Introduction

Due in large measure to the research that has been conducted at the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) United States Salinity Laboratory over the past 50 years, the measurement of electrical conductivity (EC) has become a standard soil physicochem-ical measurement both in the laboratory and in the field to address agricultural and environmental concerns. In particular, the geospatial measurement of EC with geophysical techniques, including electrical resistivity (ER), electromagnetic induction (EMI), and time domain reflectometry (TDR), has burgeoned into one of the most useful field agricultural measurements, particularly for spatially characterizing the variability of soil properties such as salinity, water content, and texture (Corwin, 2005).

The value of spatial measurements of soil EC to agriculture is widely acknowledged due to its ability to characterize spatial variability, with applications in solute transport modeling at field and landscape scales (Corwin et al., 1999), salinity mapping and assessment (Corwin et al., 2003a), mapping soil texture (Doolittle et al., 2002) and soil type (Anderson-Cook et al., 2002; Jaynes et al.,

1993), location of claypans and depth of depositional sand (Doolittle et al., 1994; Kitchen et al., 1996; Sudduth et al., 1995), soil quality assessment (Corwin et al., 2003a; Johnson et al., 2001; McBride et al., 1990), monitoring of management-induced spatiotemporal changes in soil condition (Corwin et al., 2006), delineation of site-specific crop management units (Corwin et al., 2003b, 2008) and zones of productivity (Jaynes et al., 2003, 2005; Kitchen et al., 2005), and measuring other soil properties such as soil moisture (Kachanoski et al., 1988; Sheets and Hendrickx, 1995), clay content (Sudduth et al., 2005; Triantafilis and Lesch, 2005; Williams and Hoey, 1987), shallow subsurface available soil N (Eigenberg et al., 2002), and cation exchange capacity (Sudduth et al., 2005). The above studies mostly used the noncontact electromagnetic induction (EMI) method to measure soil EC, with a few using the four-electrode contact methods that induce electrical current into the soil through insulated metal electrodes. A complete review of EC measurements in agriculture is provided by Corwin and Lesch (2005a).

As evident from the above studies, there are significant innovations in developing useful applications of soil EC measurements in agriculture. Nearly all experimental observations similar to those listed above are based on soil EC being regarded as a surrogate measure of one or more soil properties of interest. Results are dictated by the physical and chemical properties of the soil at the time of the EC measurements. However, it is not evident that the basic principles of soil EC have been adequately examined. That is particularly the case in examining the recent interests in various applications of soil EC mapping in precision agriculture. An understanding of the spatial and temporal variability of soil EC and an appreciation for its highly complex interactions with static and dynamic soil properties, particularly at low-salt concentrations, is needed. It is the intent of this chapter to highlight the important aspects of spatial EC measurements in agriculture by providing basic principles and theory of soil EC measurement and what it actually measures, standard operating procedures for conducting a field-scale EC survey, including an outlined set of EC-directed soil sampling protocols, and examples of spatial EC surveys and their interpretation.

4.1.1 Background: Dernition and brief History of Soil Electrical Conductivity

Soil EC has its historical roots in the measurement of soil salinity. In situ measurement of soil resistivity dates back to at least the latter part of the nineteenth century when Whitney et al. (1897) attempted to infer soil water content and salinity from measurements of soil resistivity using two-probe electrodes. Gardner (1898) and Briggs (1899) reported additional measurements as part of the early group of USDA scientists investigating soil temperature, salinity, and water content effects on soil resistivity. To minimize the difficulties with the unstable two-probe method, Frank Wenner (1915) introduced the theory of utilizing four equally spaced electrodes to measure earth resistivity and wrote "A knowledge of earth resistivity (or specific resistance) may be of value in determining something of the composition of earth."

Soil salinity refers to the presence of major dissolved inorganic solutes in the aqueous phase consisting of soluble and readily dissolvable salts in soil and can be determined by measuring the total solute concentration in the soil aqueous phase, more commonly referred to as the soil solution. The determination of total solute concentration (i.e., salinity) through the measurement of EC has been well established for half a century (U.S. Salinity Laboratory Staff, 1954). Soil salinity is quantified in terms of the total concentration of soluble salts as measured by the EC of the solution in dS m-1.

It is known that the EC of a pure solution (aw) is a function of its chemical composition and is characterized by Equation (4.1):

where k is the cell constant accounting for electrode geometry, X is the molar limiting ion conductivity (S m2 mol-1), M is the molar concentration (mol m-3), v is the absolute value of the ion charge, and i denotes the ion species in solution. Marion and Babcock (1976), among others, have confirmed the existence of the relationship between EC and molar concentrations of ions in the soil solution.

