4.3 - Soil Profiles and Horizons
Through the interactions of these four soil processes, the soil constituents are reorganized into visibly, chemically, and/or physically distinct layers, referred to as horizons. There are five soil horizons: O, A, E, B, and C. (R is used to denote bedrock.)
There is no set order for these horizons within a soil. Some soil profiles have an A-C combination, some have an O-E-B, an O-A-B, or just an O. Some profiles may have all the horizons, O-A-E-B-C-R. And some profiles may have multiple varieties of one horizon, such as an A-B-E-B. There are some generalized concepts of how soil layers develop with time; these are expressed below, but due to the variability of natural processes over geologic time, generalized concepts are sometimes overly general. Knowing something about the geomorphic history of the area being investigated helps unlock the landscape history the soils show.
- A: An A horizon is a mineral horizon. This horizon always forms at the surface and is what many people refer to as topsoil. Natural events, such as flooding, volcanic eruptions, landslides, and dust deposition can bury an A horizon so that it is no longer found at the surface. A buried A horizon is a clear indication that soil and landscape processes have changed some time in the past. Compared to other mineral horizons (E, B, or C) in the soil profile, they are rich in organic matter, giving them a darker color. The A horizon, over time, is also a zone of loss – clays and easily dissolved compounds being leached out – and A horizons are typically more coarse (less clay) compared to underlying horizons (with the exception of an E horizon). Additions and losses are the dominant processes of A horizons.
- B: A B horizon is typically a mineral subsurface horizon and is a zone of accumulation, called illuviation. Materials that commonly accumulate are clay, soluble salts, and/or iron. Minerals in the B horizon may be undergoing transformations such as chemical alteration of clay structure. In human modified landscapes, processes such as erosion can sometimes strip away overlying horizons and leave a B horizon at the surface. Such erosion is common in sloping, agricultural landscapes. A bulldozer preparing land for a new subdivision can also leave a B horizon at the surface. The dominant processes in a B horizon are transformations and additions.
- C: A C horizon consists of parent material, such as glacial till or lake sediments that have little to no alteration due to the soil forming processes. Low intensity processes, such as movement of soluble salts or oxidazation and reduction of iron may occur. There are no dominant processes in the C horizon; minimal additions and losses of highly soluble material (e.g., salts) may occur.
- O: An O horizon has at least 20% organic matter by mass. Two main scenarios result in the formation of an O horizon: saturated, anaerobic conditions (wetlands) or high production of leaf litter in forested areas. Anaerobic conditions slow the decomposition process and allow organic material to accumulate. An O horizon can have various stages of decomposed organic matter: highly decomposed, sapric; moderately decomposed, hemic; and minimally decomposed, fibric. In a fibric O layer, plant matter is recognizable (e.g., it is possible to identify a leaf). Sapric material is broken down into much finer matter and is unrecognizable as a plant part. Hemic is in between sapric and fibric, with some barely recognizable plant material present. It is possible to have multiple O horizons stacked upon one another exhibiting different decomposition stages. Because of their organic content, these horizons are typically black or dark brown in color. The dominant processes of the O horizon are additions of organic matter, and transformations from fibric to sapric.
- E: The E horizon appears lighter in color than an associated A horizon (above) or B horizon (below). An E horizon has a lower clay content than an underlying B horizon, and often has a lower clay content than an overlying A horizon, if an A is present. E horizons are more common in forested areas because forests are in regions with higher precipitation and forest litter is acidic. However, landscape hydrology, such as perched water tables, can result in the formation of an E horizon in the lower precipitation grasslands, as seen in the profile below. The dominant processes of an E horizon are losses.
- R: An R layer is bedrock. When a soil has direct contact with bedrock, especially close to the soil surface, the bedrock becomes a variable when developing land use management plans and its presence is noted in the soil profile description.