When to apply nitrogen and how much
- Top-dressing high rates (100 to 200kg/ha) of N at green bud-stage gave yields of more
- Top-dressing no or low rates (0 to 50kg/ha) of N at green-bud stage resulted in yields between 1.25 and 1.75t/ha.
- Top-dressing 100kg/ha at green-bud produced about $800/ha of net income – $514/ha more than applying no nitrogen.
Applying nitrogen to canola crops at the correct rate, time and growth stage can increase yields and net income and reduce the potential for nitrogen losses, a study in Victoria’s high-rainfall zone (HRZ) has found.
The GRDC-funded work, through the second phase of the More Profit from Crop Nutrition (MPCN) initiative, forms part of the National Agricultural Nitrous Oxide Research Program. The initiative seeks to improve nitrogen use efficiency of canola, wheat and sorghum in Australian cropping systems.
Research scientist Dr Rob Harris, from the Victorian Department of Economic Development, Jobs, Transport and Resources (DEDJTR) – formerly the Department of Environment and Primary Industries – says while nitrogen leaching is typically not a problem in the Victorian HRZ, his team has measured large atmospheric losses of nitrogen in soils prone to waterlogging.
“Due to the anaerobic conditions, in order to survive the soil microbes are forced to steal oxygen from the plant-available form of nitrogen – nitrate – and, in doing so, change that nitrogen into gaseous forms such as nitrous oxide,” he explains.
“We have measured annual losses of up to 35 kilograms per hectare of nitrous oxide (N2O) from around Hamilton. We haven’t measured dinitrogen gas losses but, because we’re getting these high N2O losses, we hypothosise that we might be losing a lot as dinitrogen gas as well.”
Dinitrogen gas is an atmospheric gas that makes up 78 per cent of the Earth’s atmosphere.
Dr Harris says another potential nitrogen thief is run-off.
“Unfortunately, nitrate is an anion, meaning it is negatively charged, and a lot of soil exchange sites are also negatively charged,” he says. “That means nitrate is highly mobile and wherever water is escaping from our cropping systems we can lose nitrate.”
Improve nitrogen uptake
Another aim of the project is to improve the efficiency of crop nitrogen uptake, so growers can reduce unnecessary nitrogen fertiliser costs and improve returns.
For the past two years, Dr Harris has been overseeing a trial at Southern Farming Systems’ Westmere site in southern Victoria to determine the optimal rate and timing of nitrogen and to evaluate a nitrification inhibitor fertiliser used as a coating on urea called dimethylpyrazole phosphate (DMPP).
“As cropping systems have intensified there has been an increased reliance on nitrogen fertiliser, which now constitutes the single largest variable cost for most grain growers,” Dr Harris says. “The challenge for growers is supplying the right amount of nitrogen at the right time to meet crop demand and to optimise yield and quality without oversupplying nitrogen and reducing profit.”
In 2013 and 2014, the conventional canola variety Hyola® 50 was sown at Westmere with seven different nitrogen treatments (Table 1).
The seven treatments were replicated four times, with plant emergence counts taken in early June showing about 30 to 40 plants per square metre.
Dry matter (DM) cuts were taken in early September, with 50kg/ha of urea top-dressed at the six-leaf stage giving the highest yield at 5.7 tonnes/ha. In 2013, higher DM cuts correlated with a higher yield, but Dr Harris says this remains to be seen in the 2014 results, given the abrupt, dry finish.
“In 2013 we certainly saw a healthy response up to around 100kg N/ha applied at the green-bud stage,” he says. “There was a slight response when we applied N at the six-leaf stage, but it wasn’t significant.
“We haven’t found that DMPP has been able to provide additional yield compared with conventional urea, which has been consistent with other trials conducted across Australia. The additional cost of DMPP is unable to compensate for that lack of improved yield, so at the end of the day it’s reducing profit.”
In 2013 there was a linear grain yield response in the trials up to a rate of 100kg N/ha. This was largely due to low levels of plant-available nitrogen stored in the soil profile at sowing and above-average growing-season rainfall, particularly in spring.
