Soil Formation and Profile Development – SS1 Agricultural Science Lesson Note (Second Term)

Soil formation is the gradual breakdown of rock into soil, controlled by climate, parent material, topography, living organisms and time.

Behavioural objectives

1. List the five factors that control soil formation.
2. State the elements of climate and explain how rainfall, temperature, wind and pressure help to form soil.
3. Describe how parent material and topography affect soil formation.
4. Give examples of the activities of living organisms that form soil.
5. Explain why time is needed for soil formation.
6. Name and describe the horizons of a soil profile.

What are the factors of soil formation?

There are five factors of soil formation: climate, parent material, topography, biotic factors and time.

• Climate: rainfall, temperature, wind and pressure break rocks apart.
• Parent material: the rock that supplies the minerals and decides the type of soil.
• Topography: the slope or shape of the land, which affects erosion.
• Biotic factors: living organisms that mix, weather and add material to soil.
• Time: the long period the other factors need to act.

How does climate affect soil formation?

Climate is the average weather condition of a place measured over a long period.

• Elements of climate: sunlight, temperature, wind, relative humidity, rainfall and pressure.

These elements help to form soil:

• Rainfall: running water wears away rocks during erosion and gives rise to soil. Raindrop impact can also break rocks.
• Temperature: alternate heating and cooling makes rocks expand and contract. This causes cracks and, over time, soil forms.
• Wind: strong wind, especially in desert regions, carries tiny rock particles. They hit other rocks and break them into small pieces.
• Pressure: high pressure on a hanging rock may make it fall and break into small pieces.

How does parent material affect soil formation?

Parent material is the rock from which the soil is formed.

• Effect on soil type: its physical and chemical features decide the type of soil obtained.
• Quartz: parent material that is mainly quartz gives sandy soil.
• Micas and feldspars: these give clayey soil.

How does topography affect soil formation?

Topography is the slope or shape of the land, and it decides how fast soil is worn away or built up.

• Soil erosion: the removal of soil by running water or wind.
• Sloping land: erosion is more pronounced here than on flat land.
• Wind and water: they easily wear away soil and rock surfaces on slopes.
• Bottom of the slope: washed-down particles break into smaller ones through other weathering processes.

How do living organisms affect soil formation?

Biotic factors are the microorganisms, plants and animals that take part in soil formation.

• Termites and earthworms: they mix minerals and organic matter to form soil.
• Earthworms and crickets: their burrows let air and water move through the soil. The air and water react with rock and break it down.
• Man: tillage operations break rock into tiny pieces.
• Plant roots: they penetrate cracks in rocks and cause weathering.
• Bacteria: they help fallen tree leaves decay into humus, which is rich in plant food.
• Vegetation: it adds organic matter when it dies and its cover reduces erosion.

Why does soil formation need time?

Soil formation needs a long time because rock breakdown and decay are slow processes.

• Mature soil: it takes a long time to form.
• Rock breakdown: small pieces of rock need time to disintegrate into grains of soil.
• Decay: plants take a long time to rot and become part of the soil.

What is weathering?

Weathering is the breakdown of rocks at or near the earth’s surface by climate and living things.

• Physical weathering: rock breaks into pieces without any change in its minerals.
• Chemical weathering: rock minerals change through reaction with water and air.
• Biological weathering: living things break or change rock.

How does soil form step by step?

Soil forms when weathered rock mixes with humus and slowly develops layers.

1. Rock is exposed at the surface.
2. Weathering breaks the rock into fragments and grains.
3. Small plants and microorganisms settle on the loose material.
4. Plants and animals die and decay to form humus.
5. Humus mixes with the mineral grains to make topsoil.
6. Water moves materials downwards and layers (horizons) develop over time.

What is a soil profile?

A soil profile is a vertical section through the soil from the surface down to the rock.

• Horizon: one distinct layer of the profile, differing in colour, texture and content.
• Humus: dark, decayed plant and animal matter that is rich in plant food.
• Leaching: the washing down of soluble nutrients from upper layers by water.

What are the horizons of the soil profile?

A soil profile has up to five horizons: O, A, B, C and R.

What is the difference between topsoil and subsoil?

• Topsoil: the upper, dark horizon (A) that holds humus, roots and living organisms.
• Subsoil: the horizon (B) below the topsoil, where leached materials collect.

Why is the topsoil important to farmers?

Topsoil is important to farmers because it is the layer that feeds crops.

• Nutrients: it holds the most humus and plant food.
• Roots: most crop roots grow in it.
• Organisms: it has the most earthworms and microorganisms.
• Protection: it is the layer lost first in erosion, so farmers protect it with cover crops and mulching.

Evaluation

1. Define climate.
2. Explain how temperature leads to the formation of soil.
3. Which parent materials give rise to sandy soil and which give rise to clayey soil?
4. Why is soil erosion more pronounced on slopes than on flat land?
5. Name the horizons of a soil profile and state which one is richest in humus.

Assignment

List five ways in which living organisms cause soil formation and describe briefly the role of vegetation.