When sampling feedstocks you are attempting to obtain a representative sample of a given material. You will eventually secure a one-quart to one-gallon sample (check with your lab for their preference); however the pile you are sampling from could contain 10, 100 or 1000 (or more) yards of material. As a result, pulling a sample from one location in the pile will not provide you with information that represents all of the differences in the material throughout the pile.
Questions? Ask a technical specialiast: james@highfieldscomposting.org or alex@highfieldscomposting.org