Synthesis and Characterization of Okara-Poly(acrylic acid) Superabsorbent Hydrogels for Enhancing Vegetable Growth through Improving Water Holding and Retention Properties of Soils

Okara (Ok), a byproduct of soybean milk and curd production, is a suitable candidate for developing biobased renewable materials. Herein, we report the synthesis and characterization of okara-poly(acrylic acid) (Ok-PAA) superabsorbent hydrogels by grafting and cross-linking PAA on fresh okara. The dried Ok-PAA hydrogel granules in different sizes were added into three types of soils, i.e., lawn sand, mixed soil, and potting mix, as a soil supplement. Overall, the hydrogel supplementation could improve the water holding and retention capacities much more significantly for sand and mixed soil than potting mix. Further, the hydrogels were evaluated for enhancing the growth of a green Asian leafy vegetable, the Brassica rapa subsp. chinensis var. parachinensis (choy sum) in the three soils. In mixed soil, the effect of the hydrogels for enhancing the vegetable growth and reducing the water leaching was particularly significant. Therefore, the Ok-PAA hydrogels are promising for agricultural and farming applications where the soil texture is similar to mixed soil.