To determine soil EC, the soil solution is placed between two electrodes of constant geometry and distance of separation (Bohn et al., 1979). The measured conductance is a consequence of the solution's salt concentration and the electrode geometry whose effects are embodied in a cell constant. Electrical conductance was considered more suitable for salinity measurements than resistance because it increases with salt content, which simplifies the interpretation of readings. At constant potential, the electrical conductance is a reciprocal of the measured resistance as shown in Equation (4.2):

where ECT is the electrical conductivity of the solution in dS m-1 at temperature T (°C), k is the cell constant, and RT is the measured resistance at temperature T. Electrolytic conductivity increases approximately 1.9 percent per degree centigrade increase in T. Customarily, EC is adjusted to a reference temperature of 25°C using Equation (4.3) from Handbook 60 (U.S. Salinity Laboratory Staff, 1954):

where fT is a temperature conversion factor that has been approximated by a polynomial form (Rhoades et al., 1999a; Stogryn, 1971; Wraith and Or, 1999) and by Equation (4.4) from Sheets and Hendrickx (1995):

Soil EC is determined for an aqueous extract of a soil sample. Ideally, the EC of an extract of the soil solution (ECw) is the most desirable, because this is the water content to which plant roots are exposed, but this is usually difficult and time consuming to obtain. The soil sample from which the extract is taken can either be disturbed or undisturbed. For disturbed samples, soil solution can be obtained in the laboratory by displacement, compaction, centrifugation, molecular adsorption, and vacuum- or pressure-extraction methods. Because of the difficulty in extracting soil solution from soil samples at typical field water contents, soil solution extracts are most commonly from higher than normal water contents. The most common extract obtained is that from a saturated soil paste (ECe), but other commonly used extract ratios include 1:1 (EC1:1), 1:2 (EC1:2), and 1:5 (EC1:5) soil-to-water mixtures. Unfortunately, the partitioning of solutes over the three soil phases (i.e., gas, liquid, and solid) is influenced by the soil-to-water ratio at which the extract is made, which confounds comparisons between ratios and interpretations; consequently, standardization is needed for comparison of EC measurements. For undisturbed soil samples, ECw can be determined either in the laboratory on a soil solution sample collected with a soil-solution extractor installed in the field or directly in the field using in situ, imbibing-type, porous-matrix, salinity sensors. All of these approaches for measuring soil EC are time and labor intensive; as a result, they are not practical for the characterization of the spatial variability of soil salinity at field extents and larger.

Because of the time, labor, and cost of obtaining soil solution extracts, developments in soil salinity measurement at field and landscape scales over the past 30 years have shifted to EC measurement of the bulk soil, referred to as the apparent soil electrical conductivity (ECa). The measurement of ECa is an indirect method for the determination of soil salinity because ECa measures conductance not only through the soil solution, but also through solid soil particles and via exchangeable cations that exist at the solid-liquid interface of clay minerals. The shift away from extracts to the measurement of ECa occurred because the time and cost of obtaining soil solution extracts prohibited their practical use at field scales and the high local-scale variability of soil rendered salinity sensors and small-volume soil core samples of limited quantitative value. Historically, the utility of ECa has been in identifying geological features in geophysical sciences and explorations (McNeill, 1980; Zalasiewicz, et al., 1985) and in agricultural soil salinity surveys for diagnostics, leaching, and salt loading (Corwin et al., 1996; Rhoades and Ingvalson, 1971; Rhoades et al., 1990). During the past decade, there has been an increased interest in using ECa maps to infer the spatial variability of soil properties important to crop production. In particular, there is an emerging interest in utilizing the spatial variability in ECa for the purposes of guiding soil sampling (as opposed to systematic grid sampling) and developing management zones to vary agricultural inputs.

