comments:
Are you using this as an excuse to play around electronics or are you serious about this? If you are serious, don't DIY. Buy the existing product. If it's only to mess around with electronics, reading the resistance between two metal probes in the soil is very easy to do but you need to only read for short amounts of time otherwise the probes will rust. They even sell ready-made PCBs for that, and here's a tutorial on how to use them. Also, automatic watering is a bad idea. What happens when you forget to fill the reservoir? When the system fails? When the soil stays dryer/wetter close to your sensor? Calibration will be a bitch, if you couldn't see that the plant had too much water, imagine how hard it will be to tell a computer when to stop. You can't monitor the whole pot of soil. Having a reservoir near every plants is unsightly and anything bad that happens will still be your fault. Just make the wifi alarm portion if you want to DIY, watering a plant by hand is not hard and not time sensitive enough to warrant the automatic watering.