Release kinetics of nonexchangeable potassium in calcareous soils

Release rates of nonexchangeable potassium (K) were determined for 10 intensively Cultivated soils of the Hamadan area in western Iran. Calcium (Ca)saturated soil samples were equilibrated with 0.01 M CaCl2 and 0.01 M citric acid for 2-2759 h. Plots of cumulative K+ released by different soils vs. time consisted of two parts, suggesting two different rates of K+ diffusion from soils. Four mathematical models (first-order, parabolic diffusion, power function, and Elovich equation) were used to describe cumulative K+ release. The power function and first-order equation described the K+ releases kinetics best as evidenced by the highest correlation coefficients and the lowest values of the standard error of the estimate (SE). The parabolic diffusion equation also described the data satisfactory, indicating diffusion-controlled exchange.