What are fertilisers and why are they important in agriculture?

Fertilisers are substances that are added to soil or plants to supply essential nutrients that promote plant growth and increase crop yields. They are important in agriculture because they replenish soil nutrients that are depleted by continuous farming, thus ensuring sustainable food production. Fertilisers can be organic, such as compost and manure, or inorganic, like ammonium nitrate and superphosphate.

How do fertilisers impact the environment?

Fertilisers can have significant environmental impacts if not used properly. Excessive use can lead to nutrient runoff into water bodies, causing eutrophication, which depletes oxygen in water and harms aquatic life. Additionally, the production and application of fertilisers contribute to greenhouse gas emissions. It is crucial to use fertilisers efficiently to minimize these environmental effects.

What are the differences between organic and inorganic fertilisers?

Organic fertilisers are derived from natural sources such as plant or animal waste, and they release nutrients slowly as they decompose. Inorganic fertilisers, also known as synthetic or chemical fertilisers, are manufactured and provide nutrients in a readily available form. While organic fertilisers improve soil structure and microbial activity, inorganic fertilisers offer precise nutrient content and faster results.

What role do nitrogen, phosphorus, and potassium play in plant growth?

Nitrogen, phosphorus, and potassium are the primary nutrients found in fertilisers, often referred to as NPK. Nitrogen is crucial for leaf growth and photosynthesis, phosphorus is important for root development and energy transfer, and potassium enhances overall plant health and disease resistance. A balanced supply of these nutrients is essential for optimal plant growth and productivity.

How can the use of fertilisers be optimized to reduce environmental impact?

To optimize fertiliser use and reduce environmental impact, farmers can adopt practices such as soil testing to determine nutrient needs, using slow-release fertilisers, and employing precision agriculture techniques to apply the right amount of fertiliser at the right time and place. Crop rotation and the use of cover crops can also help maintain soil fertility and reduce the need for chemical fertilisers.

Why is "Saying fertiliser directly poisons fish" a common mistake in Fertilisers?

Why it happens: Skipping the chain of events. How to avoid it: The mechanism is INDIRECT — algal bloom → decomposition → oxygen depletion → fish die. Source: 0620/42 — recurring

Why is "Saying ammonia + sulfuric acid gives ammonium nitrate" a common mistake in Fertilisers?

Why it happens: Mixing acids. How to avoid it: NH_3 + HNO_3 NH_4NO_3. With H_2SO_4 you'd get ammonium sulfate.

Why is "Linking phosphorus to leaf growth" a common mistake in Fertilisers?

Why it happens: Mixing N with P. How to avoid it: N: leaves. P: roots. K: flowers/fruit.

Chemistry of the EnvironmentCambridge IGCSE Chemistry (0620)

Fertilisers

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Fertilisers — Cambridge IGCSE 0620 Chemistry Extended (2026)

NPK fertilisers (nitrogen, phosphorus, potassium) — boost crop yield. Commonly: ammonium nitrate, ammonium sulfate, potassium chloride. Plus eutrophication risks.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

12.3 — Identify NPK as the three essential plant nutrients.
12.3 — Name common nitrogen, phosphorus, and potassium fertilisers.
12.3 — Describe production of ammonium nitrate from ammonia and nitric acid.
12.3 — Describe eutrophication.

NPK and what each element does

Nitrogen for leaves, phosphorus for roots, potassium for flowers/fruits.

Three macronutrients.

Nitrogen (N): needed for proteins, amino acids, chlorophyll. Essential for green leafy growth.
Phosphorus (P): in DNA, ATP, phospholipids. Essential for root development.
Potassium (K): regulates water balance, enzyme function. Essential for flowering and fruiting.

Common fertiliser ingredients.

Element	Common compounds
N	Ammonium nitrate, ammonium sulfate, urea
P	Ammonium phosphate, calcium phosphate, superphosphate
K	Potassium chloride, potassium sulfate

NPK ratio on bag. Commercial fertiliser bags state percentages: e.g. " $10$ - $5$ - $8$ " means $10\%$ N, $5\%$ P, $8\%$ K (by mass).

