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Potassium-Calcium Exchange in a Multireactive Soil System: II. Thermodynamics¹

ABSTRACT

Thermodynamics of K exchange were investigated in Ca-saturated samples from the Ap horizon of an Evesboro soil from Delaware. At 283 and 298 K the selectivity curves (lnk_v vs. _K) showed preference for K at low values of N_K (mole fraction of K in solution) and for Ca at higher values. This selectivity reversal may be attributed to exchange sites of varying reactivity for K and Ca ions and supports the hypothesis of the multireactive nature of the soil. Although K was selectively bound at low N_K, the soil exhibited an overall Ca preference as noted by the positive standard free energy values (G°). The standard enthalpy of exchange (H°) was negative, which indicated very strong binding of K ions with some sites of the soil. This may be associated with the presence of vermiculitic clay minerals that predominated in the <2-µm clay fraction. A thermodynamic investigation was also initiated on the various size fractions of the soil (i.e., sand, silt, and clay) and on soil that was treated with cetyltrimethylammonium bromide (CTAB) or NaOCl-DCB. These treatments explained the differences in ionic selectivity observed in the Evesboro soil.

¹ Published with the approval of the Director of the Delaware Agric. Exp. Stn. as Misc. Paper no. 1027. Contribution no. 159 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711.

² Graduate Research Assistant and 1982 Potash and Phosphate Institute Fellow, and Associate Professor of Soil Chemistry, respectively. The address for the senior author is now Dep. of Agronomy, Virginia Polytechnic Inst., Blacksburg, VA 24061.

Received for publication March 2, 1982. Accepted for publication August 10, 1983.

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