Preparation, characterization, and swelling–release behavior of sodium alginate microspheres loaded with multiple nutrients under variable environmental conditions

This study developed a sodium alginate (SA)-based slow-release microsphere (SA-NPK) for encapsulating nitrogen (N), phosphorus (P), and potassium (K) fertilizers to enhance fertilizer utilization and reduce environmental pollution. The structure and nutrient loading of the microspheres were characterized using FTIR, XRD, and SEM. These results showed that the composite fertilizer was embedded in the porous network structure of the SA microspheres in the form of crystals. Furthermore, investigating the effects of different pH values and NaCl concentrations on the swelling behavior of the microspheres and nutrient release showed that the swelling behavior of the SA-NPK microspheres exhibited good responsiveness to both pH and sodium ion concentrations, with acidic environments and elevated NaCl concentrations promoting the release of encapsulated nutrients. After 72 hours, the cumulative release amounts of N, P, and K reached 93.20–97.86%, 71.78–81.25%, and 90.48–93.45%, respectively. Moreover, soil leaching experiments demonstrated good sustained-release performance, with cumulative nutrient release rates of 82%, 62%, and 86% for N, P, and K after 15 days, respectively. The microspheres also exhibited favorable biodegradability, confirming their potential to minimize environmental impact. Overall, the SA-NPK microspheres provide a low-cost, eco-friendly, and biodegradable slow-release fertilizer system with promising applications in sustainable agriculture and horticulture. • SA-based microspheres encapsulate NPK in crystalline form, enhancing structural stability. • pH and NaCl responsiveness enables tunable nutrient release (N release 93.20–97.86%; P 71.78–81.25%; K 90.48–93.45%). • Nutrient release follows a coupled diffusion–swelling–ion exchange mechanism. • Exhibits good sustained-release performance and biodegradability in soil within 15 days. • Low-cost, eco-friendly fabrication offers a scalable and efficient controlled-release fertilizer solution.