adafruit , Englisch 
Coming soon - Soil sensor gets a square wave update
After pondering the Multi-Year Soil Sensor Thread over at https://zerocharactersleft.blogspot.com/2011/11/pcb-as-capacitive-soil-moisture-sensor.html we decided to make another prototype of our revised I2C soil sensor, this time we'll use the square wave filtering technique rather than the RC-rise-time system.