moisture sensor for sprinkler system

Has anyone seen a soil moisture sensor for use with sprinkler system timers that works wirelessly?
I have a rain sensor that defeats the timer if rain is falling, and I've seen a similar unit that also has a wind sensor. I'm looking for something similar that senses soil moisture.
Ideally (yeah, yeah, I know), I could set the system to water every day, then let the sensor override every day until the ground gets dry enough to need water. I've found lots of commercial products, but I'd like to avoid trenching and running a wire under 30' of concrete.
I'll keep my fingers crossed that someone has seen something that I missed, but I won't hold my breath.
Thanks, Steve Bell
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