Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want nitrogen and fertilisers — the nitrogen cycle, NPK labels and eutrophication — to become reliable marks instead of half-remembered “plants need nitrogen” answers.
What query it owns: how to understand and revise nitrogen and fertilisers in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the nitrogen-and-fertilisers revision-guide angle, while Tutopiya’s Nitrogen and Fertilisers subtopic page owns the learning resource and the free Nitrogen and Fertilisers quiz owns the practice.

Nitrogen is essential for plant growth — it is a key component of proteins and chlorophyll. Farmers add fertilisers to replace nutrients removed at harvest, but excess nitrogen can pollute water and cause eutrophication. Cambridge IGCSE Coordinated Science (0654) tests the nitrogen cycle, NPK fertiliser labels, and the environmental impact of agricultural runoff. This guide covers the syllabus definitions, process tables examiners expect, and the question types that appear every year.

Key takeaways

• Nitrogen fixation converts atmospheric N₂ into usable compounds (by lightning, bacteria or the Haber process).
• Nitrifying bacteria convert ammonia to nitrites and then nitrates that plants absorb.
• Denitrifying bacteria return nitrates to nitrogen gas, completing the cycle.
• NPK fertilisers supply nitrogen (N), phosphorus (P) and potassium (K) — the numbers show relative proportions.
• Excess fertiliser runoff causes eutrophication in lakes and rivers.

What are nitrogen and fertilisers in Cambridge IGCSE Coordinated Science?

Nitrogen cycling connects biology and environmental chemistry. Plants need nitrates for growth; bacteria in soil and root nodules convert nitrogen between different forms. Fertilisers add N, P and K to boost crop yields, but when rain washes unused fertiliser into waterways, algal blooms and oxygen depletion follow. The syllabus expects you to describe the nitrogen cycle, interpret NPK labels, and explain eutrophication step by step.

Read the full notes on Tutopiya’s Nitrogen and Fertilisers subtopic page before attempting questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Nitrogen fixation	N₂ → usable nitrogen compounds	”Name a process that adds nitrogen to soil.”
Nitrification	Ammonia → nitrites → nitrates	”Describe the role of nitrifying bacteria.”
Denitrification	Nitrates → N₂ gas	”Explain how nitrogen returns to the atmosphere.”
NPK fertilisers	N, P, K for plant growth	”State what the numbers on a fertiliser bag mean.”
Eutrophication	Excess nutrients → algal bloom → deoxygenation	”Describe the effect of fertiliser in a lake.”

The nitrogen cycle — key stages

Stage	Organism / process	Conversion
Nitrogen fixation	Lightning, Rhizobium bacteria, Haber process	N₂ → NH₃ / nitrates
Nitrification	Nitrifying bacteria in soil	NH₃ → NO₂⁻ → NO₃⁻
Uptake by plants	Roots absorb nitrates	NO₃⁻ used to make proteins
Decomposition	Decomposers break down dead matter	Proteins → ammonia
Denitrification	Denitrifying bacteria (anaerobic)	NO₃⁻ → N₂

NPK fertilisers — what the label means

Letter	Element	Role in plants	Deficiency symptom
N (Nitrogen)	Protein and chlorophyll synthesis	Stunted growth, yellow leaves
P (Phosphorus)	Root development, flowering	Poor root growth, delayed maturity
K (Potassium)	Enzyme activation, disease resistance	Brown leaf edges, weak stems

A label such as 15-15-15 means equal proportions of N, P and K. Higher N promotes leafy growth; higher P supports roots and flowers.

Eutrophication from fertiliser runoff — step by step

1. Rain washes excess nitrate and phosphate from fields into rivers or lakes.
2. Rapid algal growth (algal bloom) on the water surface.
3. Light blocked from submerged plants, which die.
4. Aerobic bacteria decompose dead organic matter, using dissolved oxygen.
5. Oxygen depletion kills fish and aquatic invertebrates.

Nitrogen and fertilisers in past-paper wording

Command word	What the question wants	Typical stem
Describe	Sequence in nitrogen cycle or eutrophication	”Describe the nitrogen cycle.”
Explain	Cause and effect	”Explain why farmers use fertilisers.”
State	Named bacterium or nutrient	”State one role of nitrogen in plants.”
Suggest	Environmental consequence or solution	”Suggest why fertiliser harms a lake.”
Define	Eutrophication	”Define eutrophication.”

Worked exam-style stems

1. “Explain why farmers add fertilisers to soil.” Harvesting removes nutrients; fertilisers replace nitrogen, phosphorus and potassium so crops grow faster and yields increase. Reward: nutrient removal + replacement + improved growth.
2. “Describe how excess fertiliser affects a lake ecosystem.” Nitrates/phosphates enter water → algal bloom → submerged plants die → bacteria decompose dead matter using oxygen → oxygen falls → fish die. Reward: ordered eutrophication steps.
3. “State the role of denitrifying bacteria in the nitrogen cycle.” They convert nitrates in soil back to nitrogen gas, returning nitrogen to the atmosphere. Reward: nitrates → N₂ + named role.

Practise on the Nitrogen and Fertilisers quiz.

How nitrogen and fertilisers connect to the syllabus

Nitrogen links to Plant Nutrition (mineral ions for growth) and Human Influences on Ecosystems (water pollution). The Cambridge IGCSE Coordinated Science resource hub links every ecology and chemistry subtopic.

Common mistakes students make

• Confusing nitrogen fixation (N₂ → compounds) with denitrification (nitrates → N₂).
• Saying plants absorb nitrogen gas directly (they absorb nitrates).
• Describing eutrophication without the oxygen depletion step.
• Mixing up N, P and K roles on fertiliser labels.
• Forgetting Rhizobium bacteria in root nodules of legumes as a fixation example.

When you need more support

If nitrogen-cycle and eutrophication questions keep costing marks, work through the Nitrogen and Fertilisers quiz, then get help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Why do plants need nitrogen? Nitrogen is used to make proteins and chlorophyll, which are essential for growth and photosynthesis.

What does NPK stand for on a fertiliser bag? Nitrogen (N), phosphorus (P) and potassium (K) — the three main mineral nutrients for plant growth.

What is eutrophication? Excess nitrates and phosphates in water cause rapid algal growth, leading to oxygen depletion and death of aquatic organisms.

How do I revise nitrogen and fertilisers effectively? Learn the nitrogen-cycle stages, NPK roles, and eutrophication steps, then take the quiz.

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