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Dependence of the Gapon Coefficient on Exchangeable Sodium for Mineralogically Different Soils¹

ABSTRACT

The traditional nonthermodynamic Gapon exchange selectivity coefficient (K_G) for Na-Ca exchange has previously been shown in the literature to be relatively constant for only a restricted range of Na⁺ equivalent fractions (N_Na 0.40) in the exchanger. This relationship was developed for a heterogeneous group of soils. A linear Langmuir-type plot of the traditional Gapon equation for Na-Ca exchange data of mineralogically different soils, clay minerals, and an exchange resin was used to demonstrate the applicability of a constant K_G for N_Na values >0.40 for some soils. The results demonstrate that, in general, low CEC soils are more likely to demonstrate a constant K_G for N_Na values >0.40 than are high CEC soils. The data show that a constant K_G for N_Na values >0.40 was obtained for Georgia kaolin, illite, and several soils of low CEC. However, the plot failed to predict the K_G and the CEC for soils, clay minerals, and an exchange resin of high CEC. These high CEC materials exhibited a N_Na dependent K_G. Using solution ion concentration instead of single-ion activity affected the magnitude of the K_G but did not affect the degree of constancy of the K_G for N_Na values >0.40. The relationships are of practical importance in the reclamation of sodic soils with an equivalent fraction of Na⁺ in the exchanger phase (N_Na) of >0.40.

¹ Contribution from the Dep. of Agron., Univ. of Kentucky Agric. Exp. Stn., Lexington. This investigation reported in this paper (no. 86-3-71) is in connection with a project of the Kentucky Agric. Exp. Stn. and is published with the approval of the Director.

² Associate Professor and Graduate Research Assistant, respectively, Dep. of Agron., Univ. of Kentucky, Lexington, KY 40546-0091.

Received for publication April 14, 1986.