Purpose <p>Biochar’s effectiveness in enhancing soil potassium availability has been well established in short-term pot experiments, but its long-term impact remains unclear. This study evaluates the effects of continuous biochar application on potassium supply by analyzing clay mineral composition and potassium adsorption sites in field conditions.</p> Methods <p>A nine-year field experiment was conducted to assess the effects of varying biochar application rates (1500, 3000, and 6000&#xa0;kg&#xa0;ha⁻¹), applied alone or in combination with chemical fertilizers, on potassium forms, clay mineral composition, and potassium adsorption sites in brown soil.</p> Results <p>The results demonstrated that biochar application significantly increased available and slowly available potassium by 17.21 to 51.98% and 31.73 to 54.01%, respectively, with these positive effects becoming even more pronounced when biochar was combined with chemical fertilizers. Biochar also altered soil clay minerals (particles &lt; 2&#xa0;μm in size) composition, increasing illite and transforming kaolinite, vermiculite, and chlorite, enhancing potassium release and storage. Potassium content at p-sites, e-sites, and i-sites within clay minerals increased, particularly when combined with fertilizers. Additionally, biochar improved soil properties such as alkaline-hydrolyzable nitrogen, available phosphorus, organic matter, and cation exchange capacity, thereby indirectly enhancing the retention and supply of potassium in the soil.</p> Conclusion <p>Biochar, particularly with fertilizers, significantly enhances soil potassium availability and ensures its long-term supply. This study highlights biochar as a promising strategy for potassium management in agricultural soils, warranting further research across diverse soils and conditions.</p>