need soil moisturizer level detector circuit please....

i need to build soil moisturizer level detector to complete my study project... the supply required is 12Vdc for this circuit. the function of the circuit is to detect the moisturizer of the soil... when the soil is dry, the signal (power supply) will give to the water pump to supply water to the soil... below is the my whole project block diagram: PV cell (12V solar panel) - charge controller (12Vdc) - rechargeable battery (12Vdc, 10A @ 20A) - soil moisturizer level detector (12Vdc) - inverter (12Vdc to 240Vac) - water pump (240Vac) please and hopefully someone will help me..i really appreciate it...thank you very much... you also can reach me at: true_odyssey@yahoo.com monuradli@gmail.com