Urea
Explanation
Fertilisers are substances added to the soil to supply essential nutrients required for plant growth. They are broadly classified into organic fertilisers (natural) and chemical fertilisers (synthetic).
- Cow dung is incorrect: Cow dung is a natural organic manure derived from animal waste. It is rich in organic matter and is traditionally used to improve soil structure and fertility.
- Green manure is incorrect: Green manure refers to the practice of ploughing undecomposed green plant material (often leguminous crops) back into the soil. It is a biological method used to enhance organic content and nitrogen levels.
- Urea is correct: Urea ($CO(NH_2)_2$) is a synthetic organic compound manufactured industrially from ammonia and carbon dioxide. It is a widely used nitrogenous chemical fertiliser that provides nitrogen in a form readily available for plant uptake.
- Vermicompost is incorrect: Vermicompost is an organic fertiliser produced through the decomposition of organic waste by earthworms. It is a natural process and does not involve synthetic chemical synthesis.
Key Takeaway: Urea is a synthetic chemical fertiliser characterized by high nitrogen content, whereas cow dung, green manure, and vermicompost are natural organic manures derived from biological sources.