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Kinetics of Potassium Desorption in Soil using Miscible Displacement¹

ABSTRACT

Kinetics of K desorption were conducted on samples from the Ap, A2, B21t, and B22t horizons of two Dothan (Plinthic Paleudults) soils. Aluminum- and calcium- saturated samples were equilibrated with K for 96 hours and then continuously leached with 0.01M CaCl₂ until K was not detected in the leachate. The rate of K desorption from all samples increased rapidly initially and levelled off with time. Desorption was nearly complete in approximately 3 to 4 hours for the Ap, A2, and B21t horizons, and in 8 to 9 hours for the B22t horizons. Approximately 95–98% of the adsorbed K was subsequently desorbed suggesting K adsorption-desorption in the Dothan soils was reversible. A linear relationship between time and percent K desorption indicated that diffusion was the predominant mechanism of K desorption in these soils. Diffusion-controlled exchange would be expected due to the vermiculitic clay minerals present in the soils.

Potassium desorption conformed to first-order kinetics. Apparent desorption rate coefficients (k'_d) ranged from 0.3 to 1.3 hour^–1. The magnitude of the k'_d values decreased as clay content increased in the soils. This was ascribed to increased intraparticle transport and to increased diffusion in the more clayey samples. The k'_d values were generally higher in the Al-than in the Ca- saturated samples. The effect of flow velocity on rate of K desorption was investigated using velocities of 0.5, 1.0, and 1.5 ml min^–1. The rate of K desorption increased only slightly with flow velocity.

¹ Joint publication of the Delaware Agric. Exp. Stn. and the Virginia Agric. Exp. Stn. Published with the approval of the Director of the Delaware Agric. Exp. Stn. as Miscellaneous Paper no. 890. Contribution no. 110 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711.

² Assistant Professor of Soil Chemistry, Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711; and Associate Professor and Professor of Agronomy, Agronomy Dep., Virginia Polytechnic Institute & State Univ., Blacksburg, VA 24061, respectively.

Received for publication February 25, 1980. Accepted for publication June 19, 1980.

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