By Peter McKenzie, Agricultural Consulting & Extension Services agronomist, Quirindi, NSW
Research on ammonia volatilisation supported by the Grains Research and Development Corporation (GRDC) is helping advisors and growers make more informed decisions on the cost effectiveness of fertiliser application strategies.
In simple terms, ammonia volatilisation occurs when nitrogen (N) fertiliser is lost from the soil surface as ammonia gas before it is utilised by a crop.
The extent of loss depends on a range of factors including; soil moisture, temperature, pH, naturally-occurring lime in the soil, ground cover, wind speed and direction, soil clay content, and fertiliser type.
Fertiliser loss to volatilisation can add unnecessary costs and, in some situations, impact significantly on a crop’s gross margin so it is important to have accurate estimations of potential loss when weighing up application options.
This research has significantly narrowed the knowledge gap about N fertiliser volatilisation, improving growers’ ability to quantify the risks and make management decisions that generate the greatest return per input dollar.
It is widely accepted that N fertilisers are ‘safer’ when incorporated than when surface spread, but pre‐season broadcasting and in‐season topdressing of wheat crops are practiced by many growers in the northern region.
The research results increase the management options in terms of logistics flexibility in getting N on, both pre and post season.
Splitting nitrogen application between sowing and in‐crop can allow growers to lower their financial risk on fertiliser application by letting seasonal conditions drive decisions on how much to spend on nitrogen.
That said, the highly variable nature of winter rainfall in the north means growers are often left in a “do we, don’t we” situation when it comes to fertiliser application ahead of a predicted rainfall event.
Rain is important not only for lowering the volatilisation risk for surface-spread fertilisers but also for promoting plant growth to take up the applied N.
Effective fertiliser management requires an assessment of calculated risk to determine the extent of loss if the predicted rainfall event doesn’t occur or the environmental conditions aren’t ideal.
The ammonia volatilisation research holds important implications for industry in that it enables us to more accurately quantify fertiliser losses and translate that to a cost per tonne of grain.
Previously we relied on average estimations of between 30% and 40% loss with pre-plant applications of ammonium sulfate fertiliser on vertosol soils whereas current research indicates that losses are more likely to be between 10% and 15% in favourable conditions.
The research is also enabling us to finetune fertiliser management in high calcium carbonate (lime) soils with trials showing that N losses in soils containing >10% calcium carbonate averaged >20% from ammonium sulfate applied to fallow soils.
Conversely calcium carbonate content did not affect losses from urea or other nitrogen fertilisers trialled so growers with high calcium carbonate content soils will incur less risk by tailoring their fertiliser type accordingly.
As a high input cost, fertiliser selection and application need to be managed well if growers are to maximise crop returns and this type of research is a valuable step forward in enabling industry to achieve that goal.
Sarah Jeffrey, Senior Consultant Cox Inall Communications
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