UPDATE 19th Jan: I have slightly improved this sensor by soldering an audio jack to the sensor. This provides a neater way to connect the sensor to an RF transmitter. I am thinking of adding sockets for audio jacks to my Airwick room nodes. They can then be used as soil moisture monitors if they are positioned near a plant.
I have added an experimental plant node with a soil moisture monitor:
It is based on garden bot and uses a Jeenode to transmit the moisture level. It needs a proper waterproof case.
It is available in OpenRemote.