A simple improved mixing cell model is shown to be more accurate than the standard mixing cell model when used to solve solute transport problems with a nonlinear adsorption isotherm included. The solution obtained from the improved model is found to agree closely with that calculated using a Crank-Nicolson scheme, and an exact analytical solution for solute transport subject to a nonlinear solute adsorption isotherm. Application of the improved mixing cell model was demonstrated by application to experimental data from a laboratory Ca---K exchange experiment in which a solution containing Ca was passed through a column filled with K-saturated soil. The model was used in conjunction with the experimentally determined nonlinear adsorption isotherm to predict the experimental breakthrough data. The data and the predictions agreed reasonably well, indicating that the equilibrium assumption is reasonable. However, it is suggested that an appropriate isotherm is necessary to describe well the concentration history curve.