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Chemistry · Practice book · Senior 4 · Unit 15 of 17

Fertilisers

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 15 Fertilisers

Key unit competence: Evaluate the composition, manufacture and use of different fertilisers in relation to soil fertility, crop yield and environmental impact.

Key ideas

  • Fertilisers supply plant nutrients. Organic fertilisers: farmyard manure, compost, green manure, bone meal. Inorganic (chemical) fertilisers: straight (one nutrient, e.g. urea, ammonium nitrate, superphosphate, potassium chloride) and compound/mixed (NPK).
  • Macronutrients: nitrogen (N, leaf growth and proteins), phosphorus (P, roots, flowers and energy transfer), potassium (K, fruit quality and disease resistance); secondary: Ca, Mg, S; micronutrients: Fe, Zn, Cu, B, Mn, Mo.
  • Characteristics of a good fertiliser: soluble enough to be absorbed but not easily washed away, supplies nutrients in the right amounts, does not change soil pH badly, safe to store and handle, affordable.
  • Manufacture: ammonia (Haber process) → nitric acid (Ostwald) → ammonium nitrate (NH3 + HNO3 → NH4NO3); ammonium sulfate (2NH3 + H2SO4 → (NH4)2SO4); urea (2NH3 + CO2 → CO(NH2)2 + H2O); superphosphate from phosphate rock and sulfuric acid (Ca3(PO4)2 + 2H2SO4 → Ca(H2PO4)2 + 2CaSO4).
  • Percentage of nutrient = (mass of element in formula ÷ Mr) × 100. NPK 17-17-17 means 17% N, 17% P2O5 and 17% K2O by mass.
  • Advantages of organic fertilisers: improve soil structure and water retention, release nutrients slowly, cheap and local; disadvantages: bulky, low nutrient content. Inorganic: concentrated and fast-acting but can acidify soil, cause eutrophication and be expensive. Substandard (fake or low-quality) fertilisers waste money, lower yields and may contain harmful impurities.

Common misconceptions

  • "More fertiliser always gives a bigger harvest." No: Beyond the optimum, extra fertiliser wastes money, damages plants and pollutes water.
  • "Organic fertilisers contain no nutrients." No: They contain N, P, K and micronutrients, though in lower concentrations.
  • "NPK 17-17-17 means 17% of each element N, P and K." No: The P and K values are expressed as P2O5 and K2O, not as the elements.

Worked examples

Example 1. Calculate and compare the percentage of nitrogen in ammonium nitrate and ammonium sulfate. (N 14, H 1, O 16, S 32)

Solution

NH4NO3: Mr = 80; N = 28; % N = 28 ÷ 80 × 100 = 35.0%.

(NH4)2SO4: Mr = 132; N = 28; % N = 28 ÷ 132 × 100 = 21.2%.

Ammonium nitrate supplies more nitrogen per kg; ammonium sulfate also supplies sulfur.

Example 2. Describe the manufacture of ammonium nitrate from raw materials.

Solution

Nitrogen (from air) and hydrogen (from natural gas or electrolysis of water) react in the Haber process to form ammonia.

Some ammonia is oxidised in the Ostwald process to nitric acid.

Ammonia is neutralised with nitric acid: NH3 + HNO3 → NH4NO3.

The solution is evaporated and the product is granulated (prilled).

Practice questions

  1. Name the three major plant nutrients.

    Show answer

    Nitrogen, phosphorus and potassium.

    Common mistake: Naming sodium.

  2. Give two examples of organic fertilisers.

    Show answer

    Farmyard manure, compost, green manure, bone meal (any two).

    Common mistake: Naming urea.

  3. Write the equation for the formation of ammonium sulfate.

    Show answer

    2NH3 + H2SO4 → (NH4)2SO4.

    Common mistake: Writing NH3 + H2SO4 → NH4SO4.

  4. What does NPK 20-10-10 mean?

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    20% N, 10% P2O5, 10% K2O by mass.

    Common mistake: Saying 20 kg of N.

  5. Give two characteristics of a good fertiliser.

    Show answer

    Suitable solubility, correct nutrient content, safe, affordable, does not harm soil (any two).

    Common mistake: Saying 'very insoluble'.

  6. State two advantages of inorganic fertilisers.

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    Concentrated, fast-acting, easy to apply, known composition (any two).

    Common mistake: Saying they improve soil structure.

  7. Give two environmental effects of overusing fertilisers.

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    Eutrophication, soil acidification, groundwater nitrate pollution, loss of soil life (any two).

    Common mistake: Saying ozone depletion.

  8. Why do farmers lime soil after years of using ammonium fertilisers?

    Show answer

    To neutralise the acidity they cause.

    Common mistake: Saying lime adds nitrogen.

  9. Calculate the percentage of nitrogen in ammonium sulfate.

    Show answer

    28 ÷ 132 × 100 = 21.2%.

    Common mistake: Counting one N.

  10. Give two ways to recognise substandard fertiliser.

    Show answer

    Unlabelled or damaged bags, unusual colour/texture, price too low, no certification (any two).

    Common mistake: Saying 'it smells good'.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which fertiliser contains sulfur as well as nitrogen?a. ammonia
    2. The Haber process produces:b. Ammonium sulfate
    3. A disadvantage of overusing inorganic fertilisers is:c. CO(NH2)2
    4. Urea has the formula:d. eutrophication of lakes and rivers
    Show answer

    1 → b (Ammonium sulfate) · 2 → a (ammonia) · 3 → d (eutrophication of lakes and rivers) · 4 → c (CO(NH2)2)

    • (NH4)2SO4.
    • First step in nitrogen fertilisers.
    • Runoff nutrients feed algae.
    • It is about 46% nitrogen.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    carbon dioxide  ·  nitric acid  ·  contains only nitrogen  ·  improves soil structure
    1. Urea is made from ammonia and .
    2. Ammonium nitrate is made from ammonia and .
    3. An advantage of organic manure is that it .
    Show answer

    1: carbon dioxide · 2: nitric acid · 3: improves soil structure

    • 2NH3 + CO2 → CO(NH2)2 + H2O.
    • NH3 + HNO3 → NH4NO3.
    • Adds humus.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which nutrient is important for root development and flowering?a. give lower yields and waste farmers' money
    2. NPK fertiliser supplies:b. Zinc
    3. Substandard fertilisers are harmful because they:c. Phosphorus
    4. Which is a micronutrient?d. nitrogen, phosphorus and potassium
    Show answer

    1 → c (Phosphorus) · 2 → d (nitrogen, phosphorus and potassium) · 3 → a (give lower yields and waste farmers' money) · 4 → b (Zinc)

    • P is part of ATP and DNA.
    • Three main macronutrients.
    • They may have less nutrient or harmful impurities.
    • Needed in small amounts.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    46%  ·  35%  ·  sulfuric acid  ·  acidic
    1. Superphosphate is made by treating phosphate rock with .
    2. Repeated use of ammonium fertilisers tends to make soil .
    3. The percentage of nitrogen in ammonium nitrate is .
    Show answer

    1: sulfuric acid · 2: acidic · 3: 35%

    • Makes phosphate more soluble.
    • Nitrification releases H+.
    • 28/80 × 100.

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