Here, we investigated process engineering strategies to improve the biological solubilization of K-feldspar, a low-reactivity potassium mineral, aiming to obtain a green K-fertilizer for agricultural applications. The results using submerged cultivation of Aspergillus niger showed that the biological K-solubilization of pristine K-feldspar rock released up to 15.87% (reaching 220 mg L−1 after 3 days) when using 5% glucose as carbon source and initial pH 4, without a preculture step. Besides medium acidification, the microorganism-mineral physical interaction played a key role in promoting K-solubilization. Furthermore, prior mechanical activation of the K-feldspar led to substantial increases of up to 172% in K release (reaching 326.86 mg L−1), compared to the original mineral, and reduced the K-solubilization cultivation time to only 24 h. The results demonstrated the potential of integrating mechanical activation with biological solubilization to advance on the use of low-solubility potassium minerals as K-fertilizers, thus contributing to a more sustainable agriculture. Display omitted •A green K-fertilizer was obtained from a low-reactivity potassium mineral.•Bioprocess engineering strategies improved solubilization of K-feldspar.•Physical interaction between microorganism and mineral potentiated K-release.•Mechanical activation resulted in a remarkable increase of K-released.•The strategies contribute to future developments in sustainable agriculture.