Soil water content determination using a network analyzer and coaxial probe

A small-volume dielectric constant and soil water content sensor would be d esirable in many laboratory experiments. Phase shift of the reflection coef ficient 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 densit y, and organic matter content, were tested and comparison was made with oth er dielectric methods. Synthesized time domain reflectometry (TDR), resonan t waveguide, CP, and conventional TDR measurements were in agreement for th e sand. A simple mixing model for known dielectrics accurately predicted me asurements of the apparent dielectric (K-a) for mixed solutions. A linear f unction (r(2) = 0.90), fit for the water content of all soil data, had a si ngle measurement root mean square error (RMSE = 0.039 m(3) m(-3)). The unce rtainty improved when individual linear soil calibrations (singe measuremen t RMSE = 0.012 to 0.032; r(2) = 0.95 to 0.99) were used and further improve d when repeated measurements were averaged (RMSE = 0.0073 to 0.026; r(2) = 0.97-1.00). The CP method for measuring K-a 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.