average year is probably what he would consider the base line in the high to medium rainfall, high production area of the Riverina.

“To optimise nitrogen efficiency and reduce nitrous oxide loss we need a good understanding of soil nitrogen in the system, and we therefore try to synchronise supply with peak crop demand to encourage greater fertilizer uptake,” Mr Straight said.

“Know what is under you feet so you are aware when your crop will need nitrogen.”

In one of the trials, nitrogen was applied when the crop needed it, and Mr Straight noted the great efficacy of the application.

“It went straight to the crop and not lost to the atmosphere as N2O (nitrous oxide),” he said.

Nitrogen efficiency is highly dependent on seasonal variability and the soil moisture content at the time of application, and Mr Straight pointed out an example of top dressing a crop standing in water due a wet winter.

In a wet season losses will generally be higher, and in those situations an early application assisted in maintaining the crop through the wet period.

“It may be the best strategy in terms of grain yield and quality, but may also result in elevated nitrous oxide losses,” he said.

“Applying nitrogen late still gave some response in yield, and that was because the crop was so deficient,” he said. “There were some efficiencies even though some nitrogen was lost … you just have to weigh up the potential yield and cost of urea.”

Michael Straight, Foundation for Arable Research (FAR), Research Manager, advised farmers the use of tactical nitrogen applications with the aid of a Greenseeker, allowed residual nitrogen from previous crop applications to be better identified.

“This allowed reduced fertilizer application without compromising on yield,” he said.

“In our trials we able to apply smaller amounts and we still got the same yield as if we had spread more.”

In the second year of the trial, results showed delaying nitrogen applications maintained grain yield, but it allowed ‘in-season’ decisions and use those tools such as Greenseeker and soil tests as the crop advanced.

“This allowed more accurate forecasting of when the crop demand for nitrogen is higher such as when the stem is experiencing the elongation phase,” he said.

Dr Cassandra Schefe, RiverinePlains extension officer said nitrogen was found, during the trial, at various levels through the soil profile due to the season.

“In a normal year, we found most of the nitrogen was at the top and there was diminishing supply throughout the profile,” she said.

“In wet year, it is the opposite and if crops could get down they would but that is when it comes down to physical restraints in the soil.”

Dr Schefe advised farmers if they could incremental test their soil profile for nitrogen availability, it would certainly give much more confidence about how much extra nitrogen to apply.

In a wet season, nitrogen loss because of anaerobic conditions allowing leach to the atmosphere is highest

“In those cases applying nitrogen early assisted the crop through the wet periods and that may be the best strategy in terms of yield, but may result in elevated nitrous oxide loss,” Mr Straight said.