What is Soil Profile?

A Soil Profile is a vertical section of the soil that shows its various layers or horizons. Over time, as soil matures, layers (or horizons) form a soil profile. A model of soil profile explains the horizons in a better manner. These horizons, from top to bottom, are labeled as O, A, E, B, and C horizons. The most fertile part of the soil profile is typically the A horizon, also known as topsoil. The soil profile stretches out right from the soil surface to the parent rock material. In this section, soil profile examples and soil profile diagrams are given to explain soil profile in depth.

What is Soil Profile?

The soil profile is a vertical cross-sectional view of the soil strata present in a certain area. Naturally occurring strata that run parallel to the ground’s surface are known as soil horizons. On exposure, different soil horizons become visible. Every horizon differs from other horizons in physical or chemical properties. Usually, a soil profile is examined down to a depth of around five feet. The soil at the top of the cross-section is always dark, whereas the soil below seems to be lighter in colour.

Formation of Soil

Soils are porous natural bodies made up of organic and inorganic matter and are the structural and functional basis of terrestrial ecosystems. They are created by the interaction of the Earth’s crust with atmospheric, geological, climatic, and biotic elements. Soils develop in the form of layers. These layers are called soil horizons. Soil formation is an extremely slow process. Thin soil layers typically take thousands of years to form.

Soil formation occurs gradually but slowly by the weathering process, which breaks down rocks over time. There are three types of weathering processes:

• Physical weathering: It is the degradation of rocks brought on by mechanical action. Rocks can deteriorate due to variations in temperature, frost, or abrasion from rock-on-rock collisions.
• Chemical weathering: This is due to a modification in their chemical composition. When water, air, or other substances interact with the minerals in rocks, this can occur.
• Biological weathering: Animals that burrow allow air and water into the rock, and plant roots can grow into fissures in the rock to cause it to break.

Soil formation is further aided by the accumulation of particles caused by wind, water, and gravity. These processes have the potential to proceed very slowly—many tens of thousands of years.

Factors Affecting Soil Formation

Five main factors that affect the formation of soil and produce an infinite variety of soils are:

Parent Material

Minerals form the basis of soil which are produced from rocks (parent material) through the processes of weathering and natural erosion. Water, wind, temperature change, gravity, chemical interaction, living organisms, and pressure alterations all help break down parent material. The types of parent materials and the situations under which they break down affect the properties of the soil formed. For instance, soils formed from granite are usually sandy and infertile whereas basalt breaks down to form fertile and clay soils.

Climate

It greatly influences the rate of weathering and organic decomposition. With a colder and drier climate, these processes are slow whereas with heat and moisture, they are relatively quick

Topography

The grade, shape, and length of a slope affect drainage, erosion, and deposition. The aspect of a slope determines the type of vegetation and specifies the amount of rainfall received.

Living Organisms

Plants, animals, microscopic organisms (Bacteria or Fungi), and humans interact with soil in different ways and influence soil formation. As soil forms, plants begin to grow in it. The plants mature, die and new ones replace them. Animals eat plants and their wastes and finally their bodies are added to the soil. Bacteria, fungi, and worms break down plant litter and animal waste and retain organic matter (peat, humus, or charcoal).

Time

The amount of time it takes for the above four factors to interact with each other also influences soil properties It also depends on how long the soil has been weathered.

To know more, refer to the article What are the factors responsible for the formation of Soil?

Layers of Soil

The differences between each soil horizon layer and the layers above and below it may range from very little to significant. Every horizon reveals information about that layer’s age, composition, texture, and other characteristics. The layers are divided into three categories: topsoil, subsoil, and bedrock. Soil differentiation and land-use potential are determined by horizon features.

Soil Profile has five major horizons:

The majority of the soils have five major horizons designated by a capital letter in the order (from top to down): O, A, E, B, C, and R. Further A and B horizons are subdivided into micro-layers A1, A2, A3, B1, B2, and B3.

