For reading the soil moisture, I am using a DF Robot Soil Moisture Sensor. This is a very simple sensor, and reads moisture on the soil based on resistance between the two metal elements on the sensor. It reads this based on the following scale:
- 0 ~300 : dry soil
- 300~700 : humid soil
- 700~950 : in water
I converted the units to percent, for easy readability, but as you can see, its a very simple sensor.