Application methods will be assessed in research designed to better understand the potential role enhanced efficiency fertilisers (EEFs) could play in the northern region.
Recent research by a team at the University of Queensland led by Professor Mike Bell, in collaboration with Queensland University of Technology colleagues, found that EEFs can reduce denitrification losses and leave more residual nitrogen in the soil profile than urea.
Professor Bell says EEFs have proved to be valuable in addressing nitrogen loss in the sugarcane industry and the group is investigating whether the same benefits could be obtained by the region's grain growers.
However, this is proving to be a conundrum. While early trials in irrigated maize crops grown in clay soils have shown reduced gaseous nitrogen losses, the maize crop has had difficulty in capturing the extra nitrogen that was made available via EEFs. This has led Professor Bell to question whether EEFs require different application methods.
Conventional versus enhanced fertilisers
Conventional nitrogen fertilisers, such as urea, are rapidly transformed in moist soil into nitrate, which can be lost to the environment, reducing nitrogen use efficiency (NUE) and causing environmental damage.
For example, work in the Great Barrier Reef catchment has shown that 40 to 60 per cent of applied fertiliser nitrogen can be lost from the sugarcane industry. Similar losses can be recorded in wet years on alkaline clay soils in the grains industry.
One solution is to use EEFs. These products aim to slow the transformation of nitrogen into nitrate, reducing losses from leaching or denitrification and matching the nitrate supply more closely to the demands of the growing crop.
Professor Bell says that, in theory, this means lower nitrate transformation rates and better environmental outcomes.
EEFs have different modes of action, from controlling urea release to inhibiting nitrification. Most research on them has been in cooler climates, where EEFs are broadcast or incorporated with other fertilisers.
"In the northern region we typically apply these products in concentrated bands and into warmer soil temperatures. We wanted to test how well they would do under such conditions, compared to urea."
In a trial sown in late January 2018, the team set out to determine the NUE of urea applied to irrigated maize at various rates. The results showed that while EEFs reduced nitrous oxide emissions, this did not result in significant increases in either crop nitrogen uptake or grain yield. In fact, the most common result was the 'saved' nitrogen was retained in the soil at the end of the growing season and so was possibly vulnerable to loss over the fallow.
These results have led the team to question how EEFs work in conditions typical of the northern region and, particularly, the impact of applying these products in concentrated bands.
In the northern region we typically apply these products in concentrated bands and into warmer soil temperatures. We wanted to test how well they would do under such conditions, compared to urea.
Professor Bell says the products did what they were meant to do - either slow urea release or inhibit nitrification.
"They work well in the sugarcane industry and we wanted to see how that would translate to grains using maize," he says.
"However, sugarcane takes up its nitrogen over six months. Maize takes up most of its nitrogen during a six-to-eight-week period.
"Slow release of nitrogen is therefore OK for the sugar industry as it matches crop uptake, and the reduced off-site losses are a great result for the Great Barrier Reef catchment.
"However, release that is a little too slow can severely impact maize yields, with supply not quick enough to meet the short-term, intense demand by the crop."
Improving fertiliser efficiency
There is currently an intense focus on improving the use efficiency of all fertiliser inputs in cropping systems, for the benefit of the environment and the farm bottom line. If EEFs are to be part of the solution to improving such efficiencies, the implications of different use strategies, soil types and cropping systems need to be better understood, he says.
Research conducted in 2019 and to recommence in 2020 is considering EEF applications using management approaches.
As Professor Bell asks: "Are our early results because of how we applied the products or because of our clay soils? Our current theory is that because we applied the nitrogen in concentrated bands, the combination of the band environment and root activity limited the crop's access to that nitrogen. Our questions are now asking what will happen if we change how we apply them?"
These products work differently to urea and so we may need to change our approach accordingly.
The research in 2019 tested applications one month prior to sowing and those at sowing, to see whether the release dynamics better matched crop demands.
"In 2020 we are considering placement in the soil profile, in an effort to allow better root access," Professor Bell says.
He says that the early trial work saw EEFs applied in a similar way to how urea is applied.
"We may need to rethink this. These products work differently to urea and so we may need to change our approach accordingly," he says.
Many of the benefits of EEFs have been found in systems where the fertiliser is mixed in the soil using cultivation, and also under cooler conditions.
"No one's really thought through the implications of the banding used in our tropical and subtropical row crop systems," Professor Bell says.
NOTE: This work was jointly funded by the Science with Impact Fund, the University of Queensland and the National Environment Science Program (Tropical Water Quality Hub), Australian Government.
More information: Mike Bell, email@example.com, 07 54601 140