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DIVISION S-1—SOIL PHYSICS

Soil Solution Electrical Conductivity Measurements Using Different Dielectric Techniques

Dep. of Water Resources Engineering, Lund Univ., Box 118, 221 00 Lund, Sweden

^* Corresponding author (magnus.persson{at}tvrl.lth.se)

Accurate measurements of soil solution electrical conductivity (_w) are needed in various applications. One recently developed technique that measures _w is the Sigma Probe (SP). The SP is supposed to give accurate readings only slightly dependent on water content () 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 (_a) to estimate the _w. Measurements of _w were obtained in a laboratory experiment using nine different soil types with in the range 0.05 to 0.50 m³ m^-3. In each soil type, three different _w were used (approximately 0.3, 1.2, and 3.0 dS m^-1). The linear _w–_a– model used by the SP contains only one soil specific parameter (K₀). Using this model, the SP readings were constant over the encountered range in , 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 K₀ value of 4.1 typically gave a _w that was ±20% of the true _w when _w > 1 dS m^-1. The error in the _w estimation for _w lower than 1 dS m^-1 can be much larger except in sandy soils. The TDR measurements of _w using a conventional _w–_a– model were more accurate in all soil types at all , 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.

Abbreviations: SP, sigma probe • TDR, time domain reflectometry

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