Soil moisture monitoring system based on metamaterial-inspired microwave sensor for precision agriculture applications

This paper presents a new soil moisture monitoring system for precision agriculture applications. The system consists of a metamaterial-inspired microwave sensor and an electronic back-end. The sensor was built on a biodegradable dielectric substrate, emphasizing sustainability, and utilizes a complementary split-ring resonator coupled to a microstrip line for excitation. This resonator, being a planar artificial structure with metamaterial properties, exhibits composite left/right-handed propagation, thereby displaying sensitivity to the dielectric constant of the applied medium. Accordingly, the sensing mechanism involves establishing a correlation between soil moisture content and the relative displacement of the sensor's resonance frequency, which varies as the soil's dielectric constant changes due to moisture variations. The back-end is controlled by an ESP32 module, which fulfills the tasks of signal capture, processing the data, and delivering the corresponding soil moisture. The transmission of data occurs via Wi-Fi employing the MQTT protocol. To ensure accuracy, the sensor was calibrated using a gravimetric-based method for sandy and clay soil types. A curve-fitting model was extracted, exhibiting remarkable precision (R 2 = 0.997), enabling the system to detect moisture levels from 0% to 20% for both soil types. The overall relative error was found to be 3.6% for sand and 8.3% for clay. Finally, the results obtained from the evaluation of the system in typical sandy soil conditions are presented. The system demonstrated its effectiveness by continuously detecting moisture levels ranging from 5% to 11% over a duration of 33 hours, thereby confirming its viability for practical implementation.