Relationship between the Real and Imaginary Permittivities, Temperature and Soil Water Content Measured at 50 MHz.

Soil moisture sensors generally strive to use the real permittivity as the basis for estimating soil water content from measured electrical properties of soil. It has been shown that a reasonably good general calibration can be developed for mineral soils on this basis. However, at the low measurement frequencies used by commercial sensors, there is considerable variability among soils. It has also been shown that, in general, inter-soil variability is related to the magnitude of the imaginary component. Thus, the overall calibration relationship could be improved if the imaginary component were incorporated. The problem has been that the imaginary component is highly temperature sensitive. We measured the real and imaginary components for different soils over a range of temperatures to investigate the possibility of improving calibration precision. We found that a general, a temperature corrected imaginary component could be established independent of water content. Given this, we were able to demonstrate a considerable calibration precision improvement over the general equation. We then demonstrated the use of this approach with field data.