Comparison of Electromagnetic Induction and Direct Sensing of Soil Electrical Conductivity

et al. (1989) modeled ECa as a function of soil water content (both the mobile and immobile fractions), the Apparent profile soil electrical conductivity (ECa) can be an indielectrical conductivity (EC) of the soil water, soil bulk rect indicator of a number of soil physical and chemical properties. Commercially available ECa sensors can be used to efficiently and density, and the EC of the soil solid phase. inexpensively develop the spatially dense data sets desirable for deMeasurements of ECa can be used to provide indirect scribing within-field spatial soil variability in precision agriculture. measures of the soil properties listed above if the contriThe objective of this research was to compare ECa measurements butions of the other soil properties affecting the ECa from a noncontact, electromagnetic induction–based sensor (Geonics measurement are known or can be estimated. If the ECa EM38)1 to those obtained with a coulter-based sensor (Veris 3100) changes due to one soil property are much larger than and to relate ECa data to soil physical properties. Data were collected those attributable to other factors, then ECa can be on two fields in Illinois (Argiudoll and Endoaquoll soils) and two in calibrated as a direct measurement of that dominant Missouri (Aqualfs). At 12 to 21 sampling sites in each field, 120-cmfactor. Lesch et al. (1995a, 1995b) used this direct-calideep soil cores were obtained for soil property determination. Depth bration approach to quantify variations in soil salinity response curves for each ECa sensor were derived or obtained from the literature. Within a single field and measurement date, EM38 data within a field where water content, bulk density, and and Veris deep (0–100 cm depth) data were most highly correlated other soil properties were “reasonably homogeneous.” (r 0.74–0.88). Differences between ECa sensors were more proResearch in Missouri has established direct, within-field nounced on the more layered Missouri soils due to differences in calibrations between ECa and the depth of topsoil above depth-weighted response curves. Correlations of ECa with response a subsoil claypan horizon (Doolittle et al., 1994; Sudduth curve–weighted clay content and cation exchange capacity were generet al., 1995, 2001; Kitchen et al., 1999). ally highest and most persistent across all fields and ECa data types. Mapped ECa measurements have been found to be Significant correlations were also seen with organic C on the Missouri related to a number of soil properties of interest in fields and with silt content. Significant correlations of ECa with soil precision agriculture, including soil water content (Sheets moisture, sand content, or paste EC were observed only about 10% and Hendrickx, 1995), clay content (Williams and Hoey, of the time. Data obtained with both types of ECa sensors were similar and exhibited similar relationships to soil physical and chemical prop1987), CEC, and exchangeable Ca and Mg (McBride et erties. al., 1990). Because ECa integrates texture and moisture availability, two soil characteristics that affect productivity, it can help to interpret spatial grain yield variations, at least in certain soils (e.g., Sudduth et al., 1995; Jaynes E and accurate methods of measuring et al., 1993; Kitchen et al., 1999). Other uses of ECa in within-field variations in soil properties are imporprecision agriculture have included refining the boundtant for precision agriculture (Bullock and Bullock, aries of soil map units (Fenton and Lauterbach, 1999), 2000). Apparent profile soil electrical conductivity is interpreting within-field corn rootworm (Diabrotica one sensor-based measurement that can provide an indibarberi Smith and Lawrence) distributions (Ellsbury et rect indicator of important soil physical and chemical al., 1999), and creating subfield management zones properties. Soil salinity, clay content, cation exchange (Fraisse et al., 2001). capacity (CEC), clay mineralogy, soil pore size and disTwo types of portable, within-field ECa sensors have tribution, soil moisture content, and temperature all been used in agriculture—an electrode-based sensor reaffect ECa (McNeill, 1992; Rhoades et al., 1999). In quiring direct contact with the soil and a noncontact saline soils, most of the variation in ECa can be related electromagnetic induction (EM) sensor. The earliest to salt concentration (Williams and Baker, 1982). In sensors were of the contact type and included four elecnonsaline soils, conductivity variations are primarily a trodes inserted into the soil, coupled with an electric function of soil texture, moisture content, and CEC current source and resistance meter. Hand-carried four(Rhoades et al., 1976; Kachanoski et al., 1988). Rhoades electrode sensors were initially used in salinity surveys (Rhoades, 1993), and later versions were tractor1Mention of trade names or commercial products is solely for the mounted for mobile, georeferenced measurement of purpose of providing specific information and does not imply recommendation or endorsement by the USDA or the Univ. of Illinois. ECa. The electrode-based sensing concept formed the K.A. Sudduth and N.R. Kitchen, USDA-ARS, Cropping Syst. and Abbreviations: CV, coefficient of variation; DGPS, differential global Water Qual. Res. Unit, 269 Agric. Eng. Bldg., Univ. of Missouri, positioning system; EC, electrical conductivity; ECa, apparent soil Columbia, MO 65211; G.A. Bollero and D.G. Bullock, Dep. of Crop electrical conductivity; ECa-sh, shallow (0–30 cm) apparent soil electriSci., Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801; and cal conductivity measured by Veris 3100; ECa-dp, deep (0–100 cm) W.J. Wiebold, Dep. of Agron., Univ. of Missouri, 214 Waters Hall, apparent soil electrical conductivity measured by Veris 3100; ECa-em, Columbia, MO 65211. Received 10 July 2001. *Corresponding author vertical-mode apparent soil electrical conductivity measured by Geo(SudduthK@missouri. edu). nics EM38; EM, electromagnetic induction; GPS, global positioning system; TD, topsoil depth. Published in Agron. J. 95:472–482 (2003).