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

Soil formation is the gradual breakdown of rock into soil. Five factors control it: climate, parent material, topography, biotic factors (living organisms) and time. Climate, through rainfall, temperature, wind and pressure, breaks rocks apart. Parent material decides the type of soil, slopes speed up erosion, organisms mix and weather material, and time allows these processes to work.

Behavioural objectives

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

• list the factors that control soil formation;
• state the elements of climate;
• explain how rainfall, temperature, wind and pressure help to form soil;
• describe how parent material and topography affect soil formation;
• give examples of the activities of living organisms that form soil;
• explain why time is needed for soil formation.

What are the factors of soil formation?

The factors that control soil formation are climate, parent material, topography, biotic factors (living organisms) and time.

1. Climate

Climate is the average weather condition of a place measured over a long period. The elements of climate include sunlight, temperature, wind, relative humidity, rainfall and pressure. The following elements help to form soil:

• Rainfall: Running water from rainfall gradually wears away rocks during erosion and gives rise to soil. The impact of raindrops can also break rocks to form soil.
• Temperature: The alternate heating and cooling of rocks makes them expand and contract continuously. This causes cracks in the rock and, over time, leads to the formation of soil.
• Wind: High wind velocity, most especially in desert regions, carries tiny rock particles which hit one another or other rocks. This breaks the rocks into tiny pieces to form soil.
• Pressure: High pressure on a hanging rock may cause the rock to fall and break into tiny pieces to form soil.

2. Parent material

The physical and chemical features of the parent material determine the type of soil that can be obtained from it. Parent material that is mainly quartz gives rise to sandy soil, while micas and feldspars give rise to clayey soil.

3. Topography

Soil erosion is more pronounced on sloping areas than on flat land. Wind or water easily wears away the soil and rock surfaces on slopes. As the rock particles are washed down to the bottom of the slope, they break up into smaller particles through the combined effect of other processes of weathering.

4. Biotic factors (living organisms)

The action of microorganisms, plants and animals plays an active part in soil formation. Examples:

• Termites and earthworms mix the minerals and organic matter, resulting in the formation of soil.
• The burrowing activities of earthworms and crickets allow air and water to move through the soil. The air and water react with the rock and cause it to break down.
• Activities of man during tillage operations break rock into tiny pieces to form soil.
• The penetration of plant roots causes weathering of rocks.
• The decay of fallen tree leaves, with the aid of bacteria, results in the formation of humus, which is rich in plant food.

5. Time

Time is also needed for soil formation. It takes a long time for mature soil to form. It takes time for small pieces of rock to disintegrate into grains of soil. It also takes a long time for plants to decay and become part of the soil.

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. Why does soil formation take a long time?

Assignment

1. What are the factors of soil formation?
2. What are the elements of climate?
3. List five ways in which living organisms (biotic factors) cause soil formation.
4. Describe briefly the role of vegetation in soil formation.