Standardized determination and classification of compost organic matter stability

The stability of the organic matter is a major determinant for the effect that composts will have on soil or growth substrate quality. Thus, stability testing for composts is important for optimizing the effect of compost applications. Practitioners ask for a quick, cheap and reliable method to determine compost organic matter stability. A standardized method for the determination of the intrinsic carbon mineralization rate of organic matter sources (composts, manures, etc.) was developed by means of an aerobic incubation in a suspension of the sample in a nutrient solution. The method avoids the drawbacks associated with solid state stability tests, in which the incubation conditions such as pH, aeration and moisture availability, limit the organic matter decomposition rate. The method thus reflects the intrinsic properties of the organic matter. The use of this method is not limited to advanced laboratories as the user-friendly equipment enables also other groups to apply the method, such as compost producers (to monitor and optimize their composting process) and end-users (to assess compost quality). The new test method is currently being implemented by several commercial laboratories as standard test method. It was recently also proposed in the EC-HORIZONTAL working group “stability” (CEN/TC 223 on soil improvers). Based on stability values for a range of different composts and various other organic matter sources, a classification with four stability classes for composts is proposed. The test method has also been standardized to determine the N-mineralization potential of organic matter sources and for the determination of the biochemical methane potential of organic substrates that are applied as renewable energy sources.