Fertilization of secondary successional communities generally increases biomass but reduces diversity; its impact on primary successional communities is less well understood. Following applications of a balanced fertilizer to naturally established vegetation on slate quarry waste, effects on tree growth, ground flora species and foliar invertebrates were monitored over two years. Fertilization increased tree growth, with stem basal area increasing by 130% over two growing seasons compared with a 50% increase in unfertilized trees. Several dominant ground flora species increased in cover as a result of fertilization. In contrast, cover of bryophytes was not increased. Fertilization favoured plant species associated with drier habitats, but disadvantaged less-competitive ruderal species. Tree foliar invertebrates were less abundant on the trees on slate waste than on trees in established woodland. Fertilization made tree leaves more palatable by increasing nitrogen and reducing soluble polyphenol concentrations, and increased the abundance of sap-sucking invertebrates, without changing foliar invertebrate diversity. Overall, fertilizing this primary successional site was beneficial for biodiversity; it increased abundance of dominant plant species and foliar invertebrate herbivores, yet few taxa were adversely affected during this period. Increases in tree biomass and invertebrate abundance are likely to favour the establishment of other taxa. Where the biodiversity conservation interest is principally in closed-canopy vegetation, applying fertilizer is a cheap, comparatively non-intrusive and effective way to increase biodiversity on denuded sites. However, plant species characteristic of open and infertile habitats (particularly bryophytes) and their obligate herbivorous invertebrates are likely to decrease in abundance.