Ground sensor for irrigation system?

The quoted text below is from my town's sprinkler/irrigation ordinance. The town requires all systems to be equiped with a ground sensor that measures the moisture of the ground which in turn the controller will adjust it's watering schedule accordingly.
"GROUND SENSORS WHICH MEASURE THE MOISTURE IN THE SOIL TO AVOID UNNECESSARY USE OF THE SYSTEM WHEN THE GROUND MOISTURE IS HIGH"
I have not seen a ground sensor that connects to a controller only rain sensor. Does anyone have any ideas?
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