Growers question nitrogen management strategies
Author: Sarah Jeffrey | Date: 09 Aug 2016
Saturating winter rain across parts of southern and central NSW have left some crops waterlogged and growers questioning how to approach nitrogen (N) management over the coming weeks.
The issue was a leading discussion topic among growers and advisors attending a recent series of More Profit from Crop Nutrition workshops funded by the Grains Research and Development Corporation (GRDC) and held throughout north-western, central and southern New South Wales.
The challenge facing growers is to determine whether N should be applied to water-affected crops and if so, how much and when.
A number of factors will influence the crop’s tolerance of waterlogging and therefore the viability of N applications, including crop type, stage of growth when the crop became waterlogged and N status prior to waterlogging.
Incitec Pivot Limited (IPL) technical agronomist Jim Laycock said N applications should only be considered in paddocks where water is laying when that water is receding and soil has begun to dry out.
“It’s extremely important to ensure that the crop roots are actively growing otherwise any applied N won’t be taken up,” Mr Laycock said.
“Growers should pull up individual plants from a cross-section of the paddock to check for the appearance of new white roots.
“If the roots are actively growing and an N application is being considered, timing will be critical - as it is with any in-crop N application – and the application should be made just ahead of rain to move it into the root zone. That’s of course as long as the forecast rain isn’t going to wash the applied N straight off the paddock.”
Mr Laycock said the stage of crop growth was a critical determinant in the economic value of N applications with early crops more likely to benefit than later sown crops that are at the three to four leaf stage. Waterlogging will have a greater impact on these less developed plants.
“Basically, if the crop hasn’t been impacted by waterlogging and has potential and N wasn’t applied pre-plant or during the wet period, it’s likely that an N application will deliver a benefit. Assess each paddock and crop type on a case-by-case basis,” Mr Laycock said.
“If the waterlogging has been so severe that the crop is looking yellow and currently showing no signs of active growth, then the value of an N application now is questionable.
“Continue to monitor these crops for new root growth and be prepared to apply N to these paddocks when an opportunity arises.”
He said growers wanting to plan an N strategy over the coming weeks should start by answering a few simple questions.
“Firstly, how long was the soil flooded for? If it was just a few days it is probably not as much of a concern as if it was a week. The longer the waterlogging period during winter, the greater the potential for nitrate losses.
“How long ago was the N applied? If ammonium nitrogen or urea was applied a few days before the extended wet period this N may be at risk of loss. Additional N may need to be applied to these paddocks.
“If the waterlogging event happened several months after the N was applied and the crop was well grown by this stage, the crop may well have taken up the N it needs.”
Available N and therefore application rate will be guided by estimated crop yield potential, weed emergence/density as well as whether N has already been applied this season, either in-crop, at planting or pre-planting.
It is also impacted by the quantity of N lost through leaching or denitrification as a result of waterlogging.
Nitrate leaching is a physical process that occurs with the drainage of water through the profile while denitrification is a naturally occurring process whereby in anaerobic conditions bacteria consume the oxygen molecule from soil nitrate which allows N gases to form which are subsequently lost from the soil.
Losses from denitrification are minimal in most dryland winter crop systems, but can be higher during long periods of waterlogging and as soils warm in spring.
One of the best ways to assess how much nitrate remains in the soil profile after an extended wet period is to take soil cores and test for nitrate. Importantly, as denitrification can be extremely variable, growers need to test wetter areas separately to drier parts of the paddock.
For more information on denitrification and its management, download the GRDC Denitrification Fact Sheet at this link.
A range of GRDC Updates papers on N use efficiency and N management in the northern cropping region are available in the research and development section of the GRDC website.
Jim Laycock, IPL technical agronomist
Sarah Jeffrey, Senior Consultant Cox Inall Communications
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