Soil Layers: Definition, Levels, Types, Components & Horizon

Soil Layer – Soil is part of a layer of the atmosphere in the earth’s crust that is at the top or outermost position and is part of the life and habitat of organisms or microorganisms and is composed of various minerals, organic and other inorganic materials. The role of soil is very vital in supporting life on earth because it supports the availability of nutrients for plants to develop, and plants are the basis of the food chain.

So it can be said that soil is a starting point for the source of life for all creatures on earth, without soil plants cannot survive so that the food chain will never exist. Soil has a very special structure by forming voids which generally contain air and allow plants to breathe.


Definition of Soil Layer

The layers of the soil are layers made of layers and can be specifically distinguished chemically, geologically and biologically. When a soil is cut vertically from the side, the shape of the soil layers will be clearly visible because the levels or layers do have different characteristics.

Through the vertical side, you can see the stages of soil formation. It can be said that in each layer of soil it forms a period which in the top layer of soil becomes the end result of the soil formation process, while for the deepest layer of soil which is mostly hard rock, it becomes the beginning before the soil is formed.

Each of the soil types generally has three to four different layers, which can be grouped into color appearance, physical appearance, and soil texture. Through soil texture, it can be seen from the size of soil particles, whether it is clay, sandy, loamy, contains high organic content or is in the form of sediment.

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Soil level

Soil layers are generally divided into 4 levels.


1. Topsoil

Is a layer that is 30 cm deep, often referred to as the term Top Soil. In this layer there is a lot of organic matter, humus and also produces the most fertile layer so that it is very suitable for short root crops.

The easiest way to recognize topsoil is by its darker color compared to the layer below, it can be seen that it is looser and all microorganisms live in this layer. So that there is a possibility of a process of stem residue, weathering of leaves, and other parts of living things.


2. Middle Soil Layer

Right at the bottom layer after top soil with a thickness of 50 cm to 1 meter. It is brighter in color than the layer above and this layer is formed from a mixture of weathering in the lower layer with the rest of the top soil material that is carried by water, precipitates so that it becomes denser and is also often referred to as clay.


3. Subsoil

A layer that contains a lot of rock that has begun to decay and has been mixed with sedimentary soil with the layer above it. In this layer there are still many rocks that have not been weathered and also some that have been in the weathering process of the rock itself and have the same color as the constituent rocks. It is quite deep and rarely penetrated by tree or plant roots.


4. Bed Rock Layers

This layer is the deepest layer consisting of solid rock. The types of rocks in this layer have differences from one area to another so that the resulting product from the soil will also be different.

The rocks in this layer are easy to break, but very difficult for plant roots and water to pass through, the texture is light gray-white to reddish in color. This bedrock layer can easily be seen on the walls of the steep ravines of mountainous areas.


The type of soil type in the soil layer.

In the soil layer there are several types of soil, such as:


1. Alluvial Soil

A sedimentary soil formed from silt and fine sand that has undergone soil erosion. Very many are in the lowlands, around swamps, river mouths, valleys, as well as on the outskirts of large rivers. This soil contains a lot of sand and clay, does not contain many nutrients.

Its own characteristics have a gray color with a slightly detached texture and are sensitive to erosion. Fertility levels are reaching high depending on the mother and climate. In Indonesia, alluvial soil is good soil and has been used for seasonal to annual food crops (rice fields and crops).


2. Andosol soil

The word Andosol comes from Japanese, a combination of two words (An = Black; do = Soil), so Andosol itself means a type of black soil. According to soil science, this black soil is a volcanic soil originating from a volcano. The naming of andosol is different for each country, for example in Japan itself it is called Kurobokudo which changed its name to Ando soils since 1947 by United States experts.

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The definition of andosol soil according to the Center for Research and Development of Agricultural Land Resources is a soil that has a mollic A horizon or an umbric A horizon which is after the cambic B horizon which contains fine soil fraction and is also mostly composed of volcanic ash and pyroclastic material. other vitrix.


3. Entisol Soil

Entisol soil itself is a soil that is said to be very young, since the initial stages of its development. These soils are characterized by soil mineral matter that has not yet formed a clear pedogenic horizon.


These soils occur in layers in areas of deposition of new material, in areas where erosion or deposition is faster than soil development. Such as steep slope areas, floodplains and also dunes. The main criterion for the order Entisol is the absence of organization in the soil material. These soils show little development of structure or horizon and also resemble the material in a fresh sandbank.


4. Grumusol soil

This soil is a guideline of soil formed from limestone and volcanic tuffa which in general has an alkaline nature so that there is no organic activity in it. This causes the soil to be very poor in nutrients and other organic elements. The nature of lime itself is that it can absorb all the nutrients in the soil so that high lime levels can be toxic to plants.

This grumusol soil still has properties and characteristics like the parent rock. The weathering that will occur is only changing the physique and also the texture of elements such as Ca and Mg which were previously tightly bound and also tightly attached to the parent rock so that it becomes looser which can be influenced by external factors such as climate, weather, water and others. Sometimes in this grumusol soil there is a concretion of lime with soft lime elements and develops into a thick and hard layer.


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Soil Composition Components

The components in the soil provide nutrients for plants and other microorganisms in the soil so that they can grow and develop. The growth and development of organisms will also benefit the soil. So, there exists a beneficial relationship between the soil and also the organisms. As a place for the growth of various types of living things, soil is divided into several components, as follows.



Rock is a solid material that is made naturally and also consists of a mixture of minerals and other compounds with various compositions. The rocks on the earth’s surface come from volcanic magma that comes out until it hardens on the earth’s surface due to temperature changes. The rocks on the surface of the earth then experience weathering so that it can be caused by water, wind, corrosive substances.



