EC-Sense: Radio <u>E</u> nergy <u>C</u> apture for Ultra-Low-Power Wireless Soil Moisture <u>Sens</u> ing

Understanding spatial and temporal variations in soil moisture is critical for sustainable agriculture. Contactless approaches such as ground-penetrating radar and satellite sensing are either expensive or offer limited resolution, making them less scalable. In-situ sensors provide higher resolution but often involve challenging data retrieval processes and wired infrastructure. In this work, we propose EC-Sense, a wire-free system for in-situ soil moisture sensing that employs ultra-low-power sensor Tags buried underground and reference Tags placed on the surface. EC-Sense introduces a novel sensing modality based on energy capture time (EC Time), defined as the time taken by a sensor Tag to harvest sufficient energy to activate a response. EC Time serves as a proxy for path loss in soil, which correlates strongly with soil moisture. By leveraging differential path loss—estimated from the EC Times of both surface and buried Tags—EC-Sense isolates soil-induced attenuation from environmental effects above ground. This differential sensing approach, combined with a decoupled sensing and communication architecture, enables EC-Sense to overcome limitations of existing wideband RF-based sensors. Despite using an active radio, our Tags consume only 3.7 μW on average and achieve a projected lifetime of 10 years under realistic conditions. We deployed EC-Sense in an open agricultural field for over a week and measured soil moisture at multiple depths daily, achieving 98% accuracy compared to ground truth. We further evaluate EC-Sense across three representative soil types—sandy loam, silt loam, and silty clay loam—and demonstrate reliable sensing at depths of 40 cm, 30 cm, and 25 cm, respectively, at field capacity.