Phytotoxicity of biosolids compost at different degrees of maturity compared to biosolids and animal manures

Biosolids compost is a good organic amendment but immature compost can exhibit phytotoxic behavior which can be attributed to different toxic substances. Our objective was to determine the phytotoxicity of: i) Biosolids; ii) Mix of biosolids and wood sawdust sampled a day after composting started; iii) The same material sampled at the end of the thermophilic stage; iv) cured compost; v) cow manure and vi) horse manure. A germination bioassay was carried out using Lolium perenne (ryegrass) seeds: germination and root growth percentage were determined as well as electrical conductivity, pH, phenol content and volatile organic acids. In three treatments, Ni, Pb, Zn, Cu and Cd were also determined. Ammonia volatilization was determined during biosolids composting. The germination percentage varied from 67% to 95% but the inhibition of root growth appears to be a more sensitive phytotoxicity indicator (18% to 74%. Phytotoxic effects on germinating ryegrass were mainly related to extract pH and electrical conductivity. Potentially toxic elements, volatile organic acids, phenolic compounds and ammonia were not related to germination.