Layers of Soil: Composition and Their Importance

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Soil covers the Earth’s surface, the topmost layer of the Earth’s crust on which plants grow. Soil as a natural body forms due to the complex interaction of materials, relief or topography, climate, time, and biological organisms. The main qualities of soil are its porosity, texture, degree of permeability, and time which vary with different layers of soil.

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Soil is mainly referred to as Earth or dirt. It comprises varying amounts of silt, sand, and clay soil. The proportion of these components determines if the soil is sandy, loamy, clay, or any combination of these. Soil texture has several implications for management because it affects the ability of the soil to hold water and withstand cultivation and compaction. 

What are Layers of Soil

Layers of soil are defined as the vertical section through the soil to underlying bedrocks. It shows a series of horizontal layers, with each layer showing different characteristics. These characteristics range from color to texture which varies across the layers. From the construction of buildings to various infrastructure projects, the features of soil sections are essential. The vertical soil layers show some variations due to duration and extent of weathering, geomorphic conditions, and strength of parent rocks. It also varies from place to place.

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These vertical layers show their source rocks and soil-forming processes in some soils. For example, if a massive pit (hole) of about 2 to 6m is dug vertically downwards into the ground, we can notice various layers of soil horizons. Looking at these layers from a distance gives a cross-sectional view of the ground (beneath the surface) and the soils and rocks that make up the soil profile. This cross-sectional view is called the layers of soil. It comprises layers, running parallel to the surface, called Soil Horizons.

Different Layers of Soil 

Each soil layer may be slightly or drastically different from the one above it or below it. Each layer also tells a tale about the composition, age, texture, and other characteristics of that layer. The layers have been classified into the topsoil, subsoil, and bedrock layers. There are four or five soil layers or horizons, depending on the soil formation. O, A, E, B, and C are the five soil horizons. R. designates the bedrock The horizons do not follow a set order. Some soil may contain all five horizons, while others may only include two.

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The A-horizon or topsoil contains humus or organic matter

The B-horizon or subsoil contains minerals leached from the topsoil and little organic matter

The C-horizon or weathered rock and lastly,

The D horizon or bedrock (subsoil, regolith, and bedrock).

However, there are more minor subdivisions in soil horizons. 

Steps Involved in Formation of Layers of Soil 

The formation of residual soil takes place over many years. The process involves mechanical and chemical weathering of solid rock that slowly changes into the soil. The development of residual soil generally undergoes the following steps.

  • Weathering from ice wedging or another physical process leads to the fracture of bedrock.
  • Oxygen, carbon dioxide, and water seep into the cracks, causing chemical weathering.
  • Plants, such as grass and lichens or grasses, become established and lead to biological weathering.
  • Weathered material collects until there is soil.
  • The soil develops soil horizons as each different layer becomes progressively altered. The most significant degree of weathering is in the topmost layer. Each successive lower layer gets changed slightly since water and air contact the soil is at the top.

The layer of soil plays a significant role in determining the use of the soil. Soil that has developed three layers is mature. The topsoil, subsoil, and the parent rock are the main layers of the soil. Each layer has its features.

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These characteristics of the layer of soil play a crucial role in determining the use of the soil. Soil that has developed three layers is mature. Soil is made up of distinct layers, known as horizons. Each layer has characteristics that make it different from all other layers. These characteristics play a vital role in what the soil is used for and why it is crucial.

Composition of Layers of Soil

Soil comprises of five distinct components, namely

  1. Organic soil
  2. Soil organisms
  3. Inorganic soil
  4. Soil water
  5. Soil air

Organic Soil

This soil comprises remains of decayed plants and animals that form humus soil and provide essential nutrients. Organic matter in the soil is suitable for promoting aeration, drainage, and soil moisture retention in the ground. The organic matter reduces erosion and supplies plant nutrients such as sulfur, phosphorous, and nitrogen to the plant’s root zone.

Inorganic Soil

It does not come from an animal or a plant, this type of soil forms from the most significant proportion of the solid materials which provide support and nutrient to plants.

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Soil Organisms

It includes earthworms, termites, ants, gophers, moles, millipedes, rodents, and bacterial, which are helpful to animals and plants. Many times, it can cause diseases to these animals and plants.

