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Flocculation Concentration and Sodium/Calcium Exchange of Kaolinitic Soil Clays

Dep. of Agronomy, Univ. of Georgia, Athens, GA 30602

H. Frenkel

Inst. for Soils and Water, Volcani Ctr., Bet Dagan, Israel 50250

* Corresponding author.

ABSTRACT

Clay dispersion in southeastern USA soils needs to be better characterized, because dispersion influences crusting, infiltration, and erosion of field soils. In order to study dispersion of highly weathered soils, clays separated from Ap and Bt horizons of three soil series from the Georgia Piedmont were equilibrated with solutions of varying sodium adsorption ratio (SAR) and pH in order to determine the effects of these parameters on the electrolyte concentration necessary for clay flocculation. The Ap horizon clays required from 1 to 40 mol_c m^–3 for flocculation over the range of SAR = 0 to 16 and pH range from 4.5 to 7.5. The kaolinite-dominated Cecil and Davidson clays were more dispersive than the Iredell, which contained some smectitic clay. Critical flocculation concentration (CFC) values increased with increasing pH as surface charge increased, particularly for the kaolinitic soils. Gapon selectivity coefficients increased at low pH and at low exchangeable sodium percentage (ESP) for the two kaolinitic soils, indicating a preference for Na⁺, which resulted in a greater increase in CFC when plotted against ESP than when plotted against SAR. The Bt horizon clay, despite mineralogy similar to Ap clays, was nondispersive, possibly due to the absence of dispersive organokaolinite complexes. These results confirm field observations of dispersion-related crusting and permeability decreases on these soils under rainfall, and suggest the importance of maintaining low Na contents, moderate pH levels, and high solution ionic strengths.

Research supported by Hatch funds allocated to Georgia Agric. Exp. Stn.

Received for publication July 11, 1989.

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