Compost organic matter content varied five-fold and determined compost quality across 107 composts of the North Sea Region

Composting is a widely used method to process organic waste residues. It results in a valuable product for soil application and use in growing media. The aim of this study was to investigate the variation in characteristics of composts produced in the North Sea Region, and the factors determining this variation. A total of 107 composts were categorized into two composting practices (produced on a farm or on a commercial composting facility) and three feedstock groups (manure combined with other wastes; green waste; fruit, vegetable and garden waste (fvg)), and measured for 67 physical, chemical and biological characteristics. Variation in the results was large, e.g., up to a factor 20 and 11 for total microbial biomass and potassium content, respectively, underlining the importance of compost characterization to target the intended compost use. Organic matter (OM) content varied between 14 and 73% of dry matter and was larger for Belgian composts compared to composts from The Netherlands, Denmark, Germany and Scotland. The OM content was positively correlated with total microbial biomass, cation exchange capacity and content of nitrogen (N) and phosphorus (P) of composts. Farm composts, irrespective of the OM effect, exhibited higher total microbial biomass compared to commercial composts. Compost prepared from green waste had lower N and P contents compared to compost prepared from fvg or manure waste. The study documents characteristics in composts from diverse composting practices and feedstocks, providing a benchmark and enabling targeted improvements as a first step towards tailormade compost.