Accumulation of Nitrates in Leafy Vegetables Grown Under Contrasting Agricultural Systems

Abstract

In the framework of the problem of nitrates in food and water, experiments with leafy vegetables are described and the results are presented. In pot and field trials with spinach, Swiss chard, head lettuce and corn salad it was found that fertilizing with composted farmyard manure—a slow release N-source—led to significantly lower nitrate concentrations in these vegetables compared to an N-equivalent NPK-application. These results were supported by similar findings for head lettuce, when the products of 7 paired conventional and biological growers were analyzed. The results indicate a very low risk of high nitrate concentrations in vegetables even with an overdose of composted farmyard manure (300 kg N/ha). Generally, the yields of vegetables grown with compost were lower than those with NPK fertilizers. However, if the right varieties are chosen, similar yields can be obtained. This was clearly demonstrated with two different varieties of spinach. The importance of variety was also demonstrated in preliminary trials with 12 different varieties of head lettuce.