An innovative formulation to improve spent coffee characteristics as soil fertilizer: Nutrient, chemical characterization and effects on plant germination

Spent coffee grounds (SCG) possess high organic content and various essential nutrients. However, their direct use as a fertilizer is limited due to high acidity, phenolic content, and C/N ratio. Therefore, in this study, SCG-based fertilizer (SCG_f) was developed by mixing the raw SCG (SCG_r) and zein through a granulation process and was compared with the SCG_r. According to the obtained results, in SCG_f, a reduction in the C/N ratio from 22.9 to 10.9, improved flowability from fair to good, an increase in the concentration of elements (N, C, S), nutrients (ash, fats, proteins), macrominerals (Ca, Mg, P, K, Na), microminerals (Cu, Fe, Mn, Zn), total polyphenolic content (TPC), total flavonoid content (TFC), and antioxidant activities (AOA) was noted. Additionally, a decrease in total organic acids, total chlorogenic acids, and was also observed in SCG_f. Regarding phytotoxicity test results obtained by using Sinapis alba, Sorghum saccharatum, and Lepidium sativum and treating them with 2.5%, 5%, and 10% of SCG_f and SCG_r, better germination was observed for SCG_f in S. alba (5%, 10%), L. sativum (2.5%, 5%). However, in L. sativum, 5% of SCG_r and 10% of both SCG_r and SCG_f completely inhibited seed germination. For S. saccharatum, higher germination rates were observed for SCG_f (2.5%, 10%), and at 5%, a comparable effect of both SCG_f and SCG_r was noted. SCG_f gave the longest root length at all concentrations in all plants except for 2.5% in S. alba. In conclusion, this study addresses the effective use of SCG for sustainable agriculture while mitigating environmental impacts.