Don’t Make Assumptions on N min Levels

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Don’t make assumptions on N min levels as findings from preliminary soil nitrogen measurements suggest many farms could be under fertilising their crops once again this season, whilst in other circumstances the optimum nitrogen could be much lower than expected, warns leading agronomy firm Hutchinsons.

From over 450 N-Min samples taken by Hutchinsons agronomists this spring, there is no doubt that we are seeing some interesting results, says Tim Kerr, Nutrition Manager for Hutchinsons.

The N-Min service measures the amount of nitrogen that the crop can expect to get from the soil over the entire growing season, including soil mineral nitrogen and, importantly, the additionally available nitrogen which will mineralise from the soil.

“In clay soils we would usually expect to see results after winter cereals to closely reflect the recommendations in RB209, and results show that in the west this is true. However it’s certainly not the case in the East Midlands where the results are higher than expected, which we would put down to the higher mineralisation of nitrogen over the exceptionally mild winter.”

tim kerr of hutchinsons

Mr Kerr points out that in soils where the previous crops were beans or potatoes, soils have between 15-25% less mineral nitrogen available than RB209 suggests, adding that a similar pattern is appearing on light land as well.

“Interestingly on soils following spring cereals, we would expect the SNS to be the very similar to soils following winter cereals. This year there seems to be a much bigger discrepancy, with soil mineral nitrogen levels following spring cereals showing much lower levels than RB209 predicts. This is showing up in regions from Kent to Lancashire.”

“Armed with the detail that N-Min testing provides, growers are in a much better position to decide on the optimum levels of nitrogen throughout the rotation, which will improve overall crop performance, which at current market prices, has never been more important.”

Farmacy agronomist Rob Jack, who advises growers across Suffolk and Cambridgeshire, is seeing a similar picture across many of the fields that he has sampled.  “Making sure that we are responding to the correct levels of residual N rather than assuming where soil mineral nitrogen levels should be, brings two important benefits as it avoids under fertilising soils and limiting yield where residual nitrogen is low and where relevant, prevents wasting a valuable and expensive input if it’s not actually needed.“

“On the chalky soils of Cambridgeshire we have found that by taking N-Min samples, we have avoided underfeeding our first wheats; N-Min levels in first wheats are lower than we would have expected, considering it is following a break crop, so first wheats are getting higher than normal rates of nitrogen, which in some cases are closer to those that will be applied to second wheats.”

“It’s a similar scenario in Suffolk, although levels are generally lower by 10-15kgs N/ha compared to the chalk, but we are often finding that we will need to feed first wheats as much as second wheats.”

“The winter in East Anglia has been warmer but not necessarily much wetter than normal. We have not seen the huge downpours that the north and west of the UK have had. This may explain why residual nitrogen levels are not as low as we would have first thought and are broadly similar to last year.”

Mr Jack believes that there is also a real value in growers being able to compare their individual N-Min results with regional results of similar soil type, and this is an important service to individual growers as part of the company’s Omnia Nutrition package which includes N min sampling.

“We encourage growers in a locality to confidentially pool their N-Min results, as this provides them with a snap shot of what is happening with similar soils in the region and gives context for their own individual results, which is really important. Of course this is most effective when used within a tight geographical base.”

“It also means that samples do not need to be taken in every crop in the rotation – in essence you are paying for one test but getting the pooled information from 7 or 8.

For Adam Rayner of David Rayner Farms, who grows over a 1000ha of arable crops in south Cambridgeshire, N-Min sampling on his own soils is very valuable, and even more so when he can compare his results with those in the region.

“It’s surprising what the results can show up, so it’s important to be able to get an overview of the consolidated results in the area.  We have found that nitrogen levels in second wheats are higher than we thought, and lower after break crops. By comparing this against similar soil types within the region we know that it is not an anomaly on our particular fields.”