Polysaccharide Hydrogel-Based Fertilizer Carriers: Soil-Relevant Evaluation of Nutrient Release Beyond Conventional Aqueous Testing

Polysaccharide hydrogel-based fertilizer carriers have emerged as promising alternatives to conventional synthetic systems due to their biodegradability, tunable physicochemical properties, and ability to regulate nutrient release through structure-transport interactions. However, their performance is still predominantly evaluated using simplified aqueous testing methods that fail to capture the complexity of real soil environments. This review provides an engineering-oriented analysis of nutrient release behavior from polysaccharide-based hydrogel systems, emphasizing the limitations of conventional aqueous evaluation and their implications for predicting field performance. The discussion integrates material design, transport phenomena, and environmental interactions to establish structure-property-release relationships governing nutrient delivery. Conventional aqueous testing methods are critically examined in terms of experimental configuration, performance metrics, and kinetic modeling approaches, highlighting their tendency to overestimate swelling, neglect ionic and biological interactions, and ignore external transport resistances. The influence of soil-dependent factors, including moisture dynamics, pH, ionic strength, microbial activity, and soil structure, is systematically analyzed to demonstrate their coupled effects on swelling, diffusion, and degradation-controlled release mechanisms. Comparative evidence reveals a consistent laboratory-soil mismatch, where aqueous systems predict faster release rates and shorter durations compared to soil conditions. Based on these insights, key gaps in current evaluation practices are identified, particularly the lack of soil-representative testing protocols and the limited applicability of models derived from aqueous systems. Finally, an engineering framework is proposed for soil-relevant evaluation and improved predictive modeling, aimed at supporting the rational design and scalable implementation of next-generation hydrogel-based fertilizer carriers.