Mineral Nutrition
'Plants are what they eat — the minerals they absorb determine their health and growth.' — Plant Nutrition
1. Chapter Overview
Just like animals, plants require MINERALS for growth, development, and reproduction. This chapter covers ESSENTIAL MINERAL ELEMENTS (macro and micro), their FUNCTIONS, DEFICIENCY SYMPTOMS, the NITROGEN CYCLE, and BIOLOGICAL NITROGEN FIXATION — the process by which atmospheric N₂ is converted into usable forms. Hydroponics (growing plants in nutrient solutions) is also discussed.
2. Essential Mineral Elements
Criteria for Essentiality (Arnon and Stout)
- The element is INDISPENSABLE for the plant's life cycle
- Its function CANNOT be replaced by another element
- It is DIRECTLY involved in plant metabolism (not just correcting some condition)
Classification
Macronutrients (Required in LARGE amounts, > 10 mmole/kg)
| Element | Form Absorbed | Key Functions |
|---|---|---|
| Nitrogen (N) | NO₃⁻, NH₄⁺ | Proteins, nucleic acids, chlorophyll |
| Phosphorus (P) | H₂PO₄⁻, HPO₄²⁻ | ATP, nucleic acids, phospholipids |
| Potassium (K) | K⁺ | Enzyme activator, osmosis, stomatal opening |
| Calcium (Ca) | Ca²⁺ | Cell wall (calcium pectate), signalling |
| Magnesium (Mg) | Mg²⁺ | Chlorophyll (central atom), enzyme activator |
| Sulphur (S) | SO₄²⁻ | Proteins (cysteine, methionine), coenzymes |
Micronutrients (Required in TRACE amounts, < 10 mmole/kg)
| Element | Form Absorbed | Key Functions |
|---|---|---|
| Iron (Fe) | Fe²⁺, Fe³⁺ | Cytochromes, electron transport |
| Manganese (Mn) | Mn²⁺ | Photolysis of water (photosynthesis) |
| Zinc (Zn) | Zn²⁺ | Auxin synthesis, enzyme activator |
| Copper (Cu) | Cu²⁺ | Cytochrome oxidase, plastocyanin |
| Molybdenum (Mo) | MoO₄²⁻ | Nitrogenase component (N₂ fixation) |
| Boron (B) | BO₃³⁻, B₄O₇²⁻ | Cell wall formation, pollen germination |
| Chlorine (Cl) | Cl⁻ | Photosynthesis (O₂ evolution), osmosis |
3. Deficiency Symptoms
- Chlorosis: Yellowing of leaves (N, K, Mg, S, Fe deficiency)
- Necrosis: Death of tissue (Ca, Mg, Cu, K deficiency)
- Stunted growth: N, P, K, S deficiency
- Delayed flowering: N, P, S deficiency
Critical Concentration
- Deficient: Below this level → deficiency symptoms appear
- Toxic: Above this level → TOXICITY symptoms appear
- Critical: Narrow range between deficiency and toxicity for micronutrients
Hydroponics
- Technique of growing plants in NUTRIENT SOLUTIONS (without soil)
- Used to DETERMINE essential elements and deficiency symptoms
- Soilless cultivation — commercial application (NFT — Nutrient Film Technique)
4. Nitrogen Cycle
Steps in the Nitrogen Cycle
- Nitrogen Fixation: N₂ → NH₃ (ammonia) — by bacteria/industrial/lightning
- Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (by Nitrosomonas and Nitrobacter)
- Assimilation: Plants ABSORB NO₃⁻/NH₄⁺ → organic compounds
- Ammonification: Organic N → NH₄⁺ (by DECOMPOSER bacteria/fungi)
- Denitrification: NO₃⁻ → N₂ (by Pseudomonas) — returns N₂ to atmosphere
5. Biological Nitrogen Fixation
Why is it Important?
