I would like to build a circuit to automatically measure the dryness of the soil in a hanging basket and then water it when required. Some type of soil probe and a water solenoid set up.
If I was doing this, I would use a Microcontroller. I have no experience in soil sampling, so I don't know what the proper device is, I did find this. http://www.berryhilldrip.com/Irrometr.htm#Model%20'R
You could set up the controller to monitor the output level of the sensor and set up some limits, at a low level for moisture, trigger a solenoid to a water reservoir, at the high level of moisture turn off the solenoid.