Sunday, March 12, 2023

What is soil horizon?

The soil profile is composed of a series of horizons or layers of soil stacked one on top of the other. These layers or horizons are represented by letters O, A, E, C, B and R. Their physical, chemical and biological characteristics differ from the layers above and beneath.

Soil horizons differ from one another in composition (e.g. clay or organic matter content), physical properties (e.g. color or particle size), or chemical properties such as pH or CEC (Cation exchange capacity). 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.

The O Horizon
The O horizon is the upper layer of the topsoil which is mainly composed of organic materials such as dried leaves, grasses, dead leaves, small rocks, twigs, surface organisms, fallen trees, and other decomposed organic matter. The organic material has accumulated and been modified (physically and chemically) over time, typically from the remains of plant and animals. 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.

The A-Horizon or Topsoil
This layer is rich in organic material and is known as the humus layer. This is a much thicker layer than the O Horizon, dominated by highly weathered mineral particles. This layer consists of both organic matter and other decomposed materials.

The E-Horizon
The E in "E Horizon" stands for eluviation, another word for leaching. This layer is composed of nutrients leached from the O and A horizons.

The B-Horizon or Subsoil
Zone of accumulation of Fe, Al, organic matter, clay, salts, or carbonates; may also just show a pedogenic development by changes in color or structure from the parent material or C horizon. It is the subsurface horizon, present just below the topsoil and above the bedrock. It is comparatively harder and more compact than topsoil. Minerals in the B horizon may be undergoing transformations such as chemical alteration of clay structure.

The C-Horizon or Saprolite
This layer is devoid of any organic matter and is made up of broken bedrock. This layer may accumulate more soluble compounds like CaCO3. Low intensity processes, such as movement of soluble salts or oxidization and reduction of iron may occur.

The R-Horizon
It is a compacted and cemented layer. Different types of rocks such as granite, basalt and limestone are found here.
What is soil horizon?

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