The measurement of soil ECa is primarily through the use of the geophysical techniques of ER, EMI, and TDR. Among the many advanced sensors recently introduced in precision agriculture, EMI and ER ECa measuring devices provide the simplest and least expensive soil variability measurement. Electrical resistivity introduces an electrical current into the soil through current electrodes at the soil surface, and the difference in current flow potential is measured at potential electrodes that are placed in the vicinity of the current flow. Generally, there are four electrodes inserted in the soil in a straight line at the soil surface, with the two outer electrodes serving as the current electrodes and the two inner electrodes serving as the potential electrodes (Figure 4.1). A resistance meter is used to measure the potential gradient. For a homogeneous soil, the volume of measurement with ER is roughly na3, where a is the interelectrode spacing when the electrodes are equally spaced. The most commonly used ER equipment is the Veris Soil EC Mapping System (Veris Technologies, Salina, KS). The Veris 3100 unit has six coulter electrodes mounted on a platform that can be pulled by a pickup truck. It uses a modified Wenner configuration to measure ECa by inducing current in the soil through two coulter electrodes and measuring the voltage drop across the two pairs of coulters that are spaced to measure ECa for the top 0.3 m (shallow) and 0.9 m (deep) of soil (Lund et al., 2000). The shallow and deep ECa readings at each measurement point in the field are useful in examining soil profile changes. Although soil compaction affects ECa due to the reduced porosity and increased soil particle-to-particle contact, compaction is not easily identified from a Veris ECa map, as the compacted layer represents only a small percentage of the domain of ECa measurements.

The Veris unit interfaces with a differential Global Positioning System (GPS) and provides simultaneous and geo-referenced readings of ECa. The Veris unit is designed to operate in tilled or untilled conditions, where the coulters penetrate the soil 20 to 50 mm (more penetration for drier

FIGURE 4.1 Electrical resistivity with a Wenner array electrode configuration where the interelectrode spacing is equal between current and potential electrodes: C and C2 represent the current electrodes, Pl and P2 represent the potential electrodes, and a represents the interelectrode spacing. (From Rhoades, J.D., and Halvorson, A.D., Electrical conductivity methods for detecting and delineating saline seeps and measuring salinity in Northern Great Plains soils, ARS W-42, USDA-ARS Western Region, Berkeley, CA, pp. 1-45, 1977. With permission.)

FIGURE 4.1 Electrical resistivity with a Wenner array electrode configuration where the interelectrode spacing is equal between current and potential electrodes: C and C2 represent the current electrodes, Pl and P2 represent the potential electrodes, and a represents the interelectrode spacing. (From Rhoades, J.D., and Halvorson, A.D., Electrical conductivity methods for detecting and delineating saline seeps and measuring salinity in Northern Great Plains soils, ARS W-42, USDA-ARS Western Region, Berkeley, CA, pp. 1-45, 1977. With permission.)

and looser soil surface conditions). It is good practice to map fields when they are not very dry. Soil ECa mapping with the Veris unit should not be attempted when the soil is frozen, or in the presence of any frost layers. Frozen soil has significantly different conductive properties, and the ECa data collected will not be valid. The Veris unit is a rugged and reliable system with no known difficulties in mapping fields in the spring prior to tillage and planting operations or in the fall after harvest with heavy standing and flat-lying surface residue conditions (Farahani and Buchleiter, 2004). For ease of maneuvering, fields are normally traversed in the direction of crop rows, but the resulting map is not affected by the direction of travel. On average, travel speeds through the field range between 7 and 16 km h-1 with measurements taken every second, corresponding to 2 to 4 m spacing between measurements in the direction of travel, respectively. A parallel swather (such as AgGPS Parallel Swathing Option, Trimble Navigation Ltd., Sunnyvale, CA) mounted inside the vehicle pulling the Veris unit may be used to guide parallel passes through the field at desired (i.e., 12 to 18 m) swath widths.

The direct contact method used by ECa equipment like Veris has a distinct advantage over the EMI method in that there is no possibility of ambient electrical (for instance from power lines), metallic (operator's belt buckle), or engine noise interferences. Other important advantages of ERtype methods over EMI are that there is no calibration or nulling procedures required prior to mapping, and there is no known report of any observed drift in the measured soil ECa by ER. Regular "drift runs" that involve traversing the same location in a field are needed for EMI in order to determine the drift resulting from air temperature effects on the instrument throughout the day. Because the electrodes in the ER method directly inject the signal into the soil, changes in air temperature have virtually no effect on the readings. It is noted that ECa data collected with either method are affected by soil temperature. The most obvious disadvantage of direct ER methods is their intrusiveness as compared to the nonintrusive EMI methods. The invasive ER method requires solid contact between the coulters and soil; consequently, dry conditions or irregular microtopography can prevent contact. Although the distinction between the two differing ECa measuring methods of ER and EMI is important, side-by-side measurements of soil ECa by contact electrodes and EMI methods has given highly correlated values (Sudduth et al., 2003) and has provided similar maps (Doolittle et al., 2002).

