Chapter 3: Nutrition in Plants

Learning Objectives

By the end of this chapter, you should be able to:

Identify and classify essential plant nutrients
Explain nutrient uptake mechanisms and transport in plants
Understand soil chemistry and its importance for plant nutrition
Analyze the effects of nutrient deficiencies on plant growth
Evaluate different fertilizer types and their applications

Overview

Plant nutrition involves the acquisition and utilization of essential nutrients for growth, development, and reproduction. Unlike animals, plants obtain nutrients primarily through their roots from soil solution, and through their leaves from the atmosphere. Understanding plant nutrition is crucial for agriculture, horticulture, and ecological sustainability. This chapter explores the essential nutrients required by plants, their uptake mechanisms, and the soil chemistry that supports plant growth.

Essential Plant Nutrients

Classification of Nutrients

Plants require specific mineral elements for normal growth and development. These nutrients are classified based on their quantitative requirements:

Macronutrients

Nutrients required in relatively large amounts (greater than 0.1% of dry weight)

Macronutrient	Symbol	Function	Deficiency Symptoms
Nitrogen	N	Protein synthesis, chlorophyll production, nucleic acids	Yellowing of older leaves, stunted growth
Phosphorus	P	Energy transfer (ATP), nucleic acid synthesis, root development	Purpling of leaves, poor root development
Potassium	K	Enzyme activation, water regulation, disease resistance	Yellowing leaf margins, weak stems
Calcium	Ca	Cell wall structure, membrane function, enzyme activity	Young leaf distortion, blossom-end rot
Magnesium	Mg	Chlorophyll structure, enzyme activation	Interveinal chlorosis, yellowing between veins
Sulfur	S	Amino acid synthesis, protein structure, enzyme function	General yellowing, small leaves
Silicon	Si	Cell wall strength, disease resistance, drought tolerance	Weak stems, increased susceptibility

Micronutrients

Nutrients required in small amounts (less than 0.1% of dry weight)

Micronutrient	Symbol	Function	Deficiency Symptoms
Iron	Fe	Chlorophyll synthesis, electron transport	Interveinal chlorosis in young leaves
Manganese	Mn	Enzyme activation, photosynthesis	Grey spots on leaves, poor growth
Zinc	Zn	Enzyme function, auxin synthesis	Small leaves, shortened internodes
Copper	Cu	Enzyme activation, lignin synthesis	Withered tips, poor fruit development
Boron	B	Cell wall formation, sugar transport	Death of growing points, poor fruit set
Molybdenum	Mo	Nitrogen fixation, nitrate reduction	Yellowing of older leaves
Chlorine	Cl	Photosynthesis, osmotic regulation	Wilting, chlorosis
Nickel	Ni	Enzyme activation, nitrogen metabolism	Reduced growth

Did You Know? Nitrogen is the most limiting nutrient in most soils, yet it makes up about 78% of Earth's atmosphere. Plants cannot use atmospheric nitrogen directly and must obtain it in the form of nitrates or ammonium ions from the soil!

Nitrogen Fixation Process:

N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2

This reaction is catalyzed by the enzyme nitrogenase in nitrogen-fixing bacteria, requiring significant energy input.

Nutrient Functions in Plants

Nitrogen Compounds

Proteins and Enzymes:

Component of amino acids and proteins
Essential for enzyme structure and function
Required for nucleic acids (DNA, RNA)

Chlorophyll:

Central component of chlorophyll molecule
Essential for photosynthesis and light absorption
Deficiency causes yellowing (chlorosis) due to reduced chlorophyll

Energy Transfer:

Component of ATP, NADPH, and FAD $H_2$
Essential for energy transfer in metabolic pathways
Required for electron carriers in respiration

Phosphorus Compounds

Energy Transfer:

Component of ATP (adenosine triphosphate)
Essential for energy currency in cells
Required for phospholipids in cell membranes

Nucleic Acids:

Component of DNA and RNA backbones
Essential for genetic information storage and transfer
Required for cell division and reproduction

Root Development:

Promotes root growth and branching
Essential for flowering and fruit development
Required for seed formation and germination

Potassium Functions

Enzyme Activation:

Activates over 60 enzymes in plants
Essential for protein synthesis and carbohydrate metabolism
Required for photosynthesis and respiration

Water Regulation:

Regulates water uptake and loss
Essential for turgor pressure and cell expansion
Required for stomatal function and gas exchange

Disease Resistance:

Strengthens cell walls and structures
Essential for disease resistance and stress tolerance
Required for quality improvement in fruits and grains

