Effect of nitrogen nutrition and global radiation on yield and nitrate content of lettuce grown under glass

Abstract

Abstract 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.