Fertilisers (NPK)

Fertiliser questions reward one matching exercise and one reaction. Match N, P and K to their plant functions, then explain why warming an ammonium salt with a base releases ammonia. That pair covers nearly every mark Cambridge has set on this subtopic. It is the shortest section of Chemistry of the Environment, and it overlaps usefully with salts and gas tests.

Why crops need fertilisers

Growing plants take nitrogen, phosphorus and potassium compounds from the soil; harvesting removes them for good. Fertilisers replace these elements so crop yields stay high. NPK fertilisers supply all three in soluble forms. Solubility matters, because plants absorb nutrients through their roots as dissolved ions.

N, P, K: element by element

Element	Supplied as	What it does for the plant
Nitrogen (N)	Ammonium salts (NH4+), nitrates (NO3−), urea	Making proteins; healthy leaf and stem growth
Phosphorus (P)	Phosphates (PO43−)	Root growth and development
Potassium (K)	Potassium salts, e.g. KCl, K2SO4	Healthy leaves; flower and fruit growth

The mark sits in the pairing. “Plants need nitrogen to grow” is too loose; “nitrogen is needed to make proteins / for leaf growth” scores. A question may also ask you to pick the fertiliser compound supplying two elements at once: ammonium phosphate, (NH4)3PO4, covers both N and P, and reading the formula is the skill being tested.

Ammonium nitrate, NH4NO3, is the workhorse example: highly soluble and rich in nitrogen, made by neutralising nitric acid with ammonia (NH3 + HNO3 → NH4NO3), a direct application of acids, bases and salts.

Ammonium salts and bases: the reaction that wastes nitrogen

Warm any ammonium salt with a base and ammonia gas is released:

NH4Cl + NaOH → NaCl + H2O + NH3

(Ionic version, Supplement: NH4+ + OH− → NH3 + H2O.)

This one reaction does double duty in 0620:

1. It is the lab test for ammonium ions. Warm the unknown with sodium hydroxide solution; ammonia gas turns damp red litmus paper blue. Ammonia is the only common alkaline gas, so the litmus result is conclusive. This reappears in tests for ions and gases.
2. It explains a farming decision. Farmers add lime (calcium hydroxide or calcium oxide) to neutralise acidic soil. Spread lime and ammonium fertiliser together and the base reacts with the ammonium salt, releasing ammonia to the air. The expensive nitrogen literally evaporates. So the two are not applied at the same time.

The exam question links the chemistry to the consequence, and the consequence mark needs the loss spelled out: nitrogen is lost as ammonia gas, so the fertiliser is wasted / less nitrogen reaches the crop.

Worked exam question

A fertiliser contains ammonium sulfate. (a) State the essential element supplied by ammonium sulfate and its purpose in the plant. [2] (b) A farmer also adds lime, calcium hydroxide, to the same field on the same day. Explain why this wastes the fertiliser. Include the name of the gas produced. [3] (c) Describe how damp litmus paper identifies this gas. [1]

Model answer: (a) Nitrogen (1); needed for making proteins / healthy leaf growth (1). (b) Calcium hydroxide is a base (1); it reacts with the ammonium salt releasing ammonia gas (1); so nitrogen escapes into the air and less is available to the crop (1). (c) Ammonia turns damp red litmus paper blue (1).

Mark-by-mark: (a) the element is nitrogen, not “ammonium”; the purpose mark requires the protein/leaf link. (b) three separate ideas: base identified, reaction producing ammonia named, nitrogen loss stated. Stopping after “ammonia is made” leaves the third mark unclaimed. (c) the litmus must be damp and red, turning blue.

The mistakes that cost marks

1. Function pairings shuffled. Nitrogen ↔ leaves/proteins, phosphorus ↔ roots, potassium ↔ flowers/fruit. A mnemonic helps: N for leaves at the top, P for roots at the bottom, K for what’s left.
2. “Ammonium” given as the essential element. The element is nitrogen. Ammonium is the ion that carries it.
3. The base reaction without the consequence. The farming question is marked on the loss of nitrogen to the air, not just on naming ammonia.
4. Dry litmus in the gas test. The paper must be damp (ammonia dissolves in the water film) and the change is red to blue. “Litmus changes colour” earns nothing.

How examiners want it phrased

Student wording	Mark-scheme wording
”Plants need fertiliser to grow big"	"Nitrogen is needed for making proteins and for leaf growth"
"Lime ruins the fertiliser"	"The base reacts with the ammonium salt, releasing ammonia, so nitrogen is lost from the soil"
"The gas is ammonia because it smells"	"The gas turns damp red litmus paper blue, so it is ammonia"
"NPK is plant food"	"NPK fertilisers supply nitrogen, phosphorus and potassium in soluble compounds”

Bank these marks in one revision session, then spend the saved time on the heavier air quality and climate change material. Want a tutor to confirm which topics deserve your remaining weeks? That is precisely what the free trial lesson is for.

Test yourself

Three quick questions. Answer first, click after.

Q1 (3 marks). State the purpose in the plant of each essential element: nitrogen, phosphorus, potassium.

Show answer

• nitrogen: making proteins / healthy leaf and stem growth [1] • phosphorus: root growth [1] • potassium: healthy leaves / flower and fruit growth [1]

Q2 (2 marks). Ammonium nitrate is made by neutralising an acid with ammonia. Name the acid and write the symbol equation.

Show answer

• nitric acid [1] • NH3 + HNO3 → NH4NO3 [1]

Q3 (2 marks). A fertiliser contains ammonium phosphate, (NH4)3PO4. State the two essential elements it supplies and how the formula shows this.

Show answer

• nitrogen and phosphorus [1] • the NH4+ ions contain nitrogen and the PO43− ion contains phosphorus [1]