I used my own solution. It is important to know the ph, humidity, etc. Since the composition of the soil is different, you need several of these devices to accurately determine when there is too much water and when not. You can see how I solved the irrigation in this article: Automatic watering system
The problem is accuracy. I have some ones but they are rubbish. I think they just measure the resistance between two tracks on a pointed PCB. To be cheap I think they have to do something so simple.