Effect of pH on Saturated Hydraulic Conductivity and Soil Dispersion1

The adverse effects of exchangeable sodium on soil hydraulic conductivity (K) are well known, but at present only sodicity and total electrolyte concentration are used in evaluating irrigation water suitability. In arid areas, high sodicity is often associated with high dissolved carbonate and thus high pH, but in humid areas high sodicity may be associated with low pH. To evaluate the effect of pH (as an independent variable) on A", solutions with the same SAR and electrolyte level were prepared at pH 6, 7, 8, and 9. Saturated A' values were determined at constant flux in columns packed at a bulk density of 1.5 Mg m'. At pH 9, saturated K values were lower than at pH 6 for a montmorillonitic and a kaolinitic soil. For a vermiculitic soil with lower organic carbon and higher silt content, pH changes did not cause large K differences. Decreases in A" were 1 Contribution from the U.S. Salinity Laboratory, USDA-ARS, U.S. Salinity Laboratory, Riverside, CA 92501. Received 16 Dec. 1982. Approved 9 Sept. 1983. 2 Geochemist, Soil Scientists, and Plant Physiologist, respectively. The permanent address of R. Lavado is Institute de Geomorfologia y Suelos, La Plata, Argentina. not reversible on application of waters with higher electrolyte levels. The results from the K experiments were generally consistent with optical transmission measurements of dispersion. Although anion adsorption was at or below detection limits and cation exchange capacity (CEC) was only slightly dependent on pH, differences in pH effects on A" among soils are likely due to differences in quantities of variable-charge minerals and organic matter. Additional Index Words: sodium adsorption rate, electrolyte concentration, optical transmission. Suarez, D.L. , J.D. Rhoades, R. Lavado, and CM. Grieve. 1984. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil Sci. Soc. Am. J. 48:50-55. T ADVERSE EFFECTS of high levels of exchangeable sodium on the hydraulic conductivity (K) of soils are well established. The onset of reduced K at a given soil exchangeable sodium percentage (ESP) SUAREZ ET AL.: EFFECT OF pH ON SATURATED HYDRAULIC CONDUCTIVITY AND SOIL DISPERSION 51 varies with total electrolyte concentration and soil properties (Quirk and Schofield, 1955; McNeal and Coleman, 1966; Frenkel et al., 1978; Shainberg et al., 198la). Despite these and numerous other studies, the exact levels of ESP and electrolyte concentration at which reductions in K will occur for a given soil still cannot be predicted accurately. In published laboratory column studies, neutral or slightly acidic chloride salt solutions (pH =; 6.0) were normally used with a carbon dioxide partial pressure (Pco2) near atmospheric (10"kPa). The pH values of water in the columns were likely =: 6.0 in the upper portion and 7.5 to 8.0 in the lower portions when CaCO3 was present. Arid and semiarid soils are usually calcareous in the subsurface. Sodic soils in this environment are generally associated with high pH and high dissolved carbonate and bicarbonate concentrations. With such soils, pH values can exceed 10 but are more likely in the range of 8 to 9.5 near the surface. Sodic soils may occur also under acid conditions (pH 1.3 L of each solution was passed through the column. The pH 6 solutions were maintained at pH 6 by bubbling CO2 gas into the 5-L Pyrex solution reservoir. An exit tube at the bottom of the reservoir was placed at the same height as the peristaltic pump to eliminate CO2 degassing during the pumping operation. The pH of the leachate was measured in the U tubes before degassing could occur. Influent and effluent solutions were analyzed for Ca, Mg, Na, and K by atomic absorption, alkalinity by acid titration, and Cl by AgCl titration (Rhoades and Clark, 1978). At the conclusion of the experiment, the columns were separated into 1-cm sections and analyzed for exchangeable cations. For purposes of comparison, the K data were scaled to the initial K(K,) values determined with the 100 mmolc L", SAR 20 solution, or 500 mmolc L~, SAR 40 solution. Based on six replications on the Bonsall soil, SAR 40, pH 9 treatment, log K/Kt standard deviations of 0.08, 0.18, 0.19, and 0.11 were determined for solutions of 250, 100, 50, and 25 mmol,; L", respectively.