Soil: CBSE Class 7 Science NCERT Guide

What is Soil and How is it Formed?

Soil is the uppermost layer of the Earth's crust, a complex mixture of minerals, water, air, organic matter, and countless living organisms. It's much more than just rock particles! The formation of soil is a very slow, natural process called weathering. Weathering is the breaking down of huge rocks into tiny particles through the action of wind, water, sunlight, and temperature changes. This can take thousands of years! These tiny rock particles form the mineral part of the soil. But that's not all. The other crucial ingredient is humus. Humus is the rich, dark, organic material formed from the decay of dead plants and animals. It's what makes the soil fertile by providing essential nutrients for plants to grow. So, think of soil as a living recipe: start with weathered rock particles, mix in humus, add some water and air, and you get the substance that supports our planet.

The Soil Profile: A Journey into Its Layers

A-Horizon (Topsoil)

This is the uppermost layer. It's dark in colour because it is rich in humus and minerals. It's generally soft, porous, and can hold a lot of water, making it the best layer for plant growth.

B-Horizon (Subsoil)

Found just below the topsoil, this layer has a lesser amount of humus but is rich in minerals that have washed down from the A-horizon. It is usually harder and more compact.

C-Horizon (Parent Material)

This layer consists of small pieces of broken rocks, formed from the weathering of the parent rock below. It lacks any significant organic matter.

Bedrock

This is the bottom-most, unweathered, hard, and solid parent rock. It is very difficult to dig through. Water collects above it, forming the water table.

Comparing the Main Types of Soil

Worked Example: Calculating Percolation Rate

• Problem: A farmer, Ramesh, wants to test the soil in his field. He places a hollow pipe on the ground and pours 400 mL of water into it. He observes that it takes 20 minutes for all the water to seep into the soil. What is the percolation rate of the soil in his field? Solution: Step 1: Identify the given information. Amount of water = 400 mL Time taken for percolation = 20 minutes Step 2: Write down the formula for percolation rate. Percolation rate = Amount of water (mL) / Percolation time (min) Step 3: Substitute the values into the formula. Percolation rate = 400 mL / 20 min Step 4: Calculate the final answer. Percolation rate = 20 mL/min Conclusion: The percolation rate of the soil in Ramesh's field is 20 mL per minute.

Exam Tip: Understanding Soil Erosion and Conservation

Questions about soil erosion are frequently asked in exams. Remember that soil erosion is the removal of the fertile topsoil by natural forces like strong wind and flowing water. The main cause is the removal of plants and trees (deforestation) and overgrazing by animals. When you answer, clearly separate the causes (deforestation, overgrazing, floods, strong winds) from the prevention methods (afforestation/planting trees, creating shelterbelts, terrace farming on hills, and building dams). Explaining why planting trees helps (their roots bind the soil together) will show a deeper understanding and help you score better marks.

Practice Questions with Solutions

• Q: Why is loamy soil considered the best for growing crops? A: Step 1: Identify the components of loamy soil. Loamy soil is a mixture of sand, clay, and silt, along with humus. Step 2: Explain the benefits of each component. The sand particles provide good aeration and drainage, preventing waterlogging. The clay particles help in retaining water and nutrients. Step 3: Conclude with the overall benefit. This combination provides the right water holding capacity, adequate aeration, and a rich supply of nutrients from humus, creating the ideal environment for most plant roots to grow. Final answer: Loamy soil is best for crops because it has a balanced mixture of particles that provide good drainage, water retention, and a rich supply of nutrients.
• Q: A student draws a diagram of the soil profile. Which layer should be shown as the darkest in colour and why? A: Step 1: Recall the layers of the soil profile (A, B, C horizons and Bedrock). Step 2: Identify the composition of the top layer. The topmost layer is the A-horizon or topsoil. Step 3: Explain why it is dark. The topsoil is rich in humus, which is decomposed organic matter. Humus is dark brown or black in colour. Final answer: The A-horizon (topsoil) should be shown as the darkest layer because it contains the highest amount of dark-coloured humus.
• Q: In an experiment, it took 50 minutes for 500 mL of water to percolate through a soil sample. Calculate the rate of percolation. A: Step 1: Identify the given values. Amount of water = 500 mL. Time taken = 50 minutes. Step 2: State the formula for percolation rate. Percolation rate (mL/min) = Amount of water (mL) / Time taken (min). Step 3: Substitute the values and calculate. Percolation rate = 500 mL / 50 min = 10 mL/min. Final answer: The rate of percolation is 10 mL/min.
• Q: A large number of trees are cut down in a forest area near a river. What effect is this likely to have on the soil in that area? Explain. A: Step 1: Identify the immediate effect of cutting trees. Cutting down trees is called deforestation. Trees have roots that hold the soil particles together. Step 2: Explain the consequence of removing the trees. Without the tree roots to bind the soil, the topsoil becomes loose and exposed. Step 3: Describe the role of natural agents. When it rains or the wind blows strongly, the loose topsoil will be easily carried away. This process is called soil erosion. Final answer: Cutting down the trees will lead to increased soil erosion. The loose topsoil will be washed away by rainwater into the river or blown away by the wind, making the land less fertile.

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