Miceuz of wemakethings made this Chirp, a soil moisture level sensor based on ATTINY44. When the plant dries out, it starts to chirp reminding you to water the plant:
Chirp is a plant watering alarm – as simple as that. You put it into the soil near a plant and it emits a tiny chirp when the soil is dry, reminding you to water the plant. Chirp uses capacitive humidity sensing technique that does not interfere with the soil and does not cause corrosion of electrodes unlike resistive sensing.
Chirp is Open Source and totally hackable – you can download and fiddle with the firmware or use it as I2C moisture sensor.
Special care has been taken not to make it yet another source of annoyance – Chirp will be silent when it’s dark and will restrict itself from making too much noise.
The alarm level can be set for each plant individually. As water is consumed by the plant, Chirp will detect the low moisture level and will start to emit rare short chirps. As more water evaporates, Chirp will increase the alarm rate.
Chirp can detect if it’s day or night by measuring the amount of ambient light and will not make noise at night.
You can find the project code on GitHub.