There are a number of ways to measure soil moisture. The most basic is to measure the conductivity of the soil; water conducts electricity, so the wetter the soil, the lower the resistance. However, various things such as salts, acidity and even plants roots can affect the conductivity of the soil, so this method doesn’t always give good results.
An industrial method is to use tensiometers which measure suction pressure at the tip of the sensor, replicating how hard a root would have to work to get water from the soil around it. These work well where soil is fairly moist, but less so when the soil gets dry, and are very high maintenance. They are also outside the budget of the domestic smart gardener.
We use a method called capacitive soil moisture sensing, where an electric field is set up around the sensor to measure how the soil reacts to the field. It’s the same method that’s used by other leading smart watering products. While better than most methods, it’s still not perfect (none are) … getting the installation right is critical to getting good results.