A number of species-rich vegetation types (e.g. Violion caninae and Cirsio-Molinietum communities) are present in Dutch heathlands and heathlandrelated ecosystems, besides the species-poor communities, such as the Ericion tetralicis and the Calluna-Genistion pilosae. The major causes for the variation in vegetation types are differences in soil nutrient content, soil acidity, in hydrology and in succession stage. Under nutrient-poor conditions acidity-related factors as soil pH, aluminium/calcium ratio and nitrogen form (NH4+ or ND3“) are discriminating factors. A field survey showed that vegetation types, such as the Cirsio-Molinietum and the Violion caninae were only found on sites varying in pH from 4-5 to 6-5, where both NH4+ and N03" were present and where the aluminium/calcium ratio was relatively low (generally below 5). In contrast to this, Ericion tetralicis and Calluna-Genistion pilosae communities were found on more acidic soils (pH<4-5), where NH4+ was the major nitrogen form and aluminium/calcium ratios were higher. Furthermore, ND3 “ was often not detectable. Growth and development of Cirsium dissectum (Cirsio-Molinietum), Arnica montana (Violion caninae) and Calluna vulgaris (Calluno genistion pilosae) were studied on hydro cultures with different nitrogen forms and NH4+ concentrations. C. dissectum and A. montana developed best in the absence of ammonium. When ammonium was added, both species showed signs of ammonium intoxication and mortality increased with increasing ammonium concentration. C. vulgaris developed best with ammonium as the nitrogen source. Even in the presence of high ammonium concentrations (up to 1000 pmol ~ ‘) C. vulgaris developed morphologically normal, with only a small reduction in biomass compared with growth on low ammonium concentrations.