Answer: It must be required for plant to complete its life cycle, irreplaceable by another element, and have direct physiological role.
- A It must simply be chemically detectable in plant tissue samples using standard ash analysis techniques in the laboratory
- B It must be required for plant to complete its life cycle, irreplaceable by another element, and have direct physiological role
- C It must visibly and measurably improve overall crop yield under typical field cultivation conditions across seasons
- D It must occur naturally within agricultural soil at unusually high background concentrations year-round
Correct answer: B. It must be required for plant to complete its life cycle, irreplaceable by another element, and have direct physiological role
Explanation: Criteria (Arnon and Stout, 1939): (1) The plant cannot complete its life cycle without it; (2) No other element can substitute for it; (3) It must be directly involved in plant metabolism. 17 elements meet these criteria.
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.