Eleven trials with lettuce (Lactuca sativa L.) different rates of ammonium nitrate limestone (0–2.6 kg N/100 m2) were carried out in commercial glasshouses. The objectives of the study were to find the optimum nitrogen dressing for maximum production or a suboptimum of not less than 90% of maximum production, in combination with a crop nitrate content not exceeding the official limit. The nitrate content of the crop was affected by nitrogen fertilisation and by light; an interaction between the two factors was demonstrated. Under high‐light conditions, the nitrate content of the crop was low (appr. 1000 mg NO3/kg fresh) and increased considerably (up to 2500 mg NO3) with increasing N‐applications. Under low‐light conditions (400–700 J/cm2.day), the nitrate content of the crop was high (3000–3500 mg NO3/kg fresh) even without added N, and also increased with increasing rates of N, but less markedly, by about 500 mg NO3. The interaction between N and light intensity and the fact that only a small number of trials was carried out made it impossible to justifiably establish an acceptable suboptimum rate of nitrogen application.
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Eysinga et al. (1985) studied this question.