<![CDATA[Potassium-solubilizing microorganisms have potential for improving potassium availability in agricultural soils, particularly in rubber plantations where a large proportion of soil potassium occurs in mineral forms that are not readily available to plants. This study aimed to isolate, screen, and identify native potassium-solubilizing microorganisms from rubber plantation soils in Wang Thong district, Phitsanulok province, Thailand. Rhizosphere soil samples were collected from rubber plantations managed under organic and chemical fertilizer systems. A total of 48 microbial isolates were obtained using Aleksandrov agar medium supplemented with mica as an insoluble potassium source. Among these isolates, 28 produced clear halo zones, indicating their ability to solubilize potassium from an insoluble mineral source. Based on radial clear zone formation, colony characteristics, and representation of sampling sources, 12 promising isolates were selected for quantitative evaluation in Aleksandrov liquid medium. After 14 days of incubation, soluble potassium concentrations ranged from 3.01 to 5.50 mg L⁻¹. The highest concentrations were observed in isolates No. 1, No. 10, and No. 11, with values of 4.93, 5.31, and 5.50 mg L⁻¹, respectively. These isolates were identified by 16S rRNA gene sequence analysis as Bacillus siamensis, Pseudomonas orientalis, and Bacillus subtilis. The results indicate that rubber plantation soils in Phitsanulok province harbor native potassium-solubilizing microorganisms with potential for further development as potassium-solubilizing biofertilizer candidates. Keywords: potassium-solubilizing microorganisms, rubber plantation soil, Bacillus siamensis, Pseudomonas orientalis, Bacillus subtilis, biofertilizer]]>
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