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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.

The concentrations of nitrate as nitrogen (NO3–N) in mg/L and nitrite as nitrogen (NO2–N) in µg/L were determined in water samples from 60 boreholes, 21 shallow wells and 7 springs located in the four regions of the country. For all the 88 sampling points nitrate levels vary within 0.05 plus or minus 0.00 – 28.44 plus or minus 0.80 mg/L NO3–N, out of which 10.22% have mg/L NO3–N ≥ MCL (maximum contaminant level) of 10 mg/L NO3–N. Shallow wells recorded the highest NO3–N% ≥ MCL of 20.0% followed by boreholes with NO3–N % ≥ MCL of 8.33% while for springs, the NO3–N % ≥ MCL was zero. On the other hand, NO2–N (μg/L) from all the sampling points vary within (ND/0.30 plus or minus 0.00 – 297.0 plus or minus 0.50) μg/L NO2–N, which were far below the recommended MCL of 1 mg/L (= 1000 μg/L).