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Soil formation depends on the new soil material that is obtained by deposition or denudation. Soil is one of the essential natural resources that are important for the growth of plants we feed on. Soil is responsible for holding plants firmly and providing them with nutrients. Many microorganisms such as rats, earthworms, and various other species live in the soil.
| Table of Content |
Key Terms: Soil, Minerals, Weathering, Decomposition, Organic Constituents, Humus, Denudation
What is Soil?
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Soil refers to the uppermost layer of the crust of the Earth. It is formed by the weathering of mountains continuously for over a thousand years.
- Soil is formed of 4 basic elements - minerals, water, air, and organic materials.
- The main components that are responsible for the texture of the soil are the clay, sand, and slit.
- The upper layer of the soil known as humus plays a vital role in the fertility of the soil.
- It is formed by the decomposition of organic constituents and leaves.
Formation of the Soil
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The process of soil formation depends upon denudation or deposition.
- Denudation refers to the erosion of the present rock material through agents such as ice, water, or wind.
- Deposition refers to the accumulation of materials that have been eroded from another place such as river gravels or the creation of new rocks due to volcanic action or the uplift of the ocean.
The formation of soils then starts from these materials through the process of weathering along with other physical and chemical processes.
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Weathering Processes for soil formation
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The different weathering processes for soil formation are -
Freezing and Melting:
The repetitive freezing and melting results in the formation of cracks and crevices in the rocks. In the presence of the sun, the rock surface expands. When it comes in contact with a water body, the pores of these rocks get filled with water. Water expands when it is frozen, therefore the particles are pushed further apart which breaks the rock down. When the ice again melts, the rock breaks into loose soil particles
Heating and Cooling:
In places with extreme climatic conditions, the rocks are subjected to sudden expansion and contraction. This results in the loosening of the soil particles and it increases the air content. With time, this effect reduces a solid rock to loose soil.
Wetting and Drying:
When the rocks are wet, they swell. When they are dry, they shrink. The regular wetting and drying of rocks loosen the grains of the rock.
Grinding or Rubbing:
As the waves of the sea pound the rocks along the seashore, the uppermost layer is eroded and it is fragmented into smaller rocks and then further into smaller particles.
Organisms:
Various organisms such as earthworms live within the soil. They produce nutrient-rich manure in the form of excreta. The movement of these organisms in the soil helps in the mixing and aeration of the soil.
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Process of Soil Formation
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The formation of soil occurs through the following processes -
- Accumulation of materials
Materials such as decomposing materials and organic matter that are deposited by the forces of water, ice, or wind are accumulated over time.
- The accumulation happens in the topmost layer of the soil.
- The organic materials accumulate in the poorly drained soil since the water logging prevents the same from being broken down by soil organisms or being oxidized.
- The materials accumulate in the well-drained soils when they are held by the root systems.
- The forces of water, wind, or ice contribute equally to the accumulation of new materials.
- Some plants fix atmospheric nitrogen and ammonia compounds into the soil with the help of bacteria as nitrates.
- Leaching and losses
The new particles such as organic matter, silt, clay, or other chemical compounds are leached and then eroded by plants.
- Therefore, the physical and chemical composition of the materials that are newly accumulated along with the parent material of the soil are changed.
- An example of the same is leaching out of magnesium, minerals, and some carbonates.
- Transformation and illuviation
Transformation refers to the chemical weathering of sand, slit, and formation of clay minerals. It results in the change of organic materials into decay organic matter.
- The soil takes shape through transformation.
- This improves the nutrient composition and natural drainage of the soil.
- Podsolisation and translocations
Podsolisation of the soil takes place when strong acidic solutions break down into clay minerals.
Parent Material
The mineral from which the formation of soil takes place is known as the parent material.
- All soil minerals are derived from rocks.
- The parent material is physically or chemically weathered and then transported.
- This material later deposits to form different layers of soil.
- The bedrock is usually the parent material but sometimes the soil gets transported due to factors such as the water and the wind.
