The Soil Horizon And It's Layers

What is soil horizon?

To know about soil horizon, first we should know about what is soil profile?

The soil profile is defined as a vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock. 

Now,

A soil horizon is a layer generally parallel to the soil crust, whose physical characteristics differ from the layers above and beneath.

Lets see the following picture:

Layers of soil

There are mainly 5 horizons in a soil. They are:

1.  O horizon: This is called humus layer whose depth is about 0-2 inch. This horizon is derived from dead plants and animal residues. It is dark in color and soft in texture.
2.  A horizon: This horizon is referred to as topsoil or biomantle horizon. This horizon is coarser in texture. It is found at the depth of 2-10 inch. Since it is the most biological productive layer it is called biomantle horizon. In this layer mineral matters get mixed with organic matter.
3.  E horizon: It is found at the depth of 10-15 inch. Eluviation of clay and oxides of iron and aluminium occurs in this layer. This horizon is generally lighter in color than A and B horizon.
4.  B horizon: In this horizon accumulation of clay and oxides from above horizon is occurred. It is found at the depth of about 10-30 inch.
5.  C horizon: This horizon is called regolith layer. This layer is made up of large rocks or lumps of partially broken bedrock. This horizon is devoid of organic matter due to being far down in the soil profile.

There is other horizon which is not soil but rock. This layer is called R layer. This layer is consolidated rock, with little evidence of weathering.

Reference:

Brady, N.C. and Weil, R.R., The nature and properties of soil. 13th Edition.