Smart Plant Care! Build a Qwiic Soil Moisture Sensor System
Chowdah 
Calibration
System Calibration To get any sort of useful data out of your Soil Moisture Sensor, it is advised that you calibrate it to whatever soil you plan to monitor. Different types of soil can affect the sensor, and you may get different readings from one composition tot he next. Before you start storing moisture data or triggering events based on that value, you should see what values you are actually getting from your sensor. Using the sketch above, note what values your sensor outputs when the sensor is completely dry vs when the sensor is completely submerged in a shallow cup of water. Depending on what microcontoller you're using, the operating voltage of that microcontoller, and the resolution of its analog-to-digital converter, you're results will vary.
For example, using the same circuit above, if I detach the VCC pin from D7 and attach it directly to the 5V supply on the RedBoard, you'll see the close to the following values in the serial monitor when the sensor is dry (~0) and when it is completely saturated with moisture (~880).
