Fertilisers

1. Overview

Fertilisers are chemical substances added to soil to replace essential mineral ions that are absorbed by plants as they grow. By providing these nutrients, fertilisers ensure healthy plant development, increase the speed of growth, and improve overall crop yields to support the global food supply.

Key Definitions

• Fertiliser: A natural or synthetic substance applied to soil or plant tissues to supply one or more nutrients essential to the growth of plants.
• NPK Fertiliser: A multi-nutrient fertiliser containing the three primary elements required for plant growth: Nitrogen (N), Phosphorus (P), and Potassium (K).
• Yield: The mass of food produced from a specific area of land.

Core Content

Essential Elements for Plant Growth

Plants require several elements to grow, but three are particularly important and are often depleted in agricultural soils:

1. Nitrogen (N):
   • Role: Essential for making chlorophyll and proteins. It promotes healthy leaves and rapid stem growth.
   • Source: Provided in the form of ammonium salts and nitrates.
2. Phosphorus (P):
   • Role: Essential for energy transfer and healthy root development.
   • Source: Provided in the form of phosphate compounds.
3. Potassium (K):
   • Role: Essential for the production of flowers and fruits and helps the plant resist diseases.
   • Source: Provided in the form of potassium salts (e.g., potassium chloride or potassium sulfate).

Ammonium Salts and Nitrates

Most nitrogen-based fertilisers are produced using ammonia (NH₃). Ammonia is an alkali that reacts with acids to form soluble ammonium salts. These salts are easily absorbed by plant roots when dissolved in soil water.

• Ammonium Nitrate: One of the most common nitrogen fertilisers.
  • Word Equation: Ammonia + Nitric acid → Ammonium nitrate
  • Symbol Equation: NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)
• Ammonium Sulfate:
  • Word Equation: Ammonia + Sulfuric acid → Ammonium sulfate
  • Symbol Equation: 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
• Ammonium Phosphate:
  • Word Equation: Ammonia + Phosphoric acid → Ammonium phosphate
  • Symbol Equation: 3NH₃(aq) + H₃PO₄(aq) → (NH₄)₃PO₄(aq)

NPK Fertilisers

Commercially available fertilisers are often "NPK" fertilisers. These are mixtures that provide all three essential elements in a single application. The ratios of N, P, and K can be adjusted depending on the specific needs of the crop or the soil type.

Extended Content (Extended Only)

There are no specific Supplement-only learning objectives for this sub-topic in the current syllabus.

Key Equations

Reaction	Word Equation	Balanced Symbol Equation
Formation of Ammonium Nitrate	Ammonia + Nitric acid → Ammonium nitrate	NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)
Formation of Ammonium Sulfate	Ammonia + Sulfuric acid → Ammonium sulfate	2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
Formation of Ammonium Phosphate	Ammonia + Phosphoric acid → Ammonium phosphate	3NH₃(aq) + H₃PO₄(aq) → (NH₄)₃PO₄(aq)

Symbols and Units:

• NH₃: Ammonia gas (or aqueous solution)
• NH₄⁺: Ammonium ion
• NO₃⁻: Nitrate ion
• (aq): Aqueous (dissolved in water)

Common Mistakes to Avoid

• ❌ Wrong: Thinking that Potassium's chemical symbol is P.
• ✓ Right: Remember that P is for Phosphorus and K is for Potassium.
• ❌ Wrong: Writing the formula for the ammonium ion as NH₃.
• ✓ Right: Ammonia is NH₃; the ammonium ion (found in salts) is NH₄⁺.
• ❌ Wrong: Forgetting to balance the charges in ammonium sulfate.
• ✓ Right: Because the sulfate ion is SO₄²⁻ and the ammonium ion is NH₄⁺, the formula must be (NH₄)₂SO₄.

Exam Tips

• Command Words: If asked to "State" the elements in NPK, simply list Nitrogen, Phosphorus, and Potassium. If asked to "Describe" the use of fertilisers, mention that they replace lost minerals to increase crop yield.
• Calculation Questions: You may be asked to calculate the percentage by mass of nitrogen in a fertiliser. Use the formula: $$% \text{ by mass} = \frac{(\text{Number of atoms of element} \times A_r)}{\text{Total } M_r \text{ of compound}} \times 100$$
• Naming Salts: Always look at the acid used. Nitric acid makes nitrates, sulfuric acid makes sulfates, and phosphoric acid makes phosphates.
• Solubility: Remember that all ammonium salts and all nitrates are soluble in water, which is why they are effective fertilisers—plants can only take up nutrients that are dissolved in water.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) State two elements that are provided to plants by NPK fertilisers. [2]

(b) Define the term fertiliser. [1]

(c) Ammonium sulfate, $(NH_4)_2SO_4$, is a commonly used fertiliser. State the name of the process used to manufacture ammonia, the starting material for ammonium sulfate. [1]

(d) State one disadvantage of using fertilisers. [1]

Worked Solution:

(a)

1. Nitrogen This is one of the three elements in NPK fertilisers.

2. Phosphorus or Potassium Either phosphorus or potassium is acceptable.

How to earn full marks:

• Correctly state nitrogen for 1 mark.
• Correctly state either phosphorus or potassium for the second mark.

