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2016 Soil Nitrogen Survey highlights the need for regular N testing

By Rob Jones

Rob Jones

23 May 2016

Cumulative results from more than 2000 soil samples taken on farms throughout the country this spring as part of CF Fertilisers' N-Min® testing service highlight significant seasonal variations in Soil Nitrogen Supply (SNS).

This, the company emphasises, highlights the importance of testing soils for Nitrogen content every spring to ensure that only the exact amount of additional N needed to achieve optimum yield, quality and economic return is applied.

"Most farms still rely on the standard Soil Nitrogen Supply index system in the Defra Fertiliser Manual (RB209), but these figures are simply averages and cannot account for individual field and/or crop variations," states CF Fertilisers agronomist Allison Grundy.

"Growers who rely on them will therefore only ever get their Nitrogen application right around 30 per cent of the time. To ensure that they apply the optimum level they will almost certainly need to tailor these to their own conditions and take account of seasonal variations.

"Without effective testing it is impossible to know for certain how much Nitrogen is present in the soil and available to the crop. This becomes particularly important following extremes of weather, such as the record rainfall and higher-than-average temperatures recorded in many parts of the country this winter, which could have moved Nitrogen and other nutrients further down the soil profile and, critically, out of the rooting zone.

"At the start of the year it seemed that everyone was suggesting that soil Nitrogen levels would be low this spring and it would have been easy to assume, based on rainfall, that your farm had moved into a different category in terms of the RB209-index system. 

“In fact, our data  shows that 2016 and 2015 were actually very similar and that more samples measured lower for soil Nitrogen in 2014 than in 2015 or 2016. However, it was quite comforting to see that the general profile of the distribution graphs for soil Nitrogen were quite similar, despite substantial differences in seasonal weather patterns.”

Analysis of 2000 soil N-Min results by Hill Court Farm Research Ltd, one of the few independent soil analysis laboratories, also highlighted the changes in soil N levels from 2014 to 2016

The figures highlight the importance of annual soil testing but the amount of N already taken up by the crop must not be forgotten, as this contributes to the overall SNS calculation, Allison Grundy emphasises.

"Fertiliser represents the largest single arable crop input, but also offers the greatest return on investment. However, our experience shows that most farming businesses are 20kg to 30kg/ha out in terms of the Nitrogen that they apply, either above or below the optimum level, while those that have never checked their soil Nitrogen could be out by up to 50kg/ha.

“Either they are applying too much Nitrogen, thereby risking the local environment and wasting money, or using too little and missing out on additional crop performance, either of which will make a big difference to profitability.

"At a time when margins are under extreme pressure because of low crops prices the key is to fertilise for what the soil is actually capable of producing, not what you think it can produce. To do that you have to know how much N is in the soil, how much N will become available to the crop during the growing period and how much has already been taken up by the crop.

“To calculate that we need to know plant populations and tiller numbers, together with the farm's yield and quality aspirations. We then use CF Fertilisers' N-Calc system to calculate the optimum level of Nitrogen to apply to cereal and oilseed rape crops.

"The key to optimising crop performance is never to over-estimate Nitrogen supply, so you have to be certain that the fields which are sampled are representative of the farm as a whole. Typically, on a 1000-acre farm you would take four samples and block fields that represent the whole area.

The best time to take N-Min soil samples is in the spring before the first fertiliser application and at least six weeks after the last manure has been applied, Allison Grundy says.

"N-Min testing is even more important where nitrogen-containing products such livestock manures, sewage sludge, digestate from AD plants and paper crumb, are applied. Although these are a potentially valuable source of Nitrogen, their contribution varies significantly, especially as most will be applied in the autumn and lose Nitrogen over the winter. Poultry manure, for example, is high in Nitrogen which is very available, but prone to losses.

"Applying products such as these results in greater variation in soil N, and without testing it is impossible to know how much additional Nitrogen will be required. This season, for example, the majority of manures were applied in the autumn, so over the mild, wet winter it is inevitable that some of the Nitrogen they contained was mineralised and either lost to the environment or taken up by the crop. N-Min® enables you to identify precisely what the N contribution from these sources will be. 

"All this supports what industrial businesses have been saying for years, namely that 'if you don't measure you can't manage'. Certainly, the results of our tests show that to be true when it comes to assessing how much Nitrogen to apply.

“Every season is different, so the key is not to assume that your farm will be in the same SNS category each year, especially if organic manures have been applied or if  it has rained heavily over the winter, due to likely leaching loss of soil Nitrogen. Instead, the nutrient status of your soils should be tested by taking representative samples during the spring so that any shortfall in recommended levels can be made up through spring application of N, P, K and S."