Soil Class 7 Chapter Notes | CBSE Science Revision

Key Points about Soil

• Soil is the uppermost layer of the Earth's crust, essential for life.
• It is formed by the weathering of rocks and decay of organic matter.
• Humus is the dark, nutrient-rich decomposed organic matter in soil.
• A soil profile is a vertical section through the soil, showing its different layers or horizons.
• The main soil horizons are A-horizon (topsoil), B-horizon (subsoil), and C-horizon (parent rock).
• Soil is classified into sandy, clayey, and loamy based on the proportion of particle sizes.
• Loamy soil is considered the best for plant growth due to its balanced composition of sand, silt, and clay.
• Percolation rate is the speed at which water passes through soil; it's highest in sandy soil and lowest in clayey soil.
• Soil erosion is the removal of topsoil by wind, water, or ice, leading to loss of fertility.
• Soil pollution occurs when harmful substances contaminate the soil, making it unfit for use.

Key Terms in Soil Study

Weathering

The process of breaking down of rocks into smaller particles by the action of wind, water, ice, and living organisms.

Humus

The dark-coloured, decomposed organic matter in the soil, formed from dead plants and animals. It makes the soil fertile.

Soil Profile

A vertical section through different layers of soil, showing distinct horizontal layers known as horizons.

Loam

A type of soil that is a mixture of sand, silt, and clay in roughly equal proportions, considered ideal for plant growth.

Percolation Rate

The rate at which water passes down through the soil. It is usually measured in ml/min.

Soil Erosion

The removal of the top, fertile layer of soil by agents like wind, water, and human activities.

Soil Pollution

The contamination of soil with harmful substances, often chemicals, pesticides, or industrial waste, making it unhealthy and unproductive.

Understanding Soil: Formation and Importance

Soil is one of the most vital natural resources on Earth, supporting almost all life. It's not just dirt; it's a complex mixture of rock particles, humus, water, air, and living organisms. The formation of soil is a very slow process, taking hundreds to thousands of years. It begins with the weathering of rocks, where large rocks break down into smaller pieces due to physical forces (like temperature changes, freezing water) and chemical reactions (like acid rain). These smaller rock particles then mix with humus, which is the dark, nutrient-rich organic matter formed from the decomposition of dead plants and animals by microorganisms. This humus is crucial as it improves soil fertility, water retention, and aeration.

The importance of soil cannot be overstated. Firstly, it provides anchorage and nutrients for plants, which form the base of most food chains. Without healthy soil, agriculture would be impossible, leading to food scarcity. Secondly, soil acts as a natural water filter, purifying water as it percolates through its layers before reaching groundwater reserves. It also plays a critical role in the water cycle, absorbing rainfall and preventing floods. Thirdly, soil is home to countless microorganisms and small animals, contributing to biodiversity and nutrient cycling. Earthworms, for example, aerate the soil and enrich it with their castings. Finally, soil helps regulate global climate by storing carbon. Therefore, understanding soil and protecting it from erosion and pollution is essential for environmental balance and human survival.

Soil Profile: The Layers Beneath

1. — Composed of freshly fallen leaves, twigs, and partially decomposed organic matter. Often thin or absent in cultivated soils.
2. — The uppermost layer, dark in colour due to rich humus and minerals. It is soft, porous, and holds more water, making it the most fertile layer for plant growth. Most organisms live here.
3. — Lies below the A-horizon. It is generally lighter in colour, contains less humus but more minerals like iron oxides and clay. It is harder and more compact than topsoil.
4. — Consists of small lumps of weathered rock with cracks and crevices. It lacks organic matter and lies above the unweathered bedrock.
5. — The unweathered solid rock layer beneath the C-horizon. It is the original source material from which the soil above is formed.

Comparing Types of Soil

Exam Focus: Soil Properties and Conservation

In your exams, pay close attention to the properties of different soil types (sandy, clayey, loamy) and how they relate to water retention and aeration. Be prepared to draw and label a soil profile and explain the characteristics of each horizon. Also, understand the causes and preventive measures for soil erosion and pollution. Questions often test your ability to differentiate between soil types or suggest ways to conserve soil.

Practice Questions with Solutions

• Q: What is humus and why is it important? A: Humus is the dark, decomposed organic matter in soil. It is important because it makes the soil fertile, improves its water-holding capacity, and enhances soil structure.
• Q: Which soil type has the highest percolation rate? A: Sandy soil has the highest percolation rate because of its large particle size and large spaces between particles.
• Q: Name the three main horizons of a soil profile. A: The three main horizons are A-horizon (topsoil), B-horizon (subsoil), and C-horizon (parent rock).
• Q: Why is loamy soil considered the best for plant growth? A: Loamy soil is considered best for plant growth because it has a balanced mixture of sand, silt, and clay, providing good water retention, proper drainage, and adequate aeration for roots.

Frequently Asked Questions