Air is a free substance that we often encounter anywhere, as well as in the ground. Because air can occupy the same space as water, it can make up about 2% to 50% of the volume of soil. Air is essential for root and microbial respiration, which helps to support plant growth.

Carbon dioxide and also nitrogen are important for supporting the function of plants in the soil, for nitrogen-fixing bacteria. If the soil is waterlogged, it can prevent root gas exchange which can lead to plant death, which is a common problem after flooding.



Humus is a component that is often found in soil which lies at a level of about 1% to 5%. Humus itself comes from dead plants and animals. Thus, it has a high capacity to retain or provide essential elements as well as water for plant growth.



Water can make up about 2% to 50% of the volume of soil. Water plays an important role in transporting nutrients to plants and growing soil organisms as well as to facilitate biological and chemical decomposition.



Minerals are the largest component of the soil. Soil minerals are divided into two types of minerals. First, the primary minerals usually found in sand and lakes. These primary minerals are soil materials similar to the parent materials from which they were formed. They are often round or irregular in shape. Second, secondary minerals, the flip side of primary mineral weathering that releases important ions, form more stable mineral forms such as clay.



The last component of this soil layer is microorganisms. They are found in the subsoil in high numbers but account for less than 1% of the soil volume. A common estimate is that a single thimble filled with topsoil can accommodate over 20,000 microbial organisms.

The largest part of these organisms are nematodes, earthworms, and the smallest are actinomycetes, algae, bacteria and fungi. Microorganisms are the main decomposers of raw organic matter. Microorganisms consume water, organic matter, and air to recycle raw organic matter into humus, which is rich in available plant nutrients.


Soil Horizon

The description of the 4 layers of soil as previously discussed is based on appearances taken in general, and if explained in detail then each layer of soil is still divided into several parts which are called soil horizons and are arranged in a unit called the soil profile. Each soil itself is characterized again by a different arrangement of horizons, so in general a soil profile usually consists of several horizons which can be differentiated based on physical, color, chemical and other morphological properties.

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Soil horizons that have gone through this advanced development period usually have several types of horizons which are regrouped based on soil layers to prevent soil erosion. This solum is divided into two, namely the top layer and also the bottom layer. The top layer or top soil has two horizons, namely the O horizon and also the A horizon, the soil layer at the bottom also has two horizons, namely the E and B horizons. However, a soil profile with a complete arrangement has many horizons with unique or distinctive properties and characteristics.

In general, from the top to the bottom layer of soil consists of horizons O, A, E, B, C and R, for a complete explanation as follows:

1. Horizon O

This horizon is located at the very top of the soil layer, the soil layer that contains organic matter resulting from weathering and also only contains humus. This horizon can be found in natural forests that have not been disturbed by humans. This organic horizon is a soil that contains organic matter which is more than 20 percent of the total cross section of the soil.

The O horizon is further divided into two, namely the O1 horizon which is formed from plant remains that are still visible, such as fallen flowers and leaves or like tree branches, while the O2 horizon is below O1 which is made from the remains of plant parts that are no longer shaped. because it has undergone further weathering.

2. Horizon A

For Horizon A this is a mineral soil horizon that forms at the soil surface. For this horizon occurs when there is loss of most or all of the original rock structure in the soil and shows the nature of the accumulation of organic matter that has been mixed with minerals very intensively.

This A horizon consists of various topsoil, such as organic matter with a dark color mixed with mineral grains due to the effects of the activities of organisms. The finer particles will easily dissolve and be carried to the bottom layer.

3. Horizon E

For this type of layer is below the soil surface which does not have a large enough mineral content. Horizon E is often attached to the type of Horizon A with the aim of replacing the layer. To be a differentiator between the boundaries of the horizon below, that is, with a feature that is lighter in color than horizon B.

4. Horizon B

The process of the formation of this B horizon is below the A, E, O horizons which are already experiencing development. Most to all of the original rock structure is characterized as lost in this horizon. Then one or more soil properties will be seen. Such as alluvial soil types from silicates, humus, aluminum, iron compounds, as well as carbonates in combined and singular forms.

5. Horizon C

Horizon C is the parent material layer of the soil. The process of its creation was caused by a slight pedogenic process and does not have the characteristics of the O, A, E, and B horizons. It is located in the lowest soil layer consisting of weathered bedrock.

6. Horizon D or R

This D or R horizon has the most basic bedrock layers that are created from very dense rock. In this area, the rocks have not had time to experience weathering. The rocks in the D and R horizons consist of sandstone, basalt, granite, limestone, etc.


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Soil Horizon Benefits

For the purpose of grouping soil layers in a soil profile, there are many benefits of soil horizons. To know the nature of a soil requires observations on the vertical cross section of the soil.

To determine the completeness and distribution of the soil horizon, so as to determine the characteristics of the level of development and the age of the soil. The more complete and multiple horizons of a soil, the better and older the soil is.

To be able to find out the depth of top soil that needs to be done before planting short-rooted crops such as peanuts, pulses, soybeans and rice. So that it will be known which plants will be suitable in the state of a top soil.

Soil color can reflect aerobic as well as anaerobic conditions, which if the soil color is light indicates that the conditions are aerobic and gray in anaerobic conditions. In addition, the black color can also indicate the level of organic elements to the color of the soil which will determine the level of fertility.
Soil color can reflect aerobic as well as anaerobic conditions, which if the soil color is light indicates that the conditions are aerobic and gray in anaerobic conditions. In addition, the black color can also indicate the level of organic elements to the color of the soil which will determine the level of fertility.