To elucidate the causes of the sharp decline in plant species diversity in heathland and nutrient-poor grasslands in The Netherlands, we investigated the spatial variation in plant species richness and the abundance of threatened plant species in relation to soil acidity and soil nutrient supply ratio. We selected 68 plots divided equally between species-rich and species-poor parts of the study area, and collected data on soil characteristics, above-ground biomass and vegetation composition in each plot. In addition, we used phytometers planted with Molinia caerulea tillers to measure the nutrient supply in the soil. Soil acidity was the variable most strongly correlated with plant species diversity in heathland and grassland communities. We found that increased N:P and N:K ratios in plant biomass had additional negative effects. The Red List species in the data set and all other species that had been declining sharply in the Netherlands since 1950 were growing in soils with pH >5 and most were growing in soils with balanced nutrient supply ratios.