Dry soils are prone to present rocks underneath the surface. Therefore, it is advisable to dig a few centimeters and remove any rock found in the way before hammering the sensor down in the soil. Once this is done, make sure to fill in the area around the sensor soil again.
For multi-depth, it is advisable to use a drill in order to make a hole first and then place the sensor in it. Alternatively, it is possible to dig a bigger area and then cover it back with soil after placing the sensor. In this case, it is important to add water to the soil once it is filled up again.