Peat mining destroys valuable nature areas and contributes to the greenhouse effect. This warrants the search for alternatives for peat in potting mixes. Composted biowaste could provide such an alternative. An additional advantage of (partially) replacing peat by compost is the increased disease suppressiveness. In this study, nine commercial composted biowastes were tested for disease suppressiveness using the pathosystems Pythium ultimum-cucumber, Phytophthora cinnamomi-lupin and Rhizoctonia solani-carrot. Increased disease suppression was found in compost-amended potting mixes for all three pathosystems. The level of disease suppression ranged from slight stimulation of disease to strong suppression. Suppressiveness against one disease was not well correlated with that against the other diseases. The CO2 production, a measure of general microbial activity, was the parameter most strongly correlated with the level of disease suppression.
Wetsieving the biowaste with tap water over a 4-mm sieve prior to composting yielded a compost with an 2.4-fold increase in organic matter and a twofold decrease in EC and Cl--concentration of the compost. The latter reductions allow for an increase of the amount of peat that can be replaced by compost. A linear relation was found between the amount of compost added to the potting mix and the level of disease suppression indicating the potential for increasing disease suppressiveness of potting mixes by replacing peat by high-quality composted biowastes
