Soil Horizon

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What is Soil Horizon?

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Soil horizon can be defined as the parallel layer of the soil surface. Each layer has its own composition of physical, chemical and biological characteristics, they quite differ from each of the layers above and beneath each layer. Horizons have definite physical features such as the colour and texture of each layer of the soil. These soil horizons are described both in absolute terms like in terms of the particle size distribution for texture, and in terms, they are relatively ‘coarser’ or ‘sandier’ than all the soil horizons above and below.

In our prevailing section, we will plunge deeper into the Soil Horizon Definition in detail, know about each layer of soil and its benefits. 

Horizons of Soil

In this section, we will continue to explain the soil horizons layers. 

As studied vividly on the subject ‘the types of the soil’, we know there are varied types of soil, with each one having distinct characteristics. If anyone would dig down deep into any of the soils, one can see the soil is made of layers, or the horizons (O, A, E, B, C, R). Putting the horizons together, they will structure into a soil profile. Each of the profiles informs about the nature of the particular soil which has been dug deep. Majorly these soils have three major horizons that are - A, B, C and some have an organic horizon as well denoted by O. The horizons are:

1. O Horizon Which Contains Hummus or any Organic Matter: 

This layer is mostly filled with organic content like the decomposing leaves. The O horizon is particularly thinner in most soils, while thicker in others, also the horizon may not be present at all in other types of soil.

2. A Which is the Topsoil: 

Minerals are present in this layer which are generated from the parent material with the organic matter that is being incorporated. This layer serves as a good material for the plants and other organisms to live.

3. E is Known as the Eluviated Layer: 

Here clay, minerals, and other organic matter, with a concentration of sand and silt particles of quartz are present. They are mainly found in older soils and forest soils.

4. B the Subsoil Layer:

They are quite rich in minerals, the mineral seeps down from the A or E horizons and gets accumulated in this layer.

5. C the Parent Material: 

The deposit is present at the Earth’s surface, this is the place from which actually the soil is originated. 

6. R, the Bedrock Layer: 

This layer has a mass of rock like granite, basalt, quartzite, limestone and even sandstone, this layer forms the parent material for some soil.

Thus, clearly, the Soil Horizons are explained. In the next section, we will take up each of the layers in the soil profile and determine the quality of the soil that influenced such a layer. 

Soil Profile Horizons 

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The Different Layers of Soil have different functions to do. While the soil profile is quite defined as the vertical section of the soil which forms the ground surface, this seeps downwards to where the soil meets the underlying bedrocks.

The soil is arranged in layers or in horizons. They are arranged during their formation. The layers or the horizons are basically known as the soil profile. The vertical section of the soil which is exposed by a soil pit is called the soil horizon profile. The layers of soil are easily identified by their colour and texture. In presence, there are soil particles as well. The different layers of soil are as follows:

  • The Topsoil

  • The middle is the Subsoil

  • Then, comes the Parent rock

Each of the soil layers has its respective characteristics.

Layers of Soil

The soil profile consists of the series of the horizon of soil which is layered on to one another. The horizons are represented by the letters O, A, E, C, B and R, also refer to the diagram displayed above.

The O-Horizon

The uppermost layer of the topsoil is composed of organic materials like dried leaves, grasses, and other decomposed organic matter. This layer of soil is blackish brown or dark brown in colour, the colour is for the content of organic material.  

The A-Horizon is also known as the Topsoil

This layer is also rich in organic material and commonly known as the humus layer. This majorly consists of both organic matter and other decomposed type materials. The topsoil is very soft and is thus porous in nature to hold enough air and water.

The E-Horizon

This layer has nutrients seeped down from the O and A horizons. This layer is very common in forested areas which have low clayey content.

The B-Horizon or Subsoil

This horizon is present just below the topsoil, while above the bedrock. This is comparatively much harder and more compact than the topsoil. This contains less humus, soluble minerals, and organic matter. Rather this is a site of deposition of certain minerals and metal salts like iron oxide.

The C-Horizon, Known as the Saprolite

This layer has an absence of any organic matter and is made up of broken bedrock. 

The R-Horizon

They are the compacted and cemented layer with different types of rocks like granite, basalt and limestone.

FAQ (Frequently Asked Questions)

1. Which Layer of Soil is the Best For the Cultivation of Crops?

Ans. Topsoil is best for crop cultivation. Topsoil that is the A horizon is the layer of soil that is located just below the O Horizon. This layer consists of minerals and decomposed organic matter. Due to this, the soil is very dark in colour. This layer helps many plants to grow their roots firmly. The topsoil is the topmost layer of the soil which is usually 5 to 10 inches (that is 13–25 cm). This layer has the highest concentration of organic matter and microorganisms and thus it is most suitable for crops to grow. This soil layer is where most of the Earth's biological soil activity takes place. Topsoil is made up of mineral particles and organic matter, water, and air.

2. How is the Soil Horizon Formed?

Ans. Soils Developed as a result of the interactions between the climate, living organisms, and the landscape position. The soil horizon was influenced by the parent material that is the decomposition over time. The four major soil formation processes change the parent material into soil form and develop the soil horizons. 

3. What are the Basic Components Which Make Up the Soil?

Ans. The basic components which make up the soil are its minerals, organic matter, water and also air. Typically, the soil consists of moreover 45% of mineral, 5% of organic matter, 20-30% water, and the rest 20-30% of air content in it.