Salinity: Electrical Conductivity and Methods of Soil Analysis Total Dissolved Solids Dennis L. Corwin* and Kevin Yemoto

The measurement of soil salinity is a quantifi cation of the total salts present in the liquid portion of the soil. Soil salinity is important in agriculture because salinity reduces crop yields by reducing the osmotic potential, making it more diffi cult for the plant to extract water, by causing specifi c-ion toxicity, by upsetting the nutritional balance of plants, and by aff ecting the tilth and permeability of a soil. A discussion of the principles, methods, and equipment for measuring soil salinity is presented. The discussion provides a basic knowledge of the background, principles, equipment, and current accepted procedures and methodology for measuring soil salinity in the laboratory using electrical conductivity of aqueous extracts from soil samples and measurement of total dissolved solids in the saturated soil extract. Attention is also given to the use of suction cup extractors, porous matrix or salinity sensors, elec- trical resistivity, and electromagnetic induction to measure salinity in soil lysimeter columns and small fi eld plots (<10 by 10 m). Land resource managers, producers, extension specialists, Natural Resource Conservation Service fi eld staff , undergrad- uate and graduate students, and university, federal, and state researchers are the benefi ciaries of the information provided.

Abbreviations: DI, deionized; DPPC, dual pathway parallel conductance; EC, electrical con- ductivity; ECa, apparent soil electrical conductivity; ECe, electrical conductivity of the satura- tion extract; ECp, electrical conductivity of the saturated soil paste; ECw, electrical conduc- tivity of the soil solution; EC1:1, electrical conductivity of a 1:1 soil/water ratio extract; EC1:2, electrical conductivity of a 1:2 soil/water ratio extract; EC1:5, electrical conductivity of a 1:5 soil/water ratio extract; EMI, electromagnetic induction; ER, electrical resistivity; SP, satura- tion percentage; TDR, time domain refl ectometry; TDS, total dissolved solids.

DEFINITION, SOURCES, EFFECTS, AND GLOBAL IMPACT Soil salinity refers to the total salt concentration in the soil solution (i.e., the aqueous liquid phase of the soil and its solutes) consisting of soluble and readily dis- + + 2+ 2+ − − − solvable salts including charged species (e.g., Na , K , Mg , Ca , Cl , HCO3 , NO3 , −2 −2 SO4 and CO3 ), non-ionic solutes, and ions that combine to form ion pairs (Corwin, 2003). The primary source of salts in soil and water is the geochemical weathering of rocks from the Earth’s upper strata, with atmospheric deposition and anthropogenic activities serving as secondary sources. Salts accumulate in the root zone of agricul- tural soils primarily as a consequence of the process of evapotranspiration, which is the combined processes of evaporation from the soil surface and plant transpiration. Evapotranspiration selectively removes water, leaving salts behind. Soil salinity can also accumulate as a consequence of poor irrigation water quality, poor drainage due to a high water table or low soil permeability, and topographic eff ects where an upslope recharge results in a downslope discharge of salts. In addition, salts can accumulate from saltwater spills associated with oil fi eld activities, from high rates of manure and sludge applications, and from seawater intrusion in coastal areas.

D.L. Corwin, USDA–ARS, US Salinity Lab., 450 There are a variety of agricultural impacts that salt accumulation in the root West Big Springs Road, Riverside, CA 92507-4617; zone can have. Salts in the root zone can reduce plant growth, reduce yields, and, K. Yemoto, USDA–ARS, Water Management and in severe cases, cause crop failure. Salinity impacts on plant yield occur for several Systems Research Unit, 2150 Centre Ave., Bldg. D, Suite 320, Fort Collins, CO 80526. *Corresponding reasons. Salinity limits plant water uptake by reducing the osmotic potential, mak- author ([email protected]). ing it more diffi cult for the plant to extract water. Salinity may also cause specifi c-ion toxicity (e.g., Na+ and Cl−) or upset the nutritional balance of plants. The composition Methods of Soil Analysis of the salts in the soil solution infl uences the composition of cations on the exchange 2017, Vol. 2 doi:10.2136/msa2015.0039 complex of soil particles, which subsequently infl uences soil permeability and tilth. The global impact of soil salinity is daunting. Squires and Glenn (2009) esti- Received 14 Oct. 2016. mated the global extent of saline soils to be 412 Mha, which closely agrees with Accepted 15 Feb. 2017. the FAO estimate of 397 Mha (htt p://www.fao.org/soils-portal/soil-management/ management-of-some-problem-soils/salt-aff ected-soils/more-information-on-salt- © Soil Science Society of America aff ected-soils/en/). The estimate of Szabolcs (1989) is more conservative at 352 Mha. 5585 Guilford Rd., Madison, WI 53711 USA. Of the estimated 230 Mha of irrigated land worldwide, from 20 to 50% may be salt All rights reserved. aff ected (Szabolcs, 1992; Ghassemi et al., 1995; Flowers, 1999). Ghassemi et al. (1995) −1 −1 estimated that salinization of irrigated soils causes an annual (i.e., 1–50 mmolc L ), total salt concentration (C, in mmolc L ) global income loss of US$12 billion. Soil salinity is a crucial soil is linearly related to electrical conductance of the solution by chemical property for soil health and quality that is routinely measured and monitored due to its impact on agriculture. »´ C 10 ECw@25o C [1] There are a variety of techniques to measure soil salinity. In the laboratory, the determination of soil salinity involves where ECw@25°C is the electrical conductivity of the soil three diff erent approaches: (i) the measurement of the mass of solution referenced to 25°C (dS m−1). If C is measured in mil- total dissolved solids (TDS, mg L−1), (ii) the electrical conduc- ligrams per liter, then C is related to ECw@25°C by a factor tivity (EC, dS m−1) of soil water extracts, or (iii) the proportion of 640 (i.e., C » 640 ´ ECw@25°C) for EC between 0.1 to 5.0 and composition of salt species using spectrophotometry. In dS m−1 and by a factor of 800 for EC > 5.0 dS m−1. Across a the fi eld, soil salinity is generally measured by geospatial mea- −1 broader range of salt concentrations (i.e., 10–500 mmolc L ), surements of the apparent soil electrical conductivity (ECa, the relationship between C and EC is no longer linear −1 w@25°C dS m ), which are calibrated to salinity, because ECa is a rapid, and is best fi t with a third-order polynomial or an exponen- reliable, and easy to take measurement that is easily mobi- tial equation. Another useful relationship is between osmotic lized (Corwin and Lesch, 2005a). At intermediate scales (e.g., potential (yp) and EC, where yp in megapascals is related to soil lysimeter columns to fi eld plots, ranging from 2–200 m3) ECw@25°C by a factor of −0.036 (i.e., yp » −0.036 ´ ECw@25°C for soil solution extractors, salinity sensors, capacitance sensors −1 3 ≤ ECw@25°C ≤ 30 dS m ). However, because of diff erences in or frequency domain refl ectometry, time domain refl ectom- the equivalent weights, equivalent conductivities, and propor- etry (TDR), and combinations of ECa and solution extracts tions of major solutes in soil extracts and water samples, each from soil samples are used. Here we focus on soil salinity of the above relationships is only an approximation. 3 measurements of volumes <200 m with greatest emphasis Theoretical and empirical approaches are available to on laboratory analysis of soil samples for salinity by measur- predict the EC of a solution from its solute composition. An ing EC or TDS. For a comprehensive discussion of fi eld-scale example of a theoretical approach based on Kohlrauch’s Law salinity assessment using EC , see Corwin et al. (2012) and a of independent migration of ions, where each ion contributes Corwin and Scudiero (2016). to the current-carrying ability of an electrolyte solution, is

RATIONALE FOR GENERAL PROCEDURE i cc(l0 -b ) ii [2] The most common technique for the measurement of soil EC==åå ECi 1000 salinity is laboratory analysis of aqueous extracts of soil sam- ii ples. Soil salinity is quantifi ed in terms of the concentration of −1 where EC is the specifi c conductance (dS m ), ECi is the total salts in the soil. The measurement of the total salt con- −1 ionic-specifi c conductance (dS m ), ci is the concentration of centration of the aqueous extracts of soil samples can be done −1 i the ith ion (mmolc L ), l0 is the ionic equivalent conduc- either directly through chemical analysis of the chemical con- 2 −1 tance at infi nite dilution (cm S molc ), and b is an empirical stituents that constitute the soil salinity (or mass of the TDS) or interactive parameter (Harned and Owen, 1958). An empirical indirectly through the measurement of the EC. The chemical equation was developed by Marion and Babcock (1976): species of primary interest in salt-aff ected soils include: four major cations (i.e., Na+, K+, Mg2+, and Ca2+) and four major 2 − − 2− 2− log( TSS)=+ 0.990 1.055log( EC) (r = 0.993) [3] anions (i.e., Cl , HCO3 , SO4 and CO3 ) in the soil solution, + + 2+ 2+ exchangeable cations (i.e., Na , K , Mg , Ca ), and the pre- −1 where TSS is the total soluble salt concentration (mmolc L ). cipitated salts CaCO3 (lime) and CaSO4 (gypsum). Other soil properties of concern in salt-aff ected soils include pH, water Temperature infl uences EC; consequently, EC must be content of the saturation paste, Na adsorption ratio, and referenced to a specifi c temperature to permit comparison. exchangeable Na percentage. Detailed analytical techniques Electrolytic conductivity increases at a rate of approximately −1 for measuring all of these salinity-related properties can be 1.9% °C increase in temperature. Customarily, EC is found in Sparks (1996), Dane and Topp (2002), and Corwin expressed at a reference temperature of 25°C. The EC mea- and Scudiero (2016). A chemical analysis of the chemical con- sured at a particular temperature t (in °C), ECt, can be adjusted stituents that constitute the soil salinity is too labor and cost to a reference EC at 25°C, EC25°C, using (US Salinity Laboratory intensive to be practical, particularly when large numbers of Staff , 1954) samples are involved; consequently, the salinity of aqueous o =´ [4] extracts of soil samples has been most often measured by EC. EC25 C f tt EC Even the measurement of the mass of the TDS is not as rapid as measuring the EC. where ft = 0.4470 + 1.4034 exp(−t/26.815) from Sheets and It is well-known that the EC of water is a function of its Hendrickx (1995) and is referred to as the temperature correc- chemical salt composition and total salt concentration (US tion factor. Salinity Laboratory Staff , 1954). In the laboratory, soil salinity Historically, four principal methods have been used for is invariably determined from the measurement of the EC of measuring soil salinity in the laboratory, in soil columns (i.e., soil solution extracts, where the