I have been testing the Vegitronix soil moisture sensors.
http://www.vegetronix.com/Products/VG400/
They are designed to be buried in the ground (lawn mower proof) and can operate on 3.3v.
I have some ideas for interfacing them with the OSPi using RS-485 serial which can have cables up to 1 kilometer long. The RS-485 standard also can support a large number of sensors on one serial cable. A 4 wire cable can provide power and carry data (no need to change batteries).
Dan