Fertilisers

O Level / IGCSE Chemistry · Chemistry of the environment. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll O Level / IGCSE Chemistry topics

The idea

Plants take the elements they need from the soil. When crops are harvested these elements are removed with them, so farmers add fertilisers to put the elements back and improve plant growth.

The three elements needed in the largest amounts are nitrogen (N), phosphorus (P) and potassium (K). A fertiliser that supplies all three is called an NPK fertiliser. Ammonium salts and nitrates are used as fertilisers because they contain nitrogen and dissolve in water, so plant roots can absorb them.

Questions often ask which elements a fertiliser supplies, or the percentage by mass of nitrogen in a compound, which you work out from its formula.

Rules to remember

Common mistake

Counting only one nitrogen atom when the formula contains more. (NH4)2SO4 has two nitrogen atoms, and NH4NO3 also has two, one in each ion.

Worked example

What is the percentage by mass of nitrogen in ammonium phosphate, (NH4)3PO4? (H 1, N 14, O 16, P 31)

  1. Mr = (3 × 14) + (12 × 1) + 31 + (4 × 16) = 42 + 12 + 31 + 64 = 149.
  2. Mass of nitrogen in one formula unit = 3 × 14 = 42.
  3. Percentage of nitrogen = 42 ÷ 149 × 100 = 28.2%.

Answer: About 28% nitrogen by mass.

Practice questions

Try each one, then open the answer.

1. A 25 kg bag of NPK fertiliser contains 20% nitrogen by mass. What mass of nitrogen is in the bag?

  1. A
    20 kg
  2. B
    2.5 kg
  3. C
    10 kg
  4. D
    5.0 kg
Show answer

Answer: D. 20% of 25 kg = 0.20 × 25 = 5.0 kg of nitrogen. NPK fertilisers supply the three elements nitrogen, phosphorus and potassium that plants need for growth.

2. What is the percentage by mass of nitrogen in the fertiliser ammonium nitrate, NH4NO3? (H 1, N 14, O 16)

  1. A
    35%
  2. B
    17.5%
  3. C
    46%
  4. D
    28%
Show answer

Answer: A. Mr of NH4NO3 = 14 + 4 + 14 + 48 = 80. It contains two nitrogen atoms: 28 ÷ 80 × 100 = 35%. Ammonium salts and nitrates are used as fertilisers because they supply nitrogen.

3. A farmer spreads calcium hydroxide (slaked lime) and an ammonium fertiliser on a field at the same time. Why is this a bad idea?

  1. A
    the soil becomes very acidic
  2. B
    an insoluble ammonium salt forms
  3. C
    the lime makes the fertiliser dissolve and wash away faster
  4. D
    ammonia gas is given off, so nitrogen is lost from the soil
Show answer

Answer: D. Bases react with ammonium salts to release ammonia gas, for example Ca(OH)2 + 2NH4Cl → CaCl2 + 2H2O + 2NH3. The nitrogen escapes into the air.

More questions on this topic

Keep going

← WaterAir quality and climate →All rules on one pageStuck? Ask a question