Wheat productivity assisted by high soil N
Author: Sarah Jeffrey | Date: 13 Jun 2018
The importance of maintaining adequate levels of soil nitrogen (N) to maximize farming system profitability has been reinforced in results from a four-year investment by the Grains Research and Development Corporation (GRDC) conducted across northern New South Wales and southern Queensland.
The trial work, undertaken by the Northern Grower Alliance (NGA), focussed on evaluating management strategies to improve the economic performance of key wheat varieties that are generally high yielding but with low protein achievement such as EGA Gregory and Suntop.
Conducted over multiple locations, environments and years, the trials consistently showed the efficiency of N grain recovery from soil N was ~3-4 times that of fertiliser N that was applied in the year of cropping.
Fertiliser N grain recovery levels are determined by many factors including season type and starting soil N levels.
However the trials, which were conducted at 14 sites during 2014-2017, showed that even at common commercial rates of 50-100kg N per hectare, fertiliser N grain recovery in the year of application only reached mean levels of ~15-20 per cent of total N applied.
NGA chief executive officer Richard Daniel said current `rule of thumb’ recommendations for N fertiliser budgeting were generally effective but may be misleading in situations where soil N levels are low.
“N fertiliser recommendations are frequently based on setting a target for yield and protein and then ensuring a quantity of soil and fertiliser N that is generally double that target - that is, working on a 40-60 per cent grain N recovery efficiency,” Mr Daniel said.
“However when soil N levels become low, such as following an unexpectedly high yielding crop or following a very dry fallow where mineralisation is greatly reduced, N fertiliser application rates may need to be increased to commercially impractical and uneconomic levels to achieve the expected outcome.
“In some of these situations, changing from a cereal to a legume may be a much better option. At the same time, implementing agronomic strategies to maximise the N contribution from the legume phase will be highly productive.
“The results we have been finding indicate that we need to increasingly think about our N fertiliser strategy as `feeding a farming system’ with multiple crops over time, rather than just applying to benefit the current year’s crop alone.”
One possible reason for the low observed efficiency of grain N recovery from fertiliser applied in the year of cropping may be the amount and speed of N movement in soil, according to Mr Daniel.
He said the trial work found that the movement of N was slower and shallower than expected, even when applied in a dry soil profile.
“The majority of N applied in fallow, either surface spread or incorporated to depths of ~3-5cm, was still in the 0-15cm soil segment at planting,” Mr Daniel said.
“Even by harvest, the majority of extra N has generally been found in the top 15-45cm”.
So what was the fate of the unused applied N?
“Since 2016, we have monitored the next crop grown in the rotation,” Mr Daniel said.
“To date we are certainly generating yield and protein responses in the following crop. Interestingly from the trials started in 2016, we still have additional soil N from the original fertiliser application which should be available for the crop in 2018.
“The key message from these results is that our benefit from fertiliser N application may be a little less than expected in the year of application but probably providing much longer term benefits to the system as a whole”.
To assist growers in decision making over N planning, timing and application, the GRDC has released a series of Know More videos on N management in wheat featuring Mr Daniel which are available on the GRDC’s YouTube channel and GRDC website.
For more detailed information on the NGA’s N management trial work, download a copy of Mr Daniel’s 2018 GRDC Update paper Nitrogen use (in)efficiency in wheat – key messages from 2014-17 can be downloaded from the Resources and Publications section of the GRDC website .
Northern Grower Alliance CEO
0428 657 782
Cox Inall Communications
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