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Soil profile is the vertical section of the soil with different layers or horizons till the point where it reaches the bedrock. It is formed due to the compound effect of parent rock, weather elements, soil organisms, and time. The soil horizons can be differentiated due to the change in colour, size and properties of soil particles, along with the organic matter present in them. The soil profile is important for growing crops. Soils are classified on the basis of soil texture. Soil moisture can be measured with the help of a tensiometer, electrical resistance block, time-domain reflectometry. The loss of the top layer of soil is called soil erosion. It can be controlled by forestry programmes.
Read Also: Properties of Soil
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Key terms: Soil profile, parent rock, clayey soil, soil moisture, soil texture, soil erosion, tensiometer
Soil Profile
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- The topmost layer of the soil crust with mineral particles and organic matter is called the soil.
- The vertical section of the soil from the surface to the parent rock is called the soil profile.
- The soil profile consists of a number of horizontal layers called soil horizons.
- They are designated by the letters O, A, E, C, B and R horizons.
- The soil horizons can be differentiated due to the difference in colour, texture, structure, and organic matter.
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Soil Horizons
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The soil layers are called soil horizons. They are designated by letters O, A, E, C, B and R. These layers are arranged one above the other from R horizon to O horizon,
- O-Horizon
The topmost layer of the soil profile is o-horizon. It primarily consists of organic matter. The organic matter includes plant leaves, twigs, soil microorganisms, and macroorganisms. The black-brown or dark brown colour of the o-horizon is due to the presence of organic matter and humus.
- A-Horizon or Topsoil
The layer below the o-horizon in soil profile is A-horizon, It is also called the topsoil. It consists of well-decomposed organic matter called the humus. This layer consists of both micropores and macropores to hold sufficient air and water. It is a fertile layer. Germination of seeds and penetration of tender roots takes place in this soft soil.
- E-Horizon
The layer below the A-horizon is called the E-horizon or Elluviation layer. This is most abundant in fertile forest soil.
- B-Horizon or Subsoil
The layer below the E-horizon in soil profile is called the B-horizon, This layer consists of comparatively hard soil with less humus content and available minerals. It is rich in deposited salts and iron oxides. It holds more soil water and clay content.
- C-Horizon or Saprolite
The layer below the B-horizon is called the C-horizon. It is composed of well cemented broken pieces of bedrock.
- R-Horizon
The basalmost layer of the soil profile is the R-horizon. It is a well compacted and cemented layer. The bedrock is primarily composed of basalt, granite, and limestone.
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Factors Affecting Soil Profile
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Soil and soil profile are formed due to different factors like the nature of the parent rock, climate, time, topography and soil organisms.
- Parent Rock: The nature of parent materials directly influence the kind of soil, soil profile and minerals in them.
- Climate: The process of formation of soil from parent rock by climate is called weathering. It is influenced by weather elements like sunlight, temperature, rainfall, etc.
- Time: With the passage of time a lot of weather elements and organisms influence the different physical and chemical properties of the soil profiles.
- Topography: The surface properties of the soil like elevations and slopes affect the kind of the soil profile.
- Organisms: Microorganisms decompose dead and decaying plant parts and animals to add organic matter to the soil.
Soil consists of different components like mineral particles, soil air, soil water, and organic matter. The soil profile is also influenced by soil texture and soil moisture
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Soil Texture
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The relative size of soil particle is called soil texture. On the basis of soil texture, Soils in each part of soil profile are classified as:
- Clayey Soil: This kind of soil consists of a major proportion of clay particles. The finer soil particles are so closely packed that there are fewer gaps for air. It has good water holding capacity. This kind of soil is useful for growing cotton, Bengal gram, and wheat crops.
- Sandy Soil: This kind of soil has larger soil particles. The particles are loosely packed with more intercellular spaces and less water holding capacity. Carrot, radish, watermelon, etc. can be grown in such kinds of soils.
- Loamy Soil: This kind of soil has an equal proportion of sand, silt and clay particles. They have both air spaces and water holding capacity. They are the best kinds of soils for almost all kinds of agricultural crops.
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Soil Moisture
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The water content in the soil is called soil moisture. Soil moisture is due to precipitation and groundwater. Soil moisture is of different types like gravitational water, hygroscopic water, chemically combined water, and capillary water.
- Gravitational Water: The excess water that infiltrates down to the water table of the soil due to the gravitational force is called gravitational water. It is not available to the plants.
- Hygroscopic Water: A thin layer of water adsorbed onto the surface of the soil particles is called hygroscopic water. It is also not available to plants.
- Chemically Combined Water: The mineral compounds present in the soil particles hold some water. This is called chemically combined water. It is also not available to plants.
- Capillary Water: The water present in between the micropores of the soil particles is called capillary water. It is only the available water to plants.
Importance of Soil Moisture
- Soil Moisture is essential for the growth and development of plants.
- It is required for the dissolution and transport of soil nutrients to plants.
- Constant soil moisture improves crop yields.
- Soil temperature is regulated by the presence of an optimum quantity of soil moisture.
- Soil moisture is helpful in the formation of fertile soils.
- Soil moisture at various levels of the soil profile might differ accordingly.
- The beneficial microorganisms flourish well in the presence of soil moisture.
- All physiological activities of plants occur in the presence of water. So soil moisture is very important.
How to measure Soil Moisture?
