Maturity and stability parameters of composts prepared with a wide range of organic wastes

Seven different composts were prepared in a pilot plant by the Rutgers static-pile system using a wide range of wastes: sewage sludge, poultry manure, pig slurry, olivemill wastewater, city refuse and the lignocellulosic wastes cotton waste, maize straw and sweet sorghum bagasse. Their chemical and biological properties were studied at four stages of the composting process: in the initial mixture, at the thermophilic phase, at the end of the active phase and after two months of maturation. The following maturity indices were established: C/N < 12, Cw < 1.7%, Cw/Norg < 0.55. NH4/NO3 < 0.16, and NH4-N < 0.04%, with a germination index greater than 50%. In addition, some carbon mineralization parameters could also be used as maturity indices: mineralized-C in 70 days (Cm) < 30%, rapidly mineralizable-C (CR) < 7.2% and a slow mineralization rate (CS × KS) < 0.35% day−1. Maturation indices based on humification of the organic matter and the cation exchange capacity of different composts could not be found, since the values for mature compost depended on the wastes from which the composts were made.