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FLUORINE ADSORPTION BEHAVIOR IN ALKALI SOILS: RELATIVE ROLES OF pH AND SODICITY

Alkali soils formed under the influence of sodium carbonate have both high exchangeable sodium percentage (ESP) and high pH. The relative roles of these factors on adsorption of element fluorine was studied in two soils, a calcareous and a noncalcareous soil. In one experiment, fluoride sorption was studied, at 0.1 mole/liter electrolyte strength, as a function of varying pH (5.5 to 10.4), keeping ESP (100) constant. In another experiment, the effect of varying ESP or SAR (5 to 110) on F sorption at constant pH was studied. The results showed strong pH dependence of F sorption, independent of the degree of sodium saturation. The presence of CaCO3 resulted in significant sorption of F, although its effect was negligible at pH 8.35, near its isoelectric pH 8.4, and also at pH above 10.4. The studies bring out that pH, which has an intimate relationship with the sodicity of calcareous soils, is likely to affect the whole range of physicochemical and surface properties of the soils. For this reason, pH should be considered an integral part of the definition of alkali soils, rather than sodicity alone.