We investigated the effects of several soil amendment materials, potentially useful for willow biomass crops, on the abundance, diversity, and community structure of free-living soil mites (Acari: Oribatida, Mesostigmata) in a 4-year replicated field experiment. Soil treatments included biosolids (lime-stabilized sewage sludge), chicken manure compost, urea fertilizer, black plastic mulch, and control. The predaceous Mesostigmata (40 spp.) responded differently to soil amendment treatments than did saprophagous and mycophagous Oribatida (28 spp.). The urea fertilizer had no persistent effect on mite assemblages. Plastic mulch did not affect Mesostigmata, but had a lasting detrimental effect on oribatid mites. Mesostigmatid mites benefited from application of biosolids, while Oribatida were most adversely affected by this treatment. When plastic mulch and biosolids were used together, the effect of biosolids predominated. Composted chicken manure supported abundant and diverse populations of both groups.