current-carrying capacity of <2 m3) and weighing lysimeters (2–9 m3), and at fi eld-plot (i.e., the soil solution is proportional to the concentration of ions in <200 m3) scales: (i) the EC of the soil solution at or near fi eld the solution. The total concentration of the soluble salts in soil capacity, of extracts at higher than normal water contents (i.e., is measured by the EC of the soil solution (in dS m−1). Across including saturation and soil/water ratios of 1:1, 1:2, and 1:5), a range of mixed salt concentrations commonly found in soils or of a saturation paste; (ii) in situ measurement of electrical Methods Soil Science Society of America of Soil Analysis p. 2 of 16 resistivity (ER); (iii) noninvasive measurement of EC with extracts must be made at saturation or higher water contents. electromagnetic induction (EMI); and (iv) in situ measure- The saturation paste extract is the lowest soil/water ratio that ment of EC with TDR or capacitance probes. can be easily extracted with vacuum, pressure, or centrifuga- tion while providing a sample of suffi cient size to analyze. The REVIEW OF EXISTING APPROACHES: water content of a saturation paste is roughly twice the fi eld STRENGTHS AND LIMITATIONS capacity for most soils. Furthermore, ECe has been the stan- Electrical Conductivity dard measure of salinity used in plant salt-tolerance studies. Electrical conductivity is a surrogate measure of salinity Most data on the salt tolerance of crops has been expressed in in which the salt content of the soil water extract (or soil water terms of the EC of the saturation paste extract (Bresler et al., suspension) is determined by the ability of the extract (or sus- 1982, p. 174–181; Maas, 1986); consequently, ECe is regarded pension) to conduct electricity between two electrodes, i.e., the as the standard for the measurement of soil salinity. However, greater the salt concentration in the extract (or suspension), the because greater time and eff ort is needed to obtain the satura- more current that is conducted between electrodes. To deter- tion extract, soil solution extracts of higher soil/water ratios mine the EC of a soil solution extract, the solution is placed are often found in the scientifi c literature, including 1:1, 1:2, in a cell containing two electrodes of constant geometry and and 1:5 soil/water extracts. separation distance. An electrical potential is imposed across Unfortunately, the partitioning of solutes across the the electrodes, and the resistance of the solution between the three soil phases (i.e., gas, liquid, and solid) is infl uenced by electrodes is measured. The measured conductance is a con- the soil/water ratio at which the extract is made so that the sequence of the solution’s salt concentration and the electrode ratio needs to be standardized to obtain results that can be geometry, whose eff ects are embodied in a cell constant. At applied, interpreted, and compared universally. Commonly constant potential, the current is inversely proportional to the used extract ratios, other than a saturated soil paste, are 1:1, solution’s resistance: 1:2, and 1:5 soil/water mixtures. These extracts are easier to prepare than saturation paste extracts. With the exception of k sandy soils, soils containing gypsum, and organic soils, the ECt = [5] R concentrations of salt and individual ions are approximately t diluted by about the same ratio between fi eld conditions and the extract for all samples, which allows conversions between where ECt is the electrical conductivity of the soil solution (in −1 water contents using dilution factors. The conversion of EC dS m ) at temperature t (°C), k is the cell constant, and Rt is the measured resistance (in W) at temperature t (1 dS m−1 = from one extract to another is commonly done using a sim- 1 mS cm−1 = 1 mmho cm−1, which is the obsolete units of EC). ple dilution factor. For example, if the gravimetric saturation Except for the measurement of the EC of a saturated soil percentage (SP) is 100%, then ECe = EC1:1 = 5 ´ EC1:5, or if SP = 50%, then EC = 2 ´ EC = 10 ´ EC . However, this paste (ECp), the determination of soluble salts in disturbed e 1:1 1:5 soil samples consists of three basic steps (Fig. 1): (i) prepara- is not recommended, particularly in arid and semiarid soils, tion of a soil paste, (ii) extraction of the water from the soil because of potential dissolution–precipitation reactions that paste, and (iii) measurement of the salt concentration of the may occur. At best, the use of a dilution factor to convert from extract using EC. Customarily, soil salinity has been defi ned one extract to another is an approximation. in terms of laboratory measurements of the EC of the extract Any dilution above fi eld water contents introduces of a saturated soil paste (ECe). This is because it is imprac- errors in the interpretation of data. The greater the dilution, tical for routine purposes to extract soil water from samples the greater the deviation between ionic ratios in the sample at typical fi eld water contents; consequently, soil solution and the soil solution under fi eld conditions. These errors

Fig. 1. Steps involved in the preparation of a soil solution extract: (a) preparation of a soil paste, (b) extraction of the water from the soil paste, and (c) measurement of the salt concentration of the extract using electrical conductivity (EC).

Methods Soil Science Society of America of Soil Analysis p. 3 of 16 are associated with mineral dissolution, ion hydrolysis, and has not been widely used to express soil salinity for various changes in exchangeable cation ratios. In particular, soil sam- reasons: (i) it varies throughout the irrigation cycle as the ples containing gypsum deviate the most because the Ca2+ soil water content changes so it is not single valued; and (ii) 2− and SO4 concentrations remain nearly constant with sam- the methods for obtaining soil solution samples at typical ple dilution, while the concentrations of other ions decrease fi eld water contents are too labor, time, and cost intensive with dilution. The standardized relationship between the to be practical (Rhoades et al., 1999a). For disturbed soil extract and the conditions of the soil solution in the fi eld for samples, soil solution at water contents less than saturation diff erent soils is lost with the use of soil/water ratios above can be obtained in the laboratory by displacement, compac- saturation. However, software developed by Suarez and Taber tion, centrifugation, molecular adsorption, and vacuum- or (2007) allows the accurate conversion from one extract ratio pressure-extraction methods. Figure 3 shows displacement to another, provided that suffi cient chemical information is equipment. A soil sample is placed in a large plastic syringe known (i.e., knowledge of the major cations and anions and with a displacement solution (usually containing a dye) above presence or absence of gypsum). The disadvantage of deter- the soil sample followed by compression of the syringe, which mining soil salinity using a soil sample is the time and labor causes a couple of drops of soil solution to be displaced into required, which translates into high cost. However, there is a vial beneath the syringe. The ECw of the displaced drops is no more accurate way of measuring soil salinity than with measured using a drop EC meter. For undisturbed soil sam- extracts from soil samples. ples, ECw can be determined with a soil-solution extractor (Fig. Prior to the 1950s, much of the data on soil salinity was 4a), often referred to as a porous cup extractor, or using an in obtained by using a 50-mL cylindrical conductivity cell, situ, imbibing-type porous-matrix salinity sensor (Fig. 4b). referred to as a Bureau of Soils cup, fi lled with a saturated Porous cup soil solution extractors include zero-tension soil paste to estimate soluble-salt concentrations by measur- and tension (or suction) cups. Historically, suction cups have ing the ECp. This approach was fast and easy; consequently, been more widely used. Soil solution extractors are generally it was used to map and diagnose salt-aff ected soils. When used for large soil columns, weighing lysimeters, or small fi eld Reitemeier and Wilcox (1946) determined that plant responses plots (<10 by 10 m). No single soil solution sampling device to soil salinity correlated more closely with the EC values of will perfectly sample under all conditions, so it is important the saturation paste extract, the use of the paste was discontin- to understand the strengths and limitations of a sampler to ued. A theoretical relationship between ECp and ECe has since determine when to apply certain sampling methods in pref- been developed to overcome the cell’s shortcomings. This was erence to other methods. In structured soils, suction cups do done by developing a simple method of determining the volu- not sample water in preferential fl ow paths. Zero-tension cups metric water and volumetric solid contents of the saturation will almost always sample just saturated fl ow, which is more paste, the conductance of the sample surface, and the current closely associated with preferential fl ow channels, and tension pathway of the water in the cell (Rhoades et al., 1999a). Even samplers will more effi ciently sample unsaturated fl ow within so, the relationship between ECp and ECe is complex; conse- soil aggregates. Zero-tension cups represent the fl ux concen- quently, the measurement of ECp is not recommended except tration, whereas the tension samples are approximations of in instances where obtaining an extract of the saturation paste resident concentrations. is not possible or is impractical. Figure 2 graphically illus- The basic design of a suction cup apparatus consists of trates the theoretical complexity of the relationship between a suction cup, sample collection bott le, manifold (if there is ECp and ECe based on the dual parallel pathway conductance more than one suction cup), an overfl ow trap, an applied (DPPC) model of Rhoades et al. (1989a, 1989b). vacuum, and connective tubing (Fig. 4a). The general princi- Soil salinity can also be determined from the measurement ple behind the operation of suction cup extractors is simply of the EC of a soil solution (ECw), where the water content of that suction, preferably the suction at fi eld moisture capac- the soil is less than saturation, usually at fi eld capacity. Ideally, ity, is placed against the porous cup. This suction opposes the ECw is the best index of soil salinity because this is the salin- ity actually experienced by the plant root. Nevertheless, ECw

Fig. 3. Displacement method for measuring soil salinity showing Fig. 2. Theoretical relationship between saturation extract and (a) the syringes containing soil samples and displacement solution soil paste electrical conductivities (ECe and ECp, respectively) for in a handmade press and (b) the drop electrical conductivity (EC) diff erent saturation percentages (SP) based on the dual parallel meter used to measure the EC of one or two drops of displaced pathway conductance model of Rhoades et al. (1999a). soil solution.

