Soil Horizon layer of soils approximately parallel to the soil surface with characteristics produced by soil forming processes. One soil horizon is commonly differentiated from an adjacent one at least partly on the basis of characteristics that can be seen in the field. For convenience in study and description five master soil horizons are recognised. These are designated using the capital letters O, A, E, B and C. All soil profiles contain two or more master horizons. True soil or solum consists of A, E and B-horizons.
The A-horizon is the topmost mineral horizon. It contains a strong mixture of partially decomposed organic matter, which tends to impart a darker colour than that of the lower horizons. The maximum biological activity in the mineral portion of the soil occurs in the A-horizon. The E-horizon is that of maximum leaching or eluviation of clay, iron and aluminium oxides, which leaves a concentration of resistant minerals such as quartz in the sand and silt sizes. An E horizon is generally lighter in colour than the A-horizon and is found under the A-horizon. The original form of organic matter is unrecognisable for the most part.
The characteristics of the B-horizon are controlled by different sets of conditions. It is because of the fact that it lies below the surface horizon and is not in direct contact with the hydrosphere and atmosphere.
The C-horizon is the unconsolidated material underlying the solum. It may or may not be the same as the parent material from which the solum formed. This horizon is outside the zone of major biological activities and is generally little affected by the processes that formed the horizons above it.
Organic horizon (O-horizon) is not common in all soils. It is dominated by fresh or partly decomposed organic materials. These materials result from litter derived from dead plants and animals. Organic horizons commonly occur in forested areas and are generally absent in grassland regions.
The bangladesh soils are relatively young. Well-developed profiles with ABC horizons are found in the Sara, Gopalpur and Ishwardi series of the Gangetic Alluvium, and the Sonatala and Silmondi series of the Old Brahmaputra alluvium. Organic horizons are found in basin areas and one such soil having OABC horizons occurs in the Balina series. [Sirajul Hoque]