10.2 Fertilisers

2026 Syllabus Objectives

  1. Core: State that ammonium salts and nitrates are used as fertilisers
  2. Core: Describe the use of NPK fertilisers to provide the elements nitrogen, phosphorus and potassium for improved plant growth

Why Fertilisers are Needed 🌱

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Crop plants require chemical nutrients beyond what they produce through photosynthesis. While plants make their own food from carbon dioxide and water, they need additional minerals from the soil to produce:

  • Healthy leaves
  • Strong roots
  • Flowers and fruit

When the same land is used repeatedly for growing crops, these essential minerals become depleted from the soil. Artificial fertilisers replace these lost minerals, enabling:

  • Faster plant growth
  • Larger crop yields
  • Sufficient food production for the world's growing population

Ammonium Salts and Nitrates as Fertilisers 🔑

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Types of Nitrogen-Containing Fertilisers

Two main types of compounds are used as fertilisers to provide nitrogen:

1. Ammonium Salts (containing the NH4+\text{NH}_4^+ ion)

  • Examples: ammonium phosphate ((NH4)3PO4)((\text{NH}_4)_3\text{PO}_4), ammonium sulfate ((NH4)2SO4)((\text{NH}_4)_2\text{SO}_4)
  • All ammonium salts are soluble in water

2. Nitrates (containing the NO3−\text{NO}_3^- ion)

  • Example: potassium nitrate (KNO3)(\text{KNO}_3)
  • Also water-soluble

Important: The solubility of these compounds is crucial because plants can only take up soluble nitrogen compounds through their roots.

Ammonium Nitrate - The Most Important Nitrogenous Fertiliser ⚡

Ammonium nitrate (NH4NO3)(\text{NH}_4\text{NO}_3) is the most significant nitrogenous fertiliser because:

  • It provides both NH4+\text{NH}_4^+ and NO3−\text{NO}_3^- ions
  • It contains 35% by mass of nitrogen
  • It is highly soluble in water

Production of Ammonium Nitrate

Ammonium nitrate is produced through a neutralisation reaction between ammonia solution and nitric acid:

Word equation:

ammonia+nitric acid→ammonium nitrate\text{ammonia} + \text{nitric acid} \rightarrow \text{ammonium nitrate}

Balanced chemical equation:

NH3(aq)+HNO3(aq)→NH4NO3(aq)\text{NH}_3(\text{aq}) + \text{HNO}_3(\text{aq}) \rightarrow \text{NH}_4\text{NO}_3(\text{aq})

The ammonium nitrate product can be crystallised into pellet form, making it suitable for spreading on agricultural land.

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