The earthworm Eisenia fetida is the most commonly used worm for worm-supported composting of organic residues. Within the present study, the potential of E. fetida for decomposing grass clippings, an organic waste which usually causes anoxic conditions and thus insufficient degradation in the course of common composting, was investigated. To enable a thorough investigation, the substrate-related requirements of E. fetida were studied using so-called avoidance tests. These tests provide a sensitive method for evaluating the preferences and aversions of soil animals related to substrate ingredients in a sublethal range. E. fetida favored relatively moist soil with about 70% of the WHC(max) and the most preferred concentration of fresh grass clippings within soil was 15% (v/v). Pretreatments of the grass clippings like silage, precomposting or inoculation with the fungi Trichoderma viride and Geotrichum klebahnii were investigated and point to an increased tolerance of the worms against pre-composted and inoculated grass whereas ensiled grass and remoistened hay was avoided. The optimum concentration of ammonium for E. fetida was 18 mu g NH(4)(+)-N g(-1) DW soil although the worms could withstand much higher concentrations. Lactic and acetic acid, intermediates that are quickly released from fresh lawn clippings under oxygen lacking conditions, were indicated to be the most important factors for preventing worms from tolerating higher concentrations of grass clippings.