That portion of the A horizon of a soil profile which is composed of pure humus.
There is a clear change, however, in the upper AO horizon with a marked decrease in the content of the expandable smectitic clay, as well as a reduction in the amount of chlorite as indicated by the barely perceptible 13.
The same trends are found in this soil although it is notable that the expandable smectitic phase persists throughout the profile and into the AO horizon where it remains the dominant clay mineral.