Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 31, 1999
Publication Date: N/A
Interpretive Summary: To study how water transports chemicals and how much water is taken up by plant roots, agricultural and environmental scientists need frequent measurements of soil water content. Time domain reflectometry (TDR) equipment enables scientists to make frequent, nondestructive measurements of soil water. The TDR equipment has been used successfully on sandy and loamy soils, but there have been difficulties for soils with large amounts of organic matter and/or certain types of clay. We showed that the TDR measurements for these problem soils changed more as the soil temperature changed than as the water content changed, and the temperature effect was more pronounced as the length of cable connecting to the TDR device increased. In order to use TDR to measure soil water content on these soils, users of TDR devices need to subtract this temperature effect. This finding will enable scientists to extend the use of TDR equipment to these problem soils, and beyond the laboratory into the field where the length o cables connecting to the TDR devices often needs to be quite long. This will greatly expand the use of this technology for agricultural and environmental studies.
Technical Abstract: Time domain reflectometry (TDR) has been used by soil scientists to determine soil water content from the apparent dielectric number. Soils with large surface area have much bound water with different properties than free water. At GHz frequencies, free water has a negative temperature effect but bound water has a positive temperature effect on dielectric number. At high frequencies bound water has a lower dielectric number tha free water, but the dielectric number for bound water increases as the frequency of the measurement decreases. Long coaxial cables reduce the higher frequencies of the TDR equipment. The objective of this study was to determine the interactive effect of coaxial cable length and temperature on apparent dielectric properties for samples with and without large surface areas. Two undisturbed columns of Okoboji soil (fine, montmorillonitic, mesic cumbic Endoaquoll) with a specific surface area of 286 m**2/g were compared with two packed sand samples of surface area 0.01 m**2/g. For Okoboji the positive cable/temperature interaction effect was 0.0003 and 0.0006/degree C/m for two different cable types. For the sand there was a small increase in apparent dielectric number as cable length increased, and an insignificant temperature effect. In summary temperature and cable length had a greater effect on the apparent dielectric number than did the soil water content. When calibrating samples with large surface areas, the cable length should be the same as that used for measurement, and a range of temperatures should be included to determine the amount of positive or negative temperature effect.