Meter & Testing Circuit :
Plant Moisture Meter IC 741

Stick the metal probes into a freshly watered plant and adjust R5 for a mid-scale meter deflection.
The meter will monitor the soil wetness and the meter will indicate whether it is to moist or to dry.
This circuit uses a dual power supply which could be created by two 9 volt batteries.

Source : http://home.maine.rr.com/randylinscott/nov20.htm