The shape of the lesson
By the end of the lesson, learners can
- What is in an NPK fertiliser?: Add an alkali to an ammonium salt in the simulator and test the gas given off.
- Making a fertiliser salt: Ammonia is an alkali, so it neutralises acids to give ammonium salts.
- Why plants need N, P and K: Extended.
- Nitrogen: ammonium salts and nitrates.
- Ammonium salts: ammonia + acid, one product only.
- % N: (number of N atoms × 14) ÷ Mr × 100.
- Extended: N proteins and leaves, P roots, K flowers and fruit.
One picture every idea returns to
Ammonium salts (such as ammonium nitrate and ammonium sulfate) and nitrates (such as potassium nitrate) are used as fertilisers to increase crop yields. NPK fertilisers provide the three elements plants need most: nitrogen (N), phosphorus (P) and potassium (K). Ammonium fertiliser salts are made by neutralising ammonia with an acid, for example NH3 + HNO3 → NH4NO3. Extended: plants need nitrogen to make proteins for growth (healthy leaves), phosphorus for root growth, and potassium for flowers and fruit and resistance to disease.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Feeding crops with N, P and K.. Every harvest removes nitrogen, phosphorus and potassium from the soil. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: What is in an NPK fertiliser? | Add an alkali to an ammonium salt in the simulator and test the gas given off. Project tab 2 of the lab (Nitrogen and sulfur lab, ammonium salts) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Making a fertiliser salt | Ammonia is an alkali, so it neutralises acids to give ammonium salts. | Pairs |
| 21 to 30 min | Teach and check: Why plants need N, P and K | Extended. | Corners |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why plants need N, P and K, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Plants take up ions such as nitrate and ammonium dissolved in soil water, not nitrogen gas from the air, which is why the 78% nitrogen in air does not feed most crops. | Fertilisers must be soluble. And too much fertiliser washes into rivers, where nitrates and phosphates cause deoxygenation of the water. |
Fertilisers: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Name the three elements supplied by an NPK fertiliser and give a compound that could supply nitrogen.
Write a balanced equation for the reaction of ammonia with sulfuric acid to make ammonium sulfate.
Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3. (Ar: H = 1, N = 14, O = 16)
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. (Ar: H = 1, N = 14, O = 16, S = 32)
(Extended) Explain why plants need nitrogen, phosphorus and potassium.
Explain why an ammonium fertiliser should not be added to soil at the same time as calcium hydroxide.
Answer key and marking notes
- Nitrogen, phosphorus and potassium.
- Ammonium nitrate (or ammonium sulfate, or potassium nitrate).
- 2NH3 + H2SO4 → (NH4)2SO4
- Two NH3 are needed because H2SO4 provides two H+ ions.
- Mr = 14 + 4 + 14 + 48 = 80
- Mass of nitrogen = 2 × 14 = 28
- % N = 28 ÷ 80 × 100 = 35.0%
- Mr = 2 × 18 + 32 + 64 = 132
- % N = 28 ÷ 132 × 100 = 21.2%
- Ammonium nitrate supplies more nitrogen per kilogram (35.0% compared with 21.2%).
- Nitrogen: to make proteins, for growth and healthy leaves.
- Phosphorus: for root growth.
- Potassium: for flowers and fruit (and resistance to disease).
- 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.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
An NPK fertiliser supplies three elements needed by plants. What are they?
- Anitrogen, phosphorus and potassium
- Bnitrogen, phosphorus and krypton
- Cneon, phosphorus and potassium
- Dnitrogen, phosphorus and calcium
Which compound would be a suitable nitrogen fertiliser?
- Aammonium nitrate
- Bcalcium carbonate
- Csodium chloride
- Dpotassium chloride
Which equation shows the formation of ammonium sulfate, used as a fertiliser?
- A2NH3 + H2SO4 → (NH4)2SO4
- BNH3 + H2SO4 → NH4SO4
- C2NH3 + H2SO4 → (NH4)2SO4 + H2O
- DN2 + H2SO4 → (NH4)2SO4
Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3. (Ar: H = 1, N = 14, O = 16)
- A35.0%
- B17.5%
- C60.0%
- D46.7%
A farmer wants one fertiliser compound that supplies both potassium and nitrogen. Which should he choose?
- Apotassium nitrate, KNO3
- Bammonium nitrate, NH4NO3
- Cpotassium chloride, KCl
- Dammonium phosphate, (NH4)3PO4
A crop has poor root growth but healthy green leaves. Which element is most likely to be lacking?
- Aphosphorus
- Bnitrogen
- Cpotassium
- Dcarbon
Why do plants need nitrogen?
- Ato make proteins needed for growth
- Bto make chlorophyll absorb oxygen
- Cfor flowering and fruit formation only
- Dto take in nitrogen gas through the leaves
Which fertiliser contains the greatest percentage by mass of nitrogen? (Ar: H = 1, N = 14, O = 16, S = 32, P = 31, K = 39)
- Aammonium nitrate, NH4NO3 (Mr 80)
- Bammonium sulfate, (NH4)2SO4 (Mr 132)
- Cammonium phosphate, (NH4)3PO4 (Mr 149)
- Dpotassium nitrate, KNO3 (Mr 101)
Quiz key
N is nitrogen, P is phosphorus and K is potassium (from its Latin name kalium). Krypton is Kr, a noble gas of no use as a fertiliser. Neon is Ne, also a noble gas. Calcium is Ca, which is not one of the three NPK elements.
Ammonium nitrate, NH4NO3, contains nitrogen in both its ions and is soluble. Calcium carbonate contains no nitrogen; it is used to neutralise acidic soil. Sodium chloride contains no nitrogen and harms plants. Potassium chloride supplies potassium but not nitrogen.
Sulfate has a 2− charge, so two ammonium ions are needed: (NH4)2SO4, made from two NH3 molecules each taking an H+ from the acid. NH4SO4 has the wrong formula. Ammonia neutralising an acid gives only the salt, so adding water leaves the equation unbalanced. Nitrogen gas does not react with sulfuric acid; ammonia is needed.
Mr = 14 + 4 + 14 + 48 = 80. There are two N atoms, mass 28, so % N = 28 ÷ 80 × 100 = 35.0%. 17.5% counts only one nitrogen atom. 60.0% is the percentage of oxygen (48 ÷ 80). 46.7% is the result of dividing 28 by 60, which leaves the hydrogen and one nitrogen out of Mr.
Potassium nitrate contains both K and N. Ammonium nitrate supplies nitrogen only. Potassium chloride supplies potassium only. Ammonium phosphate supplies nitrogen and phosphorus, but no potassium.
Phosphorus is needed for root growth, so a shortage shows as poor roots. Nitrogen is needed for proteins and leaf growth; the leaves are healthy, so nitrogen is not lacking. Potassium is needed for flowers, fruit and disease resistance. Carbon comes from carbon dioxide in the air by photosynthesis and is not supplied by fertilisers.
Plants use nitrogen to make proteins, which they need for growth, especially of stems and leaves. Chlorophyll absorbs light, not oxygen. Flowering and fruit are mainly linked to potassium. Most plants cannot use nitrogen gas from the air; they take in dissolved nitrate or ammonium ions through their roots.
Ammonium nitrate: 28 ÷ 80 = 35.0%. Ammonium sulfate: 28 ÷ 132 = 21.2%. Ammonium phosphate: 42 ÷ 149 = 28.2%; it has three N atoms, but the heavy phosphate ion lowers the percentage. Potassium nitrate: 14 ÷ 101 = 13.9%, since it has only one N atom and a heavy potassium atom.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.