Soil electrical conductivity as a function of soil water content and implications for soil mapping

L Apparent soil electrical conductivity (ECa) has shown promise as a soil survey tool in the Midwestern United States, with a share of this interest coming from the precision agriculture community. To fully utilize the potential of ECa to map soils, a better understanding of temporal changes in ECa is needed. Therefore, this study was undertaken to compare temporal changes in soil ECa between different soils, to investigate the influence of changes in soil water content on soil FCa, and to explore the impacts these ECa changes might have on soil mapping applications. To this end, a 90 m long transect was established. Soil ECa readings were taken in the vertical and horizontal dipoles at five points once every one to two weeks from June until October in 1999 and 2000. At the same time, soil samples were collected to a depth of 0.9 m for volumetric soil water content analysis. Soil ECa readings were compared to soil water content. At four of the five sites linear regression analysis yielded r(2) values of 0.70 or higher. Regression line slopes tended to be greater in lower landscape positions indicating greater ECa changes with a given change in soil water content. Two of the soils had an ECa relationship that changed as the soils became dry. This is an item of concern if ECa is to be used in soil mapping. Results indicated that soil water content has a strong influence on the ECa of these soils, and that ECa has its greatest potential to differentiate between soils when the soils are moist. Soil water content is an important variable to know when conducting ECa surveys and should be recorded as a part of any report on ECa studies.