Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2008
Publication Date: 6/16/2008
Citation: Logsdon, S.D. 2008. Activation energies and temperature effects from electrical spectra of soil. Soil Science. 173:359-367. Interpretive Summary: Soil moisture sensors are often designed to use electrical measurements to determine the water content of a soil. These measurements are affected by the amount and type of clay minerals salt content and temperature, as well as water content. This study was designed to measure the effects of clay minerals and temperature. It was more difficult to get an electrical charge to move through a soil from the Southeastern U.S. than through soils from the Midwest or Great Plains because of the differences in the clay minerals in these soils. Temperature effects were greater for Midwestern soils because of the type of clays and greater effect of measurement conditions (frequency of alternating current). These results help explain why soil moisture sensor results can be very different depending upon the region of the country where they are being used. This knowledge is important for soil scientists and crop consultants who measure soil water and salt content in the field and under a range of soil temperatures throughout the country.
Technical Abstract: Apparent permittivity often has soil-specific temperature responses as well as soil water responses. These variations affect dielectric sensors, often requiring site-specific calibrations. Variations of permittivity as a function of frequency and temperature can be used to calculate activation energies. The purpose of this study was to examine permittivity-temperature responses for six soils, and variation in calculated activation energies. Each of the six soils was packed into a truncated coaxial cell, and the permittivity spectra was determined for a range of water contents and temperatures. Then activations energies were calculated due to ion migration or due to rotation. The rotational activation energy was not correlated with temperature, did not vary across frequencies, and was not significantly affected by soil differences. Ion migration activation energy was significantly less for Cecil (kaolinite and quartz dominant mineralogy) than for Weld or Okoboji (smectites dominant mineralogy). There were significant positive temperature correlation for the dc electrical conductivity and for the real permittivity at low frequencies, but not at higher frequencies. Fraction of sorbed water had a significant correlation with electrical conductivity and real permittivity at a range of frequencies. Delineation of these temperature and activation energy responses is necessary for describing how various soils respond to electromagnetic measurements.