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Soil Science Society of America Journal Abstract -

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


This article in SSSAJ

  1. Vol. 51 No. 1, p. 68-72
    Received: Apr 14, 1986

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  1. V. P. Evangelou and
  2. F. J. Coale2



The traditional nonthermodynamic Gapon exchange selectivity coefficient (KG) for Na-Ca exchange has previously been shown in the literature to be relatively constant for only a restricted range of Na+ equivalent fractions (NNa ≤ 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 KG for NNa values >0.40 for some soils. The results demonstrate that, in general, low CEC soils are more likely to demonstrate a constant KG for NNa values >0.40 than are high CEC soils. The data show that a constant KG for NNa values >0.40 was obtained for Georgia kaolin, illite, and several soils of low CEC. However, the plot failed to predict the KG and the CEC for soils, clay minerals, and an exchange resin of high CEC. These high CEC materials exhibited a NNa dependent KG. Using solution ion concentration instead of single-ion activity affected the magnitude of the KG but did not affect the degree of constancy of the KG for NNa 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 (NNa) of >0.40.

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