Soil Salinity Measurement Dennis L. Corwin United States Department of Agriculture (USDA), Riverside, California, U.S.A. INTRODUCTION 20 million ha severely effected by salinity worldwide.[1] Because of these detrimental impacts, the measurement, The measurement of soil salinity is a quantification of the monitoring, and real-time mapping of soil salinity is total salts present in the liquid portion of the soil. The crucial to sustaining world agricultural productivity. measurement of soil salinity is important in agriculture because salinity reduces crop yields by 1) making it more difficult for the plant to extract water; 2) causing specific- METHODS OF SOIL ion toxicity; 3) influencing the soil permeability and tilth; SALINITY MEASUREMENT and/or 4) upsetting the nutritional balance of plants. A discussion of the basic principles, methods, and equipment Historically, five methods have been developed for for measuring soil salinity is presented. The concise determining soil salinity at field scales: 1) visual crop discussion provides a basic knowledge of the background, observations; 2) the electrical conductance of soil solution latest equipment, and current accepted methodologies for extracts orextracts athigher thannormal water contents; 3)in measuring soil salinity with suction cup extractors, porous situ measurement of electrical resistivity; 4) noninvasive matrix/salinity sensors, electrical resistivity, electromag- measurement of electrical conductance with EM; and most netic induction (EM), and time domain reflectometry recently 5) in situ measurement of electrical conductance (TDR). with TDR. Visual Crop Observation SOIL SALINITY: DEFINITION, EFFECTS, AND GLOBAL IMPACTS Visual crop observation is a quick and economical method, but it has the disadvantage that salinity Soil salinity refers to the presence of major dissolved development is detected after crop damage has occurred. inorganic solutes in the soil aqueous phase, which consist For obvious reasons, the least desirable method is visual of soluble and readily dissolvable salts including charged observation because crop yields are reduced to obtain soil þ þ þ2 þ2 2 2 2 species (e.g., Na ,K ,Mg ,Ca ,Cl , HCO3 ,NO3 , salinity information. However, remote imagery is increas- 22 22 SO4 ,andCO3 ), non-ionic solutes, and ions that ingly becoming a part of agriculture and potentially combine to form ion pairs. The predominant mechanism represents a quantitative approach to visual observation. causing the accumulation of salt in irrigated agricultural Remote imagery may offer a potential for early detection soils is loss of water through evapotranspiration, leaving of the onset of salinity damage to plants. ever increasing concentrations of salts in the remaining Downloaded by [University of California, Riverside Libraries] at 13:11 02 April 2015 water. Effects of soil salinity are manifested in loss of Electrical Conductivity of Soil stand, reduced plant growth, reduced yields, and in severe Solution Extracts cases, crop failure. Salinity limits water uptake by plants by reducing the osmotic potential making it more difficult The determination of salinity through the measurement of for the plant to extract water. Salinity may also cause electrical conductance has been well established for specific-ion toxicity or upset the nutritional balance of decades.[2] It is known that the electrical conductivity (EC) plants. In addition, the salt composition of the soil water of water is a function of its chemical composition. McNeal influences the composition of cations on the exchange et al.[3] were among the first to establish the relationship complex of soil particles, which influences soil per- between EC and molar concentrations of ions in the soil meability and tilth. Irrigated agriculture, which accounts solution. Soil salinity is quantified in terms of the total for 35%–40% of the world’s total food and fiber, is concentration of the soluble salts as measured by the EC of adversely affected by soil salinity on roughly half of all the solution in dS m2 1.[2] To determine EC, the soil irrigated soils (totaling about 250 million ha) with over solution is placed between two electrodes of constant 852 Encyclopedia of Water Science DOI: 10.1081/E-EWS 120010191 Copyright q 2003 by Marcel Dekker, Inc. All rights reserved. Soil Salinity Measurement 853 geometry and distance of separation.[4] At constant soil-solution extractor or directly in the field by using in potential, the current is inversely proportional to the situ, imbibing-type porous-matrix salinity sensors. solution’s resistance. The measured conductance is a There are serious doubts about the ability of soil consequence of the solution’s salt concentration and the solution extractors and porous matrix salinity sensors (also electrode geometry whose effects are embodied in a cell known as soil salinity sensors) to provide representative constant. The electrical conductance is a reciprocal of the soil water samples.