CBSE Class 7 Science Chapter 9 Soil NCERT Solutions

Besides rock particles, soil contains essential components that support life and influence its properties. These include:

• Minerals: These are derived from the parent rock and provide essential nutrients.
• Organic Matter (Humus): This is decomposed dead plant and animal material, which enriches the soil and improves its structure.
• Air: Pores between soil particles hold air, which is vital for the respiration of soil organisms and plant roots.
• Water: Water is held in the pore spaces and is crucial for plant growth and soil processes.

Therefore, the correct answer is minerals, organic matter, air, and water.

The water-holding capacity of soil depends on the size of its particles and how tightly they are packed.

• Sandy soil has large particles with large spaces between them, allowing water to drain quickly.
• Loamy soil has a mixture of particle sizes, offering a balance between drainage and water retention.
• Clayey soil consists of very fine particles that are packed closely together, leaving very small pore spaces. This structure prevents water from draining easily, resulting in a high water-holding capacity.

Thus, clayey soil has the highest water-holding capacity.

Here is the correct matching of items from Column I to Column II:

1. A home for living organisms (b) All kinds of soil - Soil provides habitat for a vast number of living organisms, from microorganisms to larger animals.
2. Upper layer of the soil (c) Dark in colour - The topsoil is typically dark due to the presence of humus, which is rich in organic matter and nutrients.
3. Sandy soil (a) Large particles - Sandy soil is characterized by its large particle size, which leads to large pore spaces.
4. Middle layer of the soil (e) Lesser amount of humus - Below the topsoil, the subsoil generally contains less organic matter (humus) compared to the top layer.
5. Clayey soil (d) Small particles and packed tight - Clayey soil is composed of very fine particles that are tightly packed, resulting in small pore spaces and poor drainage.

Soil formation is a gradual process that begins with the breakdown of rocks. This breakdown is known as weathering. Weathering occurs due to the continuous action of natural forces such as:

• Wind: Wind can carry abrasive particles that wear down rock surfaces.
• Water: Flowing water, rain, and ice can physically break rocks apart and chemically alter them.
• Temperature Changes: Fluctuations in temperature can cause rocks to expand and contract, leading to cracks and eventual fragmentation.

This weathering process breaks down large rocks into smaller particles. Over long periods, these small rock particles mix with humus (decomposed organic matter from dead plants and animals), air, and water. This mixture of weathered rock particles and organic matter forms the soil that we see today. Soil formation is a very slow process, taking hundreds or even thousands of years to form just a few centimeters of topsoil.

Clayey soil is highly beneficial for certain types of crops due to its specific properties:

• Fertility: Clayey soil is typically rich in humus and other organic materials, making it very fertile. This high fertility provides essential nutrients for plant growth.
• Water Retention: The fine particles in clayey soil are packed closely, creating small pore spaces. This allows the soil to retain a significant amount of water, which is crucial for plants, especially during dry periods.

Because of these characteristics, clayey soil is particularly suitable for growing crops like paddy (rice), wheat, and gram. However, for crops like lentils (masoor) and other pulses, which require well-drained soil, loamy soil is preferred.

Clayey soil and sandy soil differ significantly in their particle size, structure, and properties, which affects their suitability for different purposes. Here are the key differences: