According to a company rep, Chirp uses capacitive sensing as opposed to resistive humidity sensing.
Meaning, it does not actually make an electric contact with the soil, thereby successfully avoiding electrode corrosion and soil electrolysis – resulting in optimized accuracy and extended battery life.
On the hardware side of things, a standard AVR 6 pin ISP programming header is available on the board for programming and serial communication. The device acts as a I2C slave, so the header can be used to read the moisture and light levels. It should be noted that another microcontroller or a dev board such as Arduino can be used as I2C master to read those levels.
The alarm level is set for each plant individually, with Chirp configured to detect low moisture level and emit rare short chirps as appropriate. As more water evaporates, Chirp increases the alarm rate.
Chirp is currently available on Tindie at a $15 price point.