The impact of temperature and shallow hydrologic conditions on the magnitude and spatial pattern consistency of electromagnetic induction measured soil electrical conductivity

In situ measurement of apparent soil electrical conductivity (EC(a)) is an important precision agriculture tool for determining spatial changes in the soil properties affecting soil fertility. However, dynamic temperature and shallow hydrologic conditions also influence the measured EC(a) and need to be considered. Therefore, the impact of temperature (air, soil) and shallow hydrologic conditions (soil moisture content, water table depth) on the magnitude and spatial patterns of EC(a) was evaluated. Eighty-eight EC(a) mapping surveys were conducted at a single test plot over an interval of two years. Soil electrical conductivity was measured by electromagnetic induction (EMI) at primary field frequencies of 8190, 14610, and 20010 Hz. Because results were similar at all three frequencies, analyses focused on the 14610 Hz data. The EC(a) surveys were grouped into four time periods for analysis based upon factory recalibration of the sensor, climate changes, and a shift in instrument response. Reduced values of measured EC(a) occurred with air temperatures at or below 12 degrees C and/or soil temperatures at or below 8 degrees C. Correlation analysis within each of the four data groups showed that the average EC(a), within the test plot, was most strongly affected by near-surface volumetric moisture content (r(MC-ECa: Grp. Avg.) = 0.73), followed by water table depth (r(WTD-ECa: Grp. Avg.) = -0.42), soil temperature (r(ST-ECa: Grp. Avg.) = 0.14), and ambient air temperature (r(AT-ECa: Grp. Avg.) = -0.10). Correlation analysis of spatial EC(a) patterns between pairs of EMI surveys, conducted under a range of temperature and shallow hydrologic conditions, produced r(Spatial: ECa vs. ECa) values averaging 0.63 (standard deviation equaled 0.17), indicating that spatial EC(a) patterns remain relatively consistent over time. These EC(a) results indicate that the spatial EC(a) response is governed to a large extent by the spatial changes in soil properties and less by changes in shallow hydrologic conditions.