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Ion Activities: An Historical and Theoretical Overview¹

ABSTRACT

A knowledge of ion activities in soil-water systems is essential to a proper understanding of the physicochemical behavior of soils and to the environment of plant roots in the soil. However, with the notable exception of soil acidity, perhaps no other topic in soil chemistry has provoked such fierce arguments as the meaning of ionic activities as they apply to the soil solution. The purpose of this paper is to present an historical and theoretical overview of the use of ion activities in soil chemistry. Attempting to incorporate ionic species into a thermodynamic analysis of the soil solution is a convenient mathematical device, since an ionic species cannot be described in thermodynamic terms. Ionic species are strictly molecular or microscopic concepts. It is demonstrated in this paper that when ionic equilibria between aqueous solutions and solid phases are investigated, the division of the electrochemical potential into a term in electrical potential () and a term in chemical potential (µ) is entirely arbitrary. Additionally, the early 1950s arguments of Jenny, Marshall, Peech, Coleman, Overbeek, and Babcock dealing with junction potentials, suspension effects, and mobilities of ions is reviewed. The importance of soil solution activities in relation to nutrient uptake and plant growth is also presented.

¹ Published with the approval of the Director of the Delaware Agricultural Experiment Station as Miscellaneous Paper no. 1038. Contribution no. 153 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711. Presented at the Div. S-2 Symposium "Chemistry in the Soil Environment: Activities," Soil Sci. Soc. Am., 29 Nov. 1982, in Anaheim, CA.

² Associate Professor of Soil Chemistry.

Received for publication April 6, 1983. Accepted for publication June 21, 1983.