Soil solution electrical conductivity measurements using different dielectric techniques

Accurate measurements of soil solution electrical conductivity (or.) are needed in various applications. One recently developed technique that measures sigma(w) is the Sigma Probe (SP). The SP is supposed to give accurate readings only-slightly dependent on water content (theta) and soil type. To test the performance of the SP, it was compared with another dielectric technique, time domain reflectometry (TDR). Both techniques utilize the dielectric constant (K-a) and bulk electrical conductivity (sigma(a)) to estimate the sigma(w). Measurements of sigma(w) were obtained in a laboratory experiment using nine different soil types with theta in the range 0.05 to 0.50 m(3) m(-3). In each soil type, three different sigma(w) were used (approximately 0.3, 1.2, and 3.0 dS m(-1)). The linear sigma(w)-sigma(a)-theta model used by the SP contains only one soil specific parameter (K-0) Using this model, the SP readings were constant over the encountered range in 0, whereas the TDR estimation calculated by the same model typically increased at K-a values below the range of 10 to 15. Using the SP with a default K-0 value of 4.1 typically gave a sigma(w) that was 20% of the true sigma(w) when sigma(w) > 1 dS m(-1). The error in the sigma(w) estimation for or, lower than 1 dS m(-1) can be much larger except in sandy soils. The TDR measurements of sigma(w) using a conventional sigma(w)-sigma(a)-theta model were more accurate in all soil types at all theta, with root mean square errors that were lower by about 50% compared with the SP readings. However, this model requires soil specific parameters that have to be obtained during a calibration experiment