Physico‑chemical, microbial and phytotoxicity evaluation of composts from sorghum, finger millet and soybean straws

Purpose Composting is an environmentally sustainable alternative for bioconversion of agricultural residues into a nutrientrichproduct that can enhance soil fertility/microbial diversity and thereby improve agricultural productivity. The goal of thecurrent study was to evaluate the decomposition pattern of the agro-residues and assess the maturity and phytotoxicity ofthe composts obtained using physico-chemical, microbial and statistical analyses. The study also attempted to determine athreshold germination index (GI) to serve as a maturity index for the composts by conducting seed germination assays withtomato, chickpea and soybean seeds.Methods Three agricultural residues/straws of Eleusine coracana (finger millet), Sorghum bicolor (sorghum) and Glycinemax (soybean) were subjected to aerobic composting for a period of 60 days to study the impact of saw dust on the decompositionpattern and the ultimate compost quality/characteristics.Results The results showed efficient decomposition pattern of the agricultural residues characterized by high temperatureprofiles (up to 70 °C), high microbial activity, a sharp decrease in C/N ratio of the composting materials, i.e., from an initial41–61 to final 10–17.Conclusions Statistical evaluation of seed germination assays showed that only the compost obtained from sorghumstraw + saw dust was mature and free from any phytotoxicity as all the tested seeds showed higher and statistically significantGIs. It was difficult to attribute a single threshold GI value to indicate maturity of compost and could not further be appliedto different types of composts as different seeds responded differently to the same compost.