Compositing of Coffee Silverskin with Carbon Rich Materials Leads to High Quality Soil Amendments

Purpose Coffee silverskin (CS) is the integument covering the raw coffee bean, representing the primary waste product of the coffee-roasting industry. Despite the growing attention in seeking potential reuse of this material, the majority of CS is commonly used as a firelighter or discharged to landfills. The study aimed to test co-composting as a low-cost solution that meets the circular economy paradigms proposed by the European Union. Methods Four composting mixtures were prepared mixing CS with pruning waste and biochar at different ratios, aiming to maximize the amount of compostable CS per batch and monitored for 60 days. Results The contents of macro-, micro- and trace elements of the final composts matched the strictest requirements of the Spanish national regulation on compost quality (Class A amendments), proving that CS composts area high-value amendment rich in N and K. Despite the highly phytotoxic effect of CS raw material, the seed germination tests showed that all the mature composts exhibited phytostimulant properties allowing their harmless application to the soil. The four composts had a high water holding capacity (237-351% dw) and they are likely to promote the persistence of plant-available water in the soil. Conclusion The present study showed that composting the whole CS produced in Europe would lead to a recovery of 2420-3481 tons of nitrogen and 1873 tons of potassium, reducing the dependency on mineral fertilizers, thus meeting the growing demand for sustainable and low-cost amendments. [GRAPHICS] .