O horizon is a main part of the soil. Could you please tell me why it is not mentioned in the textbook.


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).

Field criteria:

  • 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.

From Soil Taxonomy:
O horizons or layers: Layers dominated by organic materialSome are saturated with water for long periods, or were once saturated but are now artificially drained; others have never been saturated.

Some O layers consist of undecomposed or partially decomposed litter (such as leaves, needles, twigs, moss, and lichens) that has been deposited on the surface; they may be on top of either mineral or organic soils. Other O layers consist of organic material that was deposited under saturated conditions and has decomposed to varying stages. The mineral fraction of such material constitutes only a small percentage of its volume and generally much less than half of its weight. Some soils consist entirely of materials designated as O horizons or layers.

An O layer may be on the surface of a mineral soil, or at any depth below the surface if it is buried. A horizon formed by the illuviation of organic material into a mineral subsoil is not an O horizon, although some horizons that have formed in this manner contain considerable amounts of organic matter.

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