Worked qualitative. A farmer wants to boost grass growth (leafy). Should pick a fertiliser HIGH in N. Most commercial lawn fertilisers have N as the dominant element.

N: leaves. P: roots. K: flowers/fruits.
NPK ratio on bag = % by mass of each.
Common N: ammonium nitrate, ammonium sulfate, urea.
Common P: ammonium phosphate.
Common K: potassium chloride.

Making ammonium nitrate (and other N fertilisers)

$\text{NH}_{3} + \text{HNO}_{3} \to \text{NH}_{4}\text{NO}_{3}$ . Neutralisation reaction.

Ammonium nitrate is one of the most widely used N fertilisers. Made by: $\text{NH}_{3} + \text{HNO}_{3} \to \text{NH}_{4}\text{NO}_{3}.$

This is a neutralisation between ammonia (a base) and nitric acid.

Why so popular?

Very high N content (35% by mass).
Soluble in water → easy for plants to absorb.
Cheap to make from cheap inputs (NH₃ from Haber, HNO₃ from oxidising NH₃ industrially).

Other N fertilisers.

Ammonium sulfate: $2\text{NH}_{3} + \text{H}_{2}\text{SO}_{4} \to (\text{NH}_{4})_{2}\text{SO}_{4}$ . About 21% N.
Urea $(\text{CO(NH}_{2}\text{)}_{2})$ : about 46% N — highest. Made from CO₂ + NH₃ at high pressure.

Worked. Calculate %N in ammonium nitrate.

$M_r$ : $14 + 4 + 14 + 48 = 80$ .
N atoms: 2.
$\%$ N = $(2 \times 14)/80 \times 100 = 35\%$ .

Worked. Calculate %N in ammonium sulfate.

$M_r$ : $2(14 + 4) + 32 + 64 = 132$ .
N atoms: 2.
$\%$ N = $(2 \times 14)/132 \times 100 \approx 21.2\%$ .

Cambridge tip. When asked which fertiliser is "best", consider %N AND price/availability. Ammonium nitrate has the second-highest %N (after urea) and is widely used.

$\text{NH}_{3} + \text{HNO}_{3} \to \text{NH}_{4}\text{NO}_{3}$ .
%N: ammonium nitrate $35\%$ , ammonium sulfate $21\%$ , urea $46\%$ .
Calculate %N: $(2 \times 14)/M_r \times 100$ .
Higher %N → less mass to spread for same N delivered.

Eutrophication — the dark side

Run-off carries fertiliser to lakes/rivers → algal bloom → O₂ depletion → fish kill.

The problem. Modern intensive farming applies large amounts of fertiliser. Heavy rain washes some away (run-off) into lakes, rivers, and groundwater.

The chain of events.

Fertiliser run-off enters a water body.
Excess N and P stimulate massive algal growth ("algal bloom"). Often green or blue-green.
Algae cover the surface, blocking sunlight to plants below.
Underwater plants die (no photosynthesis).
Aerobic bacteria decompose the dead plant matter, USING UP dissolved oxygen.
Fish and other aerobic organisms die from low oxygen.
Anaerobic bacteria take over → "dead zone".

Result. Once-thriving water body becomes lifeless and stinking — full eutrophication.

How to reduce.

Use less fertiliser (precision farming, soil testing).
Buffer strips of vegetation along waterways to absorb run-off.
Avoid applying fertiliser before heavy rain or in winter.
Use slow-release / coated fertilisers.

Worked qualitative. Why does the dissolved oxygen DROP during a bloom, given algae produce oxygen? Algae produce O₂ during DAY (photosynthesis). At NIGHT and after they DIE, bacteria decomposing them USE oxygen. Net effect: dissolved oxygen drops, especially after the bloom dies.

