Abstract: Quantity and intensity of potassium and its exchange with calcium in sunlight greenhouse soils are studied in Guanzhong Plain,Shaanxi. Compared to open field soils, the soils under sunlight greenhouse cultivation have different curves of quantity and intensity of potassium (Q/I). The activity ratios between K⁺, and Ca²⁺ and Mg²⁺ ions of the two greenhouse soils (i.e., manural loess soil and fluvo-aquic soil) are about 14.8 and 6.9 times higher than the corre-spongding field soils,respectively. The buffer capacities of the greenhouse soils to add K⁺ are lower than those of the field soils. The changes of Gibbs free energy of K⁺ , and Ca²⁺ and Mg²⁺ exchange ( G) of the greenhouse soils indicate there are excessive accumulation of K⁺ and Ca²⁺ ions in solution, amounts of adsorbed K and Ca²⁺ ions by the two soils under both greenhouse and field conditions are increased in a linear pattern, and the adsorption rate of K⁺ ion of the greenhouse soils and adsorption rate of Ca²⁺ ion both the greenhouse and field soils follows a exponential pattern. When the ratio of K⁺ to Ca²⁺ ions added into soils is 2:1, amounts of adsorbed K⁺ by soils are higher than those of 1:2.5 of K⁺ to Ca²⁺ ions, and Ca²⁺ ion in the greenhouse soils is desorbed. The selectivities of soils to K⁺ to Ca²⁺ ions are changed with the ratios and concentrations of K⁺ to Ca²⁺ and Mg²⁺ ions in the solutions. K⁺ ion is preferential adsorbed when the ratio of K⁺ to Ca²⁺ ions added into soils is 2:1, while, Ca²⁺ ion is preferential adsorbed when the ratio of K⁺ to Ca²⁺ ions is 1:2.5.