We present a sensing system that determines soil moisture via RF using backscatter tags paired with a commodity ultra-wideband RF transceiver. Despite decades of research confirming the benefits, soil moisture sensors are still not widely adopted on working farms for three key reasons: the high cost of sensors, the difficulty of deploying and maintaining these sensors, and the lack of reliable internet access in rural areas. We seek to address these obstacles by designing a low-cost soil moisture sensing system that uses a hybrid approach of pairing completely wireless backscatter tags with a mobile reader. We designed and built two backscatter tag prototypes and tested our system both in laboratory and \emph{in situ} at an organic farm field. Our backscatter tags have a projected battery lifetime of up to 15 years on $4\times$AA batteries, and can operate at a depth of at least 30cm and up to 75cm. We achieve an average accuracy within 0.01-0.03$cm^3/cm^3$ of the ground truth with a 90th percentile of $0.034cm^3/cm^3$, which is comparable to state-of-the-art commercial soil sensors, at an order of magnitude lower cost.
