Natural Resources

What is Soil Profile and How is Soil Formed?

Learn About Soil Profile and Different layers of soil with its Formation.

What is soil profile?

The beauty is in the story of what the soil tells. So basically soil is the upper layer of earth in which plants grow, it is a mixture of substances, composed of minerals, decayed plant and animal matter, water and air that supports life on earth together.
The main objective is to provide you with information about the economic, cultural and environmental impact of soils in our lives. The aim is to raise awareness regarding soils.

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“Soil consists of 45% minerals, 25% air, 25% water, & 5% organic matter” Source – US EPA

It is our planet that is providing plants with the help of soil and nutrients which are needed to grow and a kind of interactive relationship with microbes and fungi which will help them to enter and take up nutrients.

WHAT IS SOIL PROFILE?

The soil profile can be defined as a vertical arrangement of the soil from the bottom surface downwardly to where the soil meets the underlying rock. we all know that the soil is found in layers and those layers are arranged during the formation of soil.

There are various layers in the soil which you can see in a soil pit or on a roadside cut. These layers are called soil horizons/layers and the arrangement of these horizons in soil or the vertical section of the soil is known as a soil profile.

A soil pit is a place where the secrets of the soil are hidden. You can also say it is the sequence of layers. One can easily observe the layers of soil by their color and size of particles.

“ One cup of soil may hold as many bacteria as there are people on earth, over 6 billion” – source – NRCS

There are three main layers of soil – topsoil, subsoil, and parent rock. Each layer has its own characteristics and these characteristics play a very significant role in determining the uses of the soil. It means each layer is of different color, texture and different chemical composition.

Do check more about Natural Resources.

Basically, the soil which has three layers is called mature soil. This kind of soil takes many years to develop those three layers. The soil which contains only two layers called immature soil.

So, Let’s discuss these horizons(often called as main layers of soil) in detail-

Main Horizons of the Soil-

Soil is made up of different layers that are often called horizons- these are main layers so don’t get confused when we will discuss different layers of soil in details later in this article-

Horizon A: Topsoil

  • This layer is made up of partially decomposed material like decayed plants and animal matter. That is why the colour of the topsoil is dark brown.
  • It is also called the humus layer. Humus is that component of soil that already consists of useful minerals that are useful for the growth of a plant.
  • Earthworms, millipedes, centipedes, fungi, and bacteria are the living organisms that are found in this layer of soil.

Increasing soil organic matter content and using nutrients wisely, keeping soil surface vegetated also promoting crop rotations and reducing overall erosion can lead to an average crop yield increase of 58% –  source – FAO –

Horizon B: Subsoil-

  • The next horizon below the topsoil is horizon B i.e. subsoil. This layer is more dense and harder than the topmost The subsoil is usually paler in color than the topsoil.
  • It has less humus in it.
  • This layer is rich in minerals brought down from the topsoil.
  • It contains iron oxide in a large amount alongside with metal salts.
  • Also, when the farmers plough their fields for growing any crop, they mix the topsoil and subsoil together to bring out the best.

Horizon C: Parent rock or Bedrock-

  • The parent rock is also called bedrock which consists of rocks and stones, so it is a very hard layer.
  • It has no organic matter.
  • This lowermost layer represents the transition zone between the earth’s parent rock, topsoil, and subsoil.

HOW IS SOIL FORMED?

Now, you have an idea about what is soil profile, I would like to tell you the fact that how soil is formed? i.e. it took hundreds of years to form just a 1cm layer of soil. The soil is formed by the mixture of minerals, organic matter, gases, liquids, and many organisms, air and water, etc.

“Natural processes can take more than Five Hundreds Years  just to form only 2 centimeters of topsoil”. – Source – European Soils Portal –

The process starts from the breaking down of big rocks into smaller rocks and then turn into fine particles by the continuous action of wind and rain. It is very easy to say but this process of formation of soil is very difficult.

These big rocks are mainly broken by two types of weathering: the first one is physical weathering and the second one is chemical weathering: –

  • Physical weatheringWhen the rocks and the other landscapes break down due to some other physical factors of the environment. Physical factors like sun, ice, wind, water and the temperature which varies from time to time. It doesn’t affect the chemical composition of rocks or minerals.
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How is soil formed with physical weathering

For example – the sea waves continuously hit the rocky shoreline and then the rock breaks slowly and turns into small pieces. Physical weathering is also known as mechanical weathering.

