The porocity and the density of the sample are key parameters for estimating the efficiency of the detection system using a specific extended geometry.
Activity concentration(Bq/Kg) in soil samples depend of the moisture percentage. More coveinent to estimate the efficiency of your detection system in the same matrix and geometry of the measured samples.
It is not necessary. It can be easier and more cost effective to calibrate the system with adequate density calibration standard. It all depends on what are you doing.