Measurement of available potassium (AK) is commonly used to assess potassium (K) status of a soil, and studies in the past have shown that release of AK from 2:1 K-bearing minerals was triggered by chemical extraction methods of exchanging adjacent interlayer K with hydrated cations and the dissolving minerals. However, there is still a great deal of uncertainty in and variation between these batch experiments with respect to respective contribution of the exchange and dissolution to K release budget, which has certain important implications for predicting soil sustainability. Mechanisms of AK released from illite were studied using 2:1 K-bearing mineral dominant in soil, and four extracting methods, i.e. 0.2 mol L^-1 sodium tetraphenylboron (NaTPB), 1 mol L^-1 boiling nitric acid (HNO₃), 2 mol L^-1 cold HNO₃ and 2 mol L^-1 hot hydrochloric acid (HCl) methods. X-ray diffraction (XRD), fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) and atomic force microscopy (AFM) were used to investigate changes in structure and surface morphology of the mineral. Results show that released K accounted for 59.5% of the total K in illite in amount and basically through interlayer exchanging reaction when extracted with NaTPB, but for 1.53% ~ 2.46% when extracted with acid solutions and 88.4% ~ 94.0% of the released K was K released through interlayer exchanging reaction. A very limited amount of structure elements of Fe, Al and Si was released into the solution of NaTPB, but the situations in the acid solutions were just reverse. When NaTPB was used to extract K from illite, the surface of the illite became blurry and the interlayer space was expanded, forming a silicon-rich surface of secondary transition mineral, but did not show any solution phenomena. However, when acid solutions were used to extract K from illite, the mineral did not change much in structure, but lowered in crystallinity and displayed apparent solution phenomena. In the presence of boiling HNO₃ and cold HNO₃, a small number of elliptical dissolution pits were found randomly distributed on the surface of illite. When extracted with hot HCl, the illite showed channel dissolution on the surface, and fine mineral particles attached on large grains were dissolved in the hot HCl solution. All the findings indicate that K is released from illite mainly through cation-exchange reaction in both acid and salt solutions, and interlayer K in the K-bearing minerals is the main pool of K available to plants. The acid extracting methods, on the one hand, may underestimate soil AK supply capacity; on the other hand, they extracte some structural K that is not available to plants. Thus, the 1 mol L^-1 boiling nitric acid (HNO₃), 2 mol L^-1 cold HNO₃ and 2 mol L^-1 hot hydrochloric acid (HCl) methods are not suitable to assessment of soil AK pools.