Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Plant Nutrient and Nutrient Cycle
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Plant Nutrient and Nutrient Cycle
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Subject: Agricultural Science
Class: Senior Secondary 2
Term: 3rd Term
Week: 1
Theme: Agricultural Ecology
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List plant nutrient elements under the major classes. Recognize deficiency symptoms of different elements in crops. State the factors affecting nutrient availability in the soil Describe the different methods of replenishing lost nutrients. Illustrate the different nutrient cycle using diagrams i.e. Carbon, Nitrogen, water, and phosphorus cycles.
Plant nutrients are essential chemical elements that plants require for growth, development, and reproduction. They are typically absorbed from the soil solution through the roots or from the atmosphere.
Classification of Plant Nutrients: Plant nutrients are classified based on the quantity required by plants.
A. Macronutrients: Required in relatively large quantities.
Primary Macronutrients (N-P-K): These are the most commonly deficient nutrients in soils and are the main components of commercial fertilizers.
Nitrogen (N): Role: Essential for protein synthesis, chlorophyll formation (photosynthesis), vigorous vegetative growth, and dark green leaf colour.
Forms absorbed: Nitrate (NO3−) and Ammonium (NH4+).
Phosphorus (P): Role: Energy transfer (ATP), root development, flowering, fruiting, seed formation, disease resistance, and early maturity.
Forms absorbed: Dihydrogen phosphate (H2PO4−) and Hydrogen phosphate (HPO42−).
Potassium (K): Role: Regulation of water movement (osmosis, stomata opening/closing), enzyme activation, protein synthesis, fruit quality, stalk strength, and disease resistance.
Forms absorbed: Potassium ion (K+).
Secondary Macronutrients: Calcium (Ca): Role: Cell wall formation (structural component), cell division, root growth, and nutrient uptake regulation.
Forms absorbed: Calcium ion (Ca2+).
Magnesium (Mg): Role: Central component of chlorophyll molecule, enzyme activator, phosphorus uptake.
Forms absorbed: Magnesium ion (Mg2+).
Sulphur (S): Role: Amino acid and protein synthesis, vitamin formation, chlorophyll formation, essential for oil crops (e.g., groundnut, oil palm).
Forms absorbed: Sulphate (SO42−).
B. Micronutrients (Trace Elements): Required in very small quantities, but their absence can severely impact plant growth.
Iron (Fe): Chlorophyll formation, enzyme systems. (Forms absorbed: Fe2+, Fe3+)
Manganese (Mn): Photosynthesis, enzyme activator. (Forms absorbed: Mn2+)
Boron (B): Cell wall formation, cell division, sugar transport, flowering. (Forms absorbed: H3BO3)
Copper (Cu): Enzyme systems, photosynthesis, respiration. (Forms absorbed: Cu2+)
Zinc (Zn): Enzyme systems, auxin synthesis, growth regulation. (Forms absorbed: Zn2+)
Molybdenum (Mo): Nitrogen fixation (component of nitrogenase), nitrate reduction. (Forms absorbed: MoO42−)
Chlorine (Cl): Osmosis, stomata function, photosynthesis. (Forms absorbed: Cl−)
Nickel (Ni): Nitrogen metabolism (component of urease enzyme). (Forms absorbed: Ni2+) Deficiency symptoms manifest when a plant lacks a specific nutrient, hindering its normal physiological processes. Symptoms vary by nutrient and crop.
General Considerations: Mobile nutrients (N, P, K, Mg): Symptoms appear first on older leaves as these nutrients can be translocated from older to younger, actively growing parts. Immobile nutrients (Ca, B, Fe, Cu, Mn, Zn): Symptoms appear first on younger leaves or growing points as they cannot be moved from older tissues.
Specific Deficiency Symptoms: Nitrogen (N): General yellowing (chlorosis) of older leaves, starting from the tip and moving inwards. Stunted growth, thin stems. Small, pale green leaves. Delayed maturity.
Example: Maize plants with pale yellow lower leaves.