Run-off → algal bloom.
Algae block sunlight; underwater plants die.
Bacteria decompose plants → use up O₂.
Fish die. Dead zone.
Reduce: less fertiliser, buffer strips, slow-release.

How it’s examined

Fertilisers appear most years on Paper 4 (4-6 marks): name compounds, calculate %N, balance NH₃ + acid equation, describe eutrophication. Examiner reports flag students confusing eutrophication causes (it's run-off, not direct fertiliser application).

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (12.3); 0620/42 Oct/Nov 2024 — Q20 (fertilisers, eutrophication); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Fertilisers

Step-by-step solutions to past-paper-style questions on fertilisers, written exactly the way a tutor would explain them at the board.

1Identify the NPK elements
Core• NPK
Question
What are the three macronutrients in an NPK fertiliser, and what does each promote?
Step-by-step solution
1. Step 1
  N (nitrogen): leaf growth, protein synthesis.
2. Step 2
  P (phosphorus): root development, energy transfer.
3. Step 3
  K (potassium): flowering, fruit, disease resistance.
Answer
N: leaves. P: roots. K: flowers/fruit.
2Make ammonium nitrate fertiliser
Extended• Adapted from 0620/22 May/Jun 2024 Q14• fertiliser
Question
Write the equation for the reaction making ammonium nitrate fertiliser from ammonia.
Step-by-step solution
1. Step 1
  Ammonia + nitric acid → ammonium nitrate.
  $\text{NH}_{3} + \text{HNO}_{3} \to \text{NH}_{4}\text{NO}_{3}$
Answer
$\text{NH}_{3} + \text{HNO}_{3} \to \text{NH}_{4}\text{NO}_{3}$
3Eutrophication explained
Extended• eutrophication
Question
Explain how excess fertiliser run-off causes fish to die in a river.
Step-by-step solution
1. Step 1
  Run-off increases nutrient (N, P) levels in the river.
2. Step 2
  Algae grow rapidly (algal bloom) → block sunlight → underwater plants die.
3. Step 3
  Bacteria decompose dead plants/algae using up dissolved oxygen.
4. Step 4
  Fish die from lack of oxygen.
Answer
Algal bloom → plant death → bacterial decomposition → oxygen depletion → fish kill.

Key Definitions and Keywords — Fertilisers

Definitions to memorise and the exact keywords mark schemes credit for fertilisers answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Fertiliser
Examiner keyword
A substance added to soil to provide nutrients for plant growth.
NPK fertiliser
Examiner keyword
A compound fertiliser containing all three of nitrogen, phosphorus and potassium.
Eutrophication
Examiner keyword
Over-fertilisation of water bodies causing algal blooms and oxygen depletion.

Common Mistakes and Misconceptions — Fertilisers

The traps other students keep falling into on fertilisers questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying fertiliser directly poisons fish
0620/42 — recurring
Why it happens
Skipping the chain of events.
How to avoid it
The mechanism is INDIRECT — algal bloom → decomposition → oxygen depletion → fish die.
✕Saying ammonia + sulfuric acid gives ammonium nitrate
Why it happens
Mixing acids.
How to avoid it
$\text{NH}_{3} + \text{HNO}_{3} \to \text{NH}_{4}\text{NO}_{3}$ . With $\text{H}_{2}\text{SO}_{4}$ you'd get ammonium sulfate.
✕Linking phosphorus to leaf growth
Why it happens
Mixing N with P.
How to avoid it
N: leaves. P: roots. K: flowers/fruit.

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Past paper style quiz

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Fertilisers — frequently asked questions

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Fertilisers

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Identify the NPK elements

2Make ammonium nitrate fertiliser

3Eutrophication explained

Fertiliser

NPK fertiliser

Eutrophication

✕Saying fertiliser directly poisons fish

✕Saying ammonia + sulfuric acid gives ammonium nitrate

✕Linking phosphorus to leaf growth

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Fertilisers — frequently asked questions