An Integrated Biopolymer Hydrogels with Urea as a Carrier for Controlled Fertilizer Release and Soil Moisture Retention

Water scarcity, inefficient fertilizer application, and soil degradation remain major challenges for sustainable agricultural production. Integrated biopolymer hydrogels have emerged as promising materials for improving soil water retention and enhancing nutrient-use efficiency. This review provides a comprehensive overview of integrated biopolymer hydrogels synthesized from components such as acrylic acid, starch, and urea, with a particular focus on their applications in controlled fertilizer release and soil moisture management. These hydrogels exhibit high swelling capacity and excellent water-holding properties, enabling improved soil moisture availability under both irrigated and rainfed conditions. At the same time, the incorporation of urea within the biopolymer matrix enables gradual and controlled nitrogen release, reducing nutrient losses and enhancing plant uptake. Numerous laboratory and field studies demonstrate that hydrogel application can significantly improve plant growth, biomass accumulation, photosynthetic activity, and drought tolerance. Multi-year field experiments show notable increases in crop yield and water productivity in various cropping systems, including soybean–wheat rotations, leafy vegetables, wheat, and rice. Hydrogel amendments also contribute to improved soil hydro-physical properties and enhanced water-use efficiency, often allowing substantial reductions in irrigation frequency. In addition, recent studies indicate that hydrogel systems can improve nutrient availability and mitigate stress conditions in saline–alkaline soils. Despite these benefits, several limitations remain, including reduced hydrogel performance under high salinity or alkalinity, challenges associated with large-scale synthesis, and the need for cost-effective and environmentally sustainable production. Future research directions include the use of renewable agricultural by-products as raw materials, development of predictive models for water and nutrient release, and long-term field evaluations across diverse agroecological conditions. Overall, integrated biodegradable biopolymer hydrogels represent a promising product for improving water and fertilizer efficiency while supporting climate-resilient and sustainable agricultural systems.