Cut N dose according to soil nitrogen?

Reducing nitrogen recommendations to take account of soil mineral nitrogen levels of less than 100kgN/ha as suggested in HGCA‘s new Nitrogen for winter wheat – management guidelines booklet are being questioned by The Arable Group.

The guidelines, being distributed to levy payers this month, suggest growers should take account of soil mineral nitrogen levels, as well as the nitrogen in the crop when soil is sampled.

By deducting this total soil N supply from N required to meet the crop’s demand, growers can work out what their crop N requirement will be, the guide says.

But TAG’s Richard Overthrow says the firm’s research does not support applying less N in typical arable rotations. “Commonly soil mineral N in arable rotations is around 30-60kg/ha and it is unusual for it to be more than 100kg/ha.”

In the firm’s trials there was no relationship between soil mineral N levels less than 100kgN/ha and the optimum dose of applied N, he explains. “The simple conclusion is where soil mineral N is in double figures it is not high enough to warrant cutting back on applications.”

Typically the guide suggests for every 30kgN/ha of soil mineral N, growers should reduce the total N applied by around 50kgN/ha.

But not taking account of soil N ignores three decades of research, the guide’s author, Roger Sylvester Bradley of ADAS, says. “There are many papers showing the N requirements after break crops, such as oilseed rape and pulses are less than after cereals or sugar beet. If you don’t apply any N at all, you get 1-2t/ha more yield after break crops than after cereals. It is yield not provided for by fertiliser, and you have to have a way of dealing with that.”

He says effectively ignoring soil N below an arbitrary figure of 100kgN/ha is impractical. “You don’t always know which side of 100kgN/ha you are on, and what do you do if you’re over 100kgN/ha – suddenly start cutting back a lot?”

He acknowledges no calculation method can be perfect, but is disappointed that TAG is questioning the method used in the new booklet.

“TAG has been involved through the process and I thought we had come to a consensus. Questioning the method now risks confusing farmers and devaluing the HGCA guide and the new Fertiliser Manual, when it is published.”

Yara‘s Mark Tucker says testing soil mineral N levels are the most accurate for determining what N is in the soil. “We need to base recommendations on current knowledge, and measuring soil N is better than the field assessment method.”

But he also suggests caution on how to interpret it, even at higher soil N levels.

“What the TAG data is supporting is whether to adjust by any drastic amount. At the moment the calculation gives quite a steep change.

“We must have a system that accounts for soil N, but we have to be cautious about what we expect at the moment. We can’t be accurate to the last kilogramme.

“I’m not saying either approach is wrong, but it does expose the need for much better understanding of soil N, what the measurement means and how efficiently it is used.”

For this season he suggests growers have to recognise there are errors in any system, whether it is using a flat rate based on soil type and crop yield, the HGCA guide or the new Fertiliser Manual, when published. “That means monitoring in-season to give the ability to re-assess and react if the soil is not responding as you expected.”

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