Layers of Soil: Definition, Description with Diagram (Soil Profile)

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Layers of soil or Soil profile is defined as the vertical section through the soil to underlying bedrocks. This shows series of horizontal layers, each of the layers shows different characteristic ranging from colour to texture totally different from the other layers. From construction of buildings to various infrastructure projects, characteristics of soil sections are very important.

The vertical layers of soils show some variations due to duration and extent of weathering, geomorphic conditions, and strength of parent rocks. It also varies from place to place. In some soils, these vertical layers show some evidences of their source rocks and the soil forming processes. For example, if we are to dig a massive pit (hole) of about 2 to 6m vertically downwards into the ground, we will notice various layers of soil horizons. A look at these layers from a distance, gives a cross-sectional view of the ground (beneath the surface) and the kind of soils and rocks that make up the soil profile. This cross sectional view is called as the Soil Profile. The profile is made up of layers, running parallel to the surface, called Soil Horizons.

Each soil layer may be slightly or very much different from the layer above it or the one below it. Each layer also tells a story about the composition, age, texture and other characteristics of that layer. The layers are divided into topsoil layer, sub-soil layer and the bedrock layers. There are four or five soil layers or horizons depending on where the soil is formed.

  1. The A-horizon or topsoil contain humus or organic matter
  2. The B-horizon or subsoil contains minerals leached from the topsoil and little organic matter
  3. The C-horizon or weathered rock and lastly,
  4. The D horizon or bedrock (subsoil, regolith and bedrock).

However, there are more minor subdivisions in soil horizons. Most of the soils have six major horizons labelled as O, A, E B, C, and R, but in some soil profiles, the letter E is not used in the zonation process.

Types of soil profile or layers of soil

A typical soil layer is summarized from top to the bottom as thus:

O-Horizon – this is an organic layer made up of partially decayed plant, surface organism, twigs and fallen trees. It has about 20% organic matter and it generally occurs in undisturbed soil, such as in a forest with lots of vegetative cover. This horizon is often black or dark brown in colour due to its organic content. The O horizon is thin in some soils, thick in others and may not present at all in other soil profile.

A-Horizon – the A-horizon is often referred to as the topsoil or root zone and is the surface layer where organic matter accumulates with maximum biological activities of plants microorganisms. Over time, this layer loses clay, iron, and other materials due to leaching. This layer of the soil profile is called zone of eluviation where minerals are leached down the profile from the upper horizons of soil by the downward movement of water. A horizons are made up of sand, silt and clay with high amounts of organic matter and most vulnerable to wind and water erosion, It is the top layer soils for many grasslands and agricultural lands.

E-Horizon: this is the zone of greatest eluviation because the clay, chemicals, and organic matter are leached from the O and A horizons. This zone is often rich in nutrients that are leached from the top A and O horizons. This horizon has a very light colour horizon with lower clay content. It is common in forested lands or areas with high quality O and A horizons with high amounts of rainfall; however, this is only present in older and well-developed soils and may not be present in some soils profile. It generally occur between the A and B horizons.

B-Horizon: this is often referred to as the subsoil; it has some similarities with the E horizon. It is often called the zone of maximum accumulation of substance such as iron, aluminum oxides as well as silicate leached from A and E horizon. It is also called the illuviation zone because of the accumulation of mineral particles, organic matter, chemical substances, in the lower horizons of soil from the upper horizons as a result of the downward movement of water. The B-horizon has less organic matter and more clay than the A-horizon. When you combine A, E, and B-horizons, they are referred to as the solum. This is where most of the plant roots exist.

C-Horizon- the C-horizon is called the substratum or zone of parent materials; it has a lot of weathered parent rocks and also represent the materials formed from the top soil and sub soil. The C-horizon lacks the properties of O, A and B- horizons because it is influenced less by the soil-forming processes.

R- Horizon: this layer is the underlying bedrock, such as limestone, sandstone, or granite. It is found beneath the C-horizon. R-horizon is the zone of unweathered rocks from which the rock fragments, which eventually disintegrate into soil particles are formed, it is commonly called the bedrock.

Soil profile diagram with descriptions
Soil profile diagram with descriptions

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