In the case of EMI, EC is measured remotely using a frequency signal in the range of 0.4 to 40 kHz and primarily measures signal loss to determine ECa. The EMI measurement is made with the instrument at or above the soil surface. An EMI transmitter coil located at one end of the instrument induces circular eddy-current loops in the soil (Figure 4.2). The magnitude of these loops is directly proportional to the ECa of the soil in the vicinity of that loop. Each current loop generates a secondary electromagnetic field that is proportional to the current flowing within the loop. A portion of the secondary-induced electromagnetic field from each loop is intercepted by the receiver coil, and the sum of these signals is related to a depth-weighted ECa.

For TDR, an applied electromagnetic pulse is guided along a transmission line embedded in the soil. The time delay between the reflections of the pulse from the beginning and the end of the

FIGURE 4.2 The principle of operation electromagnetic induction. (From Corwin, D.L., and Lesch, S.M., Agron. J., 95, 455-471, 2003. With permission.)

transmission line is used to determine the velocity of propagation through soil, which is controlled by the relative dielectric permittivity or dielectric constant. By measuring the resistive load impedance across the probe, ECa can be determined. Although TDR has been demonstrated to compare closely with other accepted methods of ECa measurement (Heimovaara et al., 1995; Mallants et al., 1996; Reece, 1998; Spaans and Baker, 1993), it has not been widely used for geospatial measurements of ECa at field and larger spatial extents. Only ER and EMI are commonly used at these spatial extents (Rhoades et al., 1999a, 1999b).

Field measurements of ECa to determine soil salinity began in the early 1970s with the use of ER (Halvorson and Rhoades, 1976; Rhoades and Halvorson, 1977; Rhoades and Ingvalson, 1971). However, geospatial measurements of ECa in the field did not occur in earnest until the 1980s, primarily with the use of EMI, which had definite advantages over ER because it was noninvasive. Observational research through the 1980s and early 1990s largely correlated ECa measurements to soil properties in an effort to sort out what soil properties were measured by ECa (Table 4.1). From the late 1990s to the present, the complex spatial relationship between ECa, edaphic properties, and within-field variations in crop yield for site-specific crop management has increasingly become the focus of ECa research. However, over the past three decades and even today, measurements of ECa are often misunderstood and misinterpreted. The misconceptions regarding ECa are the consequence of incomplete knowledge of the basic principles and theory of the ECa measurement.

4.1.2 Misconceptions Surrounding the Apparent Soil Electrical Conductivity (eca) Measurement

When scientists began to take ECa measurements in the field and correlate them to soil properties, there were preconceived notions about what was being measured. Those scientists in the arid southwestern United States felt that salinity was being measured, and those in the Midwest felt water content and texture were being measured. In reality, both were correct, but each failed to acknowledge that ECa is a complex physicochemical measurement influenced by any soil property that influences electrical conductance pathways in soil. Additional research produced correlations between ECa and soil properties that were not directly measured but indirectly related to properties that were measured by ECa (Table 4.1).

With a few exceptions, the ECa related observations suggest salinity, soil water content, clay content, exchange cations, temperature, and organic matter content are the dominating soil properties affecting ECa, but the strength of the reported correlations varies widely with coefficients ranging from below 0.4 to above 0.8. Contrasting findings are evident in literature; for example, Dal-gaard et al. (2001) reported a higher correlation between ECa and clay at higher water contents, and Banton et al. (1997), in a detailed study on an experimental farm near Quebec City, found texture parameters to have a stronger correlation with ECa for dry than wet soil conditions. Johnson et al. (2001), on the other hand, found no strong correlations between ECa and a host of soil properties in a 250 ha dryland no-till field in eastern Colorado, concluding that ECa delineations were useful in identifying overall soil variability but not in producing specific maps of any individual soil property. In nonsaline fields in Missouri, depth to claypans (a sublayer with 50 to 60 percent in clay and varying in depth from 0.1 to 1 m) was found highly correlated to ECa (Doolittle et al., 1994). As ECa increased, depth to claypan decreased. A more extensive study on the Missouri claypan, however, produced ECa maps that exhibited little resemblance to the maps of measured depth to claypans (Sudduth et al., 1995), concluding that the ECa data were strongly influenced by the crop and farming system, soil water, and crop biomass at the time of ECa measurements. Additional difficulties with interpreting literature are that most of the identified soil properties that dominate ECa variability exhibit significant codependency and thus provide overlapping (or redundant), but confusing, information about ECa. Generally speaking, the degree of ECa association with a given soil property

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