The initial prototyping will involve a two-pronged probe to be inserted into the ground.
This probe works by measuring the resistance of a small current through the soil. The greater the moisture content, the more conductive the soil will become. This probe will be placed at the bottom of the solar powered sensor node, which will look like a garden solar light.
Of course, this sensor does not have to be always on, and so it will be important for the wireless node to be able to turn off the sensor between readings in order to conserve power. The total current draw of this sensor will have to be verified to manage the duty cycle.