Trials supported by the Grains Research and Development Corporation (GRDC) have surprised researchers, revealing cracking clay soils potentially lose less nitrogen to the atmosphere as ammonia than first thought.
Graeme Schwenke, NSW Department of Primary Industries (NSW DPI) soil scientist, says field trials with urea showed that ammonia loss due to volatilisation averaged 11% (5–19%) when applied to fallow paddocks, and 5% (3–8%) when applied to wheat crops.
Dr Schwenke says nitrogen losses from urea in dry pasture situations were much higher at around 25pc.
“It was surprising because in cropping situations overseas researchers have often found higher losses from cropped soils than we found here,” he said.
“The biggest factor seems to be soil type and all trials in northern NSW were carried out on medium to heavy cracking clays or vertosols.
“High clay content, high cation exchange capacity (which allows them to effectively grab on to ammonium ions that result from the breakdown of fertiliser products in the soil) and a strong buffer capacity against pH changes that can accelerate nitrogen loss, seem to be the difference.”
Dr Schwenke says the research was prompted by two issues in the northern grains region.
“One of these was growers using more crystalline ammonium sulfate, which is a byproduct from the manufacture of steel.
“At the time the crystalline form was comparable in price per unit N to urea and people were spreading it on fallow paddocks ahead of a crop but weren’t sure how much would be lost through volatilisation.
“Our research showed that volatilisation losses from ammonium sulphate were often only half that from urea, unless the soil contained calcium carbonate (lime) where the opposite was the case. Ammonium sulphate reacts with lime to greatly increase ammonia loss.
“The other trend was people becoming more interested in applying nitrogen fertiliser products post-sowing as in-crop topdressing.
“Our results show you can consider reserving some fertiliser for in-crop application later in the crop growth as losses may be less than first thought.”
To download the GRDC Update paper detailing NSW DPI research into nitrogen volatilisation, visit www.grdc.com.au/GRDC-UpdatePaper-SchwenkeEtAl2012-NitrogenVolatisation.
NSW Department of Primary Industries
02 6763 1137
Michael Thomson, Senior Consultant, Cox Inall Communications
07 4927 0805, 0408 819 666