Managing for nitrogen deficiency in Central Queensland
GroundCover™ Issue: 107 | Author: Tom Dixon
Central Queensland grain grower Colin Dunne has been cropping his land for 30 years, and has relied on legume rotations and inherent soil fertility as his main nitrogen strategy.
“I’ve trialled fertilisers several times, but have found previously that we had enough natural nitrogen in our soils,” Mr Dunne says.
But times change. In the Duaringa area – 100 kilometres west of Rockhampton, Queensland, where Mr Dunne farms – the natural store of nitrogen is being slowly depleted by continuous cropping. Flooding in 2011 and 2013 has also further accelerated the loss of soil nitrogen through denitrification.
“This year will be the first in my 30 years of farming that I’ve had to apply nitrogen, and I’m expecting it to not be the last,” Mr Dunne says.
Researchers at the Queensland Department of Agriculture, Fisheries and Forestry (DAFF) have worked in the region for more than 20 years, looking at the effects of cropping on soil nitrogen levels.
Maurie Conway, a research technician with Queensland DAFF, says growers such as Mr Dunne understand that nitrogen deficiency is becoming a problem, but he says more effort is required to raise awareness of the management options that are available to growers.
“Growers can manage their fertiliser input by testing their grain protein levels and their soil nitrogen levels to see if they have a nitrogen deficiency problem,” Mr Conway says.
Grain-protein levels are often a good indicator of nitrogen availability. If grain protein levels in wheat are below 11.5 per cent, Mr Conway says that is an indicator of nitrogen deficiency.
Growers should then confirm the extent of the deficiency by conducting soil testing, which will allow them to calculate the appropriate amount of nitrogen fertiliser they will need to add. By testing soil carbon, growers can work out their soil nitrogen levels (total nitrogen is about 10 per cent of soil carbon reading).
“I’m always encouraging growers to get their soils tested for nitrogen and other nutrients such as phosphorus, potash and sulfur,” he says. “You simply can’t manage or improve something if you don’t know what you’re starting with.”
The role of legumes
Mr Conway says that due to the flooding in recent years, growers are being encouraged to introduce more grain legumes into their rotations to put nitrogen back into the soil.
Legumes have become a key part of soil nitrogen management in Australian farming systems. They form a mutually beneficial association with rhizobia (a soil bacteria) to trap, or ‘fix’, atmospheric nitrogen and convert it into ammonium – a form of nitrogen plants can use.
“More legume crops in the system, particularly in a nitrogen-deficient system, is good because they reduce the need for nitrogen fertiliser inputs and can leave nitrogen for the subsequent crop,” Mr Conway says.
“This isn’t to say legumes will solve all your nitrogen problems, but they will go a long way in helping to manage the problem in a cost-effective and profitable way,” he says.
Mr Dunne regularly grows legumes on his property as part of a crop-sequencing strategy to boost nitrogen levels, but the combination of the floods and 30 years of cropping have countered this strategy.
“My most economic crop by far is chickpea – in some years yielding as much as wheat, and it’s twice as profitable,” Mr Dunne says.
“As we have deep soils, the long taproot on the chickpea can access deeper moisture and nutrients. Add the other benefits, including minimum spraying as well as all the soil health gains from nitrogen fixing, legumes are a real winner.”
However, even with rotations including legumes, most growers in the region are finding they still need to apply fertiliser.
But, as Mr Dunne notes, it is expensive. “So I didn’t want to waste it by spreading too much, or where I don’t need it, or applying it incorrectly,” he says.
Mr Conway says that a fertilising program should be devised in conjunction with an agronomist to plan the best strategy.
“For example, infrequent and sometimes ineffective in-season rainfall events combined with the clays of the northern region limit opportunities to top-dress nitrogen,” he says. “So a smart fertilising program can improve yield and maximise water use, as well as ultimately save you money on input costs.”
Growers are encouraged to talk to their local agronomists about smart fertilising programs, and to consult the GRDC handbook Managing legume and fertiliser N for northern grains cropping: www.grdc.com.au/GRDC-Booklet-ManagingFertiliserN
GRDC Project Code DAQ00170