Adafruit Industries Adafruit Industries STEMMA Soil Sensor - I2C Capacitive Moisture Sensor
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.
Adafruit Industries STEMMA Soil Sensor - I2C Capacitive Moisture Sensor
Usually ships in 5-7 business days. Limited availability. May not be in stock at time of order. No back orders.