Using Fertilisers (SQA National 5 Chemistry): Revision Note

NPK fertilisers

• Fertilisers are substances which restore essential elements to the soil, helping plants to grow healthily

• Plants require several nutrients, but the three most important (or "essential") elements found in fertilisers are:

  1. Nitrogen, N

  2. Phosphorus, P

  3. Potassium, K

• Each element helps the plant in a different way

  • Nitrogen, N

    • Promotes healthy leaf growth

    • A nitrogen deficiency leads to reduced overall growth

  • Phosphorus, P

    • Promotes healthy root growth

    • A phosphate deficiency leads to stunted roots

  • Potassium, K

    • Promotes the growth of healthy flowers and fruits

    • A potassium deficiency can lead to yellow leaves

• To be effective, fertilisers must be soluble in water so that they can be absorbed by the plant's roots

Nitrogen-containing salts

• Nitrogen is a crucial element for plants

• Soluble nitrogen-containing salts, used in fertilisers, are made by reacting ammonia with various acids

Ammonia, NH₃ (g)

• Ammonia is a pungent, colourless gas

• It is very soluble in water

• When it dissolves, it forms an alkaline solution called ammonia solution

  • Ammonia solution can be represented by the chemical formula NH₄OH (ammonium hydroxide)

Making ammonium salts

• Ammonia solution is an alkali

• So, it can be neutralised by an acid to produce a soluble salt

• These are known as ammonium salts

  • They are excellent nitrogen-rich fertilisers

• The general word equation for these reactions is:

ammonia solution + acid → ammonium salt + water

The key industrial reaction

• The production of ammonium nitrate uses both of the key industrial chemicals, ammonia and nitric acid:

ammonia solution + nitric acid → ammonium nitrate + water

NH₄OH (aq) + HNO₃ (aq) → NH₄NO₃ (aq) + H₂O (l)

• Ammonium nitrate supplies nitrogen

• The syllabus highlights this as the most important example of making a nitrogen-based fertiliser

Other examples of fertiliser salts

• The same neutralisation principle can be used with other acids and bases to make different fertilisers that supply other essential elements

Ammonium phosphate

• Phosphoric acid can be used to create a fertiliser that provides both nitrogen and phosphorus:

ammonia solution + phosphoric acid → ammonium phosphate + water

3NH₄OH (aq) + H₃PO₄ (aq) → (NH₄)₃PO₄ (aq) + 3H₂O (l)

• Ammonium phosphate supplies nitrogen and phosphorus

Potassium nitrate

• To create a fertiliser with potassium and nitrogen, a different alkali is needed, such as potassium hydroxide:

potassium hydroxide + nitric acid → potassium nitrate + water

KOH (aq) + HNO₃ (aq) → KNO₃ (aq) + H₂O (l)

• Potassium nitrate supplies potassium and nitrogen