Automatic Irrigation System

Hello,

I want to design an automatic irrigation system that would sense the soil humidity and would turn on and off the irrigation pump accordingly.
This is my final year project due this december. I am totally new with the embedded systems and microcontrollers. I have already ordered the Arduino Mega 2560 which i well get very soon.

I need to design a soil humidity sensor and then interface that with the board then control the motor through a relay.

Where shall i start form after i get the board and please help me with the humidity sensor pls.

Thank You.

google garduino, a number of folks have web sites up describing exactly how to do this.

Thanks a lot! :)