Biopolymer-Coated Urea as a Sustainable Controlled-Release Nitrogen Fertilizer: Design, Characterization, and Environmental Performance

Abstract

Nitrogen use efficiency (NUE) of conventional urea rarely exceeds 50% under field conditions because rapid urease-catalysed hydrolysis liberates reactive nitrogen faster than crops can assimilate it, driving losses through ammonia volatilisation, nitrate leaching, and denitrification. This investigation describes the development and laboratory-scale evaluation of a fully biodegradable, dual-layer encapsulated urea product intended to extend nitrogen availability and reduce these losses. A five-constituent biopolymer blend comprising polyvinyl alcohol (PVA), corn starch, pharmaceutical-grade gelatin, gum arabic, and glycerol   was deposited onto granular urea in five sequential spray-coating and thermal-curing cycles on a rotating pan coater, then sealed with an outer neem oil (Azadirachta indica) hydrophobic layer cured at 40°C. The finished product was characterised by scanning electron microscopy (SEM), gravimetric moisture-uptake measurement, aqueous dissolution testing over 20 days, and continuous pH monitoring. SEM examination confirmed a structurally intact, fibrous, defect-free biopolymer membrane with intimate adhesion to the granule substrate; coating weight gain measured 8.2 ± 0.4% and mean membrane thickness was approximately 85 μm by cross-sectional analysis. Bare urea dissolved entirely within 5 min, whereas the coated product released nitrogen progressively over 15–20 days; Moisture absorption at 75% relative humidity was reduced by approximately 50% compared with uncoated controls. pH medium during coated-granule dissolution remained at 7.2–7.4, substantially below the 7.8 recorded for conventional urea, suppressing the ammonium–ammonia shift responsible for atmospheric nitrogen losses. The neem-oil-sealed biopolymer system is an effective, microplastic-free, scalable platform for precision nitrogen management compatible with sustainable agricultural practice.