Table 1 Nitrogen at the More Profit from Crop Nutrition site at Westmere, Victoria, in 2014
|Dry matter yield (September 2014)
| No additional N control) (ON)
| 25kg/ha N (as urea) (TD25N)
| Green bud
| Not assessed
| 50kg/ha N (as urea) (TD50N)
| Green bud
| 100kg/ha N (as urea) (TD100N)
| Green bud
| 200kg/ha N (as urea) (TD200N)
| Green bud
| 50kg/ha DMPP (DMPP50N)
| Green bud
| 50kg/ha N (as urea) (Early TD50N
The top N result
Best results in yield (Figure 1) and net income (Figure 2) in 2013 came from topdressing 100kg/ha of urea at green-bud stage.
“Top-dressing nitrogen at 100kg/ha at green-bud stage produced almost 1.3t/ha more grain and an extra $514/ha of income compared with applying no nitrogen,” Dr Harris says.
Figure 1 Canola grain yield following N treatments at the Westmere MPCN site in 2013
Despite these results, the 2013 plots fell short of their potential yield of 3.1t/ha.
“Clearly there was insufficient nitrogen supply for the control and low-nitrogen treatments – 25kg N/ha at green bud, 50kg N/ha at green bud and 50kg N/ha at six-leaf stage – to reach their water-limited potential yield; however, the high-nitrogen fertiliser treatments – 100kg N/ha at green bud and 200kg N/ha at green bud – also fell short.
“The site experienced above-average winter rain causing extensive waterlogging and crop damage in August. Another possible explanation was a shortfall in sulfur. The basal single superphosphate application at sowing – equivalent to 15 units of phosphorus and 18 units of sulfur – may have been insufficient to meet canola demand under high nitrogen supply.
“The application of single superphosphate meant no nitrogen was applied at sowing to all treatments. A shortfall in nitrogen supply early in the growing season may have also contributed to canola not reaching water-limited potential grain yield.”
Figure 2 Net income for N treatments at the Westmere MPCN site in 2013
Deep banding vs top-dressing
Another part of Dr Rob Harris’s work on improving crop nitrogen fertiliser efficiency has been to determine which application method results in improved crop nitrogen fertiliser recovery.
At Southern Farming Systems’ site in Tarrington, Victoria, in 2013, Dr Harris and his team investigated the effects of four different treatments in a wheat crop using 15N fertiliser to track its uptake by the crop:
- 100 units/ha of 15N urea deep-banded at sowing, 10 centimetres below the seed (DB100N at GS00);
- 100 units/ha of 15N urea top-dressed at first node (TD100N at GS31);
- 50 units/ha of 15N urea top-dressed at first node (TD50N at GS31); and
- 100 units/ha of dimethylpyrazole phosphate (DMPP) coated 15N urea top-dressed at first node (DMPP100N at GS31).
Results at the end of the season (Figure 3) showed that only 23 per cent of the deep-banded nitrogen was accounted for at harvest, leaving 77 per cent unaccounted for. Of the 23 per cent, only 13 per cent of the applied N was found in the crop.
“Where we top-dressed at first node it was the opposite – there was only 20 per cent of the nitrogen fertiliser we couldn’t account for,” Dr Harris says. “Roughly half of that was found in the crop [grain and straw] and the other half was largely confined to the topsoil.
“We took soil samples at depths of 20 to 40cm and 40 to 60cm, and found very little trace of 15N fertiliser, so it didn’t look like a lot had leached. We suspect a lot of it was being lost into the atmosphere.”
Dr Harris says they also measured higher gaseous losses of nitrogen where urea had been deep-banded at sowing compared with top-dressed at first node in an adjacent trial.
“It looks like mismatching the nitrogen supply with peak crop demand has contributed to that loss,” he says. “We know the peak demand for nitrogen is in the late winter/spring period, but supplying all that nitrogen up-front is providing opportunity for the nitrogen fertiliser to escape before crop uptake.”
Figure 3 Nitrogen fertiliser at recovery at Tarrington site in 2013
Dr Rob Harris, Victorian DEDJTR,
Rethink on soil organic carbon benefits
Ground Cover Direct
GRDC Project Code