The application of potassium fertilizer by farmers is often not appropriate and causing environmental pollution. By understanding the adsorptive characteristics of potassium (K) on different soils, we can prevent excessive application of K-fertilizer that can cause environmental impact. The Gapon exchange coefficient (K G ), for exchange between K and Ca, was considered as an important factor influencing the adsorption of K. This study was conducted to compare the constant K G of five important farm soils in Taiwan. The K G and CEC were then used to predict K buffering capacity (PBC). Finally, the relationship between exchangeable K ratio (E P ) and K adsorption ratio (PAR) was examined. The results show that five soils have same trends, indicating that K G decreases with increase in K saturation. The CEC and K G of Liuying (Ly) soil are both high, so that their K buffering capacity is high. The K G and CEC of Chanjing (Cj) and Sanhua (Sh) soil show moderate values. The CEC of Erling (El) soil is high, but its K G is low, so that its K buffering capacity is moderate. On the other hand, the K G of Newniaokang (Nnk) soil is high but its CEC is low, so its K buffering capacity is also moderate. The correlations between E P and PAR of five soil show linear relationship at three treatments of CaCl 2 concentration. This study may provide an important clue to the fertilization management of K-fertilizer on the different soil properties in Taiwan.