In soil science the various distinct strata within the soil are referred to as horizons. Some strata are very common, and are particularly useful in determining soil classification. It is important to keep in mind that a soil classification does not refer to 'sandy' or 'loamy', but refers to a layered sample involving multiple horizons, which may extend from a surface layer of compost all the way down to the bedrock, passing through any number of horizons along the way.

The O horizon is usually the topmost strata, being the layer of organic material actively undergoing decomposition. The next layer down is usually the A horizon, which is comprised primarily of organic matter that is not decomposing, and is more commonly called humus or topsoil.

In many ecosystems the O horizon is not very thick, although it is usually easily identifiable in forest ecosystems. There are a number of different classification systems used to describe the O horizon, but it is often subdivided into three distinct layers (these may be called either horizons or layers; I prefer the term layers because I think that it avoids confusion).

  • L layer: The uppermost layer is comprised of relatively undecomposed plant material such as leaf litter, pine needles, and small woody debris; the original structures are easily identifiable. In some classification systems this is not considered to be technically part of the soil.
  • F layer, or O1 horizon: The layer found beneath L characterized by an accumulation of partly decomposed organic matter. The original structures are generally identifiable.
  • H layer or O2 horizon: The lowest organic horizon, characterized by an accumulation of fully decomposed organic matter in which the organic structures are mostly indiscernible.

You may sometimes see a reference to a 'P horizon', which is exactly the same as an O horizon except that it formed in waterlogged conditions. The P in this case comes from 'peat'. The P horizon may be divided into P1 and P2 in the same way as the O Horizon. In addition to those listed above, there are a number of other 'suffix symbols' used to further sub-classify soil types, but these vary greatly from one country to another.

The O horizon is an important part of soil formation in many ecosystems, although there are obviously environments without an O horizon, as in many deserts, rocky mountainous ecosystems, and beaches. The O horizon also provides a habitat for bacteria, fungi, lichen, mosses, earthworms, insects, snails, small mammals, and hundreds of other types of organisms, which help support even higher trophic levels. This is also where the majority of decomposition takes place.

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