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.
FAQs on Soil Horizon
1. What is a soil horizon?
A soil horizon is a distinct layer of soil that lies parallel to the ground surface. Each horizon has unique physical, chemical, and biological properties, such as colour, texture, and composition, that distinguish it from the layers above and below. These layers are formed over long periods through processes like weathering and the accumulation of organic matter.
2. What are the main soil horizons found in a typical soil profile?
A typical soil profile consists of several master horizons, designated by letters. The main ones are:
O Horizon (Organic): The topmost layer, composed mainly of humus and decaying plant and animal matter.
A Horizon (Topsoil): A dark-coloured layer rich in minerals and organic matter, where most biological activity occurs.
B Horizon (Subsoil): A layer with less organic matter but rich in minerals that have leached down from the layers above.
C Horizon (Parent Material): Made of large, weathered rock fragments from which the soil is formed.
R Horizon (Bedrock): The solid, unweathered rock layer that lies beneath all other horizons.
3. What are the basic components that make up soil?
Soil is a complex mixture of four basic components. Typically, it consists of about 45% minerals (weathered rock particles), 5% organic matter (decomposed plants and animals), 25% water, and 25% air. The varying proportions of these components determine the soil's texture, fertility, and ability to support plant life.
4. How are soil horizons formed?
Soil horizons are formed through a combination of processes known as soil formation or pedogenesis. It begins with the weathering of parent rock (C horizon). Over time, organic matter from plants and animals accumulates on the surface, creating the O and A horizons. Water seeping through the soil carries minerals and tiny particles downwards (a process called leaching and translocation), depositing them in the B horizon. These distinct layers develop over hundreds or thousands of years due to the interaction of climate, organisms, topography, parent material, and time.
5. Which soil horizon is most important for farming and why?
The A horizon, also known as topsoil, is the most important layer for farming and agriculture. This is because it has the highest concentration of organic matter (humus) and microorganisms. This combination makes the soil fertile, gives it a good structure for holding water and air, and provides the essential nutrients that plants need to grow. The roots of most crops are found in this layer.
6. What is the difference between topsoil (A horizon) and subsoil (B horizon)?
The primary difference between topsoil and subsoil lies in their composition and function. Topsoil (A horizon) is rich in organic matter, darker in colour, and full of life, making it the most fertile layer for plant growth. In contrast, subsoil (B horizon) is denser, lighter in colour, and has significantly less organic matter. It is mainly a zone of accumulation where minerals washed down from the topsoil are deposited.
7. Why don't all soils have the same horizons?
Not all soils have the same set of horizons because their formation is influenced by five key factors: climate, organisms, relief (topography), parent material, and time. For example, a soil in a desert will be very different from one in a rainforest. A young soil on a steep slope might only have a thin A horizon over a C horizon, while an old, mature soil on a flat plain might have well-developed O, A, B, and C horizons. The type of parent rock also plays a crucial role in determining the final soil profile.
8. What is the role of the C horizon, or parent material, in soil development?
The C horizon, or parent material, is the foundation of the entire soil profile. It is the partially weathered rock layer from which the upper soil horizons (A and B) are formed. Its chemical and mineral composition directly influences the texture and fertility of the soil that develops above it. For example, soil formed from limestone will be very different from soil formed from sandstone. The C horizon represents the starting point of soil's journey from rock to fertile ground.
