A long-term evaluation of soil moisture sensing based on wireless communication

Soil and crop monitoring is an essential component of the evolving field of precision agriculture. However, existing soil moisture monitoring methods have high hardware costs, which lead to scalability issues. Existing soil moisture sensing approaches based on Radio frequency (RF) rely on the fact that water affects the transmission of radio signals and thus the Received signal strength indicator (RSSI) values. Most of the systems in literature have been deployed for less than a week and often only use laboratory setups. We have tested a sensor system in a long-term (4 months) experiment on an outdoor test site including four contrasting soil types. Therefore, we have developed a low-cost system based on Commercial off-the-shelf (COTS) hardware that is not based on classic soil moisture sensors. Our system collects RSSI values of the three technologies: Wi-Fi (2.4 GHz), Bluetooth Low Energy (BLE), and LoRa. We observed a correlation between RSSI and soil moisture, although this correlation was weaker than that reported in the related literature. Our findings indicate that we could only reliably estimate soil moisture over short time periods of approximately 10 days.