Question
List the essential macronutrients and micronutrients for plants. What are the criteria for essentiality proposed by Arnon and Stout? Give the deficiency symptoms for nitrogen, iron, and calcium.
(NCERT Class 11 — frequently asked in NEET)
Solution — Step by Step
An element is essential if it meets all three criteria:
- The plant cannot complete its life cycle without the element
- The requirement is specific — no other element can substitute for it
- The element must be directly involved in plant metabolism (not just improving soil)
About 17 elements are considered essential for most plants.
Required in concentrations > 10 mmol/kg of dry weight:
| Element | Symbol | Key role |
|---|---|---|
| Carbon | C | Organic backbone |
| Hydrogen | H | Water, organic molecules |
| Oxygen | O | Respiration, organic molecules |
| Nitrogen | N | Amino acids, nucleic acids, chlorophyll |
| Phosphorus | P | ATP, nucleic acids, phospholipids |
| Potassium | K | Stomatal regulation, enzyme activation |
| Calcium | Ca | Cell wall (middle lamella), signalling |
| Magnesium | Mg | Centre of chlorophyll molecule |
| Sulphur | S | Amino acids (cysteine, methionine), coenzymes |
Mnemonic: C HOPKiNS CaFe Mg (the “CaFe” part bridges macro and micro).
Required in concentrations < 10 mmol/kg of dry weight:
| Element | Symbol | Key role |
|---|---|---|
| Iron | Fe | Electron transport, enzyme cofactor |
| Manganese | Mn | Photolysis of water, enzyme activator |
| Zinc | Zn | Auxin synthesis, enzyme cofactor |
| Copper | Cu | Electron transport, plastocyanin |
| Boron | B | Cell wall formation, pollen germination |
| Molybdenum | Mo | Nitrogen fixation (nitrogenase), nitrate reductase |
| Chlorine | Cl | Photolysis of water, osmotic balance |
| Nickel | Ni | Urease enzyme |
Nitrogen deficiency: General chlorosis (yellowing) of older leaves first (N is mobile — the plant reallocates it from old to young leaves). Stunted growth.
Iron deficiency: Interveinal chlorosis of younger leaves (Fe is immobile — old leaves retain their iron). Veins stay green while the lamina turns yellow.
Calcium deficiency: Distorted new leaves, death of growing tips (Ca is immobile). Blossom-end rot in tomatoes. Weakened cell walls.
Why This Works
Plants need these elements because each one participates in specific biochemical reactions that no other element can replace. Nitrogen is in every amino acid. Magnesium sits at the heart of chlorophyll. Iron is essential for cytochrome proteins in electron transport.
The mobile vs immobile distinction explains why deficiency symptoms appear in different leaves. Mobile nutrients (N, P, K, Mg) are transported from old leaves to young ones during deficiency — so old leaves show symptoms first. Immobile nutrients (Ca, Fe, B, Mn) stay put — so young leaves suffer first.
Alternative Method — Deficiency Symptom Quick Reference
For NEET, remember the mobility rule: if the deficiency appears in older leaves first, the nutrient is mobile (N, P, K, Mg). If it appears in younger leaves or growing tips first, the nutrient is immobile (Ca, Fe, S, B, Mn, Cu, Zn). This one rule lets you answer at least 2-3 questions on mineral nutrition correctly even without memorising every symptom.
Common Mistake
Students often confuse chlorosis with necrosis. Chlorosis is yellowing of leaves due to loss of chlorophyll — it’s often reversible if the nutrient is supplied. Necrosis is death of tissue — brown/black patches that are irreversible. Many deficiency questions in NEET use these specific terms. Also, don’t confuse interveinal chlorosis (Fe, Mg) with general chlorosis (N) — the pattern matters for identifying the specific deficiency.