Chapter 8: Mineral Nutrition

Introduction

Mineral nutrition refers to the study of inorganic elements required by plants for their growth, development, and metabolism. Plants obtain these essential elements primarily from soil through their roots. The science that deals with how plants get mineral nutrients and utilize them is called mineral nutrition.

Essential Mineral Elements

Criteria for Essentiality (Arnon and Stout, 1939)

An element is considered essential for plants if:

1. The plant cannot complete its life cycle without the element
2. The element cannot be replaced by another element
3. The element is directly involved in plant metabolism or structure

Classification of Essential Elements

Based on Quantity Required

• Macronutrients: Required in larger amounts (>0.1% of dry weight)
  • Primary macronutrients: N, P, K
  • Secondary macronutrients: Ca, Mg, S
• Micronutrients (Trace elements): Required in smaller amounts (<0.01% of dry weight)
  • Fe, Mn, Cu, Zn, B, Mo, Cl, Ni

Based on Mobility in Plants

• Mobile elements: Can be remobilized from older to younger tissues (N, P, K, Mg, Cl)
• Immobile elements: Cannot be easily remobilized (Ca, S, Fe, B, Cu)

Based on Metabolic Function

• Structural elements: Part of plant structures (C, H, O, N)
• Enzyme activators: Function as cofactors (K, Mg, Ca)
• Energy transfer: Involve in energy transfer (P)
• Redox reactions: Involve in electron transfer reactions (Fe, Cu)

Functions and Deficiency Symptoms of Essential Elements

Macronutrients

Nitrogen (N)

• Sources: NO₃⁻, NH₄⁺
• Functions:
  • Component of proteins, nucleic acids, chlorophyll, coenzymes
  • Involved in photosynthesis, respiration, protein synthesis
• Deficiency symptoms:
  • Chlorosis (yellowing) of older leaves
  • Stunted growth
  • Reduced leaf size
  • Premature leaf fall

Phosphorus (P)

• Source: H₂PO₄⁻, HPO₄²⁻
• Functions:
  • Component of nucleic acids, ATP, phospholipids
  • Energy transfer (ATP)
  • Cell division and enlargement
  • Root development
• Deficiency symptoms:
  • Dark green leaves often with purple/red pigmentation
  • Stunted growth
  • Poor root development
  • Delayed maturity

Potassium (K)

• Source: K⁺
• Functions:
  • Enzyme activation
  • Stomatal regulation
  • Protein synthesis
  • Osmoregulation
  • Translocation of photosynthates
• Deficiency symptoms:
  • Marginal chlorosis and necrosis of older leaves
  • Weak stems and lodging
  • Increased susceptibility to diseases

Calcium (Ca)

• Source: Ca²⁺
• Functions:
  • Cell wall structure (calcium pectate)
  • Cell membrane integrity
  • Enzyme activation
  • Cell division
  • Signal transduction
• Deficiency symptoms:
  • Distortion of young leaves
  • Death of growing points (apical meristems)
  • Poor root development
  • Blossom-end rot in fruits

Magnesium (Mg)

• Source: Mg²⁺
• Functions:
  • Central atom in chlorophyll molecule
  • Enzyme activation
  • Ribosome aggregation
  • Phosphate metabolism
• Deficiency symptoms:
  • Interveinal chlorosis in older leaves
  • Leaf curling
  • Premature leaf drop

Sulfur (S)

• Source: SO₄²⁻
• Functions:
  • Component of amino acids (cysteine, methionine)
  • Constituent of vitamins (thiamine, biotin)
  • Disulfide bonds in proteins
  • Part of coenzyme A
• Deficiency symptoms:
  • Uniform chlorosis in young leaves
  • Stunted growth
  • Delayed maturity

Micronutrients

Iron (Fe)

• Source: Fe²⁺, Fe³⁺
• Functions:
  • Chlorophyll synthesis
  • Electron transport in photosynthesis and respiration
  • Component of cytochromes and ferredoxin
  • Nitrogen fixation
• Deficiency symptoms:
  • Interveinal chlorosis in young leaves
  • Severe yellowing (iron chlorosis)

