Previous projects have demonstrated that compost and management residues are good peat replacers for container cultivation of ornamentals. Composts and management residues have a specific microbiology that potentially enhances plant resilience and disease resistance. Our goal is to further increase the sustainability of the ornamental sector by an optimal working microbiology of the growing medium and by the replacement of peat in growing media by local and sustainable alternatives such as compost and management residues, e.g., from heathlands. This leads to a lower use of chemical crop protection products and less nutrient losses. A first step is the assessment of the stability of these materials. Different batches of composts and management residues were tested for their chemical and biochemical composition, were incubated to assess the N fixation risk, and the C mineralization was assessed by CO2 flux measurements. Results on the potential for predicting the N fixation risk and the stability based on the biochemical and chemical characteristics indicate that management residues behave different from composts. The methods for stability assessment should be selected and/or combined as a function of type of material, the dry bulk density, and the risk for N immobilization.