Depends on how tied you are to "Z" networks. EcoWitt, which have a range of weather related sensors also have a soil moisture sensor. Their weather sensors all report back to a "gateway" of some kind (either a small USB device or display panel) which can send regular updates to a HE driver I now maintain. The sensors communicate to the gateway via RF, with the gateway communicating with HE via either Wi-Fi or LAN communication.
So, long story... well still long... ;-), you can get various sensors and a gateway to send moisture readings locally to HE. Plus if you want, you can join the rabbit-hole like others have recently, that is EcoWitt and local weather sensors. I feel like Laurence Fishburne.... 
Will you take the EcoWitt pill? 
