All the organic material
Everyone understands the segmentation of OM246 samples by depth. That is in the name of the test, after all.
Most people understand that OM246 testing is a mass loss on ignition of all the material. Measuring all the organic material is a key feature of this type of testing. That is what I explained in episode 5 of the Grammar of Greenkeeping show.
Measuring all the organic material is substantially different than measuring the soil organic matter. Remember, soil organic matter, by definition, does not include “undecayed plant and animal residues.”
What is thatch? From Beard’s Turfgrass Encyclopedia:
an intermingled organic layer of dead and living shoots, stems, and roots …
Thatch is undecayed shoots, stems, and roots.
Because much of the cultivation and sand topdressing work done to fine turfgrass surfaces is done for the purpose of managing the total organic material, measuring that accurately, by depth in the soil, and over time, turns out to be pretty useful.
I link to three related posts here with further explanation of this topic.