Measurement
Sampling plan
A sampling plan states, before any measurement happens, where samples will be taken, how many, at what depth or spacing, how often, and how zones were divided. It is what makes a later reading comparable to an earlier one.
Written afterwards, a sampling plan describes whatever was convenient. Written first, it is a commitment that the next round has to match.
The zones matter as much as the count. A block divided by slope, soil colour and previous use gives readings that mean something. A block sampled at random gives an average of a place that does not exist.