Evaluation of a Prototype Variable-Frequency Soil-Moisture and EC Probe

Measuring surface soil moisture is vital for understanding water availability, agricultural productivity, and climate change impacts, as well as for drought prediction and water resource management. However, obtaining accurate data is challenging due to the lack of reliable probes that work across diverse soil types and conditions. This study evaluated a prototype dielectric probe developed by Daiki Rika Kogyo Co., Ltd., Saitama, Japan, through controlled laboratory experiments. The probe measures the real and imaginary parts of dielectric permittivity over 10–150 MHz in a 5.6 cm diameter, with a 2 cm length volume, achieving a ±2% accuracy for the real part of oil–ethanol and ethanol–water mixtures (3.26–79). The imaginary part of the dielectric permittivity of aqueous solutions is convertible into electrical conductivity (EC) with reasonable accuracy. For variably saturated sand, the real part is convertible to a volumetric soil-moisture content (≥0.10 m3m−3) using a custom equation. The probe’s variable-frequency measurements reduce the limitations of fixed-frequency approaches, accounting for the EC, clay, porosity, and organic matter effects. With its VNA principle and simultaneous measurement of dielectric properties, it offers innovative capabilities for addressing water management, agriculture, and climate prediction challenges.