Northern region grain growers should focus on rate when it comes to planning their nitrogen (N) fertiliser strategies to optimise yield rather than application method or timing.
This is one of the key messages from a series of 15 trials conducted over three years as part of a research investment project by the Grains Research and Development Corporation (GRDC) and the Northern Grower Alliance (NGA), which consistently demonstrated that the rate of fertiliser N applied was the main factor impacting yield, grain quality and on-farm returns.
NGA has been heavily involved in projects focussing on N application strategies in wheat with the initial aim of assisting the management of high yielding, and frequently lower protein achieving, varieties such as EGA Gregory or Suntop .
Impact of different nitrogen application methods
One aspect of this research compared the different impacts of nitrogen application.
In Queensland and NSW, growers typically apply nitrogen fertilisers such as urea directly into the soil (banding) or broadcast it on the surface then incorporate.
This is done to reduce the potential for ammonium containing (e.g. sulfate of ammonia) or ammonium producing (e.g. urea) fertilisers volatilising into the atmosphere as the gas ammonia. Volatilisation is one of the pathways of N loss from fertiliser applied or soil N, whereby ammonia gas is produced and lost to the atmosphere.
The results from the NGA trials supported the volatilisation work done by New South Wales Department of Primary Industries (NSW DPI) senior research scientist Dr Graeme Schwenke, which indicated that the volatilisation losses from surface applied urea were frequently much lower than previously expected.
NGA chief executive Richard Daniel said the trial work consistently showed equivalent crop responses in terms of biomass, yield and grain quality from urea spread immediately post sowing, and with no physical incorporation, compared to the same rate of urea spread and incorporated by sowing.
“All sites were nitrogen responsive, with significant grain protein responses to nitrogen rate in all trials,” he said.
“Across 22 comparisons of equivalent N application rates, the crop response has been the same regardless of whether we incorporated the urea at sowing or simply surface spread immediately after sowing.
“Conditions for incorporation by rainfall have varied at every site but in at least six applications, the first recorded rainfall was not until two weeks after spreading.
“In 2015 and 2016 we also evaluated urea incorporated versus surface spread when applied during the fallow period. Again, despite clear N rate responses, there was no significant difference in crop response between incorporated versus spread.
“That is not to say we aren’t losing ‘a quantity’ of N via volatilisation when urea is simply surface spread. It is however indicating that the levels of loss have been low and were not sufficient to measure any difference in crop performance at any of the sites over the past three seasons.”
Mr Daniel said Dr Schwenke’s research indicated that volatilisation loss of N from urea applied by spreading in fallow or in crop ranged from approximately 5 to 15 per cent, indicating N loss by this pathway was not as high as previously considered.
The NGA trial work validated these results under a wide range of commercial conditions and importantly to measured the impact in actual crop yield and grain quality.
“Our trials have shown equivalent wheat yield and protein regardless of whether the urea was mechanically incorporated or simply left on the soil surface,” Mr Daniel said.
“We are not saying there is no loss of N via volatilisation. The field data is simply showing that the losses have been low enough that we couldn’t measure differences in crop yield or protein content.”
Timing of nitrogen application
Image of Richard Daniel kneeling in a wheat cropA range of N application timings were also evaluated as part of the research. A split application of N (50 per cent incorporated at sowing and 50 per cent spread at approximately GS30) was evaluated in all trials, while early and late fallow applications were included in 2015 and 2016.
Split applications of N performed more consistently than expected and provided equivalent crop responses to the same total quantity of N applied at sowing.
In trials where differences were evident, Mr Daniel said the impact appeared to be linked to other agronomic issues such as seedbed soil moisture disturbance (negative impact on establishment and yield) or breaking up of high stubble loads prior to planting (positive impact)
The results confirm that in-crop spreading can be a useful tool to capture upsides in yield and grain quality when seasons are favourable however the practice is unlikely to become standard practice in the north due to the variable nature of late winter and spring rainfall.
He said although there were minor differences in response between timings at two sites, the overall message was that impact of application timing was minor.
“These results indicate that growers should keep things simple and make fertiliser application timings that fit their own situation,” Mr Daniel said.
“The results also continue to show how effectively wheat can compensate with N applied in crop. This strategy can be used in years where stored soil moisture is low at sowing but rains post sowing provide the potential for increasing crop yields.
“Follow up rain is still needed, more to get the applied N into the soil and therefore available to the growing cereal plant, than to manage N loss through volatilisation.”
Nitrogen movement in soil profile
The research also indicated that fertiliser applied N is not moving down the soil profile as quickly as expected.
NGA trials in 2016 showed that 60-80 per cent of the N applied to a dry soil profile in November-December was still in the top 15cm at planting, with 80-90 per cent in the top 30cm.
“Although the fallow rainfall in 2015/16 was lower than average, this slower movement may well explain why we are struggling to show significant impact from varying application timing,” Mr Daniel said.
“Even at harvest, 30-60 per cent of the applied N was still in the top 30cm. In 2015, despite in crop rainfall of up to 300mm, between 60-90 per cent of the fertiliser N applied at sowing was still in the top 30cm post harvest.
“The goal generally would be to move the applied N deeper in the soil where it is less likely to be unavailable due to drying soil conditions, but also more likely to be available to the crop at later growth stages when it is often more needed.”
Nitrogen use efficiency in crop
Research into quantifying crop N use efficiency through measuring N recovery in the grain from the application of fertiliser N has shown that only 20-25 per cent of the applied N is accounted for in the grain (yield and protein).
The remaining applied N is largely sitting in the soil, although additional N in the stubble has accounted for up to 20 per cent (40kg N/ha) in these trials. One downside is that increases in N levels in the stubble can lead to more rapid breakdown of stubble cover in the paddock.
“In contrast, the efficiency of recovery of soil mineral N (Nitrate and Ammonium) has been much higher. When you consider the total of soil mineral N and fertiliser applied N, the overall efficiency is generally in the ‘normal’ 40-60 per cent range.
“Although we are not measuring high levels of efficiency from the fertiliser applied N in the year it was actually applied, we expect it is boosting the quantity of the ‘more efficient’ soil mineral N for subsequent crop(s).”
All about the nitrogen rate
In summary, Mr Daniel said the key message was that the most important factor in all these trials has to been to get the N rate as closely matched to the moisture and yield potential of the crop. The method or timing of application provided no consistent benefit to crop yield or grain quality compared to the rate of N application.
“A large focus of this work was to examine whether adjusting timing (or method) of N application could assist in the management of wheat varieties with low protein achievement,” Mr Daniel said.
“Unfortunately, the conclusions to date are that these tools are of minor impact with N rate the key driver. The work however has also helped to generate a large trial data set that indicates the losses from surface spread urea are much lower than previously expected.
“If there is one message to stress from this work, it is to keep N application simple. The benefits from adjusting timing and method appear relatively minor and growers may be better off applying when and where it suits other operations. Getting the N rate right appears much more important than timing or method.”
“Another key message from this work has been that despite significant protein (and sometimes yield) benefits with increasing N rate, the highest net returns have nearly always been achieved from using 50kg of N/ha.”