- Atmospheric N₂ is VERY STABLE (triple bond)
- Only PROKARYOTES can fix N₂ (no eukaryotes)
- ~170 million tonnes N fixed/year! (58% biological)
Requirements for Nitrogen Fixation
- Nitrogenase enzyme complex: Mo-Fe protein (sensitive to O₂)
- Reducing power: Ferredoxin (reduced), ATP (16 ATP per N₂)
- Anaerobic conditions: Oxygen INHIBITS nitrogenase
Symbiotic N₂ Fixation — Rhizobium and Legumes
- Rhizobium bacteria infect root hairs → INFECTION THREAD → form ROOT NODULES
- Bacteria convert N₂ → NH₃ → plant uses for amino acids/proteins
- Plant provides BACTEROIDS with carbon sources and anaerobic environment
Nodule Formation
- Root hairs CURL around Rhizobium
- Infection thread GROWS into root cortex
- Bacteria RELEASED into cortical cells → form BACTEROIDS
- Cells DIVIDE → visible NODULE
- LEGHAEMOGLOBIN (PINK colour) — protects nitrogenase from O₂
Free-Living N₂ Fixers
| Aerobic | Anaerobic | Cyanobacteria |
|---|---|---|
| Azotobacter | Clostridium | Nostoc, Anabaena, Gloeocapsa |
Symbiotic N₂ Fixers (Non-Legume)
- Frankia (Actinomycete) — in ALDER roots (Alnus)
- Anabaena — in the WATER FERN Azolla (used as green manure in rice fields)
6. Fate of Absorbed Nitrogen
- NO₃⁻ absorbed → REDUCED to NO₂⁻ → NH₄⁺ in cells
- NH₄⁺ → AMINO ACIDS (glutamate → glutamine pathway)
- Transamination → other amino acids
7. Common Mistakes
- Nitrogen is a MACROnutrient, NOT a micro: Many students list it wrong
- N₂ gas is INERT — cannot be used directly by plants: It must be FIXED first (into NH₃/NO₃⁻)
- Iron is a MICROnutrient despite being essential: Even small amounts are enough
- Nitrogenase is OXYGEN SENSITIVE: Leghemoglobin protects it by binding O₂
- Chlorosis can be caused by MULTIPLE deficiencies: Don't assume it's always N or Fe
8. CBSE Exam Focus
- Essential elements — macro vs micro (3-mark)
- Deficiency symptoms — chlorosis, necrosis (3-mark)
- Biological nitrogen fixation — mechanism (5-mark)
- Nitrogen cycle — diagram and steps (5-mark)
- Hydroponics and critical concentration (3-mark)
9. Self-Test (5+ Q&A)
Q1: Differentiate between macronutrients and micronutrients. A: Macronutrients: Required in LARGE amounts (>10 mmole/kg) — N, P, K, Ca, Mg, S. Micronutrients: Required in TRACE amounts (<10 mmole/kg) — Fe, Zn, Cu, Mn, Mo, B, Cl.
Q2: What is the role of leghaemoglobin in N₂ fixation? A: Leghemoglobin is a PINK protein in root nodules that binds O₂ (oxygen scavenger). It PROTECTS the O₂-SENSITIVE nitrogenase enzyme from being inactivated.
Q3: Name three free-living nitrogen-fixing bacteria. A: Azotobacter (aerobic), Clostridium (anaerobic), Nostoc/ Anabaena (cyanobacteria).
Q4: How does Rhizobium form a symbiotic relationship with legumes? A: Bacteria enter through ROOT HAIRS → infection thread → reach ROOT CORTEX → bacteroids form root NODULES. Bacteria fixes N₂ → NH₃; plant supplies CARBOHYDRATES and LEGHAEMOGLOBIN.
Q5: What is the critical concentration of an element? A: The CONCENTRATION BELOW which the plant shows DEFICIENCY SYMPTOMS. The range between DEFICIENCY and TOXICITY is narrow (especially for micronutrients).
10. Conclusion
Mineral nutrition explores the CHEMICAL needs of plants. Essential elements (17 total) are classified as macro- or micronutrients based on quantity required. Deficiency symptoms help DIAGNOSE problems in crops. The NITROGEN CYCLE is one of the most important biogeochemical cycles — driven primarily by BIOLOGICAL NITROGEN FIXATION through symbiotic Rhizobium-legume interactions. Understanding mineral nutrition is CRITICAL for AGRICULTURE — fertiliser management, crop yield, and sustainable farming practices all depend on it.
