Smart and Controlled-release Hydrogels for Fertilizer and Pesticide Delivery: A Comparative Review of Synthetic and Bio-based Systems

Abstract

Global food security depends increasingly on the efficient use of agrochemicals, yet conventional fertilisers and pesticides are characterised by poor nutrient and active-ingredient use efficiency, with substantial losses through leaching, volatilisation, and runoff. Hydrogel-based delivery systems have emerged as a versatile platform for controlling the release of nutrients and crop-protection agents in response to environmental and biological cues, offering a route towards reduced input loss, improved soil water retention, and lower ecological burden. This review critically compares synthetic polymer hydrogels, principally based on polyacrylamide and polyacrylate chemistries, with bio-based systems derived from polysaccharides, chitosan, and proteins, with particular attention to their swelling behaviour, release kinetics, and agronomic performance. Stimuli-responsive or "smart" hydrogels that modulate release in response to pH, temperature, light, redox potential, or enzymatic activity are examined as a distinct and rapidly advancing subclass applicable to both synthetic and bio-based matrices. The review further considers comparative loading capacities, release efficiencies under simulated soil and aqueous conditions, environmental fate and biodegradability, and the practical and regulatory barriers that continue to limit field-scale adoption. While synthetic hydrogels generally offer superior mechanical robustness and tunable release profiles, their persistence in soil and association with microplastic accumulation raise legitimate environmental concerns. Bio-based hydrogels, by contrast, offer biodegradability and lower toxicity but often suffer from batch-to-batch variability and weaker mechanical performance. The review concludes that hybrid and composite formulations, combining biopolymer matrices with engineered nanoscale reinforcements or stimuli-responsive moieties, represent the most promising direction for reconciling release precision with environmental compatibility. Persistent gaps remain in long-term field validation, standardised release-testing protocols, and life-cycle assessment, which together constrain confident scale-up of these technologies.