Development of a Soil Impedance Measurement System for Wide-Area Soil Measurement by Sensor-to-Sensor Measurement

It is important to measure water content and electric conductivity in agricultural soil. This study presents a four-terminal soil impedance measurement system for wide-area sensor-to-sensor measurement. While conventional two-terminal methods suffer from accuracy degradation due to parasitic components in long cables, the proposed system achieves accurate measurement by separating current supply and voltage measurement paths. A digital signal processing-based lock-in amplifier was implemented on a microcontroller with a compact 50-mm square board. Performance verification demonstrated significant improvement in capacitance measurement errors, particularly in low-resistance and low-capacitance regions, generally achieving the target relative error of within 10% and showing the feasibility of wide-area soil measurement.