Looking for a wireless soil moisture sensor
The project is very much possible but it will be easier if you attach the moisture sensor directly to the arduino and add a wifi shield or a bluetooth shield to the arduino itself and make it communicate with computer or phone. You could also program the arduino to directly water the plants if it finds that they dont have much moisture. You will rarely find a sensor that transmits data wirelessly and still be economically feasible. The Sparkfun SEN-13322 moisture sensor is good for the purpose but again connect it to arduino direclty.