Soil Science – JSS3 Intermediate Science Lesson Note (First Term)

Soil is the loose upper layer of the earth’s surface in which plants grow, made of weathered rock, humus, water and air.

Behavioural objectives

1. Define weathering and name its three types.
2. Describe how soil is formed from rock.
3. Name the three main types of soil and state the features of each.
4. List the main properties of soil.
5. State five uses of soil.
6. Name and describe the layers of a soil profile.

What is soil made of?

Soil is made of five main parts: mineral particles, humus, water, air and living things.

• Mineral particles: tiny pieces of rock such as sand, silt and clay.
• Humus: dark material formed from rotted plant and animal matter, rich in nutrients.
• Water: fills part of the spaces between particles and carries dissolved nutrients to roots.
• Air: fills the other spaces and supplies oxygen to roots and soil organisms.
• Living things: earthworms, ants, termites, bacteria and fungi that live in the soil.

What is weathering?

Weathering is the breaking down of rock into smaller pieces by weather, water, chemicals and living things.

There are three types of weathering: physical, chemical and biological.

Physical weathering

• Meaning: rock is broken into smaller pieces without any change in its chemical make-up.
• Heat and cold: rock expands in the hot sun by day and contracts at night, so it cracks and crumbles.
• Freezing water: in cold places, water in cracks freezes and expands, and splits the rock.
• Moving water and wind: rivers, rain and wind wear rock surfaces away.

Chemical weathering

• Meaning: chemicals change the minerals in the rock and make it soft.
• Rainwater: rain is slightly acidic and reacts with some minerals in rock.
• Oxygen: reacts with minerals such as iron, which rusts and weakens the rock.

Biological weathering

• Meaning: living things break rock apart.
• Plant roots: roots grow into cracks and split the rock as they thicken.
• Burrowing animals: earthworms, ants and rodents loosen and mix the ground.

How is soil formed?

Soil forms when weathering breaks rock into small particles that mix with humus over a very long time.

1. Rock is exposed at the earth’s surface.
2. Physical, chemical and biological weathering break it into small pieces.
3. Small plants such as mosses and lichens grow on the broken rock.
4. The plants and animals die and rot to form humus.
5. Humus mixes with the rock particles to make soil.
6. Layers slowly develop as more soil builds up.

What are the types of soil?

There are three main types of soil: sandy soil, clayey soil and loamy soil.

They are grouped by the size of their particles and what they are made of.

Sandy soil

• Grains: large and gritty.
• Water: drains very fast, so it holds little water.
• Nutrients: few, so it is poor for most crops.

Clayey soil

• Grains: very tiny and sticky when wet.
• Water: holds a lot and can become waterlogged.
• When dry: becomes hard and cracks.

Loamy soil

• Make-up: a mixture of sand, silt, clay and humus.
• Water: holds enough water and lets the extra drain away.
• Use: the best soil for farming.

What are the properties of soil?

The properties of soil are the features used to describe and compare soils.

• Texture: how the soil feels, such as rough, smooth or sticky.
• Porosity: the amount of air spaces between soil particles.
• Water-holding capacity: how much water the soil keeps before it dries.
• Colour: dark soil usually has more humus, and reddish soil often has much iron.
• pH level: shows whether the soil is acidic, neutral or alkaline.

How is soil pH read?

• Below 7: acidic soil.
• Exactly 7: neutral soil.
• Above 7: alkaline soil.
• Crops: most crops grow best in soil close to neutral.

Why is soil important?

Soil is important because it supports plant growth, animal life and many human needs.

1. Food: cassava, maize, yam and vegetables grow in soil.
2. Clothing: cotton and rubber come from plants grown in soil.
3. Building: sand is used for blocks and glass, and clay is used for bricks and pots.
4. Home for animals: earthworms, ants and beetles live in soil.
5. Water filter: rainwater is partly cleaned as it soaks down through the soil.

What is a soil profile?

A soil profile is the vertical section of soil showing its layers from the surface down to the rock.

• Topsoil: dark, rich top layer with humus, where most roots grow.
• Subsoil: lighter layer with less humus and more stones.
• Parent material: broken and partly weathered rock that is slowly turning into soil.
• Bedrock: solid rock at the bottom.

How can soil be protected?

Soil is protected by farming methods that stop erosion and keep it fertile.

• Mulching: covering soil with dry grass or leaves to reduce water loss and erosion.
• Crop rotation: growing different crops in turn on the same land.
• Adding manure: returns humus and nutrients to the soil.
• Planting trees and cover crops: roots hold soil together against wind and running water.
• Terracing: cutting steps on slopes so water flows slowly.

Evaluation

1. Define weathering and name its three types.
2. Give one example of each type of weathering.
3. State two reasons why loamy soil is better for farming than sandy soil.
4. Which layer of the soil profile is most important for a maize plant, and why?
5. A soil has a pH of 5. Is it acidic, neutral or alkaline?

Assignment

Draw and label a soil profile. Write one line on each layer. Then say which of the three soil types you would choose for a vegetable farm, and give two reasons.