Answer: Toxic above a limit.
- A Always harmless
- B Never absorbed
- C Toxic above a limit
- D Purely beneficial
Correct answer: C. Toxic above a limit
Explanation: Above a critical level a mineral can become toxic - excess Mn competes with Fe and Mg, causing their deficiency.
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.