Use of okara-derived hydrogel for enhancing growth of plants by minimizing leaching and locking nutrients and water in growing substrate

Abstract Water is critical for plant growth. To improve the water retention of soil, we synthesized a superabsorbent hydrogel (Gel-1) derived from the food waste okara as a soil supplement. We showed that Gel-1 amendment significantly improved growth (shoot fresh weight, digital biomass, leaf area, relative growth rate) and irrigation water-use efficiency (IrrWUE) of Asian leafy vegetable Brassica rapa. The yield was increased by 88% and 113% when the vegetables were grown in substrate supplemented with 3 and 5% (w/w) of Gel-1, respectively. IrrWUE was significantly compromised when watering frequency was increased from two to four times in two weeks to prevent wilting in the absence of Gel-1. Increasing watering frequency to >5 times in two weeks without Gel-1 amendment led to >98% nitrogen loss, adversely affecting plant growth significantly. Restoring nitrogen using Hoagland's solution improved plant growth confirmed frequent irrigation resulted in nutrient leaching, affecting plant performance. In contrast, incorporation of Gel-1 reduced leaching and significantly improved IrrWUE (>80%), under water-limited conditions. Our findings suggest Gel-1 could reduce half of the watering frequency, save ~50% of water and prevent excessive nutrient leaching, thereby enhancing growth. Potential applications of such okara-based hydrogel for agricultural purposes are discussed.