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

ABSTRACT 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. Furthermore, measurements with laboratory‐prepared soil samples showed that the proposed system's capacitance values agreed to within ±10% of those obtained using a commercial impedance analyzer (4294A). Moisture estimation was achieved within ±5% error for medium‐moisture ranges (20%–40%), whereas drier soil (∼10%) yielded larger deviations due to the small capacitance shift of 74.6 fF per 1% moisture change. This indicates that improving electrode geometry to amplify moisture‐dependent impedance variation is a promising direction for enhancing accuracy in practical deployments.