If the soil is rocky, dry, or hard, Campbell Scientific recommends using a CS650G to make pilot holes before installing the sensor into the soil. When the soil is free of rocks, moderate to very wet, and relatively soft, then it is usually possible to install the sensor rods and keep them parallel by applying gentle and steady pressure on the sensor head.
Note: Never strike the sensor head with a hammer or step on the sensor head to force the rods into the soil.