Manganese (Mn)

• Source: Mn²⁺
• Functions:
  • Enzyme activation
  • Photosystem II in photosynthesis
  • Electron transport
  • Oxidation-reduction processes
• Deficiency symptoms:
  • Interveinal chlorosis with small green spots
  • Gray spots on leaves of some plants

Zinc (Zn)

• Source: Zn²⁺
• Functions:
  • Enzyme activation
  • Auxin synthesis
  • Protein synthesis
  • Carbohydrate metabolism
• Deficiency symptoms:
  • Reduced leaf size (little leaf)
  • Shortened internodes (rosetting)
  • Interveinal chlorosis

Copper (Cu)

• Source: Cu²⁺
• Functions:
  • Enzyme component
  • Electron transport
  • Lignin synthesis
  • Pollen formation
• Deficiency symptoms:
  • Wilting and necrosis of leaf tips
  • Dieback of stems
  • Poor reproductive development

Boron (B)

• Source: H₃BO₃, H₂BO₃⁻
• Functions:
  • Cell wall structure and integrity
  • Cell division and elongation
  • Pollen germination
  • Translocation of sugars
• Deficiency symptoms:
  • Death of apical meristems
  • Brittle leaves
  • Poor flowering and fruiting
  • Corky tissues

Molybdenum (Mo)

• Source: MoO₄²⁻
• Functions:
  • Nitrate reduction
  • Nitrogen fixation
  • Enzyme component (nitrate reductase)
• Deficiency symptoms:
  • Interveinal chlorosis in older leaves
  • Leaf margin necrosis
  • Reduced nitrate reductase activity

Chlorine (Cl)

• Source: Cl⁻
• Functions:
  • Osmotic regulation
  • Electrochemical balance
  • Photosynthetic oxygen evolution
  • Disease resistance
• Deficiency symptoms:
  • Wilting
  • Chlorosis
  • Reduced leaf size
  • Bronzing of leaves

Nickel (Ni)

• Source: Ni²⁺
• Functions:
  • Urease activation
  • Nitrogen metabolism
  • Seed germination
• Deficiency symptoms:
  • Reduced urease activity
  • Leaf tip necrosis
  • Poor seed germination

Soil as a Source of Nutrients

Types of Soils and Their Properties

• Sandy soil: Good drainage, poor nutrient retention
• Clay soil: Poor drainage, good nutrient retention
• Loamy soil: Balanced drainage and nutrient retention
• Silt soil: Intermediate properties between sand and clay

Soil pH and Nutrient Availability

• Acidic soil (pH < 6.5): Favors availability of Fe, Mn, Zn, Cu, B
• Neutral soil (pH 6.5-7.5): Optimal for most nutrients
• Alkaline soil (pH > 7.5): Favors availability of Ca, Mg, Mo

Factors Affecting Nutrient Availability

• Soil pH
• Organic matter content
• Soil texture and structure
• Cation exchange capacity
• Microbial activity
• Soil moisture and aeration

Mineral Uptake by Plants

Root Architecture and Uptake Efficiency

• Root hairs increase surface area for absorption
• Mycorrhizal associations enhance nutrient uptake
• Rhizosphere influences nutrient availability

Mechanisms of Mineral Uptake

• Passive absorption: Diffusion along concentration gradient
• Active absorption: Transport against concentration gradient (requires energy)
• Ion exchange: Exchange of H⁺ for cations at root surface
• Root interception: Direct contact between roots and soil particles

Nutrient Transport Within Plants

• Xylem transport: Primarily for water and minerals from roots to shoots
• Phloem transport: Redistribution of mobile nutrients to growing regions
• Nutrient remobilization: Translocation from senescing to young tissues

Mineral Deficiency and Toxicity

Visual Diagnosis of Nutrient Disorders

• Pattern of symptom development (young vs. old leaves)
• Nature of symptoms (chlorosis, necrosis, stunting)
• Progression of symptoms

Methods to Overcome Deficiencies

• Soil application