Coffee production generates large volumes of spent coffee grounds (SCGs) as a nutrient-rich residue. Despite this potential, the direct use of SCGs as a fertilizer is limited by their acidity and secondary metabolites such as caffeine, tannins, and polyphenols, which can inhibit plant growth. This study assessed the feasibility of combining SCGs with raw cow manure as a feedstock for vermicomposting by examining their effects on the growth and reproduction of Eisenia fetida and the phytotoxicity of the resulting compost.
Four treatments and four replicates were used in a completely randomized design, testing SCG-to-dairy manure ratios of 0:100 (control), 33:67, 50:50, and 67:33. Each 20 × 20 × 9 cm container was stocked with ten mature E. fetida earthworms and maintained for one month at 21 °C and 75% moisture. Parameters monitored included pH and electrical conductivity (EC). Phytotoxicity was assessed using aqueous extracts of the vermicompost in a lettuce germination bioassay.
Vermicompost pH was 7.5 across treatments, while EC decreased with higher SCG content, ranging from 674 μS/cm in the control to 383 μS/cm in the 67% SCG treatment. Increase in earthworm biomass was higher in the 33% SCG but lower in the 50% and 67% treatments relative to the control. Cocoon production decreased with increasing SCG content. In a subsequent recovery phase, where earthworms were transferred to a manure-only environment, biomass stabilized across all groups, and cocoon production increased relative to the control, suggesting that reproductive inhibition under higher SCG levels was reversible. In the lettuce seed test, the germination rate was above 90% for all treatments (92-93%).
Overall, the results demonstrate that a 33:67 SCG-to-manure ratio provides optimal conditions for vermicomposting, enabling efficient processing of SCGs while supporting earthworm performance and producing a phytotoxin-free organic soil amendment. This highlights the potential of integrating SCGs into circular bioeconomy practices in sustainable agriculture.
