Arable Focus: Debate over soil mineral nitrogen, but still value in soil testing

Ahead of the long-awaited Fertiliser Manual RB209, which is due for publication imminently, uncertainties continue surrounding soil nitrogen supply (SNS) and its relation to nitrogen recommendations.

According to TAG’s Richard Overthrow, the HGCA’s nitrogen guidelines for winter wheat growers, published at the end of last year, say the crop can use soil mineral nitrogen (SMN) more efficiently than applied nitrogen.

“They suggest for every 20kg per hectare of soil mineral nitrogen available, then growers should reduce applied nitrogen by 30kg/ha.”

Mr Overthrow points out TAG has a difference of opinion regarding the advice, on the basis of the organisation’s nationwide trials programme - focusing purely on arable farms.

Optimum dose

“We’ve looked at the contribution of SMN levels across 50 N response curve trials, plotted against optimum doses applied. In most cases, SMN levels were between 30-80kg/ha, and for each of these trials the optimum end dose was between 200-240kg/ha of applied N.

“If there had been a contribution from the SMN, then you would have expected to see a difference in crop performance in the trials. The fact is we didn’t see a response at those levels.”

However, once SMN availability exceeds 100kg/ha, then it does have an influence over how much nitrogen fertiliser is applied, and some TAG trials have demonstrated this, says Mr Overthrow.

“It’s certainly true to say that, around 100kg/ha, SMN starts to have an affect, but if you sample ahead of applications and some fields have 30kg/ha SMN, or some have 60kg/ha or 70kg/ha SMN, then our research suggests it would not make any difference to overall nitrogen applications.

“We understand while every farmer in the country has to be catered for by the guidelines, and therefore adjustments have to be made, the majority of arable farmers in the country are likely to have less than 100kg/ha SMN on their land.”

Mr Overthrow’s advice to growers this spring is to get soils tested across a representative sample of fields, prior to making application decisions. “With all the regulation that exists as well, it’s worthwhile carrying out tests to determine nitrogen requirements as accurately as possible.”

ADAS research scientist Dr Daniel Kindred, is concerned there is a danger of causing misunderstanding about assessing soil nitrogen supply.

Expert opinion

“The approach taken in the guide, and in the Fertiliser Manual, represents a consensus of opinion of experts following wide consultation with the whole industry.

“The consensus judgement based on all evidence from 30 years of trials is that adjustments should be made for soil nitrogen supply levels below 100kg/ha,” he says.

“We have to remember that there is substantial imprecision in any prediction of soil nitrogen supply whether by look-up table or soil measurement. There is also imprecision in the measurement of N optima from experiments, which is affected by differences in crop N demand and fertiliser N recovery, as well as differences in SNS.”

Dr Kindred says he wouldn’t expect a close relationship between SMN and N optima at low SNS levels. However, the expert judgement has been, and continues to be, that the same principles of SNS and N decision making apply at lower SNS levels as at higher levels.

Prediction

In the HGCA’s ‘Nitrogen for winter wheat guide’, a basic SNS prediction can be made from a diagram incorporating previous cropping, soil type and winter rainfall. Initial readings from this can indicate whether a specific soil sample for SMN is likely to be worthwhile.

“If the prediction is that SMN levels are likely to be low, then you may want to sample a few fields occasionally to confirm this, but further soil analysis is seldom worthwhile.

“But for medium SMN predictions, it’s worth sampling a representative selection of fields and, where high or uncertain SMN is expected, it’s well worth further analysis to check.”

The guide shows growers have to accept some errors and imprecision when making decisions on nitrogen application calculations. So while it is difficult to be certain about N requirements for individual fields, the important thing is to be right with average applications across the farm and to identify any fields where N rates are very wrong.

“This means we need to monitor and review success of our N management. Routine soil nitrogen testing can be an important part of this, but the best measure we have for monitoring success is grain protein content.”

If grain protein for feed wheats is consistently around 11 per cent (or 1.9 per cent grain N percentage) across fields and years, then N management has probably been about right. Much more than this indicates that too much has been applied, while less than this indicates we’ve applied too little, says Dr Kindred.

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