Adafruit STEMMA Soil Sensor - I2C Capacitive Moisture Sensor [JST PH 2mm] : ID 4026 : $7.50 : Adafruit Industries, Unique & fun DIY electronics and kits
capacitance - Capacitive moisture sensor not quite working - Electrical Engineering Stack Exchange
Amazon.com: 2 Sets Resistive Soil Moisture Meter, Icstation Soil Hygrometer Sensor Test Kit with Corrosion Resistant Probe, Digital Analog Signal Output for Arduino Garden Plant Care Automatic Watering System : Patio, Lawn