Most low-cost soil sensors are resistive type, where there are two branches and the sensor measures the conductivity between them. These work well at first, but eventually oxidize because of the exposed metal. Even if they're golden! The resistivity measurement is increasing more and more, so you must constantly recalibrate your code. In addition, resistive measurements do not always work in soft soils.

This design is superior with capacitive measurement. Capacitive measurements use only one probe, have no exposed metal and do not introduce direct current into your installations. We use the capacitive touch measurement system integrated in the ATSAMD10 chip, which will give you a reading from about 200 (very dry) to 2000 (very wet). As a bonus, we also give you the ambient temperature of the internal temperature sensor of the microcontroller, it is not a high accuracy, perhaps good at + or - 2 degrees Celsius.

So that you can use the sensor with just about any microcontroller, we have an I2C interface. Connect a 4-pin JST-PH cable (we have some in stock) to your microcontroller or single-card computer to a 3-5V, ground, I2C SDA and I2C SCL power supply, then execute our Arduino or CircuitPython code to read the capacitive measurement and temperature. No welding is required!

Note: This is only the sensor, you will also need a 4-PH JST cable to connect it! We have some options such as one with male connector pins, female connector pins and a JST-JST cable. You can use Seeed Grove cables and cards to connect - the Grove cable looks a little different but will fit very well.

Consult the complete guide for schematics, wiring, code and more:
Adafruit STEMMA Soil Sensor - I2C Capacitive Moisture Sensor