SOIL HORIZON

The vertical section of soil consisting of various horizon sequences is termed as soil profile. Horizontal layers of soil in a profile more or less parallel to the surface are horizons. They can be described by their different morphological characteristics. Master horizons are used to describe similar appearing soil layers and are designated by capital letters. Subordinate layers or distinction within these master horizons are designated by lower case letters e.g. for subordinate distinctions of organic horizon.

Oi= organic horizon, slightly decomposed

Oe= intermediately decomposed organic layer

Oa= highly decomposed organic matter

1. O Horizon - The O horizon is a surface layer dominated by organic material in different stage of decomposition. This horizon is predominantly found in forested regions, the O horizon is composed of leaf litter in various stages of decay.

 2. A Horizon - The A horizon is the uppermost mineral layer. It may lie below the O horizon. An A horizon has a high concentration of humus and is not dominated by the migration of clay, humus, aluminum, or iron into or out of the horizon. The humus content gives it a darker color than the horizons below.

 3. E Horizon - The E horizon is a layer of eluviation where clay organic matter and iron and aluminum oxides have been leached out. Remaining material tends to be light colored and coarse textured. The E horizon is normally found below an O or an A horizon and above a B horizon.

 4. B Horizon - The B horizon is a subsurface layer showing evidence of one or more of the following processes:

 · Illuvial accumulation of alumino-silicate clay, iron, aluminum, gypsum, or silica;

· Carbonate removal; Residual concentration of sesquioxides;

 · Alteration that forms silicate clay or liberates oxides, or both, and that forms a granular, blocky, or prismatic structure with darker color developed due to accumulation of clay minerals

 5. C Horizon - The C horizon is a layer of minimal alteration. Material may be similar to or unlike that from which the other horizons formed. C horizons lack the properties of O, A, E or B horizons, and can include coprogenous earth (sedimentary peat), diatomaceous earth, saprolite, unconsolidated bedrock, and other uncemented geologic materials or materials soft enough for excavation with moderate difficulty.

6. R Layer - An R layer refers to hard bedrock. Material is cemented and manual excavation is impossible. Intrusive soils can be found in rare cracks in the bedrock. Examples of R layer material include: granite, basalt, quartzite, indurated limestone, or sandstone.