Soil – Part 4


Soil formation occurs by the gradual breakdown of hard rock into small particles. The rock may be exposed to the earth’s surface or underneath soil. The breakdown of rock into soil is a process called weathering.

The diagram below shows the events that take place from the onset of weathering until top soil is fully well formed.


Physical weathering

Physical weathering is breakdown of large soil particles to smaller ones by physical means.

It is caused by several agents such as; water, wind, temperature or chemicals.  Examples of physical weathering include

Rolling objects in running water break rocks leading to soil formation.

Strong winds will also cause fragmentation of rocks as they blow over them leading to formation of soil particles.

Temperature changes cause contraction and expansion of rocks that result into cracking of rocks which eventually breakdown into small particles of soil.

Chemical weathering

Chemical weathering is breakdown of large soil particles to smaller ones by chemical means

  • Rain water dissolves carbon dioxide in the atmosphere forming carbonic acid. This acid dissolves and weakens constituents of rock turning them into soil.
  • Some rocks contain iron, when wet the iron is oxidized by oxygen in the atmosphere. This causes the rocks to disintegrate into soil.

Biological weathering

Biological weathering is the breakdown of large rocks into small soil particles caused by living organisms. Examples of biological weathering include

· Movement of animals on rocks causes breakage of rocks into small particles of soil.

· Some organisms like lichens grow on rocks. When they die, they form humus which is a component of soil.

· Also, roots of plants can penetrate through the rock cracks, hence making them even wider. Some small rock particles of the rocks may break off, leading to formation of soil.


  1. What does soil arise from?
  2. Define weathering
  3. Name the three types of weathering
  4. Mention three agents of physical weathering
  5. During chemical weathering, a weak solution of acid acts on rocks causing them to break down in small particles. Write equation to show formation of carbonic acid from carbon dioxide and water.
  6. From your own knowledge of weathering, suggest how human can bring about weathering of rocks
  7. How can living organisms bring about break down of soil and soil formation?