Soil — Composition & Types

• Soil profile — A, B, C horizons; topsoil, subsoil, bedrock.
• Components of soil: mineral particles, water, air, humus, living organisms.
• Types of soil: sandy, clayey, loamy — properties and uses.
• Soil erosion: causes (wind, water, deforestation) and prevention.
• Percolation rate — how fast water drains through soil types.

Soil profile (cross-section from top to bottom)

Soil types and properties

Loamy soil is best for plants — balanced water retention + air + nutrients.

Humus

• Decayed plant and animal matter in topsoil.
• Dark in colour; makes soil fertile.
• Improves water retention AND aeration simultaneously.
• Earthworms, bacteria produce humus by decomposition.

Soil erosion

Causes:

• Wind — dry, loose soil blown away (desertification risk).
• Water (rain/rivers) — topsoil washed into rivers; rivers carry it as silt.
• Deforestation — tree roots no longer bind soil; heavy rain washes topsoil.

Prevention:

• Planting trees and grass (afforestation, contour ploughing).
• Terrace farming on hills reduces water runoff speed.
• Windbreaks (rows of trees) reduce wind erosion.

Percolation rate

Sandy soil > Loamy soil > Clayey soil (fastest to slowest water drainage).

A student pours 200 mL of water into 50 g each of sandy, loamy, and clayey soil in separate funnels. After 10 min, she collects: Sandy 180 mL, Loamy 140 mL, Clayey 60 mL in beakers.

Percolation rate = volume collected / time.
Sandy: 180 mL / 10 min = 18 mL/min → highest drainage (least water retention).
Loamy: 140 mL / 10 min = 14 mL/min → moderate.
Clayey: 60 mL / 10 min = 6 mL/min → slowest (most water retained).
Conclusion: sandy best for drainage; clayey best for water retention; loamy balanced → best for farming.

• Name the four horizons in a soil profile from top to bottom.
• Which type of soil retains the most water?
• Why is loamy soil best for growing crops?
• Name two causes of soil erosion.
• How does deforestation lead to soil erosion?

Open the Practice tab for graded questions on Soil Composition & Types.