Additional potassium (K) fertilization, a practice undertaken to reduce the absorption of radioactive cesium (Cs) by rice in paddy fields where radioactive Cs was deposited, is gradually being discontinued owing to cost issues. Understanding the dynamics of K in paddy fields and establishing a reliable and efficient K fertilization technique is necessary to continue reducing the absorption of radioactive Cs by paddy rice. Therefore, to help understand the factors that cause K leaching from paddy soils and the effect of K supply by irrigation water, we used a compact thumb-size column (diameter 2.1 cm x length 5 cm) and examined the K leaching from paddy soil following influents of different concentrations and cation composition ratios. The test soil (fine-grained ordinary lowland paddy soil) was collected from a paddy plot that received additional potassium application. The coefficients of variation during soil filling of the columns were less than 2 % on average and approximately 6 % for soil exchangeable K in the column tests, with sufficient reproducibility. The equilibrium content of soil exchangeable K formed after water percolation was determined by the concentration of K+ and the composition ratio of major cations --- K, Na, Ca, and Mg --- in influent water. The potassium adsorption ratio (PAR) of influent water was a valid indicator of K leaching. The effects of ion species on K leaching were in the order Mg2+ > Ca2+ > Na+ based on a 1 mmol L-1 change in their concentration and Ca2+ > Mg2+ > Na+ based on a 10 % change in their concentration, with divalent cations having a greater effect and Na+ having a smaller effect.