Excretion of urea by animals - Correct: Urea (CO(NH₂)₂) contains nitrogen. When animals excrete urea, microbes in the soil convert it into ammonia, and later into nitrates and nitrites — forms usable by plants. Thus, urea adds nitrogen to the soil indirectly. Burning of coal by man - Incorrect: Burning coal releases nitrogen oxides (NOₓ) into the atmosphere, not into the soil. These contribute to air pollution and acid rain, not direct soil enrichment. Death of vegetation - Correct: When plants die, their tissues decompose. Decomposers (bacteria and fungi) break down organic matter, releasing nitrogen compounds back into the soil. Hence, this process adds nitrogen to the soil naturally.