Transforming Invasive Water Hyacinth into High-Quality Compost: The Importance of Decomposer Type and Turning Regime

Water hyacinth has become a major environmental problem in aquatic ecosystems worldwide, including Indonesia, due to its rapid growth and extensive surface coverage. Transforming this invasive biomass into high-quality compost offers a sustainable solution for biomass management and agricultural utilization. However, information regarding the combined effects of decomposer selection and the compost aeration process on compost quality, nutrient transformation, and agronomic performance remains limited. This study was conducted in Rawa Pening, Central Java, Indonesia, to determine how decomposer type and turning regime (specifically the frequency and method of mechanically aerating the compost pile) influence the quality of water hyacinth compost and to identify the most suitable formulation for pak choi (Brassica rapa L.) growing media. The experiment consisted of four sequential stages: composting, compost quality assessment, compost selection, and plant growth bioassay. Three decomposers (EM4™, cow dung, and Stardec™) were evaluated under turning and no-turning conditions using a factorial completely randomized design. Compost quality was assessed based on physicochemical characteristics, nutrient composition, compost yield, and correlation analysis. The best compost treatment was subsequently evaluated in different soil–compost mixtures using ANOVA, PCA, and correlation analysis. The results demonstrated that turning and decomposer selection acted synergistically to determine compost performance. Cow dung consistently produced the highest compost yield, whereas Stardec™ combined with turning generated the most mature compost, characterized by the lowest C/N ratio and the highest nitrogen and phosphorus contents. Plant bioassay confirmed that the optimal growing medium consisted of 50% soil and 50% compost, which maximized vegetative growth by balancing nutrient availability with physical support for root development. No toxicity symptoms were observed, indicating that mature water hyacinth compost is safe for agricultural use. These findings provide practical guidance for selecting decomposers based on compost production objectives and promote the sustainable utilization of invasive water hyacinth.