Fertilisers: ammonium salts, nitrates and NPK
Identify the nutrient elements supplied by compounds and distinguish useful crop inputs from harmful runoff.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Identify element symbols and the difference between an element and a compound.
- Recall the environmental effect of nutrient inputs to water.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which formula contains nitrogen and potassium?
What you will learn
- State that ammonium salts and nitrates are used as fertilisers.
- Explain NPK as provision of nitrogen, phosphorus and potassium for improved plant growth.
Compounds can supply a nutrient element
Ammonium salts and nitrates are used as fertilisers. Ammonium nitrate, NH₄NO₃, contains nitrogen in both the ammonium and nitrate parts; those are different ions, not two different nitrogen elements. Potassium nitrate, KNO₃, contains both nitrogen and potassium. Recognise elements from a formula rather than assuming a compound's name gives every nutrient it supplies.
The elements are supplied in suitable compounds, rather than spreading reactive elemental potassium or nitrogen gas as though those were equivalent materials. A compound can supply more than one required nutrient, but that does not make every nitrate a complete NPK product. The identities and proportions of components need to match the intended fertiliser.
NPK refers to three nutrient elements
NPK fertilisers provide nitrogen (N), phosphorus (P) and potassium (K) for improved plant growth. N is not sodium (Na), and K is potassium rather than calcium (Ca). NPK can be provided through a suitable mixture of compounds; the initials do not represent one universal molecular formula with fixed ratios.
Appropriate nutrient provision can help crops, while excess nitrates or phosphates reaching water can contribute to deoxygenation and aquatic harm. Useful crop application and uncontrolled runoff therefore have different effects. In calculation practice here, hypothetical data state percentages of the elements explicitly; commercial fertiliser label conventions are not inferred from those invented values.
Pause and explain
Why are ammonium salts and nitrates relevant to fertilisers?
Worked example
A proposed mixture contains ammonium nitrate and potassium nitrate only. Which NPK elements are present, and which is not supplied by those formulas?
Show the worked solution
- NH₄NO₃ supplies nitrogen-containing ions.
- KNO₃ contains nitrogen and potassium.
- Neither given formula contains phosphorus, so the listed mixture does not supply all three NPK elements.
Answer Nitrogen and potassium are present; phosphorus is absent from the stated formulas.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the two named fertiliser compound groups and explain NPK.
Give me a hint
Give the nutrient elements, not invented ratios for one molecule.
Compare my reasoning
- Ammonium salts and nitrates are used as fertilisers.
- NPK supplies nitrogen, phosphorus and potassium.
- Suitable compounds can provide these elements for improved plant growth; NPK is not one universal fixed formula.
Ammonium salts and nitrates; NPK provides nitrogen, phosphorus and potassium for plant growth.
Look for these in your work
- Both compound groups are named.
- All three element names match their symbols.
Compare NH₄NO₃ and KNO₃ for NPK element content. Can either stated formula alone supply all three elements?
Give me a hint
Read the element symbols before inferring a complete product.
Compare my reasoning
- NH₄NO₃ contains N but neither P nor K.
- KNO₃ contains N and K but no P.
- Neither formula alone contains all three required NPK elements.
NH₄NO₃ supplies N; KNO₃ supplies N and K; neither given formula supplies P.
Look for these in your work
- Formula contents are read correctly.
- A partial nutrient set is not called complete NPK.
Hypothetical analytical data for 250 g of a mixture give 8% nitrogen, 3% phosphorus and 6% potassium by mass of the elements. Find each element's mass and explain why this calculation does not show that a spill into a pond is harmless.
Give me a hint
Multiply each explicitly elemental percentage by the total mass; then separate composition from environmental effects.
Compare my reasoning
- Nitrogen mass is 0.08×250=20 g; phosphorus is 0.03×250=7.5 g.
- Potassium mass is 0.06×250=15 g; the other mass is in other components, not assumed absent.
- The data describe the mixture, while nitrate/phosphate runoff can still contribute to water deoxygenation and aquatic harm.
20 g N, 7.5 g P and 15 g K; nutrient content does not establish harmless runoff.
Look for these in your work
- Each percentage uses the stated elemental basis.
- The environmental conclusion is not inferred from nutrient benefit alone.
Common mistakes
- Reading K as calcium or N as sodium.
- Calling NPK one fixed molecular formula.
- Assuming any nitrate supplies all three nutrient elements.
- Treating useful crop nutrition as evidence that unlimited runoff is harmless.
What can you explain now?
Name the three elements represented by NPK.
Compare with the explanation
Nitrogen, phosphorus and potassium.
N, P and K are element symbols; the nutrients can be supplied through suitable compounds.
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Make it stick
Name the NPK elements, read their presence in two formulas and explain why a nutrient-supplying fertiliser can still damage a water body through runoff.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.