Development of a Biofertilizer From Indigenous PGPR and Leonardite Residues for Sustainable Lettuce Cultivation and Soil Restoration

Plant growth-promoting rhizobacteria (PGPR) are beneficial microbes that can enhance crop yield and soil fertility. In this study, we developed a biofertilizer using an indigenous PGPR strain, Microbacterium maritypicum R4-3, isolated from leonardite-associated rhizosphere at the Mae Moh lignite mine in Thailand. The strain showed strong phosphate solubilization (index = 3.81), siderophore production (1.43), and IAA synthesis (3.76 µg/mL). Its identity was confirmed by 16S rRNA sequencing and phylogenetic analysis, placing it with M. maritypicum type strains. Lettuce (Lactuca sativa var. crispa) was used as a test crop. In laboratory experiments using Leonard’s jar systems, R4-3 significantly increased plant biomass, yielding 18.58 g fresh weight (FW) and 1.97 g dry weight (DW) per plant, compared to 12.62 g FW and 1.36 g DW in uninoculated controls. In pot trials with amended soils, the highest yield was obtained from 25% leonardite plus R4-3, reaching 48.68 g FW and 2.92 g DW, outperforming both controls (1.94 g FW, 0.18 g DW) and two commercial biofertilizers. Soil analysis revealed improvements in total nitrogen (0.41%), exchangeable potassium (448 ppm), and organic matter (7.89%) under R4-3 treatment. These findings highlight the potential of native PGPR combined with leonardite to enhance plant growth and restore soil fertility in degraded soils. Therefore, this study offers a sustainable and low-cost strategy for converting mining waste into effective biofertilizer inputs for organic farming and land rehabilitation.