Concept:Bush burning removes the organic layer of the soil, which is essential for its fertility.
Explanation:During bush burning, fire burns plant residues and surface litter.
This process turns organic matter into ash and gases, causing a loss of soil organic matter.
When organic matter is lost, soil nutrients and soil structure also decline.
Destroying weed seeds and adding potash are not considered harmful effects.
Sterilizing the soil is not a recognised result of bush burning.
Hence, the main harmful effect of bush burning is the destruction of soil organic matter.
Answer:A. destroys soil organic matter