Sustainable carboxymethyl starch–bentonite hydrogel seed coatings for improved germination under water-limited broadcast seeding

Broadcast seeding is widely used in agricultural production but places seeds at the soil surface, where rapid fluctuations in moisture availability often limit imbibition, resulting in poor germination, uneven stand establishment, and yield losses. Improving moisture availability at the seed–soil interface is therefore critical for enhancing the reliability of broadcast seeding systems. While synthetic superabsorbent hydrogels (SAHs) can improve soil water retention, their petrochemical origin and limited biodegradability raise environmental concerns related to microplastic accumulation, motivating the development of sustainable alternatives. In this study, carboxymethyl starch–based SAHs were synthesized via controlled carboxymethylation and epichlorohydrin (ECH) crosslinking to produce networks with tunable microstructure and viscoelastic properties. The hydrogels were systematically characterized using FTIR, XPS, SEM, and rheology, and applied as seed coatings using scalable spray- and dip-coating techniques. A low-crosslink-density formulation enabled uniform coatings and enhanced moisture uptake, with spray-coated wheat seeds exhibiting approximately 40% higher water absorption than uncoated controls. To further improve moisture buffering, bentonite clay was incorporated into selected coatings. Controlled-environment experiments simulating broadcast field conditions demonstrated that bentonite-reinforced hydrogel coatings accelerated germination and significantly increased wheat establishment under water-limited conditions, while maintaining high establishment under well-watered regimes. This study further elucidates the mechanistic link between the hydrogel's internal porosity and accelerated seed imbibition. Overall, this work presents a scalable, microplastic-free starch–bentonite hydrogel seed-coating platform that improves germination reliability and crop establishment in broadcast seeding systems, supporting more climate-resilient and sustainable agricultural practices.