Poor Nindex giving wrong picture – ADAS
By Robert Harris
NITROGEN recommendations based on the existing index system are imprecise and often result in over- or under-application.
A new "nitrogen accounting" system is needed to overcome that, says Roger Sylvester-Bradley of ADAS Boxworth. "Current nitrogen recommendations are dishearteningly poor."
Dr Sylvester-Bradley has calculated that even if each of the 24 categories of commonly-grown arable crop were fertilised with recommended amounts of N and produced average yields, the difference between the amount of N applied and that removed would range from 5-175kg/ha. The group of crops for which the difference exceeds 50kg/ha over or under the optimum include both N index 0 and index 1 examples, suggesting the current system is too coarse.
Another problem is that the current index takes no account of N carryover from the previous crop. Both under- or over-fertilising are quite common, says Dr Sylvester-Bradley.
"You would hope for an error of less than 20kg/ha. In fact, a study of 90 fields in 1992 showed less than half were within that optimum, and 20 were more than 50kg/ha out. That shows there can be inaccuracies even if current recommendations are followed."
To complicate matters further, crops only recover 40-80% of fertiliser nitrogen. On heavier soils, most of the rest is bound up in organic matter. And 20-65% of that which is taken up may be returned in crop residues. Both of these sources will be available to the following crop, but in varying amounts and at different times.
For an N accounting system to be truly accurate, both organic and atmospheric N must be allowed for. Best estimates so far suggest they could provide 40-60kg/ha – about one third from the former, the rest from the latter.
Experiments on heavier, retentive soils suggest N levels can be reduced by 30% of the total applied to a previous rape crop, and at least 10% of that applied to a previous wheat or potato crop, says Dr Sylvester-Bradley.
• Insufficient allowance for previous crop type.
• Does not allow for over- or under application in previous crop.
• Takes no account of organic matter and atmospheric supplies.