Calculating Cation Exchange in a Salt Transport Model

A cation exchange subroutine that can be expanded to include any number of cations was developed and interfaced with a water flow-salt transport model that also contained a lime and gypsum precipitation-dissolution chemistry subroutine. The exchange subroutine was required by the complete model to satisfactorily predict EC, SAR, and specific ion concentration changes with time and depth for a gypsiferous and a nongypsiferous soil irrigated with waters containing three different CaSO? concentrations at two leaching fractions. In his sudy, exchangeable Ca, Mg, Na, and K were considered. Exchangeable K was included for me with high exchangeable and soluble K soils and high K irrigation waters. The additional cation exchange coefficient values needed for K exchange and a method for their calculation is given