[6 –8] Because of their small sphere of resistance (Eq. 1): measurement, neither extractors nor salt sensors ade- quately integrate spatial variability;[9 –11] consequently, ECt ¼ k=Rt ð1Þ Biggar and Nielsen[12] suggested that soil solution samples where EC is the EC of the solution in dS m2 1 at are “point samples” that can provide qualitative measure- t ment of soil solutions, but not quantitative measurements temperature t (EC), k is the cell constant, and Rt is the 2 1 unless the field-scale variability is established. Further- measured resistance at temperature t. One dS m is equivalent to 1 mmho cm2 1. more, salinity sensors demonstrate a response time lag that Customarily, soil salinity has been defined in terms of is dependent upon the diffusion of ions between the soil laboratory measurements of the EC of the saturation solution and solution in the porous ceramic, which is affected by 1) the thickness of the ceramic conductivity extract (ECe), because it is impractical for routine purposes to extract soil water from samples at typical cell; 2) the diffusion coefficients in soil and ceramic; and 3) the fraction of the ceramic surface in contact with field water contents. Partitioning of solutes over the three [13] soil phases (i.e., gas, liquid, and solid) is influenced by the soil. The salinity sensor is generally considered the least desirable method for measuring EC because of its soil–water ratio at which the extract is made, so the ratio w low sample volume, unstable calibration over time, and must be standardized to obtain results that can be applied [14] and interpreted universally. Commonly used extract ratios slow response time. other than a saturated soil paste are 1:1, 1:2, and 1:5 soil– water mixtures. Soil salinity can also be determined from the Electrical Resistivity measurement of the EC of a soil solution (ECw). Theoretically, EC is the best index of soil salinity Because of the time and cost of obtaining soil solution w extracts, developments in the measurement of soil EC because this is the salinity actually experienced by the have shifted to the measurement of the soil EC of the bulk plant root. Nevertheless, ECw has not been widely used to express soil salinity for various reasons: 1) it varies over soil, referred to as the apparent soil electrical conductance the irrigation cycle as the soil water content changes and 2) (ECa). The apparent soil EC measures the conductance methods for obtaining soil solution samples are too labor, through not only the soil solution but also through the solid and cost intensive at typical field water contents to be soil particles and via exchangeable cations that exist at the practical for field-scale applications.[5] For disturbed solid–liquid interface of clay minerals. The techniques of samples, soil solution can be obtained in the laboratory electrical resistivity, EM, and TDR measure ECa. Electrical resistivity methods introduce an electrical by displacement, compaction, centrifugation, molecular adsorption, and vacuum- or pressure-extraction methods. current into the soil through current electrodes at the soil surface and the difference in current flow potential is For undisturbed samples, ECw can be determined either in the laboratory on a soil solution sample collected with a measured at potential electrodes that are placed in the vicinity of the current flow (Fig. 1). These methods were Downloaded by [University of California, Riverside Libraries] at 13:11 02 April 2015 developed in the second decade of the 1900s by Conrad Schlumberger in France and Frank Wenner in the United States for the evaluation of ground electrical resis- tivity.[16,17] The electrode configuration is referred to as a Wenner array when four electrodes are equidistantly spaced in a straight line at the soil surface with the two outer electrodes serving as the current or transmission electrodes and the two inner electrodes serving as the potential or [18] Fig. 1 Schematic of electrical resistivity of four electrodes (the receiving electrodes. The depth of penetration of the electrical current and the volume of measurement increase Wenner array configuration). C1 and C2 represent the current electrodes, P1 and P2 represent the potential electrodes, and a as the inter-electrode spacing, a, increases. For a represents the inter-electrode spacing. Modified from Rhoades homogeneous soil, the soil volume measured is roughly and Halverson.[15] Pa 3. There are additional electrode configurations that are 854 Soil Salinity Measurement frequently used, as discussed by Burger,[16] Telford considerably