Physicochemical and microbial characteristics of locally processed green waste composts

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 properties, 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 followed 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 0.04.0 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 forming 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. ochraceous, Cladosporium spp., and Penicillium spp. is 25% each. The results showed that the physicochemical properties 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 fertilizer 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.