Co-composting of soybean residues and leaves in Hong Kong

The goal of this project was to evaluate the feasibility of co composting o f soybean residues and leaves and the effects of turning frequency on compo st quality. Soybean residues were mixed with leaves and sawdust in 1:1:3 (w /w wet weight) for achieving a C/N ratio of about 30. Three heaps of about 4 m(3) of compost mixtures were prepared receiving a turning frequency of d aily (pile A), 3-day (pile B) and weekly (pile C) turning. Different turnin g frequencies did not significantly affect the changes in pH and volatile s olids throughout the composting period. High turning frequency caused a low er electrical conductivity and NH4-N contents as well as a shorter duration of thermophilic phase, because of a high heat loss by evaporation and vola tilization of ammonia in the pile. The highest C decomposition of 4% occurr ed in the pile with a 3-day turning period, which coincided with the higher -nitrogen content in this treatment. All treatments with different turning frequencies reached maturation at 63 days as indicated by the soluble organ ic carbon, soluble NH4-N, C/N ratio and cress seed germination index. Howev er, increasing the aeration during composting period was beneficial in acce lerating the maturation process. Taking into consideration less labour and lower operation costs as compared to daily turning, it can be suggested tha t a 3-day turning frequency would be more appropriate for reaching acceptab le quality of compost and ease in operation. (C) 2000 Elsevier Science Ltd. All rights reserved.