Agriculture – Chapter 22

Definition and Purpose

Fertilizers are natural or artificial substances containing chemical elements that improve plant growth and productivity. They work by:

• Increasing the natural fertility of soil
• Replacing elements taken from soil by previous crops
• Providing essential nutrients in readily available forms

Types of Fertilizers

Organic Fertilizers: Derived from natural sources (compost, manure, animal by-products). Nutrients are released slowly as material decomposes.

Inorganic Fertilizers: Artificially synthesized, allow rapid nutrient uptake by plants.

Macronutrients in Fertilizers

Primary Macronutrients (NPK):

• Nitrogen (N): Essential for leaf growth and green color (chlorophyll)
• Phosphorus (P): Important for root development, flowering, and fruiting
• Potassium (K): Regulates water balance, disease resistance, and overall plant health

Secondary Nutrients: Calcium, Magnesium, Sulfur

Micronutrients: Zinc, Manganese, Copper, Iron, Boron, Molybdenum (required in small quantities)

Specific Fertilizer Types and Functions

Nitrogen Fertilizers:

• Most useful during middle growth phases
• Examples: Ammonium nitrate, Calcium ammonium nitrate, Urea
• Must be applied at start of growing season

Phosphorus Fertilizers:

• Reinforce roots and stems
• Crucial at all growth stages
• Boost flowering, seed production, and fruiting
• Examples: Superphosphate, Triple superphosphate, Diammonium phosphate

Potassium Fertilizers:

• Promote root development
• Maintain healthy photosynthesis
• Limit disease spread
• Deficiency symptoms: Yellow/brown leaf edges, withering
• Examples: Potassium chloride (KCl), Potassium sulfate (K₂SO₄), Potassium nitrate (KNO₃)

Impact on Soil Health

Proper fertilizer use improves crop yields, but imbalanced application can cause:

• Soil pH changes
• Increased pest attacks
• Soil acidification and erosion
• Reduced soil organic carbon
• Harm to beneficial soil organisms
• Greenhouse gas emissions

Definition and Classification

Chemical substances used to control or kill pests including insects, fungi, rodents, and weeds.

Major Pesticide Categories:

• Insecticides: Control insects
• Herbicides: Control unwanted plants (weeds)
• Fungicides: Control fungi and their spores
• Rodenticides: Control rodents
• Nematicides: Control plant-parasitic nematodes
• Biopesticides: Derived from natural materials

Insecticides – Modes of Action

Organophosphates and Carbamates:

• Inhibit acetylcholinesterase enzyme
• Cause acetylcholine buildup
• Result in nervous system overstimulation and paralysis

Pyrethroids:

• Disrupt sodium channels in insect neurons
• Produce repetitive nerve impulses and paralysis

Neonicotinoids:

• Affect nicotinic acetylcholine receptors
• Disrupt nerve signal transmission causing paralysis

Herbicides – Modes of Action

Glyphosate:

• Inhibits protein production
• Controls broadleaf weeds and grasses

2,4-D and Dicamba:

• Mimic plant hormone auxin
• Cause uncontrolled growth and plant death

Paraquat:

• Inhibits photosynthesis
• Causes plant death

Fungicides – Modes of Action

Triazoles:

• Control biosynthesis of ergosterol
• Break fungal cell membrane integrity

Strobilurins:

• Interrupt electron transport chain in mitochondria
• Disrupt energy production

Copper-based fungicides:

• Interrupt various cellular processes in fungi

Benefits of Pesticides

• Protect plants from pests and diseases
• Increase yield and food production
• Control spread of diseases between plants, animals, and humans
• Cost-effective compared to alternative methods
• Produce higher-quality fruits and vegetables
• Contribute to food security worldwide

Risks and Negative Aspects

• Carcinogenic and neurotoxic effects on humans
• Harm to non-target organisms (beneficial insects, birds, aquatic life)
• Water contamination through runoff
• Pesticide residues on food crops
• Development of pesticide resistance in pests
• Reduction in beneficial insects and biodiversity loss
• Harm to pollinators (bees)
• Soil and water contamination from improper use/disposal

Definition and Causes

Normal rainwater: pH 5.6 (saturated with CO₂)

Acid rain: pH lower than 5.6

Caused by air pollution from sulfur oxides (SOₓ) and nitrogen oxides (NOₓ) dissolving in rainwater to form:

