Preliminary performance of rhizobium tropici and bacillus safensis on agro-waste substrates: viability and economic feasibility as alternatives to soil-charcoal carrier

Carrier materials are critical to the viability and functional delivery of microbial biofertilizers, directly influencing their efficacy in sustainable agriculture. Conventional carriers, such as soil mixed with wood charcoal, are becoming increasingly costly and scarce, prompting the need for alternative low-cost substrates. This study investigated whether locally abundant agro-wastes could serve as effective carriers for biofertilizer formulations. To assess the microbial viability of Rhizobium tropici and Bacillus safensis co-inoculated on agro-waste substrates—rice hull, coconut coir dust, coconut coir fiber, and corn cobs—and to compare their economic feasibility with traditional soil-charcoal carriers. Sterilized formulations of agro-waste carriers were prepared in three ratios and inoculated with standardized cultures of R. tropici and B. safensis. Microbial populations were quantified as colony forming units per gram (CFU/g) immediately and at 48 hours post-inoculation using serial dilution and selective agar plating. A direct cost analysis evaluated raw material expenses relative to conventional carriers. Rice hull in a ratio of 120g soil + 40g hull achieved the highest microbial population (3.1×10⁹ CFU/g), significantly surpassing the viability threshold of 106 CFU/g. Coconut coir dust also supported substantial populations up to 5.3×10⁸ CFU/g. In contrast, coconut coir fiber and corn cobs consistently failed to maintain viable counts. Economic analysis indicated that using agro-waste carriers reduced raw material costs by approximately ₱294 per 250 packets compared to soil-charcoal mixtures. Rice hull and coconut coir dust are effective, economical carrier substrates for biofertilizer formulations containing R. tropici and B. safensis. These findings support the integration of agro-waste carriers into localized biofertilizer production systems, promoting sustainable and cost-efficient agricultural practices.