  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.
