A very simple circuit using IC LM3915 to measure the moisture level in soil. (Just for hobby thing, not for Industrial standard measurements).
The IC LM3915 is a dot/bar display driver monolithic integrated circuit that sense analog voltage levels and drives ten LEDs and provides a logarithmic 3dB/step. This IC operates with minimum 3V to maximum 25V DC bias.
Construction & Working
Here the LM3915 connected with 9V battery and ten LEDs are connected to the output terminals, use different colored LED for different level of outputs. The test probe is connected between divider resistors R2 and R3 through VR1 and R1 resistors, the probe brings ground to the reference voltage at pin6 and makes the variations to the reference voltage, depends on the reference voltage LEDs are drive to glow.
Use slew less single strand copper wire (or insulation removed copper wire) as a test probe and it could measure the moisture level in the soil. Place the test probe ground pin and test probe pin closely (like in centimeters).
Refer Datasheet for detailed operation, key features and typical applications of IC LM3915.