Soil Water

It contains 25% of the total volume of soil found between the pore spaces of the soil. Soil water has a significant role as it helps dissolve plant nutrients and aid in growth of the plant.

Soil Air

It is the content of air in the soil. It contains the same gases as in the atmosphere, but in different amounts, because the constituent parts of the soil modify it. Soil air has almost the same proportional volume as soil water, and it can be located between the soil particles. It is vital because it aids the respiration of soil organisms.

Five Factors of Layers of Soil Formation

The whole soil, from the surface to its lowest depths, develops naturally as a result of these five factors, namely

  •  parent material,
  •  relief or topography,
  •  organisms (including humans),
  • climate
  • Time.

From the surface to its lowest depths, the whole soil develops naturally as a result of these five factors.

Parent material

It is the base material from which soil is formed. The chemical composition and mineral materials are made up of quartz, giving rise to poor sandy soil.

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Biological organisms

Vegetation, animal inhabitants, and human populations can significantly affect the formation of soils. Roots open up the earth for more aeration. When the plants die, it adds minerals to the ground. The plant’s bodies are changed or decomposed in soils. Soil organisms such as Earthworms pass soil materials through their gullets. Consequently, as the material passes through the worm, they are ground and mixed up with the organic matter. This soil is deposited as cast on the surface.

On the other hand, humans are also part of the biological influx that affects soil formation. Human influence can be as severe as wholesale removal or burial (by urbanization) of an entire soil profile. It can be as subtle as a gradual modification of organic matter by agriculture or soil structure by irrigation.

Climatic weathering

Rocks exposed to the weather break down to form soils. Mechanical and chemical weathering processes are involved; hence, leaching affects soil formation.

Topography or Relief

Relief or topography affects soil formation. It is characterized by the terrain’s inclination (slope), elevation, and orientation. Additionally, the topography determines the rate of precipitation or runoff and the formation or erosion of the surface soil profile. Soils on hill tops (contour ploughing) are usually thin because of active soil erosion. The soil in the valley is thicker and heavier since the valley is more fertile for plant growth; the soil in the valley also receives eroded soil from the upland.


If a piece of rock is broken down and exposed to the factors above, changes can be seen over a long period, and the rock will eventually become soil. The states of the soil system vary with time, i.e., they are not stable. Some might take longer, while others may take a short time to break down.

Process of Formation of Layers of Soil

The process of soil formation involves changing soil parents materials into the soil by following a series of various steps. The method includes the operation of leaching, eluviation, illuviation, humification, and mineralization. The process is dependent on the climate. However, there are specific processes that are necessary for soil formation, and they include

  1. Humifications

    It is the breakdown of rotten organic waste to form humus. This process involves the decomposition or partial decomposition of organic matter animal and vegetation origin in the soil. This process plays an essential but complicated role in maintaining the fertility of the soil, and it is much more noticeable in wet and warm environments than it is in dry areas or cold regions.

  2. Mineralization

    This process involves the breakdown of organic waste into mineral matter content. The inorganic substance possesses specific chemical compositions and definite physical and chemical properties with the exemption of rocks. Sometimes, these chemical properties are broken down further into mineral or inorganic substances. A dead leaf is mineralized to form carbon dioxide. The mineralization process is vital because it sets free nutrients beneficial for the growing plants in the soil, which can be used again.

  3. Leaching

     This process involves the loss of nutrients from the soil caused by water flowing through it. The water flow deprives the soil of nutrients, and it may pollute watercourses. Leaching involves the removal of organic and mineral salt from the upper part to the lower layer of the soil by the percolating rainwater. On the decomposition of rocks, carbonates and salt are produced.

  4. Illuviation

    This process involves the movement of chemicals and particles from the topsoil into the subsoil. It involves leaching materials from the higher or upper layer of the soil and then depositing the materials leached down to the layer down in the same layer of soil.

  5. Eluviation

     It is the process by which fine particles are removed in suspension, such as clay from the soil. It is very similar to leaching, but the significant difference is that materials removed are solid particles.


The soil is the medium for growth of plants. Additionally. it makes nutrients, moisture, water and minerals available to the plants. It supports the roots of plants and helps them stay erect for growth. Without soil, there would be no plants. Consequently, without plants, there will be no animals and no life on earth.

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