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Kinetics of Potassium Exchange in a Paleudult from the Coastal Plain of Virginia¹

ABSTRACT

The kinetics of K adsorption from solution to exchangeable phases were investigated on the Ap, A2, B21t, and B22t horizons of Dothan soil (Plinthic Paleudult) from two locations in Virginia. These soils are loamy sands in the upper horizons with clay content increasing with depth, are slightly acidic in the surface with pH decreasing with depth, have CEC's ranging from 3.4 to 8.6 meq/100 g in the four horizons and contain considerable quantities of chloritized vermiculite and kaolinite in all horizons. Potassium adsorption with time was evaluated on Al- and Ca-saturated samples from each horizon using 5, 25, and 100 µg/ml K solutions equilibrated for 0, 1, 2, 24, 96, and 192 hours. Equilibrium in K exchange was reached in 2 hours with the 5 and 25 µg/ml solutions and in about 24 hours with the 100 µg/ml solution. This slow rate of K exchange was attributed to diffusion-controlled exchange, which reflects the relatively high amount of vermiculitic material in these soils. Adsorption rate coefficients (k_a) were calculated from reaction time vs. quantity of K sorbed using a modified form of the Freundlich equation. The magnitude of the k_a values decreased with increasing ionic strength, which conforms to Bronsted's activity rate theory. The similar magnitude of the k_a values from horizon to horizon suggests that similar exchange reactions were taking place in all horizons.

¹ Contribution from the Dep. of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Presented to Div. S-2, Soil Sci. Soc. Am., 7 Dec. 1978, in Chicago, Ill.

² Former Graduate Research Assistant, Associate Professor, and Professor of Agronomy, respectively. The senior author is presently Assistant Professor of Soil Chem., Dep. of Plant Sci., Univ. of Delaware, Newark, DE 19711.

Received for publication January 11, 1979. Accepted for publication September 27, 1979.

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