Valorization of Thai Purple Yam (Dioscorea alata L.) Starch into a Biodegradable Hydrogel-Based Nitrogen Fertilizer...

<b>Background and Objective:</b> Improving fertilizer-use efficiency and soil water retention is essential for sustaining crop production under increasing water scarcity. This study aimed to valorize starch extracted from Thai purple yam (<i>Dioscorea</i> <i>alata</i> L.) into a biodegradable hydrogel-based nitrogen fertilizer with slow-release and water-retention functionalities. <b>Materials and Methods:</b> Yam starch hydrogels were synthesized through citric acid crosslinking and loaded with calcium nitrate to produce a starch-hydrogel slow-release fertilizer (SH-SRF). The swelling behavior, soil water-retention capacity, nitrate-release characteristics, biodegradability and plant-growth performance under water-deficit conditions were evaluated using lettuce and tomato as model crops. <b>Results:</b> The optimized hydrogel exhibited a swelling ratio of 5.94 g/g and significantly improved soil water retention, maintaining 9.87% residual moisture after 7 days compared with 0.44% in unamended soil. In a soil-column system, conventional Ca(NO₃)₂·4H₂O released nearly all nitrate within 10 days, whereas SH-SRF extended nutrient release to 30 days, with cumulative NO₃^- release of 1.68%, 91.23% and 100% after 1, 20 and 30 days, respectively. The hydrogel showed partial biodegradability with 17.3% mass loss after 20 days. Under water-deficit conditions, SH-SRF markedly enhanced seedling performance. Lettuce treated with SH-SRF showed improved growth and biomass, whereas plants in the control and conventional fertilizer treatments failed to survive. A similar trend was observed in tomato, where SH-SRF produced superior growth compared with other treatments. <b>Conclusion:</b> These results demonstrate that Thai purple yam starch can be converted into a biodegradable hydrogel fertilizer that simultaneously regulates nutrient release and improves soil moisture retention, offering a sustainable strategy for crop production in water-limited agricultural systems.