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Nitrogen Nutrition Effects on Nitrate Accumulation of Soil‐Grown Greenhouse Butterhead Lettuce

Leafy vegetables are the major source of dietary nitrate intake of humans. Nitrates can have many detrimental effects on human health besides some beneficial effects. A further reduction in nitrate concentration thus can represent added value for vegetable products rich in carotenoids, vitamins C and E, selenium, dietary fiber, plant sterols, and so on. Though nitrate accumulation is governed by a plenitude of factors (i.e., genetic setup, environmental conditions and nutrients), the factor most readily controllable is nitrogen nutrition. This study examined the dependence of butterhead lettuce crops' nitrate concentration on soil mineral nitrogen (N) content. It was shown that the effect of the soil's mineral N content at harvest was strongly associated with the nitrate concentration of lettuce at a low to intermediate mineral N content (<100 kg N ha−1). This demonstrates the importance of N‐fertilization practices. Results of 24 comparative experiments, based on two different N‐fertilization recommendation procedures, did show that an N application difference at the start of the growing period has an effect on the nitrate concentration at harvest. A lower N application at either recommendation resulted in 71% of the experiments (17/24) having a lower leaf nitrate concentration. The head weight was negatively affected in only 2 of these 17 experiments. Minimizing the N input and hence lowering the soil N content at harvest points to the possibility of further reducing the nitrate concentration level in greenhouse lettuce, while having no significant negative effect on economic yield.