Use of composts for habitat restoration offers advantages in terms of efficient use of resources. Chemical amendment of compost to reduce its pH and P availability was investigated in order to improve suitability for use in reclamation of blocky quarry waste to acidic grassy heathland. The effect of these amendments was observed on competition between two grass species: Agrostis capillaris and Festuca ovina. A factorial, pot-scale greenhouse experiment was set up using two composts (one a mixture of green waste and catering waste, and the other a mixture of green waste and sewage sludge). In addition, two soils were collected from upland acidic grassland to provide a natural comparison. S0 was applied to reduce soil pH, and Fe(OH)3 from a coal waste treatment plant was applied to counteract the expected increase in P availability due to acidification of the composts by S0. Addition of S0 significantly reduced soil solution pH and addition of Fe(OH)3 significantly reduced soil solution P concentration. In one compost S0 reduced the biomass of F. ovina while increasing that of A. capillaris, whereas Fe(OH)3 had no significant effect on the biomass of either species. Although S0 and Fe(OH)3 did adjust the chemical properties of the soil solution, Fe(OH)3 did not bind P strongly enough to make it unavailable to plants. Further work is required, however, the use of chemically amended composts provides a sustainable sink for organic wastes and we conclude from this study that they have great potential for large-scale restoration of blocky waste tips.