A soil horizon is a layer that runs parallel to the soil surface and has different physical properties than the layers above and below it. There are normally three or four horizons in each soil type. Horizons are usually defined by visible physical characteristics, such as colour and texture.
These can be characterised in absolute terms (for example, particle size distribution for texture) as well as relative to the surrounding material, i.e. ‘coarser’ or’sandier’ than the horizons above and below. As water flows through the soil, it dissolves and eliminates nutrients.
Most soils, especially in temperate regions, follow a same basic pattern of horizons, which is typically depicted in diagrams as a ‘ideal’ soil. Each primary horizon is identified by a capital letter, which may be followed by a number of alphanumerical modifiers emphasising the horizon’s most notable qualities.
While the general O-A-B-C-R sequence appears to be universal, there is some diversity in classification systems around the world. Furthermore, the precise definition of each main horizon may vary slightly – for example, the US system includes horizon thickness as a distinguishing trait, whereas the Australian system does not.
It is important to note that no single system is more correct than the others — Their value as artificial structures stems from their capacity to consistently and accurately depict local situations. In addition, many subtropical and tropical places include soils like oxisols or aridisols that have substantially different horizons than a “ideal” soil, or none at all.
B: A B horizon is a zone of accumulation, also known as illuviation, and is often a mineral subsurface horizon. Clay, soluble salts, and/or iron are typical materials that accumulate. Minerals in the B horizon might be experiencing changes, such as chemical changes in clay structure. Erosion can occasionally tear away underlying horizons in human-modified environments, leaving a B horizon at the surface. Erosion like this is frequent in agricultural regions with sloping terrain. A B horizon can be left at the surface by a bulldozer preparing ground for a new development. Transformations and additions are the most common processes in a B horizon.
C: A C horizon is made up of parent materials like glacial till or lake sediments that haven’t been altered much by soil formation processes. Low-intensity activities such as soluble salt transport or iron oxidation and reduction may occur. In the C horizon, there are no dominating activities; however, there may be minor additions and losses of highly soluble material (e.g., salts).
O: Organic matter makes up at least 20% of the bulk of an O horizon. The establishment of an O horizon is caused by two primary scenarios: saturated, anaerobic conditions (wetlands) or excessive leaf litter generation in wooded regions.
Anaerobic conditions inhibit the breakdown of organic material and enable it to accumulate. Highly degraded organic materials (sapric), moderately decomposed organic matter (hemic), and slightly decomposed organic matter (fibric) can all be found in an O horizon.
Plant stuff can be seen in a fibric O layer (e.g., it is possible to identify a leaf). Sapric material has been broken down into much finer materials and can no longer be identified as a plant component. Hemic is in between sapric and fibric, with some barely discernible plant matter.
Multiple O horizons can be piled on top of one another, each with a distinct decomposition stage. These vistas are often black or dark brown in hue due to their organic composition. Organic matter additions and transformations from fibric to sapric are the most important processes in the O horizon.
E: The hue of the E horizon is lighter than that of the A horizon (above) or the B horizon (below). If an A horizon is present, an E horizon has a lower clay content than an underlying B horizon and frequently has a lower clay content than an overlaying A horizon. Because forests are in locations with greater precipitation and forest litter is acidic, E horizons are more abundant in forested areas. However, in the lower precipitation grasslands, landscape hydrology, such as perched water tables, might result in the creation of an E horizon, as seen in the profile below. Losses are the most important processes in an E horizon.
R: Bedrock is a R layer. When a soil comes into direct contact with bedrock, particularly near the surface, the bedrock becomes a variable in land use management plans, and its existence is mentioned in the soil profile description.