Effect of Exchangeable Sodium on Soil Electrical Conductivity-Salinity Calibrations 1
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This study was undertaken to ascertain if soil sodicity significantly influences soil electrical conductivity (<r.) salinity calibrations. Laboratory columns of Fall brook (Typic Haploxeralfs) and Yolo (Typic Xerorthents) soils were adjusted to various levels of salinity and sodicity by leacbing them with solutions varying in electrical conductivity (O and sodium adsorption ratio (SAR); then cr, was measured using four-electrode techniques. Calibrations obtained between <rw and a, over the <rw range 1 to 20 were compared at different levels of SAR ranging between 0 to 80 or 400. The calibrations between a, and <r. were found to be insignificantly influenced by variations in SAR over the range studied. Normal variations in the exchangeable Na contents of typical saline, arid-land soils should not cause any serious misdiagnosis of soil salinity based on measurements of bulk a. and use of a,-a, calibrations. Additional Index Words: soil sodicity, earth resistivity, sodium adsorption ratio. Bottraud, J.-C, and J.D. Rhoades. 1985. Effect of exchangeable sodium on soil electrical conductivity salinity calibrations. Soil Sci. Soc. Am. J. 49:1110-1113. T AND INSTRUMENTATION for measuring soil electrical conductivity (o-a) have been substantially advanced since 1971 when <ra was first shown applicable to the determination of soil salinity in the field (Rhoades and Ingvalson, 1971). Theories of the measurement and interrelations among the various soil parameters involved have been described, improved instrumentation and circuitry have been developed, and commercial units are available for measuring aa using four-electrode, electromagnetic induction and time domain reflectrometry methodology. It has been shown that o-a and soil salinity, in terms of the electrical conductivity of the soil solution (<rw) or of the soil extract ((re), are closely related. Accurate and simple methods have been developed for calibrating soil salinity and aa. Methods for predicting such calibrations have also been developed. Applications of the method for measuring, mapping, and monitoring field salinity, detecting the presence of a shallow water table, detecting saline seeps, determining leaching fraction, and scheduling and controlling irrigations have been developed and demonstrated. Reviews of much of the above are given elsewhere (Rhoades, 1976, 1978, 1985; Rhoades and Corwin, 1984; Rhoades and Oster, 1985). The following equation(s) of Rhoades et al., (1976) has been shown to be valid for saline soils: [1] [2] T = a® + b, where 0 is volumetric soil water content, <rw is electrical conductivity of the soil water, <rs is apparent electrical conductivity of the solid phase (primarily due to surface conductance and exchangeable cations), and T is a transmission coefficient (pore geometry factor of value < 1) which is linearly dependent upon 0 (when 0 is greater than some minimum; 0t) with a and b being empirical parameters appropriate for the particular soil. This relation has been found to describe observed data quite well, except where <rw and 0 are atypically low for arid soils. Under such conditions the <ra<rw relation becomes curvilinear at o-w levels of less than about 4 dS m~' (or at <re levels of less than about 2 dS m~') depending on soil clay content and soil type (Shainberg et al., 1980; Nadler and Frenkel, 1980; Nadler 1982). From the above, it may be assumed that surface conductance is generally insignificant relative to <rw in moist, saline soils and should not interfere seriously in their <ra-<rw calibrations. But little data are available to establish limits in this regard. Likewise, little data are available to establish the specific influence of exchangeable sodium on the <ra-crw relation. One would expect <rs to increase with exchangeable sodium, especially as <rw is reduced and the outward extent of the double layer influence is increased. On the other hand, as double-layer effects are manifested, reductions in pore geometry could occur with clay swelling; hence, T could be simultaneously reduced. Thus the two processes—increase in <TS and decrease in T— could be offsetting, or at least partially so. Early unpublished attempts of the second author to evaluate the influence of exchangeable sodium percentage (ESP) on o-a-<rw relations yielded inconsistent, inconclusive results. In a subsequent attempt (Shainberg et al., 1980), some hint of an ESP effect was evidenced but the variability of the data limited its conclusiveness. This study was undertaken using improved techniques and more critical methods of statistical analysis in an attempt to gain more conclusive data and to establish the influence, if any, of ESP on <ra-o-w calibrations and the conditions under which ESP might produce a misdiagnosis of salinity level from <ra measurements. MATERIALS AND METHODS The electrical conductivities of two California soils, Fallbrook-B (USSL soil no. 3677; fine-loamy, mixed thermic Typic Haploxeralfs) and Yplo (USSL soil no. 3416; finesilty, mixed, nonacid, thermic Typic Xerorthents) were studied as a function of exchangeable sodium percentage (as deduced from the sodium adsorption ratio, SAR = Na/ [(Ca + Mg)/2], where the solute concentrations are expressed in mmolc L) and pore solution electrical conductivity. Some properties of these soils are given in Table 1. The Yolo soil is dominated by montmorillonitic clay while the Fallbrook soil is dominated by kaolinitic clay; thus, surface conductance and clay swelling effects would be more likely to occur with the Yolo soil. Seven columns of Fallbrook soil and 14 columns of Yolo soil were prepared by packing approximately 300 g of <2 ' Contribution from the U.S. Salinity Laboratory, USDA, ARS, Riverside, CA 92501. Received 17 Jan. 1985. Approved 23 Apr. 1985. 2 Visiting Research Scientist and Research Leader, Soil and Water Chemistry, U.S. Salinity Laboratory, Riverside, CA. This research was supported in part by a grant from the French Ministere des Affaires Estrangeres. 1110 Published September, 1985