120 Compost Science & Utilization Spring 2012 Introduction Of the four compost types (Green waste, mixed solid waste, sewage sludge, mushroom compost), green waste compost (GWC) is a biodegradable waste that originates from almost pure plant materials such as garden trimmings or organics collected from veg- etable and fruit markets (Anon 2007a). It improves the physical and chemical properties of the soil (Dalal et al. 2009), enhanced plant growth (Keeling et al. 2003; Ali et al. 2007), remediates contaminated soil (van Her- wijnen et al. 2007, 2008; Alvarenga et al. 2009; Hartley et al. 2009), and suppression of some soil borne dis- eases (van Deer Gaag et al. 2007; Lozano et al. 2009). In Oman, land is the major nonrenewable re- source facing the challenging threat of soil degrada- tion. Sustainable agriculture must be environmentally safe and produce adequate amount of quality food with minimum of purchased fertilizers and rely, as much as possible on the renewable resources of the farm itself (Sarwar et al. 2008; Chitravadivu et al. 2009). This is especially important in that 90% of the farms that exist in the third world, agricultural inputs are of- ten not available or affordable (Chitravadivu et al. 2009). Thus, compost is one of the important low cost inputs for meeting nutrient requirements for plant growth (Zameer et al. 2010). In Oman, green waste composts are imported from other countries with high cost and economic burden. Therefore, the compost in- dustry was locally developed aiming at production of high quality composts with affordable prices to the small village farmers. The present study was conduct- ed to evaluate the physical, chemical and microbial properties of locally produced green waste composts in order to determine their ability to meet the accept- able standards. Compost Science & Utilization, (2012), Vol. 20, No. 1, 120-127 Physicochemical and Microbial Characteristics of Locally Processed Green Waste Composts S.A.F. El-Nagerabi 1 , A.E. Elshafie 2 and H.S. AlRawahi 2 1. Department of Biological Sciences and Chemistry, College of Arts and Sciences, Nizwa University, Nizwa, Sultanate of Oman 2. Department of Biology, College of Science, Sultan Qaboos University, Sultanate of Oman *E-mail contact: nagerabi@unizwa.edu.om or nagerabi@hotmail.com Four locally composted green waste composts (GWCs) namely Almukhasib, Growers, Plantex, and Super were screened to determine whether they meet the standards. All composts showed normal physical proper- ties, except for the bad smell from sulfur reducing bacteria in Almukhasib compost, and light brown color Plantex. The germination indexes of the composts comparable to the standard (90%) were 98% for Plantex fol- lowed by Growers (77%), and 5% for both Super and Almukhasib. The chemical and physical properties vary considerably as follows: pH 3-10.5 (standard 5-8), electrical conductivity (EC) 0.4-10.2 mScm -1 (standard 0.0- 4.0 mScm -1 ), moisture content (MC) 29-43.7% (standard 35-60%) and water holding capacity (WHC) 92-200%. Wide ranges in the chemical properties were expressed as total nitrogen concentration 5705-16401 mgkg -1 (standard <500 mgkg -1 ), organic matter 17-67.6% (standard 35%). Although, there are significant variations in the concentration of the heavy metals among the tested composts, the concentrations of these metals (Zn, Ni, Pb, Hg, As, Cd, and Cr) were lower than the recommended levels. The average of the bacterial colony form- ing unit per gram dry weight ranged between 464-2292 cfu/g, whereas the fungal cfu were 14-4308 cfu/g (standard < 1000 cfu/g). The most probable number (MPN) for coliform bacteria was 64-1549 cfu/g dry weight. Aspergillus niger was the predominant fungus recovered from all compost samples (100%) followed by A. fumigatus (50%), A. sparsus (50%), yeasts (50%), A. flavus (37.5%), and the remaining A. restrictus, A. ochra- ceous, Cladosporium spp., and Penicillium spp. is 25% each. The results showed that the physico-chemical prop- erties and microbial contamination of the screened composts were considerably varied and did not meet many of the acceptable limits in Oman, which render them unsafe for handling or unsuitable as direct fertil- izer for plant growth or for soil bioremediation, which suggests mixing the composts with vermiculite and soil in order to improve their general characteristics. Therefore, there is evident need for urgent development of proper composting techniques, standard laboratory testing methods for high quality control measures, and adopting strong legislation. The use of certified and high grade mature green waste compost is a priority.