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Coupled Diffusion of Exchangeable Cations in Soil

Soil Science Dep., Univ. of Florida, Gainseville, Fl 32611

* Corresponding author.

ABSTRACT

Although coupling of transport processes in soil has been studied for some time using the principles of irreversible thermodynamics, its importance with respect to coupled diffusion of cations in soil has not yet been fully determined. The purpose of this study was to evaluate the importance of coupled diffusion of cations in a sandy soil in which the pH-dependent charge from organic matter represented a major proportion of the total cation-exchange capacity. The degree of coupling was evaluated by determining what effect counterdiffusion of exchangeable K and Ca had on the distribution of exchangeable Na when the latter was initially at a uniform concentration throughout the system. Coupling was evidenced by a Na flux that was counter to, and ranged from 16 to 28% as large as, the K flux. The simultaneous diffusion of Ca, K, and Na was modeled by assuming that the coupling resulted from a diffusion potential arising from the counterdiffusion of K and Ca. Intrinsic diffusion coefficients required in the model were estimated from conductance data. Predicted and measured cation concentrations showed a highly significant linear correlation, with predicted concentrations averaging 96% of measured concentrations for all three cations.

Published as Florida Agric. Exp. Stn. Journal Series no. R-01737.

Received for publication January 7, 1991.