Fertilisers

1. Ammonium salts and nitrates as fertilisers

• Ammonium salts and nitrates are widely used in agriculture because they supply essential nitrogen to plants in a form that can be absorbed quickly by their roots.
• Examples of ammonium salts used as fertilisers:
  • Ammonium sulfate – (NH₄)₂SO₄
  • Ammonium nitrate – NH₄NO₃
  • Ammonium phosphate – (NH₄)₃PO₄
• Nitrate fertilisers include:
  • Sodium nitrate – NaNO₃
  • Potassium nitrate – KNO₃
  • Calcium nitrate – Ca(NO₃)₂
• Why nitrogen is important for plants:
  • Nitrogen is a major component of amino acids, proteins, nucleic acids (DNA and RNA), and chlorophyll.
  • Nitrogen deficiency in plants leads to yellowing of leaves (chlorosis), stunted growth, and poor crop yields.
• These compounds dissolve easily in water, making nitrogen available to plants rapidly, but this also increases the risk of leaching into waterways.

2. NPK fertilisers

• NPK fertilisers are formulations that provide three essential nutrients to plants:
  • N → Nitrogen – for leafy growth and synthesis of proteins and chlorophyll.
  • P → Phosphorus – usually provided in the form of phosphate ions (PO₄³⁻), essential for root development, seed and fruit production, and energy transfer through ATP.
  • K → Potassium – usually provided in the form of potassium ions (K⁺), essential for regulating water balance in plant cells and activating enzymes in photosynthesis and respiration.

2.1 Sources of each nutrient in NPK fertilisers:

• Nitrogen sources:
  • Ammonium nitrate (NH₄NO₃)
  • Ammonium sulfate ((NH₄)₂SO₄)
  • Urea (CO(NH₂)₂) – which releases ammonia upon hydrolysis
• Phosphorus sources:
  • Calcium phosphate (Ca₃(PO₄)₂) from phosphate rock
  • Ammonium phosphate ((NH₄)₃PO₄)
  • Superphosphate (Ca(H₂PO₄)₂) produced by treating phosphate rock with sulfuric acid
• Potassium sources:
  • Potassium chloride (KCl)
  • Potassium sulfate (K₂SO₄)
  • Potassium nitrate (KNO₃)

3. Advantages of NPK fertilisers

• Provide a balanced mix of essential nutrients in one product.
• Increase crop yield by accelerating plant growth and improving overall health.
• Correct specific nutrient deficiencies in soil.
• Can be tailored for different crops and soil types.

4. Disadvantages and environmental impact

• Eutrophication: Excess fertiliser can be washed into rivers and lakes by rainwater, causing nutrient overload, algal blooms, and oxygen depletion, killing aquatic life.
• Soil degradation: Overuse can lead to changes in soil pH, reducing fertility over time.
• Groundwater contamination: Nitrate leaching into water supplies can cause health problems such as methemoglobinemia (blue baby syndrome) in infants.
• Dependency: Crops may become dependent on artificial fertilisers, reducing natural soil fertility.

5. Sustainable fertiliser practices

• Controlled application: Applying the right amount at the right time reduces waste and runoff.
• Slow-release fertilisers: Coated granules release nutrients gradually.
• Organic fertilisers: Compost and manure supply nutrients more slowly but improve soil structure.
• Crop rotation and nitrogen-fixing plants: Such as legumes, which naturally replenish soil nitrogen.