Synthesis of Chitosan-Based Composite Hydrogels for Slow-Release Fertiliser Applications in North Central Nigerian Soils

Conventional fertilisers applied in Nigerian smallholder agriculture suffer from high nutrient losses through leaching and volatilisation, reducing use efficiency and contributing to water body eutrophication. This study synthesises chitosan-based composite hydrogels crosslinked with genipin and loaded with urea and di-ammonium phosphate as a slow-release fertiliser delivery system for application in North Central Nigerian soil conditions. Chitosan was isolated from locally available shrimp shells sourced from Kwara State, and composited with clay minerals to form structural matrices. Hydrogels were characterised by FTIR spectroscopy, X-ray diffraction, scanning electron microscopy, swelling ratio determination, and thermal analysis. Nutrient release profiles in distilled water, simulated soil solution, and actual Nigerian Alfisol were measured over 60 days and compared with uncoated conventional fertilisers. A pot trial with maize was conducted to compare crop growth and nitrogen use efficiency under slow-release versus conventional fertiliser treatments. Results show that urea release from hydrogel composites followed a prolonged diffusion-controlled mechanism, achieving 85 percent nitrogen release over 45 days compared to three days for uncoated urea. Phosphate release was similarly extended. Maize pot trials showed that slow-release hydrogel fertiliser plots achieved comparable final biomass to conventional-fertilised plots while requiring 30 percent less applied nitrogen. Soil total nitrogen and available phosphorus were better retained at 45 days in hydrogel-treated plots. The findings support the commercial development of biopolymer-based slow-release fertilisers from Nigerian bio-waste chitosan. Keywords: chitosan hydrogel, slow-release fertiliser, nutrient release, North Central Nigeria, soil amendment