New nitrogen management guidelines for winter wheat published by the HGCA take growers step-by-step through calculating a N recommendation and judging its success.
But, in a departure from both the yet-to-be published Fertiliser Manual and its predecessor RB209, it calculates the crop’s N requirement rather than using look-up tables to find a value.
“We’ve never had a way of calculating the crop’s N requirement before in the UK, we’ve always gone for look up tables,” guide author Roger Sylvester Bradley of ADAS told Farmers Weekly after speaking at the HGCA Agronomist’s conference.
|Crop N requirement = (Crop N demand – Soil N supply) / Fertiliser N recovery|
Those tables were based on finding a soil N index from previous cropping, soil type and over-winter rainfall, and then looking up a recommendation based on that index, he explained.
“We’ve done some analysis of look-up tables and, frankly, they’re just not that accurate,” he said. “We’re using the same rules but have worked out a way of putting it into an equation.”
Essentially that equation (see box) calculates the amount of N that would be needed to satisfy the crop’s total N demand after taking into the amount of N in the soil.
Growers using the guide will be taken through a series of steps that first judge crop N demand, then assess soil N supply, before calculating the crop’s N requirement.
Yield expectations are used to help determine the crop’s N demand, he notes, another departure from previous practice. “It was something we used in the 1980s. The concern is whether it will cause people to over-fertiliser based on aspirational yields.”
Soil sampling guide
|The new guide put a much greater emphasis on soil sampling to determine soil supply than ever before, Prof Sylvester Bradley said.
The old field assessment method was downplayed, he said, with a single diagram based on previous cropping, soil type and excess winter rainfall indicating whether the soil nitrogen supply was going to be low, medium or high.
Where the result suggests the soil nitrogen supply is high it suggests growers check with a soil mineral N test, while testing barometer fields is a good idea where a medium level is judged likely.
Only if the table suggested the SNS was likely to be low was a test unlikely to be worthwhile, he said.
That was why reviewing the success of N applications was important, he added. “The guide highlights farmers need to be good detectives. Unlike herbicide or fungicide applications it is not easy to see when you get N applications wrong, unless it is grossly over or under.”
The best way of judging optimal N use was to monitor grain protein levels, particularly for feed wheats, where the target should be 11%, he said. “It is correct about 70-80% of the time.”
That meant it was best to average results over several fields or years to draw conclusions, and then factored into the next season’s plans.
New N guidelines