The aim of this research is the definition of optimal conditions for the existence and therefore the conservation of species-rich grasslands. Four long-term experiments in the Ardennes region (Wallonia) are being monitored since 1995. Each experiment consists of 20 plots and was established on different soils (rich and poor in nutrients). Five different cutting-regimes (1 or 2 cuts per year, in June, July or August) are being applied in the different plots. Each year botanical releves are taken (with the Braun-Blanquet coverage scale) and each two years soil samples are analysed. Yield and forage quality are also taken yearly. As result of this research, positive influence of low quantities of exchangeable phosphorus on plant diversityhas been highlighted. This correlation could be partly explained by the influence of soil phosphorus content in the amount of soil available nitrogen for the plants, but partly also by the interactions of these two nutrients in plant metabolism. In high fertility soils, nitrogen is the major element controlling plant diversity. It has nevertheless been shown, by a chemical analysis of these elements in the standing biomass, that phosphorus is related to plant diversity independently to nitrogenin low fertility soils.