Methods Soil Science Society of America of Soil Analysis p. 4 of 16 sensors are generally used in soil columns (i.e., <2 m3) and weighing lysimeters (i.e., 2–9 m3) because of their relatively small volume of measurement. There are various advantages and disadvantages to mea- suring EC using soil solution extractors or soil salinity sensors. The obvious advantage is that ECw is being measured, but this is outweighed by the disadvantages. Even though the sample volume of a soil-solution extractor (10–100 cm3) is roughly an order of magnitude larger than a salinity sensor (1–2 cm3), both measure very limited sample volumes; consequently, there are serious doubts about the ability of soil solution extractors and porous-matrix salinity sensors to provide representative soil water samples, particularly at fi eld scales (England, 1974; Raulund-Rasmussen, 1989; Smith et al., 1990). Soil heteroge- neity signifi cantly aff ects chemical concentrations in the soil solution. Because of their small sphere of measurement, both solution extractors and salt sensors do not adequately inte- grate spatial variability (Amoozegar-Fard et al., 1982; Haines et al., 1982; Hart and Lowery, 1997). Biggar and Nielsen (1976) suggested that soil solution samples are “point samples” that can provide a good qualitative measurement of soil solu- tions but not adequate quantitative measurements unless the fi eld-scale variability is adequately established. Furthermore, salinity sensors demonstrate a response time lag that is depen- dent on the diff usion of ions between the soil solution and the solution in the porous ceramic, which is aff ected by (i) the thickness of the ceramic conductivity cell, (ii) the diff usion Fig. 4. Schematic of instruments for obtaining soil solution extracts coeffi cients of both soil and ceramic, and (iii) the fraction of at water contents less than saturation, including (a) a soil solution the ceramic surface in contact with soil (Wesseling and Oster, extractor (source: Corwin, 2002a) and (b) a porous-matrix salinity 1973). The salinity sensor is generally considered the least sensor (source: Corwin, 2002b). desirable method for measuring ECw because of its low sam- ple volume, unstable calibration with time, and slow response capillary force of the soil at fi eld capacity, causing soil solution time (Corwin, 2002b). Soil solution extractors have the draw- to be drawn across the porous wall of the cup as a result of back of requiring considerable maintenance due to cracks in the induced pressure gradient. The imbibed solution is stored the vacuum lines and clogging of the ceramic cups with algae in a sample collection chamber. This approach for extracting and fi ne soil particles. Both solution extractors and salt sensors soil solution is viable when the soil-water matric potential is are considered slow and labor intensive. greater than about −30 kPa. Obtaining the EC of a soil solution when the water The salinity sensor consists of a porous ceramic substrate content is at or less than fi eld capacity, which are the water with an embedded platinum mesh electrode, which is placed contents most commonly found in the fi eld, is considerably in contact with the soil to measure the electrical conductiv- more diffi cult than obtaining extracts for water contents at or ity of the soil solution that has been imbibed by the ceramic above saturation because of the pressure or suction required (Fig. 4b). The method assumes that the salt concentration in to remove the soil solution at fi eld capacity and lower water the ceramic is in equilibrium with the ionic constituents of the contents. The EC of the saturated paste is the easiest to obtain soil solution. A housing covers the sensor, with a stainless- followed by the EC of extracts greater than SP, followed by the steel spring between the housing and sensor, which ensures EC of extracts less than SP. However, it is ECe above all others good physical contact of the ceramic and soil. The salinity sen- that is most preferred; consequently, either measuring ECe or sor contains a thermistor designed to temperature correct the being able to relate the EC measurement to ECe is critical. electrical conductivity readings. Both the electrolytic element and thermistor of a salt sensor (Fig. 4b) must be calibrated for Electrical Resistivity proper operation. Calibration is necessary because of (i) the Because of the time and cost of obtaining soil solution variation in water retention and porosity characteristics of extracts and the lag time associated with porous ceramic cups, each ceramic and (ii) the variation in electrode spacing, both developments in the measurement of soil EC shifted in the of which cause the cell constant to vary for each salt sensor. 1970s to the measurement of the EC of the bulk soil, referred The calibration provides a functional relationship between the to as the apparent soil electrical conductivity (ECa). Apparent conductance measured by the salinity sensor (Cse) and the EC soil electrical conductivity provides an immediate, easy-to- of the adjacent soil solution preferably at fi eld capacity (ECw), take measurement of conductance with no lag time and no i.e., Cse = a + bECw, where a and b are coeffi cients of the linear need to obtain a soil extract. However, ECa is a complex mea- relationship. The calibration can change with time, so periodi- surement that has been misinterpreted and misunderstood cal recalibration is necessary. The reliability and limitations by users in the past due to the fact that it is a measure of the of salinity sensors were addressed by Aragüés (1982). Salinity electrical conductance of the bulk soil and not just a measure Methods Soil Science Society of America of Soil Analysis p. 5 of 16 −1 of the conductance of the soil solution, which is the desired (dS m ), respectively; and ECs is the electrical conductivity measurement because the soil solution is the soil phase that of the surface conductance (dS m−1). Equation [6] is not eas- contains the salts aff ecting plant roots. The techniques of ER, ily parameterized, but soil ECa is easily converted into ECe by EMI, and TDR measure ECa. The most comprehensive body substituting the following equations, originally developed by of research concerning the adaptation and application of geo- Rhoades et al. (1989b), into Eq. [6]: physical techniques to the measurement of soil salinity within the root zone (top 1 to 1.5 m of soil) was conducted by scien- PW´rb tists at the US Salinity Laboratory. The most recent reviews of q=w [7] this body of research were provided by Rhoades et al. (1999a), 100 Corwin (2005), and Corwin and Lesch (2005a, 2013). q = q+ [8] Because ECa measures the conductance of the bulk soil, it ws 0.639w 0.011 measures anything conductive in the soil. Apparent soil elec- trical conductivity is a measure of the conductance through r q= b [9] three pathways in the soil (see Fig. 5): (i) solid–liquid (Pathway ss 2.65 1), (ii) liquid (Pathway 2), and (iii) solid (Pathway 3) conduc- tance pathways. Because of these pathways of conductance, ECs =- 0.019( SP) 0.434 [10] ECa is infl uenced by a complex interaction of several edaphic properties including salinity, water content, texture (or SP), r bulk density ( b), cation exchange capacity, clay minerology, æöæö ç EC´r ´ SP÷ ç SP ÷ organic matt er, and temperature. When interpreting ECa data, EC ==ç eb ÷ EC ç ÷ [11] weç ÷ ç ÷ these infl uencing factors must be kept in mind. çèø100´qwg÷ çèø100´q ÷ There are a variety of models that relate ECa to edaphic properties. However, the most practical model is the DPPC where PW is the percentage of water on a gravimetric basis, −3 model of Rhoades et al. (1989b), later extended by Lesch and rb is the bulk density (Mg m ), SP is the saturation percent- Corwin (2003). This model is based on multi-pathway paral- age, ECw is the average electrical conductivity of the soil lel electrical conductance (Fig. 5). The DPPC model has been water assuming equilibrium (i.e., ECw = ECws = ECwc), and shown to be applicable to a wide range of typical agricultural ECe is the electrical conductivity of the saturation extract situations (Corwin and Lesch, 2003). The DPPC model demon- (dS m−1). The DPPC model is a module in the ESAP soft- strates that ECa can be reduced to a nonlinear function of fi ve ware by Lesch et al. (2000), which can be downloaded from soil properties: salinity as measured by ECe, SP, volumetric htt p://www.ars.usda.gov/Services/docs.htm?docid=15992. soil water content, rb, and soil temperature. The DPPC model Electrical resistivity was originally used by geophysi- of Rhoades et al. (1989b) is cists to measure the resistivity of the geological subsurface. 2 Electrical resistivity methods involve the measurement of the (qss +q ws) EC ws EC s resistance to current fl ow across four electrodes inserted in a ECa = +( qw -q ws)EC wc [6] qssEC ws +q ws EC s straight line on the soil surface at a specifi ed distance between the electrodes (Corwin and Hendrickx, 2002). The electrodes where qw = qws + qwc = total volumetric water content are connected to a resistance meter that measures the potential 3 −3 (cm cm ); qws and qwc are the volumetric soil water content gradient between the current and potential electrodes (Fig. 6). 3 −3 in the soil-water pathway (cm cm ) and in the continuous These methods were developed in the second decade of the 3 −3 liquid pathway (cm cm ), respectively; qss is the volumetric 1900s by Conrad Schlumberger in France and Frank Wenner 3 −3 water content of the surface conductance (cm cm ); ECws in the United States for the evaluation of near-surface ER and ECwc are the specifi c electrical conductivities of the (Rhoades and Halvorson, 1977; Burger, 1992). soil-water pathway (dS m−1) and continuous liquid pathway

Fig. 6. Schematic of four-electrode probe electrical resistivity used to measure apparent soil electrical conductivity (modifi ed from Corwin and Hendrickx, 2002). Schematic shows the electrical re- Fig. 5. Schematic illustrating the three conductance pathways sistivity method with an array of four electrodes: two outer current for the apparent soil electrical conductivity (ECa) measurement: electrodes (C1 and C2) and two inner potential electrodes (P1 and solid–liquid (Pathway 1), liquid (Pathway 2), and solid (Pathway 3) P2). When the electrodes are equally spaced, the electrode array (modifi ed from Rhoades et al., 1989b). is referred to as a Wenner electrode confi guration.

