A Future for Vegetable Cultivation Within the Flemish Manure Decree?

Abstract

Most vegetables, despite N fertilization recommendations, are heavily fertilized, leaving high NO3 −‐N residues in the soil profile at harvest. This contrasts with the Flemish Manure Decree stating that the NO3 −‐N residue in the soil profile (0–90 cm) must remain below 90 kg ha−1 during a fixed curfew (i.e., October 1st until November 15th). This study was conducted to examine the feasibility of this prevailing standard for properly fertilized open‐air vegetables (cauliflower, leek, lettuce, etc.). During 3 consecutive yr (i.e., 2000–2002), a scientifically sound N fertilization recommendation, based on an Nmin measurement in the soil profile, was given for 136 experimental fields. Yield and product quality, as well as the NO3 −‐N residue, were determined for each plot, and results were compared with a control plot. The results showed that during the curfew, only 44.9% of the experimental plots remained below 90 kg NO3 −‐N ha−1 as demanded by the Flemish Manure Decree. For the years 2000 and 2001, even at the lowest NO3 −‐N‐residues, no significant loss of yield and quality was observed for the harvested vegetables. However, results of the year 2002 showed distinct losses for both yield and quality, especially for cauliflower and leek. The results further indicated that for the vegetables mentioned above, which have a limited rooting depth and root development, the excessive NO3 −‐N rates are partly linked with a high organic matter content in the soil and/or a labile organic matter pool, giving rise to large uncertainties in N fertilization recommendations. Thus, it appears to be rather difficult to meet the needs of the market for fresh vegetables, especially during late summer and early autumn, and meet the requirements of the manure decree. Furthermore, lowering the NO3 −‐N residue in the soil after harvest will require specific measures and will result in an increase of the production cost for Flemish vegetables.