A frequency-response permittivity sensor for simultaneous measurement of multiple soil properties: Part I. The frequency-response method

A microcontroller-based frequency-response permittivity sensor for simultaneously measuring multiple soil properties was developed and tested in the laboratory. The sensing system consists of a sensor probe designed based on a modified Wenner-array structure, hardware for measuring frequency-response data, and software for controlling the hardware. The frequency responses of the sensor were measured within the frequency range of 200 Hz to 100 MHz in soil samples at combinations of three soil textures (very fine sandy loam, silt loam, and silty clay loam), five gravimetric water contents (12% to 22%), five salinities (1.90 to 11.15 dS m(-1)), and five bulk densities (1.05 to 1.30 g cm(-3)). Frequency-response tests showed that both the magnitude and phase parts of the frequency-response data can be good indicators of multiple soil properties. A higher water content and a higher salinity yielded a lower magnitude ratio and a higher phase difference at all frequencies. The phase difference showed the potential to measure water content in saline soils and to measure salinity in wet soils. The R-2 values achieved by the models for predicting water content under 7.89 dS m(-1) salinity using the magnitude ratio and the phase difference obtained at 600 Hz were 0.982 with root mean square error (RMSE) of 0.005 m(3) m(-3) and 0.978 with RMSE of 0.005 m(3) m(-3), respectively. The R-2 values achieved by the models for predicting salinity under 0.243 m(3) m(-3) water content using the magnitude ratio and the phase difference obtained at 20 MHz were 0.969 with RMSE of 0.328 dS m(-1) and 0.982 with RMSE of 0.328 dS m(-1), respectively. These results demonstrated that the sensor developed in this study is capable of measuring water content and salinity as accurately as commonly used soil moisture sensors, including TDR and capacitance sensors, and traditional four-electrode electrical conductivity sensors when only one variable (water content or salinity) of the soil is varied.