Methods Soil Science Society of America of Soil Analysis p. 6 of 16 Even though two electrodes (one current and one poten- tial electrode) can be used, the stability of the reading is improved with the use of four electrodes. With two electrodes, measurements are obscured by electrode polarization result- ing from the interaction of the charged electrode surface with free charges in the electrolyte forming an electrical double layer on the electrode surface with a large capacitance. Four electrodes mitigate the problem of electrode polarization by providing a second pair of electrodes to measure the voltage across the soil. The resistance is converted to EC using Eq. [5], where the cell constant, k, in Eq. [5] is determined by the electrode confi g- uration and distance. The depth of penetration of the electrical current and the volume of measurement increase as the inter- electrode spacing increases. The four-electrode confi guration is referred to as a Wenner array when the four electrodes are equidistantly spaced (inter-electrode spacing = a). For a homo- geneous soil, the depth of penetration of the Wenner array is a and the soil volume measured is roughly pa3. There are additional four-electrode confi gurations that are frequently used, as discussed by Burger (1992), Telford et al. (1990), and Dobrin (1960). The infl uence of the inter-elec- trode confi guration and distance on ECa is refl ected by ïïìü 1000ïïf t EC o = íý [12] a,25 C p ïï-- + 2 Rt îþïï11( rr12 1 1 R 1 1 R 2) where ECa,25°C is the apparent soil electrical conductivity tem- −1 perature-corrected to a reference of 25°C (dS m ) and r1, r2, R1, and R2 are the distances (in cm) between the electrodes as shown in Fig. 6. For the Wenner array, where a = r1 = r2 = R1 = R2, Eq. [12] reduces to ECa = 159.2 ft/aRt and 159.2/a represents the cell constant (k). There are a variety of commercially developed four- electrode probes, refl ecting diverse applications. Burial and insertion four-electrode probes are used for continuous moni- toring of ECa and to measure soil profi le ECa, respectively (Fig. 7a and 7b). These probes have volumes of measurement roughly the size of a football (i.e., about 2500 cm3), while bed- ding probes with small volumes of measurement of roughly 3 25 cm were used to monitor ECa in seed beds (Fig. 7c). However, these probes are no longer commercially available. Only the Eijelkamp conductivity meter and probe are com- Fig. 7. Examples of various four-electrode probes: (a) burial probe, mercially available, which is similar in use and basic design to (b) insertion probe, and (c) bedding probe. the insertion probe in Fig. 7b. Rhoades and van Schilfgaarde (1976) and Austin and Rhoades (1979) provided details for the integrates the high level of local-scale variability often associ- construction of a four-electrode probe and circuitry for read- ated with ECa measurements. ing the four-electrode probe, respectively. Because ECe is regarded as the standard measure of Electrical resistivity is an invasive technique that requires salinity, a relation between ECa and ECe is needed to relate good contact between the soil and four electrodes inserted ECa to salinity. The relationship between ECa and ECe is lin- −1 into the soil; consequently, it produces less reliable measure- ear when ECa is >2 dS m and is dependent on soil texture, ments in dry or stony soils than a noninvasive measurement as shown in Fig. 8. Rough approximations of ECe from ECa −1 −1 such as EMI. Nevertheless, ER has a fl exibility that has proven (in dS m ) when ECa ≥ 2 dS m are: ECe » 3.5ECa for fi ne- advantageous for fi eld application, i.e., the depth and volume textured soils, ECe » 5.5ECa for medium-textured soils, and −1 of measurement can be easily changed by altering the spac- ECe » 7.5ECa for coarse-textured soils. For ECa < 2 dS m , the ing between the electrodes. A distinct advantage of the ER relation between ECa and ECe is more complex. In general, at −1 approach is that the volume of measurement is determined ECa ≥ 2 dS m salinity is the dominant conductive constituent; by the spacing between the electrodes, which makes a large consequently, the relationship between ECa and ECe is linear. −1 volume of measurement possible. For example, a 1-m inter- However, when ECa < 2 dS m , other conductive properties electrode spacing for a Wenner array results in a volume of (e.g., water and clay content) and properties infl uencing con- measurement of >3 m3. This large volume of measurement ductance (e.g., bulk density) have greater infl uence. For this Methods Soil Science Society of America of Soil Analysis p. 7 of 16 Fig. 9. Schematic showing the operation of an electromagnetic in- duction conductivity meter (adapted from Corwin et al., 2012).

