Effect of pH on Saturated Hydraulic Conductivity and Soil Dispersion

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 < 6.) in humid environments, where CaCO3 is absent. El-Swaify (1973) found an anion effect on K for several tropical soils and interpreted this as due primarily to the different pHs measured. If pH has an effect on dispersion and hence on soil K, the published threshold relations between soil ESP and electrolyte levels are applicable only to slightly acidic conditions and not to the more prevalent arid-land sodic-soil conditions of higher pH. In the present study, we examine the effect of pH (at fixed electrolyte concentration and ESP level) on the saturated hydraulic conductivity of three mineralogically different arid land soil types using laboratory soil columns. We also examine the relationship between optically measured dispersion and pH. MATERIALS AND METHODS Hydraulic Conductivity Studies Three soils of different properties were chosen for study (Table 1). The Fallbrook and Bonsall soils have similar textures, whereas the Arlington soil is higher in silt content. All three soils are low in organic carbon content (determined using the method of Allison, 1960), with Arlington having the lowest value. The soil clays are predominantly kaolinite, vermiculite, and montmorillonite for Fallbrook, montmorillonite-vermiculite intermediate (according to x-ray analysis), mica and kaolinite for Bonsall, and vermiculite, mica and kaolinite for Arlington. The soils are noncalcareous and exhibit relatively low salt release by mineral weathering (e.g., see Fig. 1, Shainberg et al., 1981b). Low salt release is a necessary criterion in this study, because mineral weathering (largely release of Ca and Mg) alters solution composition and total solute concentration during flow measurements. Surface areas were measured using the method of Cihacek and Bremner (1979). Mixtures of Na and Mg salts rather than Na and Ca salts were used to prepare the solutions at each pH to prevent CaCO3 precipitation at high pH and low SAR. The effects of Mg on K are similar to those of Ca over the range of SAR used (McNeal et al., 1968). Two sets of solutions were prepared, SAR 20 and SAR 40, by mixing salts of Mg-Na-ClHCOj. The solutions were prepared to have total electrolyte levels of 8,15, 25, 50, 100,250, and 500 mmoU L-' (mmoles of charge per liter) and pH levels of 6, 7, 8, and 9 by adjusting the C1/HCO3 ratio and Pco2The pH 6 and 9 solutions were the same except that the pH 6 solutions were equilibrated at Pco2 = 97 kPa and the pH 9 solution at 35 Pa Pco2 (atmospheric CO2). The composition and Pco2 necessary to achieve the desired pH for each solution were calculated from the equilibrium constants listed by Suarez (1977); prepared solutions were within 0.03 units of the desired pH. In a parallel experiment, the K values of one treatment (SAR 20, pH 8) and soil (Fallbrook) were determined using Na-Ca salts for comparison. For the K measurements, plastic cylinders (5.5 cm i.d.) were packed with 300 g of sieved (< 2 mm), air-dried soil to a bulk density of 1.5 Mg m~. All columns were slowly wet from below by capillary rise, and then the same solution was applied from above using a peristaltic pump with constant flow of 17 nL s~'. The K values were determined by measuring the head developed in each column under constant flow with mercury or water manometers. The effluent from the columns flowed through a U tube and into a fraction collector. Each column was initially leached with the most concentrated solution of the desired pH and SAR treatment. When the K of the column and the composition and pH of the effluent had stabilized, the next more dilute solution of the same SAR and pH was applied. Usually > 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.