A Small‐Scale Matric Potential Sensor Based on Time Domain Reflectometry

Measurements of soil matric potential (ψ) are needed in many soil science applications. In the present study, a small matric potential sensor having a length of 30 mm and a diameter of 9.6 mm was developed. The sensor consists of two coils made of lacquer‐coated copper wires embedded in gypsum. The dielectric constant of the gypsum ( K gypsum ) was measured with time domain reflectometry (TDR). The gypsum probes were tested in a sandy loam soil using a pressure extractor to obtain the relationship between K gypsum and ψ, allowing inference of the soil ψ in equilibrium with the sensor. It was shown that the gypsum probe could give ψ estimates with a root mean square error (RMSE) of around 50 kPa in the range of −20 to −1500 kPa. Most of the sensitivity of the sensor was in the −50‐ to −1000‐kPa range.