The Features of different Soil Horizons:

Horizon O

It is also known as the Organic matter horizon. It is a layer of fresh or partially decomposed plant and animal debris that has formed on the soil’s surface. It typically occurs in places with undisturbed soil, like forests. The organic content of this horizon causes it to appear dark brown or black in colour.

Horizon A

It is also known as the Topsoil or root zone. It is a humus-rich layer where nutrient, organic matter, and biological activity are highest (i.e., roots, earthworms, and micro-organisms are active. Because of the organic elements, it is typically darker than other horizons. This is the layer that is most vulnerable to wind and water erosion. Eluviation is a term given to the downward movement of water that carries organic materials, chemicals, and mineral particles from the higher horizons of soil to the lower horizons. Horizon A, often referred to as the horizon of eluviation, is the top layer of soil that is susceptible to leaching and from which certain soil elements have been extracted.

Horizon E

It is known as the Leaching Layer or zone of greatest eluviation. It is a thin layer between A and B horizons. This horizon typically appears in heavily precipitated sandy forest soils. Due to the leaching of organic waste, chemicals, and clay, the E horizon has a relatively light colour.

Horizon B

It is also known as the Subsoil. This layer is rich in clay and retains more moisture but is less fertile than topsoil. It has less biological activity and a lighter colour than A horizon. It’s the layer that contains large trees’ roots. Illuviation is the term used to describe the accumulation of organic matter, chemicals, and mineral particles from the higher horizons of soil into the lower ones due to the downward movement of water. Horizon B, also referred to as the horizon of illuviation, is the intermediate layer where the materials that leached from horizon A have been redeposited. Compared to the A horizon, the B horizon contains more clay and less organic materials.

The A, E, and B horizons collectively are referred to as the Solum. The majority of plant roots sprout in this area. The top two soil layers, known as the A and B horizons, are crucial for plant growth.

Horizon C

Also known as Substratum, this layer acts as the parent substance to create soil. It contains the weathering rock beneath which the horizons A and B form. Since it is less affected by the processes that generate the soil, it lacks the characteristics of the A and B layers. Because of its deep burial in the soil profile, it is devoid of organic materials.

Horizon R

Also known as Bedrock. It is the soil profile’s deepest soil horizon. This layer consists of granite, sandstone, or limestone. The colours match those of the region’s native rock.

Soil Profile Diagram

Below is a diagram of the layers of the Soil profile:

Importance of Studying Soil Profile

The study of the soil profile is crucial because it serves as a historical record of all the processes that have formed the soil and serves as the foundation for pedagogical research. The thin layer of material that covers the surface of the Earth is called soil, and it is created as rocks weather. It is mostly composed of organic and mineral-based components, water, air, and living things, all of which interact slowly yet continuously. The majority of plants are the primary source of food for people, animals, and birds and receive their nutrients from the soil. Therefore, soil is essential to the existence of the majority of land-dwelling organisms.

FAQ’s – Soil Profile

1. What makes up a Soil Profile?

A soil profile is a vertical section of the soil that shows its various layers or horizons. These horizons, from top to bottom, are labeled as O, A, E, B, and C horizons.

2. What is Soil Profile Affected by?

The soil profile is influenced by a variety of factors, both natural and human-induced. The key factors are Climate, Time, Topography, Parent Material and living organisms.

3. What is the pH Value of Soil?

The pH value of soil is a measure of its acidity or alkalinity. Values below 7 indicate acidic soils, and values above 7 indicate alkaline soils. The pH of soil can influence nutrient availability, microbial activity, and overall soil health.

4. Which Part of the Soil Profile is Fertile?

The most fertile part of the soil profile is typically the A horizon, also known as topsoil. The A horizon is the uppermost layer of soil.

5. What are the Colors of the Soil Profile?

The colors of the soil profile can vary widely. O Horizon is dark brown to black in colour, A Horizon is dark brown, E Horizon is grey or pale yellow, B Horizon has shades of red to grey and C Horizon has the colour of bedrock.