Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2000
Publication Date: N/A
Interpretive Summary: Soil scientist, agricultural engineers, and private companies have been developing and evaluating methods for soil water content measurement for nearly a century, yet the most commonly used method remains the laborious, time consuming, and destructive method of oven drying. Thus, the development of new techniques using modern instrumentation is warranted. We evaluated a rapid and easy electromagnetic technique whereby the measurement could be made on a small sample using a probe and network analyzer instrument. A simple linear calibration can be employed and the instrument's accuracy improves when measurements are repeated at a number of locations on the sample. These findings may be useful for a broad range of agricultural, scientific, and engineering applications in which soil water content information is essential.
Technical Abstract: A small-volume dielectric constant and soil water content sensor would be desirable in many laboratory experiments. Phase shift of the reflection coefficient in soil and various solutions was measured with a coaxial probe (CP) and a network analyzer operating at a frequency of 795 MHz. The CP had a measurement depth <1 cm. Five soils, varying widely in texture, bulk density, and organic matter content, were tested and comparison was made with other dielectric methods. Synthesized time domain reflectometry (TDR), resonant waveguide, CP, and conventional TDR measurements were in agreement for the sand. A simple mixing model for known dielectrics accurately predicted measurements of the apparent dielectric (Ka) for mixed solutions. A linear function (r2 = 0.90), fit for the water content of all soil data, had a single measurement root mean square error (RMSE = 0.039 m3m-3). The uncertainty improved when individual linear soil calibrations (singe measurement RMSE = 0.012 to 0.032; r2 = 0.95 to 0.99) were used and further improved when repeated measurements were averaged (RMSE = 0.0073 to 0.026; r2 =0.97 to 1.00). The CP method for measuring Ka is fast, simple, linear, easily repeated, and reasonably accurate, indicating that this instrumentation is useful for studying dielectric behavior of soil and various solutions and for rapid determination of soil water content in a small sample.