Stability of a Calcareous Saline-Sodic Soil During Reclamation

General guidelines to reclaim saline or sodic soils do not adequately consider variables such as pH and the presence of organic matter that are known to affect soil stability. Poor structural stability of sodic, saline-sodic, and high-pH soils adversely influences crop yields; promotes piping, tunneling, and slope erosion; and can accelerate the failure of water conveyance systems. We evaluated six soil teats, used to measure physico-cbemical properties, for their suitability to evaluate the structural stability of a calcareous, saline-sodic soil under reclamation. The stability tests were wilting point, plastic limit, coefficient of linear extensibility (COLE,& water content at 0.03 MPa, liquid limit, and dispersion index. The range of electrical conductivity (EC) studied was 0.5 to 20 dS m-l, sodium adsorption ratio (SAR) 0 to 400 (mmol L_‘)OJ, and pH 8.4 to 10.5. The results obtained indicate that the amount of water necessary for a soil to Sow under standard conditions for the liquid limit test decreased an average of 25% when the EC decreased from 40 to 2 dS m-t. The liquid limit and EC showed a linear correlation (R2 = 0.785); therefore, the liquid limit was considered to be an appropriate index to evaluate the physical properties of a soil under a leaching process. Plasticity index and available water were more useful in the evaluation of the mechanical properties of the soil when we used amendments. Liquid limit combined with the water content of the soil at 0.03 MPa was the most useful tool for evaluating soil structural stability during reclamation. I T IS WELL KNOWN that sodic soils are highly dispersive. Dispersion causes loss of soil structure and reduction in hydraulic conductivity and increases soil erodibility (Quirk and Schofield, 1955; McNeal et al., 1968; Rhoades and Ingvalson, 1969; Frenkel et al., 1978; Perry and Postol, 1977; Pupisky and Shainberg, 1979; Suarez et al., 1984; Yousaf et al., 1987). This dispersive effect is specially pronounced for sodic soils of high pH (>8.5). Also, clay dispersion promotes piping and tunneling erosion (Sherard and Decker, 1977). Sodic soils have an adverse effect on crop production and in the maintenance of canals and irrigation systems. Characterization of the physico-chemical soil properties to ensure a suitable state is necessary to avoid both agricultural and structural problems. The term alkaline has been used in the literature in relation to soils with high alkali metal content (Na) in the exchange complex. In most cases, high pH is also associated with these soils. Allison (1964) and van Beek and van Breemen (1973) made a clear distinction between I. Lebron and D.L. Suarez, U.S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92501; and F. Alberto, Consejo Superior de Investigaciones Cientificas, E.E. Aula Dei, Apdo. 202, 50080 Zaragoza, Spain. Received 11 May 1993. *Corresponding author (lebron@ucrvms). Published in Soil Sci. Soc. Am. J. 58:1753-1762 (1994). the terms sodic soil and alkaline soil. Alkaline soils have a pH >7 (van Beek and van Breemen, 1973) and sodic soils have high Na content (normally >15%) in the exchange complex. Sodic soils do not necessarily have alkaline pH values, and alkaline soils do not necessarily have high exchangeable Na. Gibbs (1945) established that soils having a plasticity index <O. 1 kg kg-’ and liquid limit <0.30 kg kg-’ were erodible. The liquid limit represents the minimum amount of water that a small soil sample needs to flow under standard conditions. The plastic limit is the water content at which the soil starts to loose cohesion due to the absence of water. Plasticity index is the difference between the liquid limit and the plastic limit values. These three values are known as the Atterberg limits (American Society for Testing Materials, 1985). Sherard (1953) realized that earthen dams with a plasticity index <0.05 kg kg-’ broke down in a few years; and, for a given plasticity index, the soils with higher liquid limit had more resistance to piping. Cole and Lewis (1960) reported similar observations in Australia. Aitchison (1960) proposed the use of clay dispersibility as an index to classify the susceptibility of soils to erosion. Clay dispersibility is the ratio of the weight of clay dispersed in deionized water and the weight of clay dispersed with sodium hexametaphosphate. Although the introduction of the dispersibility concept (Aitchison, 1960) changed the diagnostic criteria of most engineering recommendations, it was not universally accepted. Resendiz (1977) maintained that the clay activity index (plasticity index/ clay percentage by weight) is an appropriate index to determine the stability of soil. He found that soils with clay activity index values of 0.003 to 0.01 were susceptible to piping. However, Sherard and Decker (1977) showed that most clay soils are within this range of clay activity index and a soil can go from a dispersed to a flocculated state and still stay within this range. Agronomists and soil scientists have centered their efforts on tests such as hydraulic conductivity, aggregate stability, and clay dispersion. Many of the guidelines for the reclamation of saline and sodic soils given by various researchers during the last 50 yr (U.S. Salinity Laboratory Staff, 1954; Quirk and Schofield, 1955; Abbreviations: PC, electrical conductivity; ECSE, electrical conductivity of the saturation extract; SAR, sodium adsorption ratio; COLE, coefficient of linear extensibility; ESP, exchangeable sodium percentage; ESR, exchangeable sodium ratio; pIAP, negative log of the ion activity product (a~, X ace,); PZNPC, point of zero net proton charge; Treatment A, natural soil; Treatment B, Ca-saturated soil; Treatment C, leached soil; &, zeta potential; ANOVA, analysis of variance; MANOVA, multivariate analysis of variance.