Answer: Extra nitrogen.
- A Extra potassium
- B Extra phosphorus
- C Extra carbon
- D Extra nitrogen
Correct answer: D. Extra nitrogen
Explanation: Legume root nodules fix atmospheric nitrogen, leaving the soil richer in nitrogen after the crop.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.