Spray-Drying Microencapsulation of <i>Bacillus megaterium</i> in PVA/Cationic Starch/Zinc Oxide for Promoting Growth and Zinc Availability in Soybean Plants

Zinc (Zn) is essential for plant development and its deficiency can reduce agricultural productivity. Nutrient-solubilizing microorganisms offer a promising solution to enhance the zinc availability for plants. However, directly applying these microorganisms in the field presents challenges such as cell viability loss. Here, we developed a formulation using poly(vinyl alcohol) (PVA), cationic starch (CS), and zinc oxide (ZnO) for microencapsulating Bacillus megaterium via spray drying. Our results showed that B. megaterium effectively solubilizes zinc oxide. The PVACS-ZnO matrix provided a favorable environment for the growth and development of B. megaterium, releasing cells in quantities exceeding initial inoculation (10 log10 CFU/g). Additionally, it protected the cells against adverse field conditions, maintaining bacterial viability after heat (50 °C/48 h), UV light (95% after 180 min), and fungicide/insecticide exposure (99% after 2 h), unlike free bacteria. Accelerated shelf life tests indicated prolonged stability of PVACS-ZnO microspheres, with double the estimated shelf life (14 months) compared to free bacteria (6 months). In greenhouse experiments, the formulation increased aerial and root biomass of soybean plants, and enhanced phosphorus and zinc absorption. These findings indicate that PVASC-ZnO formulations offer a promising strategy for encapsulating microorganisms and enhancing zinc availability, resulting in an effective and environmentally friendly biofertilizer product.