Improving soil fertility is essential for enhancing agricultural productivity, with potassium (K) being a crucial nutrient for plant growth. In soil, potassium is predominantly found in insoluble forms, making it unavailable to plants. This study aims to identify and evaluate fungal strains capable of solubilizing insoluble potassium sources to improve soil fertility. Soil samples were collected from paddy fields in Pudukkottai, Tamil Nadu, and potassium-solubilizing fungi were isolated using modified Aleksandrov agar and Potato Dextrose Agar media. Ten fungal isolates were screened for their ability to solubilize potassium, with KSF1 demonstrating the highest solubilization index (SI) of 1.51. Quantitative analysis revealed that KSF1, identified as Aspergillus niger, released the highest amount of potassium from mica (42.16 mg/25 ml) on the 10th day of incubation. This isolate showed superior solubilization capabilities compared to others. The findings suggest that Aspergillus niger KSF1 is a highly effective potassium solubilizer, offering a sustainable approach to enhancing soil fertility and supporting agricultural productivity.