Duration of composting of yard wastes affects both physical and chemical characteristics of compost and plant growth

Windrows of municipal yard and landscape waste at three commercial composti ng sites in California were sampled at approximate to 3-week intervals thro ugh 12 to 15 weeks of composting to observe changes in physiochemical and b iological characteristics of importance to horticulture. Initial C,N,P,and K content averaged 30%, 1.3%,0.20%,and 0.9%, respectively, Carbon concentra tion declined rapidly through the first 6 to 9 weeks, while N, P, and K rem ained relatively stable throughout the sampling period, Few viable weed see ds were found in any compost, A high level of phytotoxicity, as measured by a tomato (Lycopersicon esculentum Mill) seed bioassay, was observed at onl y one site; overall, the degree of phytotoxicity declined with compost age, Short-term net N immobilization (in a 2-week aerobic incubation) was obser ved in nearly all samples, with an overall trend toward decreased immobiliz ation with increased compost age. In a 16-week pot study in which fescue (F estuca arundinacea Shreb,) was grown in compost-amended soil, net N mineral ization averaged only 2% to 3% of compost total N content. Neither composti ng site nor duration of composting significantly affected either N minerali zation rate or fescue growth. Growth of vinca (Catharanthus roseus Don.) in a blend of 1 compost : 1 perlite increased with increasing compost age. Ov erall, at least 9 to 12 weeks of composting were required to minimize the u ndesirable characteristics of immature compost.