Bentonite-Polymer Seed Encapsulants with Microparticles for Agricultural and Reforestation Using UAVs

Abstract Uncrewed Aerial Vehicles (UAVs) offer a promising solution for efficient and cost-effective reforestation, particularly on challenging terrains inaccessible by traditional methods. This study explores soft hydrogel technology as a seed encapsulant for UAV dispersal, emphasizing criteria such as on-site preparation feasibility, a limited (5-hour) processing window, and deployment capabilities from a UAV at 3 m height. The research utilising a Psyllium Husk Mucilage (PHM) hydrogel was deemed to be impractical due to rapid degradation and prolonged stabilisation requirements. Hence, the current study employed a blend of bentonite micro-clay, carboxymethyl cellulose (CMC), and sodium alginate (SA), simplifying processing requirements, and potentially improving the permeability with the degradation process, due to cracking. Growth and germination of all seeds were evaluated under 50% and 90% soil field capacity (FC), mimicking dry and moist soil conditions, and compared with conventional planting (C-Planted) and surface sowing (C-Surface). While the encapsulants successfully germinated agricultural seeds overall they were worse than C-Planted at 50% FC, and C-Surface at 90% FC. Encapsulant growth performance for beans was also concluded to be reduced compared to C-Planted at 50% FC. Cucumbers exhibited improved growth with encapsulants at both moisture levels. For non-agricultural native species like C. refractus and E. coolabah, CMC encapsulants initially showed low germination rates, though the addition of additives and microparticles (CMC-AB) notably enhanced germination outcomes. For A. stenophylla the application of microparticles (CMC-B) and CMC-AB reduced growth properties, whilst CMC-AB largely had no effect on the growth properties of C. refractus. The reduced germination rates with encapsulants were attributed to potential seed entrapment within the gel matrix, limiting emergence. In conclusion, while bentonite clay-based hydrogel encapsulants show promise for agricultural and non-agricultural seed germination and growth enhancement, their efficacy varies across species and soil moisture conditions. The study underscores the need for further optimization to maximize their potential in UAV-based reforestation efforts. Graphical Abstract