Under the current version of the Sustainable Nitrogen Management Programme (PGDA III), growing vegetables is prohibited for a period of two years after the destruction of permanent pasture. However, many organic market gardeners set up on former grasslands in the search for soils rich in organic matter and free from phytopharmaceutical products.
To assess the acceptability of this ban, which is considered a hindrance to the development of organic market gardening, the CRA-W cross-functional Research Cell in Organic Agriculture (CtRAb) monitored four neo-market gardening situations established on former permanent grasslands throughout the 2016 and 2017 growing seasons.
The study confirmed that destruction of former pasture releases significant amounts of mineral Nitrogen (N). In the first year after the destruction, the supply in N is of the order of 150 to 200 kg/ha for mown grassland, and 200 to 300 kg/ha for unmown grassland. Where there has been manure or extensive livestock presence in previous years, the supply can be as high as 500 kg/ha.
Due to this high N input, many of the crops monitored exceeded potentially leachable nitrogen (PLN) thresholds. The establishment of crops with a high affinity for N, and the maintenance of permanent soil cover, are the main guidelines for efficient management of nitrogen fertility in plots cultivated after grassland. With this in mind, the establishment of intercrops (CIPAN) and relay crops can take up mineralised N efficiently at the back end of the season.
In 2017, 130 of the 173 organic market gardeners listed in Wallonia cultivated an area less than 2 ha. Organic farming market gardening established on former grassland therefore covers a very small area. Moreover, the critical period for nitrate leaching is only a two-year transitional stage. It therefore seems appropriate to relax the legislation in this matter, while ensuring that restrictions still apply to the areas concerned and that market gardeners are guided by advisory bodies towards suitable technological pathways.
A results sheet and good agronomic practice leaflet are available on the site: http://www.cra.wallonie.be/fr/implantation-de-cultures-de-legumes-apres-prairie-permanente