Plant Nutrition and Essential Nutrient Elements
Plant nutrition questions test Arnon's criteria, the 17 essential elements, their functions, and how mobility inside the plant explains where deficiency symptoms appear first.
Core rules
- Essential elements: C, H, O, N, P, K, Ca, Mg, S (macronutrients) plus Fe, Mn, Zn, Cu, B, Mo, Cl, Ni (micronutrients). Arnon's criteria: indispensable, specific, and directly involved in plant metabolism.
- Mobile elements (N, P, K, Mg) show deficiency in older leaves; immobile elements (Ca, Fe, B, and largely Mn, Zn, Cu) show it in young leaves and growing points.
- Key functions: Mg is the central atom of chlorophyll; Mo serves nitrate reductase and nitrogenase; Fe is in cytochromes; B is needed for cell wall and pollen tube growth; Cl is required for oxygen evolution in photosystem II; Ni is the cofactor of urease.
- Deficiency signatures: N general chlorosis of older leaves, P purpling, K marginal scorch, Mg interveinal chlorosis of older leaves, Fe interveinal chlorosis of young leaves, Ca blossom-end rot and dead growing points, B hollow stem/brown heart, Zn little leaf, Cu exanthema, Mo whiptail.
Worked example
A crop yields 4 t/ha grain containing 1.5% N and 2 t/ha straw containing 0.5% N. Nitrogen removed = (4000 × 0.015) + (2000 × 0.005) = 60 + 10 = 70 kg N/ha. That is the removal figure to compare with a fertilizer dose, not the dose itself; recovery efficiency and soil supply must be added, so 70 kg removal may justify 100–140 kg applied N.
Traps that cost marks
- Confusing critical concentration (the value below which deficiency occurs) with the sufficiency range.
- Forgetting luxury consumption, where tissue content rises without yield response.
- Ignoring antagonism: excess K depresses Mg and Ca uptake; high P can induce Zn and Fe deficiency.