Low-Cost and Low-Frequency Interface for Soil Moisture Monitoring

Soil impedance monitoring is crucial for various applications, but traditional impedance analyzers, while capable of providing detailed frequency-dependent data, are costly and difficult to integrate into low-cost monitoring systems. Moreover, interpreting soil impedance remains challenging due to the heterogeneous nature of soil and the variability of its electrical properties. This paper presents a simplified, low-cost, low-complexity soil monitoring interface mainly based on internal MCU peripherals and minimizing the usage of external circuitry. GPIO ports are used to generate low-frequency square wave driving signals while internal pull-up and pull-down resistors or external fixed resistors are used to measure soil impedance across different resistance ranges. The system minimizes polarization effects on the sensor electrodes, enabling stable measurements over time. The system was tested with resistive loads in the (100 Ω, 200 kΩ) range and a measurement error lower than 2% was achieved. Then, tests with soil samples at different volumetric water contents were performed, proving the system effectiveness for real-time monitoring and IoT integration. Finally, the front end was integrated into an IoT monitoring system for smart agriculture, demonstrating its usability over long measurement periods.