Fig. 8. Relationships between apparent and saturation extract coil of the instrument, and the sum of these signals is amplifi ed electrical conductivities (ECa and ECe, respectively) for represen- and formed into an output voltage, which is related to a depth- tative soil types found in the US Northern Great Plains (modifi ed weighted EC . The amplitude and phase of the secondary fi eld from Rhoades and Halvorson, 1977), including sandy loam (SL), a silty clay loam (SiCL), clay loam (CL), sandy clay loam (SCL), will diff er from those of the primary fi eld as a result of soil prop- loam (L), silty clay (SiC), and clay (C). erties (e.g., clay content, water content, salinity), spacing of the coils and their orientation, frequency, and distance from the soil −1 surface (Hendrickx and Kachanoski, 2002). reason, it is recommended that below an ECa of 2 dS m , the relation between ECa and ECe be established by calibration. The most commonly used EMI conductivity meters in The calibration between ECa and ECe is established by mea- soil science and in vadose zone hydrology are the Geonics suring the ECe of soil samples refl ecting a range of ECa values EM-31 and EM-38 (Geonics Ltd.) and the DUALEM-2 and collected throughout the volume of measurement for the (Dualem Inc.). The EM-38 has had considerably greater ECa technology used (i.e., ER or EMI). application for agricultural purposes because the depth of There are also low-cost insertion and burial sensors that measurement corresponds roughly to the root zone (gener- measure ECa through two-probe ER to obtain soil salinity ally 1–1.5 m). When the instrument is placed in the vertical and measure dielectric permitt ivity of the surrounding soil or coil confi guration (EMv, i.e., coils are perpendicular to the medium through capacitance–frequency domain technology soil surface), the depth of measurement is about 1.5 m, and to obtain the volumetric water content. These sensors include in the horizontal coil confi guration (EMh, i.e., coils are par- Decagon’s 5TE and GS3 sensors. Both sensors measure electri- allel to the soil surface), the depth of the measurement is cal conductivity, volumetric water content, and temperature. 0.75 to 1.0 m. The EM-31 has an intercoil spacing of 3.66 m, The sensors consist of three prongs. which corresponds to a penetration depth of 3 and 6 m in The Decagon 5TE uses a two-electrode array (i.e., two the horizontal and vertical dipole orientations, respectively. Both depths extend well beyond the root zone of most agri- screws on the surface of the sensor) to measure the ECa of the surrounding medium, with an accuracy of ±10% from cultural crops. However, the EM-38 has one major pitfall, 0 to 7 dS m−1, which is adequate for most fi eld, greenhouse, which is the need for fi eld calibration (i.e., zeroing), which and nursery applications. Special considerations must be the DUALEM-2 does not require. The dual-dipole EM-38 and made for salt-aff ected soils outside the sensors’ upper limit of DUALEM-2 are multiple core orientation instruments that −1 −1 23.1 dS m , and user calibration is required above 7 dS m . obtain both the EMv and EMh ECa measurements simulta- Using an oscillator operating at 70 MHz, the sensor measures neously. Further details about and operation of the EM-31 the dielectric permitt ivity of the surrounding soil or medium and EM-38 equipment are discussed in Hendrickx and to determine the volumetric water content. A thermistor Kachanoski (2002). Information concerning the DUALEM-2 located underneath the sensor overmold in contact with one can be found at htt p://www.dualem.com/documents.html of the sensor prongs provides the soil temperature. The GS3 (accessed 1 Sept. 2016). For a detailed discussion of EMI con- is similar to the 5TE in most respects, but the GS3 provides an ductivity equipment, protocols, methodology, and guidelines increased range of accuracy in the measurement of ECa (±10% for three-dimensional mapping of the spatial variability of −1 from 0–10 dS m ). soil properties using ECa–directed sampling at the fi eld scale and larger spatial extents, see Corwin and Scudiero (2016). Electromagnetic Induction Apparent soil electrical conductivity measured by EMI at −1 Apparent soil electrical conductivity can be measured non- ECa < 1.0 dS m is given by (McNeill, 1980) invasively with EMI. Electromagnetic induction is used on fi eld æö plots or larger spatial extents. A transmitt er coil located at one 4 ç H s ÷ ECa - ç ÷ [13] end of the EMI instrument induces circular eddy-current loops 2 ç ÷ 2pm0 fs èøç H p ÷ in the soil, with the magnitude of these loops directly propor- tional to the electrical conductivity in the vicinity of that loop where ECa is measured in siemens per meter; Hp and Hs are (Fig. 9). Each current loop generates a secondary electromag- the intensities of the primary and secondary magnetic fi elds netic fi eld that is proportional to the value of the current fl owing at the receiver coil (A m−1), respectively; f is the frequency of within the loop. A fraction of the secondary induced electro- the current (Hz); m0 is the magnetic permeability of free space magnetic fi eld from each loop is intercepted by the receiver (4p10−7 H m−1); and s is the intercoil spacing (m). Methods Soil Science Society of America of Soil Analysis p. 8 of 16 Both ER and EMI are rapid and reliable technologies for Time Domain Refl ectometry and Capacitance Sensors the measurement of ECa, each with its advantages and disad- Time domain refl ectometry and capacitance sensors are vantages. The primary advantage of EMI over ER is that EMI is grouped together because both measure the dielectric per- noninvasive, so it can be used on dry and stony soils that would mitt ivity of the surrounding porous medium or soil. Time not be amenable to invasive ER equipment. The disadvantage domain refl ectometry determines the characteristics of elec- is that ECa measured with EMI is a depth-weighted value that trical lines by observing refl ected waveforms. An impulse is nonlinear, whereas ER provides an ECa measurement that is of energy is propagated into the soil, followed by the sub- more linear with depth. More specifi cally, EMI concentrates its sequent observation of the energy refl ected by the porous measurement of conductance throughout the depth of penetra- medium. An analysis of the shape, duration, and magni- tion at shallow depths, while ER is more uniform with depth. tude of the refl ected waveform determines the nature of Because of the linearity of the response function of ER, the ECa the impedance variation of the porous medium. In contrast, for a discrete depth interval of soil, ECx, can be determined with capacitance sensors determine the dielectric permitt ivity of the Wenner array by measuring the ECa of successive layers by the soil by measuring the charge time of a capacitor, which increasing the inter-electrode spacing from ai−1 to ai and using, uses the soil as a dielectric. for resistors in parallel (Barnes, 1952), Time domain refl ectometry was initially adapted for use in measuring water content, q (Topp et al., 1980, 1982; Topp EC- EC aiiiaa a-1 i-1 and Davis, 1981). The TDR technique is based on the time for EC== EC - [14] x aaii-1 a voltage pulse to travel down a soil probe and back, which is aaii- -1 a function of the dielectric permitt ivity (e, F m−1) of the porous where ai is the inter-electrode spacing, which equals the depth medium being measured. Later, Dalton et al. (1984) demon- of sampling, and ai−1 is the previous inter-electrode spacing, strated the utility of TDR to also measure ECa. The measurement which equals the depth of previous sampling. This approach of ECa with TDR is based on the att enuation of the applied is an estimation of the conductance profi le and has been signal voltage as it traverses the medium of interest, with the improved with the use of inverse modeling. Improved accu- relative magnitude of energy loss related to ECa (Wraith, 2002). racy in conductance profi ling is possible by combining inverse By measuring e, q can be determined through calibration modeling with EMI instrumentation having dual-geometry (Dalton, 1992). The value of e is calculated as (Topp et al., 1980) receivers at multiple separations from the transmitt er (e.g., 2 DUALEM-421) or electrical resistivity tomography. This was æö2 æö ç ct ÷ ç la ÷ discussed in greater detail by Corwin and Scudiero (2016). e=ç ÷ =ç ÷ [16] èø2lèøç lvp ÷ Measurements of ECa by ER and EMI at the same loca- tion and throughout the same volume of measurement are not where c is the propagation velocity of an electromagnetic comparable because of the nonlinearity of the response func- wave in free space (2.997 ´ 108 m s−1), t is the travel time (s), l tion with depth for EMI and the more linear response function is the real length of the soil probe (m), l is the apparent length for ER. An advantage of ER over EMI is the ease of instrument a (m) as measured by a cable tester, and vp is the relative veloc- fi eld calibration. Field calibration of the EM-31 and EM-38 is ity sett ing of the instrument. The relationship between q and e more involved than for ER equipment. However, there is no is approximately linear and is infl uenced by soil type, rb, clay need to fi eld calibrate the DUALEM-2. content, and organic matt er (Jacobsen and Schjønning, 1993). There is also a low-cost insertion and burial sensor that By measuring the resistive load impedance across the measures ECa through an electromagnetic fi eld, Delta-T probe (Z ), EC can be calculated as (Giese and Tiemann, 1975) Devices’ WET-2 sensor. Like the Decagon sensors, the L a WET-2 sensor is a three-prong sensor that measures electri- e00cZ cal conductivity, volumetric water content, and temperature. ECa = [17] However, in contrast to the Decagon sensors, the WET-2 lZL sensor applies a 20-MHz signal to the central rod, which −12 −1 where e0 is the permitt ivity of free space (8.854 ´ 10 F m ), produces a small electromagnetic fi eld within the soil. The −1 water content, electrical conductivity, and composition of the Z0 is the probe impedance (W), and ZL = Zu[(2V0/Vf) − 1] , soil surrounding the rods determines its dielectric proper- where Zu is the characteristic impedance of the cable tester, V0 ties. The sensor detects these dielectric properties from their is the voltage of the pulse generator or zero-reference voltage, infl uence on the electromagnetic fi eld and calculates the and Vf is the fi nal refl ected voltage at a very long time. The value of ECa can be referenced to 25°C by water content and pore-water electrical conductivity (ECpw, −1 dS m ). For all three sensors (i.e., 5TE, GS3, and WET-2), - EC = K fZ 1 [18] ECpw is derived from Hilhorst (2000) using a cLt −1 epwEC a where Kc is the TDR probe cell constant (Kc [m ] = e0cZ0/l), ECpw = [15] which is determined empirically. e -e = a ECa 0 For a parallel plate capacitance sensor, the capacitance where e is the real portion of the dielectric permitt ivity of (C, F) is a function of the dielectric permitt ivity (e) of the soil pw between the capacitor plates: the soil pore water (unitless), ea is the real portion of the dielec- tric permitt ivity of the bulk soil (unitless), ande is the real ECa=0 eA portion of the dielectric permitt ivity of the soil when the ECa C = [19] is 0 (unitless). S

