The dependence of nitrate content of vegetables on biological peculiarities and growth conditions

Field and greenhouse vegetables experiments (1972...1992) have indicated, that the nitrate content of vegetables depends, in addition to fertilization also on several other factors e.g. on the biological peculiarities of vegetables and on growth conditions. There was a major dependency between the nitrate content and species. According to the level of nitrate accumulation (in dry matter), the vegetables grown in Estonia can be divided into the following groups: 2500 mg/kg - chinese cabbage, spinach, lettuce, celery and parsley; 1000...2500 mg/kg - beetroot, radish, winter radish; 400...1000 mg/kg - white cabbage, rhubarb, leek, pumpkin, marrow; 100...400 mg/kg - white cabbage, carrot, turnip, cucumber, cauliflower, celery, root parsley, scorzonera; 100 mg/kg - tomato, sweet pepper, onion, garlic, pea, bean, brussels sprouts. The nitrate content of vegetables depended also on the variety. Early varieties contained more nitrates than late ones. The F1- hybrids of white cabbage contained relatively little nitrates. The different parts of the plant accumulated nitrates differently. During the growth period the nitrate content of plants decreased. The instability of weather conditions of different years during the main growth period affected considerably the nitrate content in vegetables. In years rich in precipitation and poor in sunlight, vegetables contained more nitrates than in years with little or normal amounts of rainfall and lots of sunshine. The accumulation of nitrates was favoured by a shady growing place, as well as by insufficient light in greenhouses. Intense additional lighting in winter months reduced the nitrate content of greenhouse vegetables. The nitrate content was also affected by soil type, the occurence of plant diseases, pests and weeds, the use of some herbicides, sprinkling, etc. In greenhouse and under provisional plastic covers, the vegetables contained more nitrates than the field vegetables at the same growth stage.