Soils are composed of layers called horizons. The actual geological material each horizon is composed may vary due to deposition of materials by wind, water and other processes. The horizons are defined by both a number code and a letter code. The following letter are used to define a soil O, A, E, B, C, and R.
The number code relates to the number of that specific horizons that occurs in a given soil. Additionally lowercase letters are used to describe a specific characteristic of that horizon such as an accumulation of clays. Soils will vary due to enviromental conditions and geolological materials in how deep each horizon is and whether each horizon is present in a given soil.
The O horizon is the zone where the organic materr accumulates. This horizon can be subdivided into three specific horizons. The Oi horizon is where you can tell what the specific material was (fibric). The Oe horizon is where decompostion has started to occur (humic). You will be able to determine that the material was from plants but not from what specific plants the material is compose of. The Oa horizon is where the material has decompose to a state that the origin of the material can no longer be determined (sapric).
The A horizon is the first or topmost mineral horizon in the soil. It has This horizon is where humus accumulates so it generally has the highest organic matter content of any of the horizons. It is also a zone of leaching where water carries dissolved and suspended materials down into lower horizons. This is where the highest amount of biological activity occurs and also is where roots of most plants are most present.
The E horizon which stands for eluvial is where significant amounts of clays, iron, and other constituents of the soil have leached out. This layer when it occurs will generally underlie the A horizons. The material that is left is generally resistant materials of silt and sand size particles. It is lighter in color than the A horizon due to lower organic matter content.
The B horizon is the zone of illuviation or where materials accumulate. This mineral horizon will underlie the above horizons. In hunid climates, this is where the clays and iron and aluminum oxides leached from the above horizons tend to accumulate. In more arid climates, this where calcium carbonate, calcium sulfate, and other salts accumulate.
The C horizon is where the unconsolidated rock materials occur. This horizon is largely uneffect by the biological activity, weathering and other processes which occur in the upper horizons. It may or may not be composed of the same geological materials that the upper horizons.
The R horizon is the consolidated hard rock that underlies the other horizons. There is generally little evidence of weathering in this horizon. Besides, the distinctively master horizons, there are horizon that can occur which are transition layers between the different horizons. They are given two alphabet letter to describe this transition layer with the first letter being which horizon dominates. For example an AB horizon would be a transition layer where the characteristics of A dominate this horizon. A slash (/) is used when both horizons equally or which horizon dominates varies within the transition layer.
The following subcase letters may be used to describe a distinct horizon within the soil.