The sample must represent its matrix so as to offer a complete frame of its qualitative status.

A soil sample to be analysed must guarantee:

- that the sample chemical composition has not been modified through heating, washing or contamination by the drilling tools;

- that the planimetric position and depth have been accurately detected;

- that the sample has been preserved correctly, up to the time of delivery to the lab.

However, given the soil uneven nature, it is difficult to extend the characteristics of a single sample to a large portion. Each sample represents exclusively the soil portion around the pick-up point whose size depends on many factors; for this reason, the sampling efficacy depends on the quantity of samples: the higher the number of samples, the higher its accuracy.