Phosphorus (P): Purplish discoloration of leaves (especially older ones) or stems, particularly on the underside. Stunted growth and poor root development. Delayed flowering and maturity, poor seed/fruit development.
Example: Cassava leaves turning purplish.
Potassium (K): Yellowing or browning of leaf margins (edges) of older leaves, known as "scorching" or "firing." Weak stems, lodging (falling over). Poor fruit development and quality. Increased susceptibility to diseases.
Example: Plantain leaves with burnt-like edges.
Calcium (Ca): Symptoms appear on younger leaves and growing points (immobile nutrient). Distortion or curling of young leaves, often with dark spots. Death of growing points (meristematic tissue) of roots and shoots. Blossom-end rot in fruits like tomatoes.
Example: Tomato fruits with dark, sunken spots at the bottom.
Magnesium (Mg): Interveinal chlorosis (yellowing between the veins) of older leaves, while veins remain green. Leaves may become brittle and curl upwards.
Example: Cocoa leaves showing yellowing between the main veins.
Sulphur (S): General yellowing of younger leaves (sometimes confused with N, but N affects older leaves first). Stunted growth. Poor nodulation in legumes.
Iron (Fe): Pronounced interveinal chlorosis of younger leaves, often leading to almost white leaves, with veins remaining green.
Example: Citrus trees with very pale yellow young leaves but green veins.
Boron (B): Death of terminal buds, resulting in rosette appearance. Thick, brittle young leaves. Cracked stems and fruits (e.g., "heart rot" in sugar beet, "internal cork" in apples). The ability of plants to absorb nutrients from the soil is influenced by several factors: Soil pH: This is the most critical factor. Each nutrient has an optimal pH range for its availability.
Acidic soils (low pH): Micronutrients like Fe, Mn, Zn become more available and can reach toxic levels. P, Ca, Mg, Mo become less available.
Alkaline/Basic soils (high pH): P, Fe, Mn, Zn, Cu, B become less available. Mo becomes more available. Most nutrients are optimally available in slightly acidic to neutral soils (pH 6.0-7.0).
Organic Matter Content: Decomposing organic matter releases nutrients (N, P, S, micronutrients) into the soil. It also improves soil structure, water holding capacity, and cation exchange capacity (CEC), which holds onto nutrients.
Soil Texture: Clay soils: Have higher CEC and retain nutrients better but can become compacted, limiting root growth and aeration.
Sandy soils: Have low CEC, poor nutrient retention, and are prone to leaching.
Soil Moisture and Aeration: Adequate moisture: Essential for dissolving nutrients and their transport to plant roots.
Waterlogging (poor aeration): Reduces oxygen, inhibits root respiration and nutrient uptake, promotes denitrification (loss of N).
Drought: Reduces nutrient mobility and uptake.
Microbial Activity: Soil microbes are crucial for nutrient cycling.
Decomposers: Break down organic matter, releasing nutrients (mineralization).
Nitrogen fixers: Convert atmospheric N2 into usable forms (e.g., Rhizobium in legumes).
Nitrifying bacteria: Convert ammonium to nitrate.
Denitrifying bacteria: Convert nitrates back to atmospheric N
2. Presence of Antagonistic or Synergistic Elements: Antagonism: High levels of one nutrient can reduce the uptake of another (e.g., excess K can inhibit Mg uptake; excess P can inhibit Zn uptake).
Synergism: The presence of one nutrient can enhance the uptake of another (e.g., N enhances P uptake).
Soil Temperature: Affects microbial activity, decomposition rates, and root metabolic processes (which influence nutrient uptake).
Tillage Practices: Excessive tillage can deplete organic matter, destroy soil structure, and increase nutrient loss. No-till or minimum tillage can preserve nutrients. Sustainable agricultural practices aim to maintain or improve soil fertility by replenishing nutrients removed by crops or lost through other means.
Application of Organic Fertilizers: Compost: Decomposed organic matter (crop residues, animal waste, kitchen scraps). Improves soil structure, water retention, and slowly releases a wide range of nutrients.
