Composted recycled organic matter suppresses soil-borne diseases of field crops.

• Suppression of soil-borne diseases of horticultural crops by composts has been attributed to the activities of antagonistic microorganisms. The potential for abiotic disease suppression in field-grown crops was explored using glasshouse-based bio-assays for five pathogens of agricultural crops and eight formulations of recycled organic matter compost. • The effects of compost formulation and postmaturation processing were investigated by comparing disease severity on recycled organic matter composts with a commercial seedling compost. Comparisons were made within native and autoclaved series of composts. Autoclaving perturbed compost chemistry without the addition of other materials. • The severity of autoclaving-induced changes to compost chemistry were related to formulation and processing issues. Extractable carbon content, nitrate-N content and pH all contributed to the suppressive capacity of recycled organic matter composts. There may also be a biotic component to the suppression observed. • For each pathogen the influential chemical characteristics of suppressive composts were the same as those identified for soils in other studies. There is potential for the suppressive capacity of recycled organic matter composts to be improved.