Once the New Year is over, farmers’ thoughts turn to nitrogen applications (amongst many other things). NIAB TAG has become particularly interested in nitrogen dose since the introduction of NVZs, increased nitrogen costs and the full realisation of its environmental impact.
In the first couple of years of cross compliance there were many queries from members on the dose of nitrogen for wheat following oilseed rape. The then RB 209 (7th edition) stated that the dose should be as low as 100 kg N/ha following a dry winter and this was the value used by The Environment Agency inspectors to check compliance with the NVZ regulations.
It was clear to everyone that this recommendation was far too low and that following it would lead to sub-optimal crops and margins. To give The Environment Agency credit, they also soon realised that this recommendation was far too low and their inspectors accepted higher doses where justified.
The most recent (8th) edition of Defra’s RB 209 significantly increased the recommendations for wheat following oilseed rape but the issue rankles with advisers and farmers who still consider them to be too low.
One basic assumption in RB 209 is that 100% of Soil Nitrogen Supply (SNS = Soil Mineral Nitrogen plus N in the crop in February plus any allowance for mineralisation) is used by the crop but only around 60% of the applied fertiliser nitrogen is utilised. There are historical reasons for assuming 100% efficiency of use of SNS. In former years, there was more organic material in soils that would release nitrogen during the season, but those days are long gone. Nowadays the majority of arable land has lower organic matter levels and does not receive organic amendments and manures.
This assumption means that for every additional kg/ha of SNS, the recommended dose of applied nitrogen is reduced by around 1.5 kg/ha. Hence, after crops that may leave higher nitrogen residues, as assumed after oilseed rape, the recommended optimum applied dose for feed wheat is reduced disproportionately.
NIAB TAG approached HGCA with its concerns and they financed two projects on the subject (HGCA Project Report 490 - establishing best practiced for estimation of SNS and HGCA Research Review 58 - SNS testing: practice and intepretation). The field studies concluded that the efficiency of use of SNS is typically much less than 100%. In fact, my analysis of the data generated in the field studies suggests that it is below 50% for long term arable soils that do not receive organic manures or amendments.
This means that instead of reducing the applied N dose by around 1.5 kg/ha for every additional unit of SNS the figure may well be around 0.75 kg/ha. I’ve tried this with a retrospective look at around 70 NIAB TAG nitrogen dose trials in wheat that have taken place over the past 10 years or so. These were all carried out on long-term arable soils where no organic manures or amendments were used.
The conclusions are firstly that the Field Assessment Method in RB 209 provides a more accurate prediction of the economic optimum for feed wheat than using the SNS measurement method. Secondly, the accuracy of predicting the optimum is improved by changing the recommendation tables on the basis of an assumed efficiency of use of SNS of 50% rather than 100%. However, this is still no more accurate than using one fixed amount of nitrogen in all the trials!
The most disturbing element is how bad all the recommendation systems are, only predicting within 50 kg N/ha of the optimum in around 60% of the trials. Obviously, we’re a long way from an accurate prediction system. There may be many explanations for this and perhaps this is a subject for a future blog.
At the moment, we have to be content that progress has been made and I hope that the results of the two HGCA funded projects are incorporated into the next version of RB 209. At least it will allow us to catch up with, amongst others, the Australians and the New Zealanders who have for sometime been assuming an efficiency of use of Soil Mineral Nitrogen of between 30-60%.