Each plant has its own water requirements. The soil moisture content can be measured by this moisture meter to give you a good idea whether or not your plant needs water.
 
This moisture meter uses the different conductivity between the two metals at the probe tip to work.
 
Directions for use -
 
Wipe the soil moisture probe with a cloth before and after each use to ensure that the sensor is clean, then insert about 2-4 inches deep to test moisture in the soil.
Designed for use in houseplant compost mixes (not hard garden soils).
Never put this moisture meter direct into water.