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Previous research has demonstrated that composts (COM) and woody residues from nature management (MR) are potential peat replacers for growing media, but their compositions are highly variable. Our goal is to make growing media more sustainable by optimizing the selection of local and sustainable alternatives for peat. Different batches of COM and MR were incubated to assess the microbial activity based on (1) the N drawdown risk, (2) the C mineralization and (3) the inoculation efficiency of a commercially available biocontrol fungus. The various batches were characterized based on biochemical, chemical (pH, available and total nutrients) and microbiological biomass analysis. COM and MR were scored based on chemical or stability characteristics to assess their suitability to replace peat, lime and fertilizers in growing media. This score allowed for a clear differentiation between the materials; MR received higher scores on average than COM. Five composts were further tested for the effect of storage after blending with an acidic MR, acidification with elemental S, or removal of the finer fraction. One batch of chopped soft rush was acidified with elemental S. Blending and acidification were the most effective treatments as they resulted in a clear increase of the suitability score.