<![CDATA[Water hyacinth and banana pseudostem are abundant aquatic and agricultural biomasses with contrasting physicochemical properties that may provide complementary nutrient and carbon sources during composting. However, the effects of progressively substituting lignocellulosic banana pseudostem with nutrient-rich water hyacinth under a constant manure input remain insufficiently understood. This study evaluated how different water hyacinth–banana pseudostem ratios influence the nutrient composition and C/N characteristics of compost. Four 6-kg feedstock formulations were investigated: X1 (4 kg banana pseudostem + 1 kg water hyacinth + 1 kg goat manure), X2 (3 + 2 + 1 kg), X3 (2 + 3 + 1 kg), and X4 (1 + 4 + 1 kg), using five independent 90-L composting units per formulation (n = 20). The mixtures were composted for 30 days and turned every two days. Total N, P, and K, organic C, and C/N ratio were determined at the end of composting. Feedstock formulation significantly affected total P (F(3,16) = 6.583, p = 0.004, η² = 0.552), with the highest concentration observed in X2 (0.794 ± 0.167%). Total N and K were numerically highest in X3, although differences among formulations were not statistically significant (p = 0.091 and p = 0.095, respectively). Organic C (p = 0.816) and C/N ratio (p = 0.256) also showed no significant differences, with mean C/N ratios ranging from 11.04 to 12.87. In contrast, aggregate N + P₂O₅ + K₂O differed significantly among formulations (F(3,16) = 10.255, p < 0.001, η² = 0.658), with X2, X3, and X4 exceeding X1 but not differing significantly from one another. These findings reveal a threshold-type rather than monotonic nutrient response, indicating that increasing water hyacinth beyond the X2 proportion did not produce proportional improvements in aggregate nutrient content. Co-composting water hyacinth, banana pseudostem, and goat manure therefore provides a promising nutrient-recovery strategy for managing invasive aquatic biomass and agricultural residues, although maturity assessment, contaminant screening, and soil–plant validation are required before field-scale application.]]>
