More than ninety percent of the potassium (K) in the soil is found in the form of silicate minerals that are insoluble and cannot be used by plants directly. K-solubilizing bacteria (KSB), as biofertilizers, are capable to dissolving silicate minerals and release insoluble K into soluble forms, hence increasing soil fertility and plant growth.
In this study, twenty bacterial isolates were obtained from wheat rhizosphere soils and grown on Aleksandrov medium containing orthoclase as a K-source. Eight isolates showed ability to solubilize orthoclase based on inducing clear zones around their colonies. The solubilization index of bacterial isolates KSB1, KSB2, KSB3, KSB4, KSB5, KSB6, KSB7and KSB8 were 3.60, 2.72, 3.69, 3.67, 3.16, 2.8, 2.73, and 2.78, respectively. The highest solubilization index was recorded by the strain KSB3 and the lowest by the strain KSB2. Determination of quantitative K solubilization was carried out in broth Aleksandrov medium. The isolated KSB3 and KSB4 showed the highest K⁺ solubilizing abilities, whereas recorded (29.57, 69.36) and (30.64, 68.42) mg L^-1 after 20 and 30 days from incubation, respectively, in the broth Aleksandrov medium.
The isolated KSB3 and KSB4 were identified by the cell morphological, physiological, and biochemical characteristics. Also, they were identified as Bacillus subtilis (accession no. OR856236) and Bacillus velezensis (accession no. OR856237), respectively; based on 16S rDNA sequences.