hi, this can be done with a simple arduino motor control along with the humidity sensor interfaced to it . And also in the tank you can put water level indicator so that the system can do its irrigation process accordingly.
Control all the process by your cellphone. This can be done by using bluetooth module or wifi module and a customized android app.
Use humidity sensor. This will allow your system to irrigate the plants only when the soil gets dry.
Set a time on which the system starts irrigating. This can be on daily basis or hour basis. This can be done by using delay function inside a loop in the arduino source code or by using an external timer and taking input from it.
The basis model is attached along with this comment.
You can exclude water tank from this model.
Feel free to ask anything. 😃