Acid rain can have pH as low as 2.1 (more acidic than lemon juice or vinegar)

Impact on Soil Chemistry

Soil Acidification:

• Lowers soil pH, making it more acidic
• Causes leaching of essential nutrients (Ca²⁺, Mg²⁺, K⁺, Na⁺)
• Replaces these nutrients with hydrogen ions (H⁺)

Chemical Reactions in Soil:

This reaction removes calcium from soil

This reaction releases aluminum ions, which are toxic to plants

Effects on Plants and Crops

• Direct damage to waxy leaf surface, increasing water loss
• Increased susceptibility to disease and environmental stress
• Reduced nutrient availability due to leaching
• Aluminum toxicity damaging root systems
• Difficulty in water absorption by plants
• Negative impact on sensitive crops (soybeans, wheat, peanuts)
• Reduced nitrogen fixation in legumes like soybeans
• Disruption of soil microbial communities
• Overall reduction in crop yield

Basic Concepts

Genetic engineering (genetic modification): Technique to change an organism’s genes to enhance its capabilities.

Can range from changing single DNA base to deleting/inserting whole DNA regions.

The Genetic Engineering Process

1. Identification: Scientists identify desired trait (drought tolerance, pest resistance)
2. Isolation: Gene responsible for trait is identified and isolated from source organism
3. Insertion: Isolated gene is inserted into target plant’s genome using various techniques
4. Testing: Genetically modified plant is grown and tested for desired traits and safety

Examples of Genetically Modified Crops

Herbicide-Tolerant Crops:

• Soybeans, corn, cotton engineered to tolerate specific herbicides (e.g., glyphosate)
• Enables weed control without crop damage

Bt Crops:

• Contain gene from Bacillus thuringiensis bacteria
• Produce protein toxic to specific insects
• Examples: Bt corn, Bt cotton
• Reduces need for chemical pesticides

Disease-Resistant Crops:

• Engineered for resistance against viruses, fungi, bacteria
• Example: Rainbow papaya resistant to papaya ringspot virus

Nutritionally Enhanced Crops:

• Example: Golden rice produces beta-carotene (Vitamin A precursor)
• Addresses vitamin A deficiency in developing countries

Benefits of Genetically Modified Crops

• Increased crop yields
• Reduced production costs
• Reduced pesticide use
• Improved food composition and quality
• Pest and disease resistance
• Improved food safety
• Medical benefits for growing population

Risks and Concerns

• Limited information on long-term human health impacts
• Potential for antibiotic-resistant diseases
• Reduction in agricultural biodiversity
• Development of disease-resistant pests
• Unforeseen consequences from cross-pollination with wild relatives

Temperature Effects

Heat Stress:

• Causes dehydration in plants
• Affects photosynthesis, respiration, and water absorption
• Can cause slow growth, reduced yield, and crop failure

Cold Stress:

• Can kill young plants or damage structures
• Results in reduced or complete yield loss

Precipitation Changes

Optimal Rainfall:

• Provides water and nutrients for plant growth
• Maintains soil moisture and fertility

Heavy Rainfall:

• Leaches essential nutrients from soil
• Reduces nutrient availability to crops

Drought Conditions:

• Cause moisture stress
• Lead to plant wilting and reduced yields

Extreme Weather Events

Storms and Strong Winds:

• Cause physical damage (uprooting, branch damage, lodging)
• Speed up moisture loss from leaves
• Worsen drought stress

Hail:

• Punctures leaves, stems, and fruits
• Causes tissue damage and photosynthetic failure
• Can cause significant yield loss or complete crop destruction

Heat Waves:

• Put crops under extreme heat stress
• Speed up water loss through transpiration
• Cause leaf burning, reduced flowering, and lower yields

Impacts on Food Security and Storage

• Crop failures disrupt food supply chains
• Lead to food shortages and price increases
• Affect regions heavily reliant on local agricultural production
• Reduced crop yields impact food storage facilities
• Smaller quantities available for processing and distribution
• Can lead to increased food waste
• Threaten overall food security

Developing Resilient Crop Varieties

Climate change necessitates development of crops resilient to changing environmental conditions through:

• Breeding or genetic engineering
• Traits like drought tolerance, heat tolerance, disease resistance
• Improved water and nutrient use efficiency