Methods Soil Science Society of America of Soil Analysis p. 9 of 16 where A is the area of the capacitor plates (m2) and S is the dis- or from a sample of displaced soil solution. The most common tance between the plates (m). The charge time on the capacitor laboratory technique for measuring soil salinity is the analysis is a simple linear function of the dielectric permitt ivity of the of aqueous extracts of soil samples using either direct or indi- surrounding soil expressed by rect analysis. Direct analysis involves the measurement of the total salt concentration through the chemical analysis of the t individual chemical constituents that constitute the soil salin- e= [20] éù ity for a soil sample or through the residue weight of the soil (RA S)lnëû( V-- Vf) ( V if V ) solution or aqueous extract following evaporation to dryness where t is the time it takes to charge the capacitor (s) from a after fi ltration. The total salt concentration is the sum of the chemical constituents. Indirect analysis involves the measure- starting voltage of Vi to a voltage V with an applied voltage Vf, and R is the series resistance (W). ment of the EC of the aqueous extract of a soil sample. The complex permitt ivity (i.e., dielectric permitt ivity) The primary chemical species in salt-aff ected soils + + 2+ 2+ of soil measured by dielectric sensors (i.e., TDR and capaci- include: four major cations (Na , K , Mg , and Ca ) and − − 2− 2− tance sensors) is the sum of the soil real (e¢) and imaginary four major anions (Cl , HCO3 , SO4 , and CO3 ) in the (e²) permitt ivity (i.e., e = e¢ + ie², where i is the imaginary part, soil solution, exchangeable cations (i.e., Na+, K+, Mg2+, and 2 2+ i = −1). Water content is related to e¢ only, while e² changes Ca ), and the precipitated salts CaCO3 (lime) and CaSO4 according to salinity, soil temperature, and the operating fre- (gypsum). Soluble fertilizers also contribute additional ions + − quency of the dielectric sensor. A drawback of both TDR and that constitute the salinity, including NH4 and NO3 . To capacitance sensors is that the interpretation of their output determine the contribution of these anion and cation con- centrations to soil salinity, a variety of approaches are used: can be problematic at high salinity (e.g., the Decagon ECH2O- 5TE capacitance probe has an upper limit of about 10 dS m−1). ion chromatography, colorimetric methods, fl ame-atomic Consequently, capacitance sensors and TDR are primarily absorption spectroscopy, titrimetric methods, induced cou- used to measure water content and have not been the instru- pled plasma-atomic emission spectroscopy, and induced ments of choice for the measurement of salinity, whether in coupled plasma-mass spectrometry. the laboratory or in the fi eld. However, improved calibration Soil salinity can also be measured as TDS by evaporating models and dielectric sensors with higher operating frequen- a known fi ltered volume of water to dryness and weighing cies are extending the range and accuracy of soil water content the residual solids. However, this method is subject to error and salinity determination (Scudiero et al., 2012). as a consequence of incomplete fi ltration of clay platelets and The advantages of TDR for measuring ECa include (i) a inclusion of dissolved organic compounds in the evaporite. relatively noninvasive nature because there is only minor Rhoades (1996) provided the tedious procedure for determin- interference with soil processes, (ii) an ability to measure ing TDS based on residual weight following evaporation to both soil water content and ECa, (iii) an ability to detect small dryness after fi ltration. There is no exact quantitative relation- changes in ECa under representative soil conditions, (iv) the ship between EC and TDS because the chemical constituents capability of obtaining continuous unatt ended measurements, and salt species infl uence the conversion values. However, and (v) a lack of a calibration requirement for soil water con- there is an approximation when NaCl is the dominant salt, tent measurements in many cases (Wraith, 2002). Capacitance in which case TDS (mg L−1) is obtained by multiplying EC sensors have many of the same advantages. However, because (dS m−1) by a factor of 500 to 670; the factor commonly used c and l in Eq. [16] are constant and a fi xed length, respectively, is 640. TDR measurements are considered less susceptible to soil con- In most instances, the laboratory analysis of the individ- ditions than capacitance sensors. ual chemical constitutes that comprise soil salinity or of the Soil ECa has become one of the most reliable and fre- residue-weight after evaporation is too resource intensive to quently used measurements, particularly to characterize fi eld be practical, particularly when large numbers of samples are variability, due to its ease of measurement and reliability involved; consequently, the salinity of aqueous extracts of soil (Corwin and Lesch, 2003). Although TDR has been demon- samples has been most often measured by EC. Electrical con- strated to compare closely with other accepted methods of ECa ductivity is measured in the laboratory using a conductivity measurement (Heimovaara et al., 1995; Mallants et al., 1996; meter. In lysimeter columns, fi eld plots, and entire fi elds, EC Spaans and Baker, 1993; Reece, 1998), it is still not suffi ciently is most commonly determined from the measurement of the −1 simple, robust, or fast enough for the general needs of fi eld- bulk soil EC or ECa (dS m ). scale soil salinity assessment (Rhoades et al., 1999a). Only ER Partitioning of solutes across the three soil phases (i.e., and EMI have been adapted for the georeferenced measure- gas, liquid, and solid) is infl uenced by the soil/water ratio at ment of ECa at fi eld scales and larger (Rhoades et al., 1999a, which the extract is made, so the ratio must be standardized to 1999b). Capacitance sensors and TDR are most often used in obtain results that can be applied and interpreted universally. soil columns or small fi eld plots where spatial variability is not Customarily, soil salinity has been defi ned in terms of labo- a particular concern, thereby minimizing the need for numer- ratory measurements of the ECe because it is impractical for ous sensors to characterize variability. routine purposes to extract soil water from samples at typical fi eld water contents. One widely used technique is to obtain PROCEDURE FOR LABORATORY MEASUREMENT an extract by vacuum fi ltration of a saturated soil paste made OF SALINITY FROM SOIL SAMPLES with distilled water. Commonly used extract ratios other than Soil salinity is quantifi ed in terms of the total salt concen- the saturation extract from a saturated soil paste are 1:1, 1:2, tration of the soil solution of a soil sample. The soil solution and 1:5 soil/water mixtures. However, extracts at these ratios may be obtained from an aqueous extract of the soil sample provide only relative salinity because the soils are adjusted to Methods Soil Science Society of America of Soil Analysis p. 10 of 16 unnaturally high water contents not found in the fi eld. Soil · a balance or scale accurate to 0.1 g salinity can also be determined from the measurement of the · a vacuum-line extraction rack (see Fig. 1) consisting EC of the soil solution at some defi ned fi eld water content of a Buchner funnel, vacuum fl ask, fi lter paper (e.g., (ECw). An example would be the EC at fi eld capacity, which Whatman no. 1 medium fl ow), and extract bott les for represents the water content of the soil 2 to 3 d after irrigation collecting and storing the solution extract sample. when free drainage is negligible. Ideally, ECw is the best index of soil salinity because this is a salinity actually experienced by Reagent: the plant root. Nevertheless, ECw has not been widely used for · Sodium hexametaphosphate [(NaPO3)6] solution, 0.1%. two reasons: (i) it varies throughout the irrigation cycle as the Dissolve 0.1 g of (NaPO3)6 in DI water and dilute the soil water content changes; and (ii) methods for obtaining soil solution to 100 mL. solution samples are too labor and cost intensive to be practi- Method: cal for most applications. Using a laboratory marker and labeling tape, label the Soil solution extracts can be obtained by various means. lidded plastic container with a unique ID code for the sam- For disturbed samples, the soil solution can be obtained in ple. Using the marker pen, label the oven-safe weighing tin the laboratory by displacement, compaction, centrifugation, with the same ID code. Weigh out 350 to 450 g of air-dried molecular adsorption, and vacuum- or pressure-extraction soil (ground to pass 10-mesh sieve, i.e., <2.0 mm) of known methods. For undisturbed samples, EC can be determined w water content into a lidded plastic container. Make certain either in the laboratory from a soil solution sample collected that the 350 to 450 g of air-dried soil subsample is obtained with a soil-solution extractor (see Fig. 4a) or directly in the from a homogeneously mixed soil sample to assure that it is fi eld using in situ, imbibing-type porous-matrix salinity sen- representative of the entire soil sample. Add incremental vol- sors (see Fig. 4b). The following detailed procedures focus on umes of DI water to the soil, stirring the sample with a spatula the measurement of soil salinity through the analysis of aque- after each increment of DI water. Add suffi cient DI water until ous extracts of disturbed soil samples using indirect analysis nearly saturated. Allow the mixture to stand covered for a of the EC, which is the prevalent and most common means couple of hours to allow the soil to imbibe water and the read- of determining soil salinity. The soil-water extracts from dis- ily soluble salts to dissolve. Add more water with continued turbed soil samples include