- Decaying plant is a kind of organic matter
- Organic matter is a source of nutrients in soil
- So, decaying plant is a source of nutrients in soil

- Decaying plant is a source of nutrients in soil
- Fertile soil has a high number of nutrients
- Hence, decaying plant can make the soil fertile

So, decaying plant can make the soil fertile
Question: What is the source of the organic matter needed for most fertile soils?
Context: - Decaying plant is a kind of organic matter 
- Organic matter is a source of nutrients in soil 
- Fertile soil has a high number of nutrients