Abstract: 【Objectives】 Long-term fertilization has changed contents of soil readily available and slowly-available potassium (K) and soil pH, while all these changes affect availability of soil K to plants. In order to instruct the optimum fertilization to improve soil fertility, a long-term fertilization experiment started from 1990 (Qiyang experimental station located in Hunan province) was involved to explore soil K availability to plants. 【Methods】 Treatments of no fertilization (CK), mixed application of chemical nitrogen and phosphorus fertilizer (NP), application of NP combined with chemical potassium fertilizer (NPK), and application of NPK combined with manure (NPKM) were selected. The bio-exhausting method was used to exhaust soil K for five periods, twenty days for each period after the planting of maize. After each harvest, the contents of plant K and the soil readily available K and slowly-available K were determined. For each period the N and P but K nutrients were applied to soil. 【Results】 1) The contents of soil readily available K were decreased with the planting periods. The long-term different K fertilization changed the contents of soil readily available K. The contents of soil readily available K arrived at the lowest level?? after the second planting period with CK, while the NP nearly arrived at this level after the fifth planting period. However, after the fifth period, the contents of soil readily available K were decreased by 45.2 and 41.7 mg/kg under CK and NP, and the decreased ratios were 65.9% and 60.4% respectively, while the contents under NPK and NPKM were decreased by 171.0 and 390.3 mg/kg with the decreases ratios of 73.4% and 81.3% respectively. 2) After the 22 years?? fertilization the contents of soil slowly-available K were in the order of NPKM NPK NP CK. The contents of soil slowly-available K with CK and NP were decreased by 54.0 and 77.0 mg/kg with the decreased ratios of 46.0% and 53.8% respectively, while the contents under NPK and NPKM were decreased by 41.0 and 55.0 mg/kg with the decreased ratios of 27.2% and 22.5% respectively after the fifth planting period. 3) The amounts of K uptake by maize were in the order of NPKM NPK CK NP. The amount of K uptake by maize with NPKM was 3.1 times as high as that with NPK. Furthermore the amount under NPK was 1.9 times as high as that under NP. 4) The Elovich equation showed that the soil K availability to plant well matched the real value. The first order kinetic equation can simulate the treatments of CK and NPKM better than the Elovich equation. The Elovich equation can simulate the treatments of NP and NPK better than first order kinetic equation, judging from R2 and standard error (SE). 5) There was a multiple linear regression between the content of soil readily available K (X1), slowly-available K (X2), total K (X3) and soil pH (X4) and the amount of K uptake by maize (y). The equation was y=-438.90+0.33X1+0.22X2+22.14X3+18.44X4 (R2=0.995). There was a significant positive correlation between X1 and y (P0.05). The stepwise multiple regression equation was y=-107.36+0.38X1+25.16X4 (R2=0.985). There was an extremely significant positive correlation between X1 and y (P0.01), and a significant positive correlation between X4 and y (P0.05) respectively. 【Conclusions】 The K supply capacity was in the order of NPKM NPK CK NP based on the amounts of K uptake by maize with planting periods. There was a significant positive correlation between the content of soil readily available K and soil pH and the amount of K uptake by maize respectively.