saturation, 1:1 soil/water ratio, stirring to bring it to saturation and to get a uniformly satu- 1:2 soil/water ratio, and 1:5 soil/water ratio extracts as well rated paste. Characteristic signs of a saturated paste are: (i) the as the EC of the saturated soil paste. For a detailed discus- surface of the paste will glisten, (ii) the saturated paste fl ows sion of measurements of solute concentrations (including slightly when the plastic container is tipped, (iii) the paste soil salinity) from undisturbed volumes of soil ranging from slides freely off a smooth spatula, (iv) running a thin stick or soil columns to fi eld plots (i.e., 2–200 m3) using soil solution spatula through the paste will create a groove that will hold extractors, salinity sensors, and TDR, see Dane and Topp its shape (if additional DI water is added and the groove folds (2002, p. 1253–1536). in on itself, then the point of saturation has been exceeded), and (v) the paste consolidates easily and the groove folds in Electrical Conductivity of Soil Extracts on itself by tapping the container. Be sure there are no dry Saturation Extract clumps of soil and the soil paste is homogeneous. Seal the con- Soil solution samples at water contents typically found in tainer with the snap-tight lid and place in cold storage (4°C) the fi eld are too time, labor, and cost intensive to be practi- overnight. If microbial activity is a concern with regard to the cal; consequently, aqueous extracts of soil samples are taken saturation extract composition, then thymol (C10H14O) can be at higher than normal water contents (i.e., saturation and added to the paste (Carlson et al., 1971). above) for routine soil salinity analysis. Standardization of the In the morning remove the saturated paste from cold soil/water extract ratio is necessary to obtain results that can storage; let the paste sit until it reaches room temperature. be interpreted and compared because the absolute and rela- Recheck the paste for saturation: (i) free water should not col- tive amounts of solutes are infl uenced by the soil/water extract lect on the soil surface, (ii) the paste should not have stiff ened, ratio (Reitemeier, 1946). The saturation extract has become the and (iii) the paste should still glisten. Stir the soil paste with standard because it is the lowest water content for most soils a spatula to ensure that it is homogeneous. Record the weight that provides suffi cient extract for determination of chemical of an empty weighing tin. Add a subsample (40–50 g) of the composition and analysis, causing it to be the soil water con- saturated soil paste to the weighing tin and record the weight tent for the measurement of soil salinity (i.e., ECe) used in all of the weighing tin with the wet subsample. Place the tin into plant salt tolerance studies (Maas and Hoff man, 1977; Maas, the laboratory oven set to 105°C. Oven dry for 24 h, allow the 1986, 1990; Grieve et al., 2012). weighing tin to cool, and weigh the weighing tin with the Materials needed: oven-dried subsample. (Note: If ECp is desired, then see below for detailed procedural information.) Calculate the saturation · 500-mL (or greater) capacity plastic containers with snap- water percentage (SP) of the saturated paste using tight lids · deionized (DI) water WW- W SP== 100w s 100 water [21] · a spatula WWs - Wsoil · oven-safe weighing tins where W is the weight of the weighing tin (g), Ws is the dry · labeling tape weight of the oven-dried saturated soil paste subsample · marker pen including weight of weighing tin (g), Ww is the wet weight of Methods Soil Science Society of America of Soil Analysis p. 11 of 16 the saturated soil paste subsample including weight of weigh- for collecting and storing the solution extract sample. ing tin (g), Wwater is the weight of the water (g), and Wsoil is the weight of the oven-dried soil (g). Reagent: Apply vacuum suction and add the saturated soil paste to · Sodium hexametaphosphate [(NaPO3)6] solution, 0.1%. the Buchner funnel fi tt ed with Whatman no. 1 fi lter paper (if Dissolve 0.1 g of (NaPO3)6 in DI water and dilute the the extract is turbid, then a more highly retentive fi lter paper solution to 100 mL. such as Whatman no. 5 may be needed). Make sure to evenly Method: and completely cover the fi lter paper with the soil paste. As Determine the gravimetric water content of the air-dried moisture is removed from the soil paste, cracks in the soil soil by drying a subsample at 105°C for 24 h: gravimetric water cake will form. If left unchecked, air will be drawn through content of air-dried soil = (g of air-dried soil − g of oven-dried the cracks in the soil cake and through the extraction fl ask soil)/g of oven-dried soil. Weigh out 350 to 450 g of air-dried into the vacuum system. Air drawn through the soil cake will soil ground to pass a 10-mesh sieve (<2.0 mm) and transfer to prematurely dry the cake at the cracks. Likewise, air travel- a labeled fl ask or bott le. Make certain that the 350 to 450 g of ing through the extraction fl ask will cause extract to evaporate. air-dried soil subsample is obtained from a homogeneously To prevent these from happening, the extraction process mixed soil sample to assure that it is representative of the should be monitored from time to time, and cracks in the soil entire soil sample. Add the required amount of DI water to cake should be smoothed with a spatula. Once the extract bring the soil and water to an equal weight for a 1:1 extract, is obtained, add 1 drop of 0.1% (NaPO ) solution for each 3 6 taking into account the water content in the air-dried soil when 25 mL of extract, which prevents the precipitation of CaCO . 3 determining the amount of DI water to add. For example, an Extracts that are turbid containing soil particulates will need air-dried sample containing 2% water (i.e., a gravimetric water to be fi ltered a second time. The second fi ltration is achieved content of 0.02 g g−1) on an oven-dried basis can be adjusted by decanting the extract through a highly retentive, slow-fl ow to a 1:1 soil/water ratio by adding 98 mL of water to 102 g of fi lter paper (Whatman no. 5) in a laboratory funnel. air-dried soil. For the 1:2 soil/water extract, the weight of the Samples that contain substantial clay may require the water should be two times the weight of the soil, and for the transfer of the unextracted soil paste to another extraction fun- 1:5 soil/water extract, the weight of the water should be fi ve nel or may require multiple days to extract. In the former case, times the weight of the soil. At a soil/water ratio of 1:5, no cor- the soil paste at the bott om of the soil cake dries, sealing the rection for the water content of the air-dried soil is generally extract above from gett ing through. Transfer the unextracted needed because the water content is so high that the correction soil to another extraction funnel and continue extracting. becomes insignifi cant. In the latt er case, cover the top of the Buchner funnel with Stopper the container and shake it on a mechanical shaker, Parafi lm, a watch glass, and a wet paper towel, turn off the preferably an end-over-end shaker, for 1 h to dissolve soluble vacuum, and leave it overnight. In the morning, unwrap and salts. In the absence of a mechanical shaker, shake the con- continue extracting. tainer vigorously by hand for several minutes at least fi ve to The soil extract should be placed in cold storage (4°C) six times at 30- to 60-min intervals. Filter the suspension with when not being analyzed. a highly retentive fi lter paper (Whatman no. 5). If fi ltration is too slow with the highly retentive fi lter paper, then fi lter with 1:1, 1:2, and 1:5 Soil/Water Extracts Whatman no. 1 fi lter paper (if extract is turbid, then fi lter again Even though the saturation extract is the standard, there with a more highly retentive fi lter paper such as Whatman are several other soil/water extracts that are used including 1:1 no. 5). Once the fi nal extract is obtained, add 1 drop of 0.1% and 1:5 soil/water extracts. The 1:2 soil/water extract has found (NaPO3)6 solution for each 25 mL of extract. All extracts occasional use in European countries and is recommended should be stored at 4°C until analyzed. in place of 1:5 soil/water extracts for soils with appreciable amounts of gypsum. The reason for using soil/water extracts Comments Regarding Soil Extracts that are generally above saturation is because the procedure is Selection of the mass of soil to use to obtain a water faster, enabling the water extraction of large numbers of soil extract depends on several factors: (i) the volume of extract samples in less time with less labor. needed for chemical analyses as determined by the number Materials: and type of analyses conducted and (ii) the salt content of the · 500-mL (or greater) air-tight fl ask or bott le soil. As a general rule, 25 to 30% of the water in a saturation soil paste can be removed by the vacuum extraction and fi ltra- · DI water tion process. · spatula As discussed by Reitemeier (1946), salinity and chemi- · oven-safe weighing tins cal constituent errors due to mineral dissolution, dispersion, hydrolysis, and cation exchange increase as the soil/water · labeling tape ratio increases from 1:1 to 1:5. Even though extract ratios of · marker pen 1:1, 1:2, and 1:5 require less labor and usually provide greater extract volume, they are less representative of fi eld soil water · balance or scale accurate to 0.1 g compositions and contents. Sonneveld and van den Ende · mechanical shaker (1971) suggested the use of a 1:2 soil/water extract because it · vacuum-line extraction rack (see Fig. 1) consisting of a (i) is closer to the saturation extract ratio, (ii) provides greater Buchner funnel, vacuum fl ask, medium to highly reten- extract volume, and (iii) is faster. However, 1:2 soil/water tive fi lter paper (Whatman no. 1–5), and extract bott les extracts are not as common as 1:1 and 1:5. Gypsiferous soils Methods Soil Science Society of America of Soil Analysis p. 12 of 16 are particularly problematic at various extract ratios. Due to Method: dissolution reactions, the EC of extracts of gypsiferous soils Remove soil solution extracts from 4°C cold storage and decreases less as the soil/water ratio increases from 1:1 to 1:5. bring to room temperature. Calibrate the EC instrument using Gypsum dissolution exaggerates the soluble salt concentra- the calibration standard solution. Calibration procedures vary tion at lower soil water contents, especially at water contents by instrument; consequently, following the manufacturer’s found