Picture of Automatic Watering System
For making this circuit you will need basic knowledge of electronics, also you need to know how to make a PCB. If you don't know how to make a PCB and you want to know how, we recommend you to go to google and you tube and search "How to make a pcb" before you continue with this tutorial.

What we need?
The material we need to make this circuit its really little. I found all the material really easy. You will need:

nand shmith IC (I use 74S132)
a npn transistor
3- 1K resistor
1- 1.2 M resistor
1- 10 K resistor
1- potenciometro 50K
two leds
one switch
one electrolitic capacitor 10uF 100V
one diode  1n4007 or similar
4 header terminals with two screws
one relay of 12v
a copper plaque of 7X7 CM

Step 1: Schematic

Picture of Schematic
What this circuit makes is measure the resistance of earth between the probes. IF the earth is wet the resistance of the soil get smaller, that makes that the circuit became close and the water pump goes off. The circuit its really efficient so you can use it with batteries and they will last a really long time.

This is the schematic circuit: