Submitted to: Applied Engineering in Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/21/2005
Publication Date: 4/4/2006
Citation: Allred, B.J., Ehsani, R.M., Saraswat, D. 2006. Comparison of electromagnetic induction, capacitively-coupled resistivity, and galvanic contact resistivity methods for soil electrical conductivity measurement. Applied Engineering in Agriculture. 22(2):215-230. Interpretive Summary: The soil profile properties that affect soil fertility are often strongly correlated to soil electrical conductivity. Consequently, soil electrical conductivity measurements in an agricultural field can potentially be used to gauge spatial changes in soil fertility, which can be very important with regard to applying precision agriculture techniques. There are three near-surface geophysical methods available for rapid, continuous measurement of apparent soil electrical conductivity in agricultural fields. These methods are electromagnetic induction, capacitively-coupled resistivity, and galvanic contact resistivity. Testing of all three methods under variable shallow hydrologic conditions was conducted at two adjacent test plots having silt loam to silty clay loam soils. Research results indicate that electromagnetic induction, capacitively-coupled resistivity, and galvanic contact resistivity methods are all capable of determining the lateral soil electrical conductivity patterns present in agricultural fields. Changes in shallow hydrologic conditions (soil surface volumetric moisture content and shallow water table depth) were not found to have an overwhelming impact on lateral soil electrical conductivity patterns, which suggests that soil electrical conductivity patterns are mostly likely governed by spatial changes in soil profile properties, at least for fine-grained glacial sediment derived soils common throughout the Midwest U.S. The magnitude of the measured soil electrical conductivity varied depending on the geophysical methods utilized, therefore, care and caution are obviously warranted when integrating soil electrical conductivity measurement results from different geophysical methods in order to make specific assessments, say for example, evaluating changes in agricultural field transient conditions (nutrient levels, soil wetness, etc.) or determining the similarity in soil texture between two farm locations. In conclusion, all three geophysical methods tested were able to provide useful information on lateral and vertical soil electrical conductivity trends in an agricultural field, and as such, may be of value to those employing precision agriculture techniques.
Technical Abstract: In situ measurement of apparent soil electrical conductivity (ECa) can be an important tool in regard to precision agriculture. There are three near-surface geophysical methods available for rapid, continuous measurement of ECa in agricultural fields. These methods are electromagnetic induction (EMI), capacitively-coupled resistivity (CCRes), and galvanic contact resistivity (GCRes). Testing of all three methods under variable shallow hydrologic conditions was conducted at two adjacent test plots having silt loam to silty clay loam soils. Different operational modes for each of the three geophysical methods were evaluated, including three primary electromagnetic field frequencies (8190, 14610, and 20010 Hz) used for the EMI method, four spacing distances (0.625, 1.25, 2.5, and 5.0 m) between the two dipoles employed with the CCRes method, and two different Schlumberger electrode array lengths (0.7 and 2.1 m) utilized for the GCRes method. Consequently, there was a total of nine geophysical method - operational mode combinations. Similarities and differences between the nine method - operational mode combinations were examined with respect to measured areal ECa patterns for the two agricultural test plots and in regard to test plot ECa averages, medians, and standard deviations. Furthermore, electrical conductivity variation as a function of soil depth was assessed from survey data obtained by each of the three near-surface geophysical methods along with electromagnetic vertical sounding measurements. Based on spatial correlation analysis, the areal ECa patterns measured by the nine method - operational mode combinations showed substantial similarity regarding one another, with one exception. The exception was the short electrode array mode of the GCRes method, which when paired with the various modes of the other two geophysical methods, exhibited an average correlation coefficient, that ranged between only 0.30 and 0.45. All other average values for pairs of different geophysical method - operational mode combinations ranged between 0.62 and 0.97. Spatial correlation coefficients, for both test plots, between the same method - operational mode combination but at two times in which hydrologic conditions differed ranged from 0.55 to 0.95 for eight of the nine method - operational mode combinations, with the GCRes short electrode array having values of 0.32 and 0.58. Electromagnetic vertical sounding measurements along with results obtained by EMI, CCRes, and GCRes surveying, when combined, indicate for both test plots that from the surface to a depth of a little over 2 m, soil electrical conductivity on the whole first increases and then towards the bottom of the interval decreases. By and large, the results of this research project demonstrate that the three geophysical methods tested can all provide useful information on the lateral and vertical distribution of soil electrical conductivity.