The presence of soil moisture can be measured by using tensiometers, electrical resistance blocks, and time domain reflectometry.
Tensiometers: A tensiometer is a device used to measure soil water tension. It consists of a cylindrical pipe about an inch in diameter. A porous ceramic cup is attached to one end and a vacuum gauge is attached to the other end. It measures the soil moisture in negative pressure.
Electrical Resistance Blocks: The electrical resistance blocks consist of two electrodes linked by a lead wire. It is extended from the deeper layers of the soil to the surface. These blocks measure the tension of the soil water.
Time Domain Reflectometry (TDR): This device is used to measure the quantity of soil moisture. It consists of steel rods placed deep in the soil. The rods pass on electrical signals. Through these signals, we can measure the water content of the soil.
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What is Soil Erosion?
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- The loss of the topmost layer of soil is called soil erosion.
- The process of soil erosion is carried out by water, wind and ice.
- Bare soils without a proper vegetation cover and a network of roots are prone to soil erosion.
- Deforestation is one of the most important causes of soil erosion.
- As deforestation is the cause of soil erosion, programmes like afforestation, reforestation, and social forestry control soil erosion.
- A thick mat of grassroots prevents soil erosion to a great extent.
Things to Remember
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- Soil profile is the vertical section of the soil containing different layers.
- The topmost layer of the soil crust with mineral particles and organic matter is called the soil.
- Soil and soil profiles are formed due to different factors like the nature of the parent rock, climate, time, topography and soil organisms.
- The relative size of soil particles is called soil texture. On the basis of soil texture, soils are classified as sandy soils, clayey soils and loamy soils.
- The water in the soil is called soil moisture. It can be measured by tensiometer, electrical resistance block, time-domain reflectometry
- Soil moisture is of different types like gravitational water, hygroscopic water, chemically combined water, and capillary water.
- The loss of the top layer of soil is called soil erosion. It can be controlled by vegetation cover.
Read More: NCERT Solutions for Class 7 Chapter 9 Soil
Sample Questions
Ques. What is the difference between soil and soil profile? (2 Marks)
Ans. The fertile topmost layer of the earth's crust is called soil. The soil profile is the vertical section of the soil with different layers or horizons,
Ques. What are the different layers in the soil profile? How can they be demarcated? (3 Marks)
Ans. The soil horizons or layers are designated by letters O, A, E, C, B and R horizons. The soil horizons can be differentiated due to the difference in colour, texture, structure, and organic matter.
Ques. Describe the organic horizon in the soil profile? (2 Marks)
Ans. The topmost layer of the soil is the o-horizon. It primarily consists of organic matter. The organic matter includes plant leaves, twigs, soil microorganisms, and macroorganisms.
Ques. What is topsoil? Describe it in your own words. (2 Marks)
Ans. The A-horizon is also called the topsoil. It consists of well-decomposed organic matter called the humus. Germination of seeds and penetration of tender roots takes place in this soft soil.
Ques. Why is the E-horizon called so? Where is it present? (2 Marks)
Ans. The layer below the A-horizon is called the E-horizon. It is called so because the majority of salts are leached down from here. This is most abundant in fertile forest soil.
Ques. What is regolith? Which soil layer has regolith? (2 Marks)
Ans. The parent rock is called regolith. The basalmost layer of the soil profile is the R-horizon. It is a well compacted and cemented layer. The bedrock is primarily composed of basalt, granite, and limestone.
Ques. What is soil texture? (2 Marks)
Ans. The relative size of soil particles is called soil texture. On the basis of soil texture, soils are classified as sandy soils, clayey soils and loamy soils.
Ques. What is soil erosion? (2 Marks)
Ans. The process of loss of the top fertile layer of soil is called soil erosion. The agents that enhance soil erosion are wind, water, and ice. It is meant for loss of soil fertility. It is mostly due to the lack of vegetation on the soil.
Ques. What are the three devices to measure soil moisture? (5 Marks)
Ans. The presence of moisture in the soil can be measured by using three devices tensiometers, electrical resistance blocks, and time domain reflectometry.
- Tensiometers
A tensiometer is a device to measure soil water tension. It measures the soil moisture in negative pressure. It consists of a cylindrical pipe about an inch in diameter. A porous ceramic cup is attached to one end and a vacuum gauge is attached to the other end.
- Electrical Resistance Blocks
The electrical resistance blocks consist of two electrodes linked by a lead wire. These blocks measure the tension of the soil water. It is extended from the deeper layers of the soil to the surface.
- Time Domain Reflectometry (TDR)
This device is used to measure the quantity of soil moisture. It consists of steel rods to place deep in the soil. The rods pass on electrical signals. Through these signals, we can measure the water content of the soil.
Ques. What is soil moisture? What are its different forms? (5 Marks)
Ans. The water in the soil is called soil moisture. Soil moisture is of different types as follows:
- Gravitational Water
The excess water that infiltrates down to the water table of the soil due to the gravitational force is called gravitational water. It is not available to the plants.
- Hygroscopic Water
A thin layer of water adsorbed onto the surface of the soil particles is called hygroscopic water. It is also not available to plants.
- Chemically Combined Water
The mineral compounds present in the soil particles hold some water. This is called chemically combined water. It is also not available to plants.
Capillary Water
The water present in between the micropores of the soil particles is called capillary water. It is the only available water to plants.
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