Optimization of nutrient release behavior of modified starch/bentonite coated urea fertilizer in soil using response surface methodology

Efficient nitrogen management is essential for sustainable agriculture as conventional urea rapidly dissolves and causes nutrient losses. This study optimized the formulation of a slow-release urea fertilizer (SRUF) coated with modified starch and bentonite clay using response surface methodology (RSM). A central composite design examined the effects of bentonite amount, starch concentration, and urea particle size on nitrogen release in soil. Analysis of variance showed all main factors significantly influenced nutrient release (p < 0.05), with an excellent model fit (R2 = 0.9997). The optimal conditions were 24.97% bentonite, 9.49% starch, and 2.92 mm particle size yielding a predicted 45.01% release profile after 30 days which closely matching the experimental value (45.56%). Compared with raw urea, the optimized SRUF provided sustained release behavior for over 30 days, demonstrating its potential to enhance nitrogen-use efficiency, and reduce environmental losses through a biodegradable, cost-effective fertilizer system.