RESIDUAL SOIL NITRATE UNDER INTENSIVE AGRICULTURE

From November 1991 until January 1992, a 956 km2 area of the loamy san d to sandy loam region of West-Flanders (Belgium) was sampled for resi dual soil nitrate, i.e. the amount of nitrate nitrogen in the soil pro file at the end of the growing season. In total, 296 soil samples were taken to a depth of 90 cm (in layers of 30 cm) on arable land only. O n basis of the rate of the mineral nitrogen losses occuring during thi s extended period, all residues present on the date of 21 November 199 1 were estimated. This date corresponded with the start of the samplin g campaign and is close to the end of the growing season. The residues were lognormally distributed around a mean of 177 kg NO3--N/ha with a minimum of 24 and a maximum of 881 kg NO3--N/ha. Classifying the resi dues into groups according to the preceding crop, the lowest residues appeared after beets (a median of 59 kg NO3--N/ha). Between cereals, m aize, potatoes, and vegetables, no significant differences were found. Each of these four groups had a median residue of about 170 kg NO3--N /ha. The high amounts found after cereals were due to a continuing min eralization of the soil organic matter during the three months between harvest and sampling time. This residual soil mineral nitrogen must b e limited to minimize the NO3--N drainage losses during winter which o therwise enhance the nitrate concentration in ground and surface water s.