Nitrogen

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Abstract
During 1983–5, 46 trials were conducted on commercial vegetable crops in Eastern England to test the value of sap and soil nitrate tests for predicting N top‐dressing requirement. This paper summarises the results of the 20 brussels sprout trials. Optimum rates of top‐dressed N (Nopt) showed no simple relationship with sap or soil nitrate measured just before top‐dressing, nor with an index based on previous cropping, or the growers’ own practice. A multiple regression of Nopt on soil nitrate (0–60 cm) and an estimate of mineralization following top‐dressing accounted for 30% of the variance of Nopt. Consideration of average yield deficits resulting from various strategies shows that, from the point of view of growers’ profits, the safest approach is to apply a top‐dressing of 120 kg N. ha‐1 in all circumstances.

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