Environmental Hazard: Soil Erosion – JSS3 Basic Science Lesson Note (First Term)

Soil erosion is the washing away or wearing away of the topsoil and sub-soil from land by water or wind.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define soil erosion;
2. describe the three stages of soil erosion;
3. state the causes of soil erosion;
4. list ways of controlling or preventing soil erosion;
5. state the effects of soil erosion.

What is soil erosion?

Soil erosion is the removal of topsoil and sub-soil from one place and its movement to another by water or wind.

• Topsoil: the dark upper layer of soil, rich in humus and nutrients, where most plant roots grow.
• Sub-soil: the layer below the topsoil, which holds less humus and fewer nutrients.
• Erosion: the wearing away and carrying off of soil particles from the land surface.
• Bare soil: soil without plant cover, which erodes faster than soil covered by plants.

What are the agents of soil erosion?

An agent of erosion is anything that detaches and carries soil away, and the main agents are water, wind and human activities.

What are the stages of soil erosion?

Soil erosion has three stages: detachment, movement and deposition.

1. Detachment: the topsoil is detached from the rest of the soil by raindrops, running water or wind.
2. Movement: the topsoil is transported to another area by water or wind.
3. Deposition: the topsoil ends up and is deposited in a new place, such as a river bed, a valley or the foot of a slope.

What are the types of water erosion?

There are four common types of water erosion: splash, sheet, rill and gully erosion.

• Splash erosion: raindrops hit bare soil and knock soil particles loose.
• Sheet erosion: water removes a thin, even layer of topsoil over a wide area.
• Rill erosion: running water cuts many small, shallow channels in the soil.
• Gully erosion: running water cuts deep, wide channels that are hard to repair.

What causes soil erosion?

Soil erosion is caused by natural factors and by human activities that leave soil bare or loose.

Natural causes

• Heavy rainfall: strong rain detaches soil and produces fast-running water.
• Strong wind: wind blows away dry and loose soil.
• Steep slopes: water runs faster downhill and carries more soil.
• Loose, sandy soil: its particles are easily carried by water and wind.

Human causes

• Overgrazing: too many animals eat up the grasses or plants of an area and leave the soil bare.
• Bush burning: setting bush on fire destroys plant cover and exposes the soil.
• Deforestation: cutting down trees removes roots that hold the soil together.
• Building and construction work: clearing and digging loosen the soil and remove plant cover.
• Wrong farming methods: ploughing up and down a slope and leaving fields bare after harvest.
• Other human activities: mining disturbs the soil, and oil spillage kills plants that protect it.

How can soil erosion be controlled or prevented?

Soil erosion is controlled by covering the soil, holding it in place and slowing down running water and wind.

• Mulching: covering the soil surface or heaps with plant materials such as dry grass.
• Avoiding overgrazing: keeping only the number of animals the land can feed.
• Avoiding bush burning: leaving plant cover on the land.
• Public education and awareness campaigns: teaching people the causes and dangers of erosion.
• Afforestation: planting trees on bare land, so that roots hold the soil.
• Planting cover crops: crops such as melon and beans cover the soil and protect it from the agents of erosion.
• Making terraces and contours: step-like ridges across a slope slow down running water.
• Planting windbreaks: rows of trees reduce the speed of wind across farmland.
• Crop rotation: growing different crops in turn keeps the soil covered and healthy.
• Digging drainage channels: channels lead excess water away safely.

What are the effects of soil erosion?

The effects of soil erosion are the loss of fertile soil, damage to land and property, and harm to water and food supply.

• Loss of soil fertility: the nutrient-rich topsoil is carried away.
• Water pollution: eroded soil gets into streams and rivers and makes the water dirty.
• Soil acidity: it can increase soil acidity, which affects the well-being of plants.
• Low crop yield: poor soil gives smaller harvests and can lead to food shortage.
• Gullies: deep channels damage farmland, roads and buildings.
• Siltation: deposited soil fills river beds and can make flooding more likely.

Evaluation

1. Define soil erosion.
2. Name the three stages of soil erosion.
3. State four causes of soil erosion.
4. List four ways of controlling soil erosion.
5. State two effects of soil erosion.

Assignment

Explain any three ways of controlling soil erosion and state two effects of soil erosion.