O horizons: are soil layers with a high percentage of organic matter. Typically within a woodland area there are three distinct organic layers: one of leaves, pine needles and twigs (Oi); underlain by a partially decomposed layer (Oe);and then a very dark layer of well decomposed humus (Oa).
Greater than 20-30% organic matter (less if high clay content)
Dark or dark-reddish brown, nearly black, color - colors can be misleading and should only be used when other field criteria are observed.
Low strength, greasy feel, light weight when dry, may have a high fiber content.
Typically a very dark surface horizon. When observed buried beneath a mineral horizon, this may signify a disturbed site where the original soil was buried by fill material.