Duration of Composting of Yard Wastes Affects Both Physical and Chemical Characteristics of Compost and Plant Growth

Abstract

Windrows of municipal yard and landscape waste at three commercial composting sites in California were sampled at ≈3-week intervals through 12 to 15 weeks of composting to observe changes in physiochemical and biological characteristics of importance to horticulture. Initial C, N, P, and K content averaged 30%, 1.3%, 0.20%, and 0.9%, respectively. Carbon concentration declined rapidly through the first 6 to 9 weeks, while N, P, and K remained relatively stable throughout the sampling period. Few viable weed seeds were found in any compost. A high level of phytotoxicity, as measured by a tomato (Lycopersicon esculentum Mill.) seed bioassay, was observed at only one site; overall, the degree of phytotoxicity declined with compost age. Short-term net N immobilization (in a 2-week aerobic incubation) was observed in nearly all samples, with an overall trend toward decreased immobilization with increased compost age. In a 16-week pot study in which fescue (Festuca arundinacea Shreb.) was grown in compost-amended soil, net N mineralization averaged only 2% to 3% of compost total N content. Neither composting site nor duration of composting significantly affected either N mineralization rate or fescue growth. Growth of vinca (Catharanthus roseus Don.) in a blend of 1 compost : 1 perlite increased with increasing compost age. Overall, at least 9 to 12 weeks of composting were required to minimize the undesirable characteristics of immature compost.