Nutrient Uptake and Transport

Root Systems and Nutrient Absorption

Root Structure for Nutrient Uptake:

Root Feature	Description	Function
Root Hairs	Thin, tubular extensions of epidermal cells	Increase surface area for absorption
Cortex	Parenchyma tissue between epidermis and stele	Storage, radial transport
Endodermis	Inner layer of cortex with Casparian strip	Selective barrier, regulates uptake
Xylem	Vascular tissue for water and mineral transport	Long-distance transport

Nutrient Absorption Mechanisms:

Passive Absorption

Diffusion: Movement down concentration gradient
Mass Flow: Movement with water flow due to transpiration
Ion Exchange: Exchange between root H⁺ ions and soil cations

Factors Affecting Passive Uptake:

Soil concentration: Higher soil levels increase uptake
Transpiration rate: Higher rates increase mass flow
Root surface area: More area increases absorption

Active Absorption

Requires energy (ATP) from plant metabolism
Against concentration gradient from low soil to high root concentrations
Specific membrane transporters for different ions

Active Transport Examples:

Potassium uptake: H⁺/K⁺ antiport mechanism
Nitrate uptake: H⁺/N $O_3$ ⁻ symport mechanism
Phosphate uptake: Specific phosphate transporters

Energy Requirements:

\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}

\text{Energy} + \text{Nutrient}_{out} \rightarrow \text{Nutrient}_{in}

Transport Pathways in Plants

Symplastic vs. Apoplastic Transport:

Transport Type	Pathway	Mechanism	Selectivity
Symplastic	Through cytoplasm via plasmodesmata	Cell-to-cell transport	Selective, regulated
Apoplastic	Through cell walls and intercellular spaces	Non-selective flow	Non-selective, fast

Long-Distance Transport:

Transport System	Direction	Function	Components
Xylem	Roots to shoots	Water and mineral transport	Vessels, tracheids, xylem parenchyma
Phloem	Sources to sinks	Organic nutrient transport	Sieve tubes, companion cells, phloem parenchyma

Xylem Transport:

Driven by transpiration pull and root pressure
Moves water and minerals from roots to shoots
Unidirectional flow (roots to shoots)
Passive process requiring minimal energy

Phloem Transport:

Driven by pressure gradients from source to sink
Moves sugars, hormones, and organic compounds
Bidirectional flow (both directions possible)
Active loading requires energy expenditure

Pressure Flow Hypothesis:

Loading of sucrose into phloem sieve tubes at source
Osmotic water entry creates high pressure
Pressure-driven flow to sink regions
Unloading of sucrose at sink
Water exit reduces pressure, completing cycle

Soil Chemistry and Nutrient Availability

Soil Components

Soil Composition:

Component	Percentage	Function
Mineral Matter	45%	Provides structure, nutrients
Organic Matter	5%	Nutrients, water retention, microbial activity
Water	25%	Medium for nutrient transport
Air	25%	Root respiration, microbial activity

Soil Texture:

Sand: Large particles, good drainage, low nutrient retention
Silt: Medium particles, moderate drainage, moderate nutrient retention
Clay: Small particles, poor drainage, high nutrient retention
Loam: Balanced mixture, ideal for most plants

Soil Structure:

Aggregates: Clumps of soil particles
Porosity: Air and water holding capacity
Bulk density: Weight of soil per unit volume

Soil pH and Nutrient Availability

pH Scale:

Acidic pH: Less than 7.0
Neutral pH: 7.0
Alkaline pH: Greater than 7.0

Optimal Soil pH:

Most plants: pH 6.0-7.0 (slightly acidic)
Acid-loving plants: pH 4.5-5.5 (e.g., blueberries, rhododendrons)
Alkaline-tolerant plants: pH 7.5-8.5 (e.g., asparagus, beets)

pH Effects on Nutrient Availability:

pH Range	Available Nutrients	Limited Nutrients
Acidic (<6.0)	Aluminum, manganese, iron	Calcium, magnesium, phosphorus
Neutral (6.0-7.0)	All nutrients readily available	None significant
Alkaline (>7.5)	Calcium, magnesium, sodium	Iron, manganese, zinc, phosphorus

pH Adjustment Methods:

Lime (CaC $O_3$ ): Raises pH for acidic soils
Sulfur or elemental sulfur: Lowers pH for alkaline soils
Organic matter: Buffers pH and improves structure

Cation Exchange Capacity (CEC)

Definition: The ability of soil to hold and exchange positively charged ions (cations)

