Theory:

The soil is formed in layers during the formation. Each layer of the soil is called a horizon. Soil Profile is the depiction of the vertical arrangement of these soil horizons below the surface of the earth. Each horizon runs parallel to each other and has distinct characteristics and properties. It extends from the top soil to the parent materials. It plays a vital role in maintaining the nutrients of the soil. There are five horizons represented using the letters O, A, E, B C and R.
 
Typical-Soil-Profile1.png
 
O horizon – upper layer of topsoil – Organic horizon consisting mainly decomposed plant and animal matters – darker in colour.
 
A Horizon – Top Soil layer – high humus content and minerals – Plant roots penetrate up to this layer.
 
E Horizon – consist of nutrients leached away from above O and A horizons.
 
B Horizon – Sub Soil Layer – found below the topsoil – comparatively hard and has less humus content.
 
C Horizon – layer of weathered bedrock and lies below the sub soil layer – made up of fragments of stones and rock.
 
R Horizon – unweathered parent material – compacted and cemented layer.
Reference:
https://www.researchgate.net/figure/Typical-Soil-Profile_fig3_334454668