under fi eld conditions. Inaccuracies due to dissolution instructions is recommended. No single probe can mea- reactions can be dealt with through the use of the ExtractChem sure the entire range of soil solution extracts; consequently, software of Suarez and Taber (2007), which enables the con- the appropriate EC probe must be selected that will cover version of EC from any soil/water extract ratio to the EC of the the range of ECs for the soil solution extracts of interest. An −1 saturation extract (i.e., ECe). However, ExtractChem requires EC probe range of 0.001 to 50 dS m is generally suffi cient. knowledge of the SP and major cations and anions, which is Calibration is done using one or two calibration standard solu- considerable work to obtain, making the preparation of satu- tions depending on the EC instrument. Rinse the conductivity ration extracts more appealing in many cases. probe thoroughly before and after calibration using DI water, Air-dried soil should be used rather than oven-dried soil and carefully blot the probe dry using a Kimwipe. Choose a when preparing extracts for the determination of EC. This calibration standard solution that approximates the range of is due to the fact that heating to 105°C converts a portion of your extracts. If the range is broad, then select a calibration standard solution that is 65% of the full range. Gently agitate the gypsum (i.e., CaSO4×2H2O) to CaSO4×½H2O, which has a higher solubility in water than gypsum. Furthermore, other the calibration standard solution and add about 5 mL of solu- salts and minerals are aff ected by heating. tion to a shell vial. With the EC meter set to an appropriate Special consideration should be given to the preparation range, dip the probe into the shell vial. The probe should not of saturation extracts from peat or muck soils, very fi ne-tex- rest at the bott om of the vial, and the tip of the probe should be tured soils, and very coarse-textured soils (Prichard et al., submerged about 2.5 cm. Gently move the probe up and down a few times to dislodge any air bubbles. Allow the reading to 1983). Peat and muck soils need to remain moist after sam- equilibrate. Adjust the calibration control until the display pling; consequently, they should not be air dried like mineral reads the value of the standard solution used. Using the wash soils. Once brought to near saturation, both peat and muck bott le, rinse the outside and inside of the conductivity probe; soils must sit overnight to obtain a more accurate saturation use the 100-mL beaker to catch the rinse water. Shake off the point by adding DI water and remixing the following morn- excess water and gently pat dry with a Kimwipe. The conduc- ing. In the case of very fi ne-textured soils, DI water should tivity meter should be recalibrated if changing ranges. For this be added with a minimum of mixing to bring the sample to reason, it is recommended to set aside extracts that are of a nearly saturation. The wett ed soil should sit overnight. In the diff erent range to be analyzed later. morning, DI water should be added and mixed to bring the Gently agitate the extract sample and pour about 5 mL of soil to saturation. This will help obtain a more defi nitive end- soil extract into a shell vial. Dip the probe into the shell vial. point for saturation and reduce the likelihood of overwett ing. The probe should not rest at the bott om of the vial, and the Similarly, caution should be taken to prevent the overwett ing tip of the probe should be submerged about 2.5 cm. Gently of very coarse-textured soils. An indicator of oversaturation is move the probe up and down a few times to dislodge any air the presence of any free water standing on the surface of the bubbles. Allow the reading to equilibrate and record the EC soil paste. The eff ect of oversaturation has a small eff ect on the reading of the extract. Use the wash bott le and Kimwipes to EC but can result in appreciable error in the SP. e rinse and dry the probe between extracts. Many EC meters Electrical Conductance of Soil Extracts have automatic temperature correction, which adjusts the EC to 25°C. If the meter does not have any means of tempera- Materials: ture correction, then the temperature of the extract must be · EC meter obtained and the EC must be adjusted to 25°C using Eq. [4]. If the EC reading is erratic, it may be necessary to clean the · calibration standard solution (if available) probe. Follow the cleaning instructions provided by the man- · wash bott le with DI water ufacturer. This often requires soaking the probe in a cleaning · Kimwipes solution overnight. · shell vials Electrical Conductance of Saturated Soil Paste · 100-mL beaker Materials: · EC meter Reagent: · calibration standard solution (if available) · If a calibration standard solution is not available, then make a 0.010 M KCl reference solution. Dissolve 0.7455 · wash bott le with DI water g of reagent-grade anhydrous KCl (i.e., KCl should be · Kimwipes dried at 110°C for 2 h) in CO2–free distilled or DI water (EC < 0.001 dS m−1) and make to a volume of 1.0 L in a · 100-mL beaker glass-stoppered borosilicate glass volumetric bott le. The reference solution EC is 1.413 dS m−1 at 25°C. If the soil Reagent: solution extract has an EC signifi cantly higher than 1.413 · If a calibration standard solution is not available, then dS m−1, then a higher EC calibration standard solution make a 0.010 M KCl reference solution. Dissolve is needed. 0.7455 g of reagent-grade anhydrous KCl (i.e., KCl Methods Soil Science Society of America of Soil Analysis p. 13 of 16 should be dried at 110°C for 2 h) in CO2–free distilled or · muffl e furnace (capable of operation at 600 ± 25°C) DI water (EC < 0.001 dS m−1) and make to a volume of 1.0 L in a glass-stoppered borosilicate glass volumetric · desiccator −1 bott le. The reference solution EC is 1.413 dS m at 25°C. · drying oven (capable of operation at 180 ± 2°C) If the saturated soil paste has an EC signifi cantly higher than 1.413 dS m−1, then a higher EC calibration standard · analytical balance (accuracy ±0.1 mg) solution is needed. · fi ltration apparatus (capable of removing particulates >0.45 mm in diameter) Method: · distilled deionized (DDI) water Conductivity of the soil paste is determined before the soil solution is extracted from the saturated soil paste. Use an Method: EC probe that is easy to clean and doesn’t require the soil paste Heat an evaporation dish for 1 h to 600 ± 25°C. Cool and to fl ow into the probe. A probe should be selected that will store the evaporation dish in a desiccator. Precisely measure enable good contact with the saturated soil paste. the volume (in mL) of the soil saturation extract needed to yield Gently agitate the calibration standard solution. Follow on evaporation a minimum of 25 mg of residue. Pass the mea- the same EC instrument calibration procedure described sured volume of extract through a fi lter membrane, followed above for soil extracts. by three successive 15-mL volumes of DDI water. Weigh the Remove the saturated soil paste from overnight 4°C cold evaporation dish. Transfer the complete fi ltered sample to an storage; let the paste sit until it reaches room temperature. evaporation dish. Heat the evaporation dish to evaporate the Stir the soil paste to make it homogeneous just prior to mea- fi ltered sample but do not bring it to a boil and don’t heat to suring the ECp. Dip the probe into the soil paste, moving the the point where the fi ltered sample is dry. When the fi ltered probe around to ensure good contact with the soil paste. Wait sample is nearly dry, transfer it to a 103°C drying oven and for reading to equilibrate and record measurement. Use the complete the evaporation. Dry the evaporation dish and resi- wash bott le and Kimwipes to rinse and dry the probe between due for 1 h at 180 ± 2°C, cool in a desiccator, and weigh the pastes. Many EC meters have automatic temperature correc- evaporation dish and residue. Repeat the 1-h drying, cooling tion, which adjusts the EC to 25°C. If the meter does not have in the desiccator, and weighing cycle until a constant weight is any means of temperature correction, then the temperature att ained (or until the weight loss is <4% of the previous weight of the paste must be obtained and the EC must be adjusted or 0.5 mg, whichever is less). Record the weight of the dried to 25°C using Eq. [4]. If the EC reading is erratic, it may be residue plus dish. The TDS (in mg L−1) is calculated by necessary to clean the probe. Follow the cleaning instructions provided by the manufacturer. This often requires soaking the AB- TDS= 1000 [22] probe in a cleaning solution overnight. Vse

Total Dissolved Solids in Soil Saturation Extract where A is the weight of the dried residue plus the evapora- Total dissolved solids refers to any salts, minerals, met- tion dish (mg), B is the weight of the evaporation dish (mg), als, cations, or anions dissolved in a soil saturation extract, and Vse is the volume of soil saturation extract (mL). composed primarily of inorganic salts consisting of the major ACKNOWLEDGMENTS cations (i.e., Na+, K+, Ca2+, and Mg2+), major anions (i.e., Cl−, − − 2− 2− We wish to acknowledge the research of numerous scientists NO3 , HCO3 , CO3 , and SO4 ), and small amounts of over the past four decades who have contributed to the current organic matt er that are dissolved in water. These inorganic and knowledge and understanding culminating in the development of organic substances are dissolved in water in molecular, ion- protocols and guidelines for measuring soil salinity. We specifi cally ized, or microgranular suspended form. The amount of TDS in acknowledge the work of Jim Rhoades, whose close collaboration a soil sample is measured by weighing the residue remaining throughout Dennis Corwin’s career has been infl uential. The diligent and conscientious technical support of Nahid Vishteh, Clay Wilkinson, after evaporating a soil or water sample that has been fi ltered. and Harry Forster is also acknowledged and appreciated. Product The general methodology consists of fi ltering a soil sample identifi cation is provided solely for the benefi t of the reader and does that has been brought to a specifi ed water content (usually not imply the endorsement of the USDA. saturation) through a membrane fi lter (usually 0.45-mm pore size), evaporating the fi ltrate to dryness in a ceramic weighing REFERENCES dish placed in a drying oven at 103°C, and fi nally drying to a Amoozegar-Fard, A., D.R. Nielsen, and A.W. Warrick. 1982. 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