Australia - GRDC Updates explore the current state of the northern soil nitrogen bank

Date:01-31-2023

After three consecutive wet years and multiple high production seasons in the north, experts are warning growers that the old rules of thumb for calculating nitrogen (N) rates won’t cut it this season.

The current state of northern soils in terms of the level of N contribution from major soil pools and what growers can do to rebuild their pool and reduce losses will be the focus of Back Paddock Company Consultant Agronomist, Chris Dowling’s presentations at this year’s Northern Update series, hosted by the Grains Research and Development Corporation (GRDC).

Dr Dowling says over the last two seasons, northern growers have had most of their accumulated soil mineral nitrogen wiped out by high crop yields, waterlogging and leaching.

″What we’ve seen is a major shift in weather patterns,″ he says.

″During the first few years of the recent drought, when growers didn’t expect it to go on for as long as it did, they were optimistic and applied nitrogen as usual.″

″Low yields during the drought saw soil nitrogen levels build up to a point that was in many cases quite extraordinary and has been able to support multiple high production years with relatively low fertiliser N requirements.″

″Now though, after a few huge harvests and ongoing wet conditions, growers are at a point where they’re back to relying on the fertiliser N efficiency as a major determinant of productivity. There is no longer a well-distributed soil mineral N buffer.″

Dr Dowling says an ideal situation for growers would be for organic matter and stored soil mineral N pools to supply at least 70 per cent of a crop’s requirement, so fertiliser decisions only affect 30 per cent of total crop N requirement.

″Currently, many growers would be in the situation where 50 per cent of their N requirements will need to come from fertiliser. This increases the risk to profitability from variations in N cost, grain prices and weather events,″ he says.

Dr Dowling says growers should consider the protein content of last year’s cereal crops and conducting a soil test to confirm what contribution can be expected from soil mineral and organic N sources this year.

″If by the time we get to March/April, soil mineral nitrogen is low (less than 40 kg/ha) in that top 60cm, growers will have a heavy reliance on fertiliser N efficiency during the season to support crop requirements,″ he says.

″The challenge here, is that with such big stubble loads, some applied nitrogen will be tied up by microbes trying to break down the stubble and poor distribution of mineral and fertiliser N down the soil profile will lower uptake efficiency.″

Dr Dowling says during the Updates series, he will discuss the necessity for growers to go back to the drawing board and consider a long-term N strategy so they’re less at the mercy of challenging N fertiliser prices and large swings in weather.

″It’s time for growers to target getting back to where annually their soil is supplying 70 per cent of their crop N requirements,″ he says.

″We’ll look into ways we can build that pool - which can include looking at the value of strategic use of pulse crops or pasture ley opportunities.″

″If we want to get to a more stable system, we need to create and utilise more soil mineral and organic nitrogen, which frequently have a higher use efficiency by crops.″

Dr Dowling says he would also discuss fertiliser application and how in some cases, growers will have to sacrifice some fertiliser and mineral N to increase soil organic carbon and nitrogen in the longer term.

GRDC Northern Grower Relations Manager, Vicki Green, says current N prices are having huge impacts on growers’ budgets and feedback has shown that growers want to be as efficient as possible with their N applications.

″It’s important to understand how to get the best return on N during the season, especially considering the significant investment growers are making to support crops,″ she says.

″We have tailored the GRDC updates to provide regionally relevant news and research to northern growers to help them respond to the unusual seasonal conditions they’ve been experiencing and get the most out of their inputs.″

″I would encourage all growers who have concerns about crop N requirements to register for an event and come and hear the latest research, development and extension on N efficiency.″