Soil Horizon Layers

Along with wind and water, soil is the major natural resource that supports life on earth. It is so important that a whole branch of study called soil science has developed around it. It is made of three components - minerals, organic matter and the living organisms that live in its upper layers. Soil is formed by the weathering action of natural elements like wind, water, glaciers and change in temperature. Soil can be formed from the rocks lying below or from rocks present somewhere far away. These agents of weathering progressively break rocks into finer grains that are laid in layers to form the soil.

What is Soil Horizon?

Soil is made of distinct layers that lie one above the other, parallel to the soil surface. Each distinct layer is called a soil horizon. A vertical cross-section of a soil known as the soil profile reveals the various horizons of the soil.

Each soil horizon is the result of a number of geological, chemical and biological processes that have been taking place for over thousands of years. Hence, the soil horizons are best formed and delineated from each other in older soils. The various soil horizons are identified on the basis of physical features, mainly their color, texture and particle size. Though the soil composition varies from place to place, most soils conform to a general pattern consisting of six horizons.

Soil Horizon Layers

The soil contains the following physically distinct horizons from top to bottom:

O Horizon
The letter 'O' stands for organic. As the name suggests, this horizon is rich in organic material of plant and animal origin. These materials are generally in various stages of decomposition. This decomposed organic material is called the humus that gives this horizon its characteristic dark color.

A Horizon
This is also known as the 'topsoil', and it is the topmost layer of the mineral soil. However, as it lies just below the O horizon, this layer also has some amount of humus in it. Hence, it is darker in color than the layers lying below it. This layer is also known as the 'biomantle' as it is the A horizon in which most of the biological activities take place. Soil organisms like earthworms, fungi and bacteria are mainly concentrated in this layer. The soil particles in this region are smallest and finest as compared to the lower horizons of the soil.

E Horizon
This layer lies below the A horizon and above the B horizon. It is light in color and contains mainly sand and silt. It is poor in mineral and clay content as these are lost to the lower layer by the process of leaching. Hence, this horizon is also called the layer of eluviation (leaching).The soil particles of this layer are larger in size than those in the A horizon but smaller than those in the underlying B horizon.

B Horizon
This is referred to as the 'subsoil'. This lies just below the E horizon and is rich in clay and minerals like iron or aluminum. Though this layer has a higher mineral content than the topsoil, some organic material may reach this layer from the layers above by the process of leaching. Plant roots may reach this layer. However, the B horizon is reddish or brownish due to the oxides of iron and clay.

C Horizon
This layer is also known as regolith. The C horizon is mainly made of large rocks or lumps of partially broken bedrock. This layer is least affected by weathering as it lies deep within the soil and is inaccessible to the soil-forming agents. Hence, the rocks in this layer have changed very little since their origin. Plant roots do not reach so deep down to this layer. The C horizon is typically devoid of organic matter.

R Horizon
This is the bedrock. It is the deepest soil horizon in the soil profile. Unlike the above layers, this horizon does not consist of rocks or even boulders. It is made of continuous mass of bedrock. Digging through this layer is very difficult.

Study of the soil horizon is the firs step towards soil taxonomy. These layers help us to understand the geological events of the past, and the properties of the soil in general.