Hydrogel nanocomposites incorporating humic acid extracted from vinasse as eco-friendly slow-release fertilizer

Herein, the study developed a slow-release hydrogel fertilizer using humic acid (HA) extracted from sugar-industry waste (vinasse), which contains approximately 11% HA. HA was recovered by an alkaline extraction method and then coated with biopolymer hydrogels to improve controlled nutrient release and soil performance. One coating combined natural cellulose with chitosan (CS-CEL). Two additional coatings were prepared from polyvinyl formal resin (PVR) or polyvinylpyrrolidone (PVP), each mixed with chitosan. The materials were characterized by FTIR, XRD, SEM, TEM, EDX, and GC/MS, confirming key functional groups and structural features that support soil-enhancement applications. Nutrient release behavior was evaluated by monitoring electrical conductivity (EC) and the concentrations of potassium, phosphate, and HA in sandy soil. The CS-CEL and CS-PVP composites delivered the strongest performance, achieving slow nutrient release into sandy soil with EE% of 85.24% and 78.53% for K + and 86.41% and 72.24% for P 2 O 5 , respectively. Application of CS-CEL raised the water-holding capacity of sandy soil from 16% to 72%. Water absorption capacities reached 73.066 for CS-CEL and 60.019 for CS-PVP. Measured degradability ranged from 44.64% to 57.98%, indicating that the hydrogels combine gradual biodegradation with sufficient structural stability for sustained nutrient delivery. These results demonstrate that coated HA derived from vinasse offers a practical route to enhance both fertility and water retention in sandy soils while enabling controlled, long-term nutrient release.