Design of a Capacitive Probe for Laboratory Analysis of Dry and Wet Soil Electrical Properties

Understanding soil electrical properties is essential for estimating the soil water content and salinity through electrical measurements on properly designed sensors. In recent years, several low-cost soil water content sensors have been developed for geotechnical and agronomic applications, but for their validation, a characterization process is required. This is typically performed with liquids whose electrical properties are well-documented in the literature. Conversely, the electrical properties of soil are inherently unknown a priori, as they are influenced by a wide range of environmental and physical factors. This work presents a new system based on cylindrical capacitors that can be used to estimate the soil electrical properties in both wet and dry conditions. The analytical model adopted for the estimation is derived from Gauss’s Law. Its accuracy is validated against Finite Element Method (FEM) simulations, showing excellent agreement (R2 = 0.9999). The impact of the electrode insulating coating in the design is also evaluated and measurements at low frequency, 10–100 kHz range, are performed in ground coffee samples.