Purpose: Potassium (K) in soil minerals is an important factor affecting rice production in red paddy soil of subtropical China under intensive agricultural production. The long-term application of green manure might affect the soil K availability through accumulative effects of microbial activity on K-bearing minerals. This study aims to investigate the effect of green manure applications (GMs) on soil K availability in relation to soil minerals.Materials and methods: The soil mineralogy under 36 years of GMs, including milk vetch application (MV), rape application (RA), and ryegrass application (RY), was studied by X-ray diffraction (XRD) measurement, with a winter fallow treatment (WF) as the control. Fast available K (FAK), non-exchangeable K (NEK), and structural K (SK) were extracted with NH4OAc, HNO3, and HF-HClO4 solution, respectively. All the extracted K was determined by flame photometry. The K release or retention was measured by a new method of potassium Q/I curve.Results and discussion: The GMs significantly decreased the SK, which was associated with weakened diffraction intensity of feldspar and mica. However, the NEK under GMs increased from 90.9 to 100.1–104.8 mg kg−1 compared with that under WF, which was in line with the higher content of illite under GMs. The soil clays contained much kaolinite and illite, leading to easy leaching of K. The GMs obviously improved K retention in the soil, especially the MV and RY, which could be ascribed to the large amounts of illite with poorly crystallized structure under GMs.Conclusions: Long-term GMs, especially MV and RY, can improve the availability of SK and K retention capacity in the soil in relation to soil mineralogy, and thus can be an effective alternative to enhance soil K availability in red paddy soil.