State that ammonium salts and nitrates are used as fertilisers

Chemistry of the Environment – Fertilisers

Learning Objective

State that ammonium salts and nitrate salts are used as fertilisers.

Why Fertilisers Are Needed

Plants require nitrogen (N) to synthesise three essential biomolecules:

• Proteins – the building blocks of enzymes and structural material.
• Nucleic acids (DNA & RNA) – store and transmit genetic information.
• Chlorophyll – the pigment that captures light energy for photosynthesis.

In many soils the natural supply of usable nitrogen is insufficient for optimal crop growth, so fertilisers are applied to increase the nitrogen available to plants.

NPK Fertilisers – Supplying Nitrogen, Phosphorus and Potassium

Commercial fertilisers are sold as “NPK” blends. The three numbers on the bag represent the percentage by mass of:

• N – elemental nitrogen.
• P – phosphorus expressed as phosphorus pentoxide (P₂O₅).
• K – potassium expressed as potassium oxide (K₂O).

These three macronutrients are all essential for plant growth:

• Phosphorus (P) – vital for energy transfer (ATP), nucleic acids and root development.
• Potassium (K) – regulates osmotic balance, activates enzymes and improves water‑use efficiency.

Key Types of Nitrogen Fertilisers

The two main groups of nitrogen‑containing fertilisers are:

• Ammonium salts – supply nitrogen as the ammonium ion (NH₄⁺).
• Nitrate salts – supply nitrogen as the nitrate ion (NO₃⁻).

Common Ammonium‑Based Fertilisers

Common Nitrate‑Based Fertilisers

How Ammonium and Nitrate Are Used by Plants

Both NH₄⁺ and NO₃⁻ are absorbed through the root system.

1. Ammonium (NH₄⁺) can be directly incorporated into amino acids via the glutamine synthetase pathway.
2. Nitrate (NO₃⁻) is first reduced in two enzymatic steps:
   • NO₃⁻ → NO₂⁻ (catalysed by nitrate reductase)
   • NO₂⁻ → NH₄⁺ (catalysed by nitrite reductase)
   The resulting NH₄⁺ then follows the same assimilation pathway as that taken up directly.

Quantitative Example (AO2 Skill)

A farmer buys a 50 kg bag of urea (46 % N by mass). How much elemental nitrogen does the bag contain?

Calculation:

Mass of N = 0.46 × 50 kg = 23 kg of nitrogen.

This amount can be compared with the crop’s nitrogen requirement to decide the appropriate application rate.

Environmental Considerations

Excess nitrogen fertiliser can lead to:

• Leaching of nitrate into groundwater and surface water.
• Eutrophication – algal blooms that deplete dissolved oxygen and harm aquatic life.

Mitigation Strategies (as required by the syllabus)

• Split‑application – apply smaller doses at key growth stages.
• Slow‑release or coated fertilisers – release nitrogen gradually.
• Cover crops or green manures – capture residual nitrogen.
• Precision farming – soil testing, GPS‑guided spreading, and variable‑rate technology.

Safety Note (Laboratory & Field)

• Ammonium nitrate is a strong oxidiser and can become explosive when mixed with combustible material or confined under heat. Store in a cool, well‑ventilated area away from fuels.
• Handle all fertilisers with gloves and eye protection to avoid skin irritation and inhalation of dust.