Spruce Sensors are optimized to be fully inserted into the soil. However they will still work as long as they are at least about half-way in, but will report somewhat lower moisture levels.
If you are placing the Spruce Sensor into a lawn that will be mowed, you should insert the sensor fully into the soil so that the mower blades can pass over the sensor without impact.
In this case, you may need to adjust the setpoint for moisture in the Smartapp to adjust for the lower moisture levels – otherwise, the Spruce system may overwater.