  • Chemical weathering – Prime cause here is Acid rain. When the changes occur in the chemical compositions of the rocks or minerals due to exposure to the environment will be called as chemical weathering.
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how is soil formed with chemical weathering

It includes chemical changes that may get decompose, dissolve or may get breaks down the various parts of rocks or other landscapes. These changes are the result of other chemicals and minerals that seep into rocks, usually as gases or in rainwater.

  • A number of natural forces work to break the parent rock into tiny particles of soil. This natural force includes wind, water, sun’s heat, plants, and animals, etc. then these pieces also break down to form sand and slit, finally into finer particles and the process continues till then these particles form the top layer of the soil.

“Trees that are mature can have as many as 5 million root tips” – Source – Soil Food Web

FIVE SOIL FORMATION FACTORS

As we are discussing what is soil profile we should also know about the five soil formation factors which are as follows-

  • CLIMATE- The climate is a very important factor in soil formation. It determines many things like, how much water will drain through different layers of the soil profile. Water is needed to convert the minerals into clay and pass on that clay to the lower layers. Moisture and temperature affect the soil. pH (potential of hydrogen) is highly influenced by climate.
Learn about – Climate change causes and effects here

“Soil needs 4,000 Gallons of water to produce one bushel of corn”– source – Audubon

  • ORGANISMS- Soil microorganisms and plants provide organic matter and transfer nutrients to parent materials. Every other group of organisms has different effects on them. Like for example trees in the forest have different root systems compare to grassland when it comes to producing organic matter.
  • RELIEF OR TOPOGRAPHY – it gets influenced where all other materials and water gather together. For example: if the water is running down the hill it is clear that the bottom of the hill will receive more water, so, parent material will have more leaching of water than the parent material on the top.
  • PARENT MATERIAL- Parent material is generally the underlying geological material where all the soil horizons/layers are formed. All parent materials are different when it comes to physical and chemical properties. For example- parent material like basalt lava much differs from the beach sand.

“Soil supports our planets biodiversity and they host a quarter of the total” source- FAO

    • TIME- A soil took many years to form. As time is the universal factor of everything, here also new soil takes time to form and give us the best of what we deserve.

DIFFERENT LAYERS OF SOIL

Besides the main horizons there are other layers of soil usually six layers which are- O, A, E, B, C, R: –

  • LAYER O (HUMUS)- The top organic layer which is made up of defoliation and hummus is known as O layer.
  • LAYER A (TOPSOIL)- The next layer below the O layer is called topsoil means layer A. It is the layer found between layer O and layer E. Seeds develop and the roots of the plant grow in this dark-colored It is a mixture of humus and mineral particles.
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source – pmfias.com
  • LAYER E (ELUVIATION LAYER)– This layer consists of sand and silt. When the process of eluviation occurs, the water drips through the soil causes loss of minerals and clay. This layer is light in colour and it is found below layer A and above layer.

“  1 Tablespoon of soil has more organisms in it than there are people on the earth” – Source- Audubon –

  • LAYER B (SUBSOIL)– This layer is known as subsoil. When water drips through the soil from the upper layers (layers above subsoil), it takes along minerals and clay with it like calcium carbonate, iron, and aluminum oxides, etc. in the subsoil.
  • LAYER C (REGOLITH)– This layer is the second last layer of the soil. The minerals and clay could not reach this layer. That is why it consists of slightly broken-up bedrock or parent rock. Very little organic material is found in this layer, so the roots of the plant do not penetrate it. This layer is also called regolith.
  • LAYER R (BEDROCK)- This layer is called the lowermost layer of the soil. So this is the layer that is made up of just rocks and stones. It is the hardest layer of the soil. It also has no organic material in it like layer C. One can only find the unweathered rocks in this layer.

Knowledge is important and soil layers and factors knowledge will help you all to have healthy conversations in-home, class and work areas. Feel free to write about your views and thoughts in the comment section.

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