Predicting exchangeable cation distributions in soil by using exchange coefficients and solution activity ratios

Abstract

A simple model which predicts the distribution of surface-applied chemicals in soil following leaching is described. The applied cation is considered to reach equilibrium between the solution and exchange phases prior to moving into the underlying soil layer. The distribution between the solution and exchange phases is defined by the Gapon relationship. The Gapon exchange coefficient is derived from exchange isotherms determined using a batch technique. Movement of the applied cation through successive layers and cation exchange with the soil continues until the activity ratio in solution equals the equilibrium activity ratio determined from the exchange isotherm. The applied cation remaining in solution is then considered to move 'freely'. A number of physical and chemical processes have been omitted in developing the model to retain simplicity and to ensure that the required parameters can readily be obtained using simple and well established laboratory procedures. In particular, only a binary cation exchange system is considered, and it is assumed that the rate of exchange is instantaneous. The inputs to the model are the amount of cation added, the Gapon exchange coefficient and corresponding solution activity ratio, the equilibrium activity ratio, the initial exchangeable and solution cation concentrations, and the average volumetric moisture content of soil during leaching. The simple model satisfactorily predicted the distribution of exchangeable K+ and Mg2+, and percentage of applied K+ or Mg2+ leaching 'freely' in the soil solution in homogeneous columns of soil having varying texture, clay mineralogy, and initial cation composition.