How to: soil moisture measurement with WaterMark?
You can google 
"Arduino-based system for soil moisture measurement"
and the author about this paper is "Vuk Radman" .

Now, I am also try to find the method using Arduino to connect WaterMark 200ss Sensor. The paper I share to you is what I have found from GoogleScholar, but I haven't used this method yet. Nevertheless, I think the method mentioned in the paper is feasible. Hopefully, this information can help you.