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Weathering Agents
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The different weathering agents are -
Glacier
As the glaciers move from one place to another, the soil is pushed further with them. This drifted material is then deposited miles away from the place where it is formed. When the glaciers melt, a huge amount of soil is left behind, a part of which is carried by the stream to a different place.
Water
As the water flows, the particles of the soil are also transported along with it. The smallest particles travel a large distance while the heavier particles, such as rock and sand get settled earlier. Soil that is deposited along the banks of the river is known as alluvial soil. Alluvial soil is rich in mineral content. Rainfall is responsible for washing off the soil in the exposed lands.
A huge amount of soil is transported from one place to another through the air. The wind carries loose soils from one place to another.
Things to Remember
- Soil is an essential resource that provides plants with nutrition.
- Soil formation is also referred to as Pedogenesis.
- This is started by weathering and variations according to the weather conditions.
- Glaciers, Water, and Winds are the weathering agents.
- Processes such as freezing, heating, wetting, and grinding are responsible for weathering which leads to soil formation.
Sample Questions
Ques 1: What are the values for the ideal pH for the soil? (3 marks)
Ans: The ideal value of the pH level for the soil ranges from 6.0 and 7.0. The soil having pH levels from 3.5 to 6.0 are acidic and for the pH value of 7.0 and above, the nature is alkaline. The natural pH of the soil in the areas with higher rainfall ranges from 5 to 7 whereas in the driest areas the pH level of the soil is around 6.5 to 9.
Ques 2: List the effects of fertilizer on the soil. (5 marks)
Ans: The excessive use of fertilizers in the soil can lead to many problems such as:
- Soil Degradation
- Soil Compaction
- Nitrogen Leaching
- It reduces the content of organic matter in the soil
- The humus content is also affected
- Fertilizers can alter the pH level of the soil
- It can also increase the release of greenhouse gasses
- The minerals and nutrients of the soil are affected
Ques 3: For the formation of soil, what are the important factors? (3 marks)
Ans: The 5 most important factors that are responsible for the formation of the soil are:
Ques 4: The soil formation is a slow process. Explain. (2 marks)
Ans: Soil formation is a slow process. Although this process is continuous due to the gradual breakdown of the rocks because of weathering. Weathering can be a biological, chemical, or physical process. The rock material is transformed into the soil slowly but gradually over the years.
Ques 5: How can the soil be protected? (4 marks)
Ans: Protection can be done in various ways:
- Using Terrace Farming
- Planting a rain garden
- Restoring Wetlands
- Re-establishing Forest Covers
- Crop Rotation
- Residue Management
- Strip Cropping
Ques 6: Why should Soil be saved? (3 marks)
Ans: Soil is the basis for the sustenance of humans. It helps in regulating the climate and preserve clean water. The degradation of the soil can reduce agricultural yields and threaten the livelihood of the farmers. Healthy soil acts as a foundation and supports the plants, keeping them upright for their growth and supporting the roots. It also maintains aeration for the plants at adequate levels and provides oxygen to the plant roots, insects, and microbes.
Ques 7: Name 3 main nutrients present in the soil. (3 marks)
Ans: For the growth of plants, the soil is the major source of nutrients. These are as follows:
- Nitrogen (N)
- Potassium (K)
- Phosphorus (P)
Ques 8: Explain Soil Profile. (3 marks)
Ans: Soil formation is a slow process. It is the uppermost thin layer of the earth which is made up of fine rock material and organic matter which has been decayed.
Soil Profile:
- Humus: This is the uppermost layer of the topsoil, formed by the decomposition of animal and plant matter.
- Topsoil: It is the topmost layer of soil which is about top 5-10 inches. Most of the biological activity of the earth occurs here.
- Subsoil: It is a layer beneath the topsoil, and consists of silt, sand, and clay. It is made of soluble minerals and weathered rocks. The rainwater is collected here.
- Substratum: A layer of weathered rocks that is beneath the layer surface. This usually consists of the parent material of the soil.
- Bedrock: It is the bottom layer in the soil profile which is made up of unweathered parent rocks.
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