Evaluation of Default, Soil-Specific, and Clay Content Correction Calibration Functions for Dielectric Sensors in Soils with Differing Properties

Correct soil water content measurement is important for various environmental and agricultural applications. The aim of this study was to evaluate the performance of two commercially available dielectric sensors, the SM150T (Delta-T Devices) and TRIME-Pico 32 (IMKO). We evaluated the manufacturer's calibration functions and the two-point calibration function recommended by Delta-T Devices. The coefficients of the two-point calibration showed a statistically significant dependence on clay content. Because the soils tested contained a wide range of clay contents, a clay content correction function was developed. Soil-specific calibration functions were also developed. In general, measurement errors were the largest when the manufacturer's calibration was used, but were smaller for the SM150T [root mean square error (RMSE) of 0.011 to 0.051 m(3) m(-3)] compared to the TRIME-Pico 32 (RMSE of 0.030 to 0.060 m(3) m(-3)). The errors decreased after applying the clay content correction function for TRIME-Pico 32. For both sensor types, the two-point calibration reduced the errors, and the soil-specific calibration yielded the lowest error rates.