IGCSE · Practice questions · Fertilisers

Put back what crops take.

Six original Cambridge-style questions on fertilisers, including a percentage-by-mass calculation.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

N, P, K and the salts that carry them.

Nitrogen: ammonium salts and nitrates.
Ammonium salts: ammonia + acid, one product only.
% N: (number of N atoms × 14) ÷ M_r × 100.
Extended: N proteins and leaves, P roots, K flowers and fruit.

Recall

[2 marks]

Name the three elements supplied by an NPK fertiliser and give a compound that could supply nitrogen.

Nitrogen, phosphorus and potassium. ✓
Ammonium nitrate (or ammonium sulfate, or potassium nitrate). ✓

Recall

[2 marks]

Write a balanced equation for the reaction of ammonia with sulfuric acid to make ammonium sulfate.

2NH₃ + H₂SO₄ → (NH₄)₂SO₄ ✓
Two NH₃ are needed because H₂SO₄ provides two H⁺ ions. ✓

Calculation

[3 marks]

Calculate the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃. (A_r: H = 1, N = 14, O = 16)

M_r = 14 + 4 + 14 + 48 = 80 ✓
Mass of nitrogen = 2 × 14 = 28 ✓
% N = 28 ÷ 80 × 100 = 35.0% ✓

Calculation

[3 marks]

A farmer can buy ammonium nitrate (35.0% N) or ammonium sulfate. Calculate the percentage of nitrogen in ammonium sulfate, (NH₄)₂SO₄, and state which fertiliser supplies more nitrogen per kilogram. (A_r: H = 1, N = 14, O = 16, S = 32)

M_r = 2 × 18 + 32 + 64 = 132 ✓
% N = 28 ÷ 132 × 100 = 21.2% ✓
Ammonium nitrate supplies more nitrogen per kilogram (35.0% compared with 21.2%). ✓

Explain

[3 marks]

(Extended) Explain why plants need nitrogen, phosphorus and potassium.

Nitrogen: to make proteins, for growth and healthy leaves. ✓
Phosphorus: for root growth. ✓
Potassium: for flowers and fruit (and resistance to disease). ✓

Explain

[2 marks]

Explain why an ammonium fertiliser should not be added to soil at the same time as calcium hydroxide.

Calcium hydroxide is an alkali; it reacts with ammonium salts to release ammonia gas. ✓
The nitrogen escapes into the air as ammonia, so it is lost to the plants. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.