Microbial–Alginate Composite Fertilizer Coatings: Rapid Fabrication, Nutrient Release Kinetic Modelling, and Agronomic Performance

This study aimed to develop and validate a microbial–alginate composite coating for controlled-release fertilizers by integrating formulation optimization, nutrient-release modelling, and agronomic evaluation to support precision fertilization. Sodium alginate was reinforced with Bacillus subtilis (0.5 g log-phase biomass), supplemented with Copper (II) ions (Cu²⁺) and glycerol, and applied to fertilizer granules via multilayer immersion followed by either 24-hour or 30-minute drying. Coating thickness and morphology were characterized microscopically. Nutrient-release behaviour was experimentally quantified and simulated using an integrated kinetic model. Agronomic performance was assessed using water spinach (Ipomoea aquatica). Coating thicknesses were consistent across drying regimes (0.48–1.16 μm), confirming rapid drying as an effective processing strategy. The kinetic model showed strong agreement with experimental data (R² > 0.95), accurately capturing diffusion- and microbially mediated nutrient release. Two-layer microbial-coated fertilizers significantly increased biomass accumulation compared with uncoated controls, indicating improved nutrient availability and synchronized release. The microbial–alginate composite coating provides a scalable, eco-friendly platform for precision nutrient delivery, integrating controlled release with biological functionality to advance sustainable fertilizer systems.