SOIL HORIZONS AND THEIR CHARACTERISTICS - GEOGRAPHY - 2025

The horizons of soil are layers present within each type of ground floor, which have different characteristics even though they are contiguous. These characteristics make each one have a particular color, so a clear division is created between the two layers, generating a visual division from one to the other.

These divisions on the ground can be seen in two different ways. The first is visually - it's easy to see the difference with the naked eye. The second is in regards to its structure, since each type of horizon has a different composition and texture according to the material that makes it up.

There is a general classification for horizons, where each letter represents a different horizon. This general classification consists of five types of horizon: O, A, B, C and R. The system has some changes depending on the region under study, but in general terms it is used to classify all soil horizons in the world.

Soil horizons and their characteristics

It is important to differentiate a soil horizon from each layer that makes up the planet. Soil horizons are present only in the soil; Each type of soil in the world has a different division of horizons, but each region usually presents a specific pattern.

For example, in tropical regions around the world the soil is divided in very similar ways and the same measurement systems can be used to study this type of soil.

In turn, the reason why these horizons exist is due to the presence of different types of color, texture, consistency and structure of the soil. The deeper the part of the ground that is being studied, the more each characteristic of the horizon in question varies.

ID

Soil horizons can be identified according to a number of characteristic factors. These horizons lie parallel to the soil surface, and are specifically divisions found within each soil profile.

Each soil profile studied is divided by several horizons, which present different characteristics from each other. Each type of horizon is assigned a letter to be able to identify it when conducting a study.

For example, the topmost layer of the horizon is called the "O horizon." In this case, as this part of the soil refers to organic materials such as leaves, the letter represents the word “organic”. Each letter in the OABCR classification represents a distinctive feature of each horizon.

Types

While there are mainly five specific types of horizons represented in the OABCR classification, in some cases additional letters are used to describe other horizons. Some of these horizons are not very common, but they are present in specific regions and cannot be ignored from the classification.

Horizon O

The O horizon is the most superficial layer of the soil, which can always be seen with the naked eye without the need to excavate or extract soil. This layer consists of all the organic material found on the soil surface, such as leaves and peat.

Some O horizons have been saturated with water for long periods of their existence (which occurs in the soil of ancient lakes that are already dry). Other O horizons are still saturated with water today, such as the bottom of the world's lakes.

These horizons are characterized by being formed of organic material that is not completely decomposed.

Horizon P

A P horizon has, like O, an organic composition. However, these types of horizons exist only in regions that have been exposed to floods at some point in their existence. A P horizon can be classified as a subdivision of O, but it is not always present in all types of soil profiles.

In this case, the P refers to peats, the Anglo-Saxon word for "peat." Peat is an organic plant material, rich in carbon and with a fairly spongy composition. It is the material used in the development of organic components; it is present in the P horizons with abundance.

Horizon A

The A horizon is the one that is present just below the O horizon. It is composed of all kinds of mineral substances.

Usually the layer is formed by the presence of rocky material, but not in its original structural form, but destroyed. For this reason, minerals are usually accompanied by an organic layer that keeps them fixed to the soil.

The organic material present in this horizon is not dominant in the area, as it is in horizons B and C; however, it does exist in abundance in conjunction with mineral material.

In many cases the characteristics of this organic material are the product of cultivation, grazing or other types of alterations that occur in the soil.

Horizon E

An E horizon is usually made up of silicates, a type of salt made up of oxygen and silicone. In these horizons the mineral and organic matter is almost completely "eluted", a word that represents the initial of the horizon.

These types of layers are usually present in old soils, which have been affected by the passage of time. The E horizons are created between the A and B horizons.

In many types of soils (particularly those with the presence of animals), the E horizons present a layer of stone at the base that separates it from the B horizon.

These horizons usually have a great loss of minerals such as clay, iron or aluminum, which simply leaves behind a layer of salt and silicone that is low in organic or mineral content.

Horizon B

These types of horizons contain within themselves the material that makes up the soil. It is common to refer to these horizons as subsoils, as they have a high concentration of material and organic that accumulates in this layer as a result of leaching (seepage).

These types of horizons commonly have a large amount of clay, iron, aluminum, humus, or silicone. These can be present individually or collectively (that is, a grouping of two or more of these minerals can exist in a B horizon).

These horizons also often show signs of a lack of carbonates and oxides. This makes the consistency of this horizon very different from that of the other contiguous horizons. This difference is clearly noticeable with the naked eye, since the colors of a B horizon are very different from those of an E horizon.

Horizon B is usually the last horizon that plant roots reach; below this horizon there is no organic plant material on the surface.

Even so, there is little organic content in this horizon because the roots that reach the B horizon are accompanied by a large amount of other minerals.

Horizon C

This horizon is the one that is located directly below horizon B. It is characterized by the lack of mineral and organic properties of the rest of the upper horizons, and few soil movement processes affect its characteristics.

All this means that the human or animal movement that occurs in the O horizon causes very little change to the C horizon. This means that there is a large amount of rocks with little erosion as a consequence of the lack of external movement. The rest of the horizons present rock fragments, while the C contains larger rocks.

This layer forms over time, when the base rock of the soil deteriorates and fragments, creating large stones that rise to the upper horizon. The C horizon is one of the deepest in the classification and does not present organic matter in its composition.

Horizon R

The R horizon is contiguous to the C horizon and is characterized by being mostly made up of a single rock layer that is not divided. This rock is the so-called bedrock, which supports all the upper layers and is not easily eroded.

When this bedrock is broken, its fragments become part of the C horizon. The bedrock is so solid that it cannot be excavated without using specialized machinery.

Horizon L

This type of horizon only exists in areas where there has been, at some point, a layer of water on the surface. They are created as a result of water seepage over time.

It is made up of remains of sedimented peat and loam. It is not very common and is not found in all soil profiles on the planet.

Other classifications

It is possible to find different classifications of horizons when a horizon presents characteristics of two horizons at the same time. For example, when an A horizon exhibits characteristics of a B horizon in combination with its own, that horizon is often referred to as the “AB horizon”.

The description systems for each horizon and the number of letters used vary according to the country and the study group that uses it.

References

1. Soil Secret Agents, University of Sheffield, (nd). Taken from Sheffield.ac.uk
2. Soil Horizons, New Zealand Soils, (nd). Taken from nzsoils.org.nz
3. Horizons; PR Owens, EM Rutledge in Encyclopedia of Soils in the Environment, 2005. Taken from sciencedirect.com
4. How to Recognize Soil Horizons, New Zealand Soils, (nd). Taken from nzsoils.org.nz
5. Soil Horizons, Forest Floor Soil Web, (nd). Taken from soilweb.ca