Updated in early 2020 to give a better range of values from soil moisture sensing
Determine your soil moisture via innovative capacitive measurement instead of resistive!
No longer worry about your sensor oxidizing requiring repetitive re-calibration
Most low cost soil sensors are resistive style, where there's two prongs and the sensor measures the conductivity between the two. These work OK at first, but eventually start to oxidize because of the exposed metal. Even if they're gold plated! The resistivity measurement goes up and up, so you constantly have to re-calibrate your code. Also, resistive measurements don't always work in loose soil.
This design is superior with a capacitive measurement. Capacitive measurements use only one probe, don't have any exposed metal, and don't introduce any DC currents into your plants. Adafruit uses the built in capacitive touch measurement system built into the ATSAMD10 chip, which will give you a reading ranging from about 200 (very dry) to 2000 (very wet). As a bonus, you see the ambient temperature from the internal temperature sensor on the microcontroller, it's not high precision, maybe good to + or - 2 degrees Celsius.
To make it so you can use the sensor with just about any microcontroller, Adafruit added an I2C interface. Connect a 4-pin JST-PH cable (not included) to your microcontroller or single board computer to 3-5V power, Ground, I2C SDA and I2C SCL and load the code. No soldering required.
Item Specifics Size: 3.0" x 0.6" x 0.3" Weight: 4.0g