A series of field and pot trials were carried out to determine the effects of growing wheat and oilseed rape in soils supplemented with green waste composts and provided with additional fertilisers. It was shown consistently that the response of wheat and rape to compost and fertiliser applied together was greater than the responses to the individual additives, but only when very stable compost was used (>10 months processing). Experiments with N-15-labelled fertiliser showed that wheat was able to utilise the applied N more efficiently when cultivated in the stable compost. The enhanced growth was also demonstrated in hydroponic culture of oilseed rape with water extracts of green waste compost in the presence of compound fertiliser. However the effect was rapidly lost at higher dilutions of compost extract (>3). It was concluded that water-extractable growth promoters are present in stable green waste compost, but these only have measurable activity at high concentrations. The identity of the growth promoting factors remains to be found, but the literature suggests that water-extractable humic substances or cytokinins may be involved. (C) 2003 Elsevier Ltd. All rights reserved.