Soil Water Content Derived from Frequency-Dependent Permittivity using a Portable Network Analyzer and Open-Ended Dielectric Probe.

Soil constituency plays a significant role in the dielectric permittivity-water content relationship which informs electromagnetic sensor-based readings. Sensor output generally provides a bulk determination of permittivity representing an unspecified frequency. The effective frequency can vary with soil properties including salinity, clay content and water content. The objective of this work was to utilize a portable network analyzer and open-ended dielectric probe for mobile determination of soil water content from frequency-dependent permittivity. Open-ended dielectric probes were made from a variety of coaxial fixtures and tested using an Anritsu portable network analyzer. After analyzer calibration using an open, short and load, the measured reflection coefficient was determined for two reference liquids. Bilinear analysis was used to determine bilinear coefficients representing the physical measurement system and thus providing permittivity determination of unknown materials. An HP network analyzer and dielectric probe provided reference permittivity measurements for calibration. Dielectric spectra of ‘lossy’ soils demonstrate varying permittivity with frequency and suggest measurements made at multiple fixed frequencies could potentially be used to estimate soil texture. Reliable and rapid mobile measurement of soil water content at the field-scale could greatly enhance many agronomic activities such as development of reclamation strategies, reducing fertilizer application and optimizing fresh and saline water use. This capability will require improvements in sensor design, signal analysis and novel application of the technology before real-time field-scale measurements for precision agriculture are readily available.