Components of CEC:

Clay particles: Negative charges on surfaces
Organic matter: Negative charges from functional groups
Humus: Decomposed organic matter with high CEC

Cation Exchange Process:

H⁺ ions released by roots into soil solution
Cations (C $a^2$ ⁺, M $g^2$ ⁺, K⁺, N $H_4$ ⁺) bind to exchange sites
H⁺ exchange for plant-available cations
Nutrient uptake by root system

Common Exchangeable Cations:

Cation	Charge	Relative Availability
Calcium (C $a^2$ ⁺)	+2	High
Magnesium (M $g^2$ ⁺)	+2	Moderate
Potassium (K⁺)	+1	High
Sodium (Na⁺)	+1	Low (can be toxic)
Hydrogen (H⁺)	+1	Low

Fertilizers and Soil Management

Types of Fertilizers

Organic Fertilizers:

Type	Composition	Advantages	Disadvantages
Manure	Animal waste	Improves soil structure, slow release	Variable composition, possible pathogens
Compost	Decomposed organic matter	Improves soil, balanced nutrients	Bulky, slow-acting
Green Manure	Growing plants plowed in	Improves soil, nitrogen fixation	Requires time and space
Bone Meal	Ground bones	Slow phosphorus source	Expensive, slow-acting
Fish Emulsion	Processed fish waste	Quick nutrient release	Odor, variable analysis

Inorganic Fertilizers:

Type	Analysis (N-P-K)	Advantages	Disadvantages
Ammonium Nitrate	34-0-0	High nitrogen content	Acidifying, potential leaching
Urea	46-0-0	High nitrogen content	Can volatilize if not incorporated
Superphosphate	0-20-0	Phosphorus source	No nitrogen or potassium
Potassium Chloride	0-0-60	High potassium content	Can increase soil salinity
Complete Fertilizer	Balanced (e.g., 10-10-10)	Balanced nutrition	May not address specific deficiencies

Fertilizer Application Methods

Soil Application:

Broadcast: Even spreading over entire area
Band: Concentrated placement near seeds or roots
Side-dressing: Application along rows of growing plants
Incorporation: Mixing into soil before planting

Foliar Application:

Spray application directly to leaves
Quick nutrient uptake for fast response
Effective for micronutrients and correcting deficiencies
Limited to small amounts due to leaf absorption limits

Fertigation:

Fertilizer application through irrigation systems
Precise delivery of nutrients
Reduced labor and material waste
Requires proper irrigation equipment

Nutrient Management Strategies

Soil Testing:

Routine analysis of nutrient levels
pH determination for lime/fertilizer requirements
Organic matter content assessment
Cation exchange capacity evaluation

Fertilizer Recommendations:

Soil test-based application rates
Crop-specific nutrient requirements
Timing considerations for optimal uptake
Placement methods for efficient use

Environmental Considerations:

Nutrient runoff prevention
Groundwater protection from nitrate contamination
Air quality protection from ammonia volatilization
Soil health maintenance through sustainable practices

Deficiency Symptoms and Diagnosis

Visual Deficiency Symptoms

Nitrogen Deficiency:

General chlorosis starting with older leaves
Stunted growth and reduced vigor
Reduced tillering in grasses
Smaller leaves and thin stems

Phosphorus Deficiency:

Purpling of leaves due to anthocyanin accumulation
Stunted growth with dark green to purple foliage
Poor root development and delayed maturity
Reduced flowering and fruit set

Potassium Deficiency:

Marginal chlorosis and necrosis on older leaves
Weak stems and lodging in cereals
Reduced disease resistance and stress tolerance
Poor fruit quality and development

Calcium Deficiency:

Death of growing points (apical necrosis)
Distorted young leaves and malformed fruits
Blossom-end rot in tomatoes and peppers
Root tip dieback

Magnesium Deficiency:

Interveinal chlorosis in older leaves
Yellowing between veins while veins remain green
Pattern resembles iron deficiency but affects older leaves
Reduced photosynthetic efficiency

Micronutrient Deficiencies:

Iron: Interveinal chlorosis in young leaves
Zinc: Small leaves, shortened internodes
Manganese: Grey spots on leaves
Boron: Death of growing points

Diagnostic Methods

Visual Assessment:

Pattern recognition of deficiency symptoms
Progression timing (young vs. old leaves affected)
Symptom specificity to help identify the deficient nutrient

Tissue Testing:

Leaf or stem analysis for nutrient content
Critical values comparison for optimal ranges
Trend analysis over time to monitor changes