OPTIMIZATION OF MICROORGANISM-ENRICHED BIO-COATED CLAY GRANULES FOR SUSTAINABLE PLANT GROWTH USING RESPONSE SURFACE METHODOLOGY

Abstract

This study developed and optimized a microbial-enriched compost powder designed to enhance the growth performance of Red Cos lettuce using Response Surface Methodology (RSM). Three plant growth–promoting bacteria—Azotobacter sp., Bacillus spp., and Azotobacter vinelandii—were incorporated into compost powder and applied to high-porosity fired clay granules as the growth medium. A Box-Behnken Design was employed to evaluate individual and interactive effects of microbial concentrations on plant dry weight. ANOVA confirmed that the quadratic regression model was highly significant (p < 0.0001), with strong predictive capacity (R² = 0.9973; Adjusted R² = 0.9925) and an insignificant lack-of-fit. All microbial factors significantly influenced biomass production, with A. vinelandii exhibiting the most significant effect, followed by Azotobacter sp. and Bacillus spp. Response surface and contour plots revealed clear synergistic interactions among the variables and identified a distinct optimal region. The optimal formulation—0.19% Azotobacter sp., 1.84% Bacillus spp., and 0.39% A. vinelandii—yielded the highest observed dry weight (2.14 ± 0.04 g), consistent with model predictions. Confirmation experiments showed that unbalanced microbial levels led to inferior growth responses, underscoring the importance of optimized ratios. The findings establish that microbial consortia integrated into compost powder and applied via porous clay granules can substantially improve plant biomass while supporting environmentally sustainable production systems. This optimized formulation provides a promising basis for further development of microbially enhanced growth media for leafy vegetables.