Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2012
Publication Date: 11/16/2012
Publication URL: http://handle.nal.usda.gov/10113/57960
Citation: Zu, J., Ma, X., Logsdon, S.D., Horton, R. 2012. Short, multi-needle FDR sensor suitable for measuring soil water content. Soil Science Society of America Journal. 76:1929-1937. Interpretive Summary: Knowing how much water is in the soil is important for agriculture, especially for irrigation timing. This laboratory study was the first step in developing an inexpensive sensor for soil water content, that would not be sensitive to salt level. By selecting the right range of frequency (for alternating current), the effect of salt on the measurement was minimized. The short sensors provided rapid, stable measurements of soil water content. This information is important to scientists who want to evaluate soil water content in soils with high salt content. After further testing in the field, the sensor would be of interest to farmers and advisors interested in appropriate irrigation application.
Technical Abstract: Time domain reflectometry (TDR) is a well-established electromagnetic technique used to measure soil water content. TDR sensors have been combined with heat pulse sensors to produce thermo-TDR sensors. Thermo-TDR sensors are restricted to having relatively short needles in order to accurately measure soil thermal properties. Short needle lengths, however, can limit the accuracy of the TDR measurement of soil water content. Frequency domain reflectometry (FDR) sensors are an alternative to TDR sensors that can provide an inexpensive measurement of soil water content. The objective of this paper is to determine whether short FDR sensors can accurately measure soil water content. We designed and constructed a short FDR sensor. For four soil types over a range of water contents, temperatures, and salt contents, we measured soil dielectric spectra with the short FDR sensor. A vector network analyzer (VNA) was used to obtain soil dielectric spectra in the 1MHz-3GHz frequency range. The ideal frequency of a short FDR sensor is the frequency at which the permittivity is not altered by changing temperature or salt content. The 47MHz-200MHz range was an ideal frequency range for measuring soil water content, and 70MHz was the frequency least influenced by temperature and salt content. Thus, for the four soil types, the 70MHz frequency was useful for measuring soil water content, and the apparent permittivity did not change with changing temperature and salinity. The short FDR sensor provided quick, continuous, stable and cheap measurements of soil water content. Because of the promising performance of the short thermo-FDR sensor in laboratory studies, sensors should be evaluated in a future field study.