Nitrogen fertilization of vegetables. Industrial vegetables occupy 2% of the utilized agricultural area in the Walloon region; of nearly 15,000 ha., 73% is given over to the growing of legumes. Nitrogen inputs are low or zero for legumes. Experiments have shown that an appropriate supply of nitrogen to broad beans and beans contributes to a better start for the crop without increasing soil nitrogen residue after harvest. The mineralization of crop residues is responsible for the increase in soil nitrogen residue. The only way to reduce the Potentially Leachable Nitrogen (PLN: an environmental indicator in Wallonia) in the beginning nitrate leaching period is to employ good management of catch crops. The carrot has a crop cycle similar to sugar beet and has a low level of nitrogen fertilization. PLN is generally not a problem in carrots. Spinach is a nitrogen demanding crop, whose needs are well known as a result of previous experiments. Spinach is harvested during intensive growth and so its leaves sometimes have high soil nitrogen residues. The plant's short crop cycle allows double cropping. After harvesting winter and spring spinach, the mineralization of crop residues contributes to a good start for the next crop. After harvesting autumn spinach, either planting a catch crop classified as a nitrate trap or allowing the regrowth of spinach represent solutions for reducing PLN.
