The first requirement for all master horizons other than O horizons is that they are composed mostly of mineral material. It is very important to be able to differentiate organic from mineral material. Dark color of the A horizon is due to the mixing of humus with mineral. If the A is more than 2-3 inches thick, it has probably been plowed, look for an abrupt boundary such as the photo above. Some pedons in wooded areas have an A horizon that formed over an old Ap (most of the Northeast was farmed in the past).