Farmyard Manure (FYM): Animal excreta mixed with bedding materials. Rich in N, P, K, and micronutrients. Improves soil structure and microbial activity.
Green Manure: Crops (usually legumes like cowpea, sunhemp, mucuna) grown specifically to be plowed back into the soil while still green. They add organic matter and, in the case of legumes, fix atmospheric nitrogen.
Bio-fertilizers: Living microorganisms (e.g., Rhizobium for legumes, Azotobacter for non-legumes) that enrich the soil by fixing atmospheric nitrogen or solubilizing soil phosphorus. Application of Inorganic (Chemical)
Fertilizers: Manufactured synthetic compounds containing specific nutrients in concentrated forms (e.g., Urea for N, Superphosphate for P, Muriate of Potash for K). Provide nutrients quickly to plants but can have environmental impacts if over-applied (e.g., leaching, soil acidification).
Examples common in Nigeria: NPK fertilizers (e.g., NPK 15:15:15), Urea (46% N), Single Superphosphate (SSP).
Crop Rotation: Growing different crops in sequence on the same land.
Legumes in rotation: Introduce nitrogen into the soil through symbiotic nitrogen fixation. Different crops have varying nutrient demands and rooting depths, preventing depletion of specific nutrients from specific soil layers.
Mulching: Applying a layer of organic material (straw, leaves, wood chips) on the soil surface. Suppresses weeds, conserves soil moisture, moderates soil temperature, and slowly releases nutrients as it decomposes.
Cover Cropping: Growing crops (e.g., legumes, grasses) primarily to cover the soil, prevent erosion, suppress weeds, and improve soil fertility. These are often not harvested but incorporated into the soil.
Liming: Application of calcium-containing materials (e.g., agricultural lime - CaCO3) to acidic soils to raise pH. This makes P, Ca, Mg, and Mo more available and reduces the toxicity of Al and Mn.
Shifting Cultivation: A traditional method where land is cleared, cultivated for a few years, and then left fallow for a longer period (bush fallowing) to allow natural vegetation to regenerate and restore soil fertility. Less feasible with increasing population pressure.
Organic Agriculture: A holistic farming system that promotes environmental health, biodiversity, and soil fertility by avoiding synthetic pesticides, fertilizers, growth regulators, and genetically modified organisms. It relies on practices like crop rotation, green manure, compost, and biological pest control.
Importance: Environmental sustainability: Reduces pollution, conserves natural resources, maintains biodiversity.
Soil health: Builds soil organic matter, improves soil structure, enhances microbial activity.
Food quality: Produces food free from synthetic chemical residues.
Farmer income: Can command premium prices for organic produce.
Ecosystem services: Supports beneficial insects, provides habitat.
Diagnosing Crop Problems in Local Farms/Gardens: Students can apply their knowledge of deficiency symptoms to identify potential nutrient issues in common Nigerian crops like maize, yam, cassava, or vegetables grown in their community or school garden. For example, if a maize farm shows yellowing of older leaves, students can suspect nitrogen deficiency and suggest applying compost or urea. This empowers them to provide practical advice to local farmers or household gardeners. Sustainable Farming Practices and Food Security: The understanding of nutrient cycles and replenishment methods directly relates to promoting sustainable agriculture in Nigeria. Students can learn to advocate for and implement practices like composting, crop rotation (especially with legumes like cowpea), and mulching. This knowledge is crucial for maintaining soil fertility, reducing reliance on expensive imported synthetic fertilizers, minimizing environmental pollution (e.g., from runoff causing eutrophication in rivers like River Niger or Benue), and ultimately ensuring long-term food security for the nation.
Environmental Awareness and Conservation: The study of nutrient cycles highlights the interconnectedness of living organisms and the environment. Students can grasp the consequences of unsustainable practices, such as bush burning contributing to the carbon cycle (releasing CO2) or excessive use of nitrogen fertilizers leading to groundwater contamination and changes in the nitrogen cycle. This fosters a sense of responsibility towards environmental conservation and encourages eco-friendly agricultural practices.