(b)

1. A substance added to soil This defines the basic purpose of a fertiliser.

2. To provide nutrients for plant growth This clarifies the reason for adding the substance.

How to earn full marks:

• State that it's a substance added to the soil.
• State that it's to provide nutrients for plant growth.

(c)

1. Haber process This is the name of the industrial process used to produce ammonia.

How to earn full marks:

• Correctly state Haber process.

(d)

1. Eutrophication / Water pollution / Algal blooms / Harmful to aquatic life These are some of the negative consequences of fertiliser runoff.

How to earn full marks:

• State a valid disadvantage of using fertilisers.

Common Pitfall: Remember that NPK fertilisers provide Nitrogen, Phosphorus, and Potassium. Students often confuse the symbols for these elements, especially Potassium, which is represented by K. Also, be specific when stating disadvantages; simply saying "pollution" isn't enough – you need to specify the type of pollution or its effect.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) Explain why plants need nitrogen. [2]

(b) Describe how an NPK fertiliser improves plant growth. [3]

(c) Suggest why repeated applications of a fertiliser containing only ammonium salts can make the soil acidic. [1]

Worked Solution:

(a)

1. Nitrogen is needed to make proteins Proteins are essential for plant structure and function.

2. Nitrogen is needed to make chlorophyll Chlorophyll is essential for photosynthesis.

How to earn full marks:

• State that nitrogen is needed for protein synthesis.
• State that nitrogen is needed for chlorophyll production.

(b)

1. Nitrogen promotes leaf and stem growth. Nitrogen is essential for vegetative growth.

2. Phosphorus promotes root growth. Phosphorus is important for root development.

3. Potassium promotes flower and fruit development. Potassium is crucial for reproductive growth and disease resistance.

How to earn full marks:

• Describe how nitrogen promotes leaf and stem growth.
• Describe how phosphorus promotes root growth.
• Describe how potassium promotes flower and fruit development.

(c)

1. Ammonium ions release $H^+$ ions in the soil, lowering the pH. This explains the acidic effect of ammonium salts.

How to earn full marks:

• State that ammonium ions release hydrogen ions.

Common Pitfall: When explaining the need for nitrogen, be sure to mention specific compounds like proteins and chlorophyll, not just general growth. Also, remember the specific roles of each element in NPK fertilisers: Nitrogen for leaves/stems, Phosphorus for roots, and Potassium for flowers/fruits.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A farmer is deciding between using ammonium nitrate ($NH_4NO_3$) and potassium nitrate ($KNO_3$) as fertilisers on their wheat crop.

(a) Explain why both ammonium nitrate and potassium nitrate are considered fertilisers. [2]

(b) Ammonium nitrate can be produced by reacting ammonia with nitric acid. Write a balanced chemical equation for this reaction, including state symbols. [3]

(c) Describe one environmental problem that can arise from the overuse of nitrate fertilisers and explain how this problem occurs. [3]

Worked Solution:

(a)

1. Ammonium nitrate contains nitrogen in the form of ammonium ions ($NH_4^+$) and nitrate ions ($NO_3^-$). This explains the nitrogen content of ammonium nitrate.

2. Potassium nitrate contains nitrogen in the form of nitrate ions ($NO_3^-$) and potassium ions ($K^+$). This explains the nitrogen and potassium content of potassium nitrate.

3. Nitrogen is essential for plant growth (protein/chlorophyll synthesis) and potassium is important for flower/fruit development. Nitrogen and potassium are macronutrients for plants.

How to earn full marks:

• State that ammonium nitrate contains nitrogen.
• State that potassium nitrate contains potassium and nitrogen.
• State that nitrogen and potassium are essential for plant growth.

(b)

1. $NH_3(aq) + HNO_3(aq) \rightarrow NH_4NO_3(aq)$ This is the balanced chemical equation for the reaction.

2. Correct reactants and product. Correct formula for each compound.

3. Balanced equation. Correct stoichiometric coefficients.

4. Correct state symbols. All state symbols are correct.

How to earn full marks:

• Write the correct chemical formulas for the reactants and product.
• Balance the chemical equation.
• Include the correct state symbols.

(c)

1. Eutrophication This is the environmental problem.

2. Excess nitrates leach into waterways (rivers/lakes). This is how the problem begins.

3. This leads to excessive algal growth (algal blooms). This is the consequence of increased nitrate levels.

4. Algae block sunlight, preventing photosynthesis by aquatic plants. This explains the impact on aquatic plants.

5. When algae die, they are decomposed by bacteria, which use up oxygen in the water. This explains the oxygen depletion.

6. The reduced oxygen levels kill aquatic organisms. This is the consequence of oxygen depletion.

How to earn full marks:

• State eutrophication as the environmental problem.
• Describe how excess nitrates leach into waterways.
• Explain how this leads to algal blooms.
• Explain how algal blooms cause oxygen depletion.
• Explain how oxygen depletion kills aquatic organisms.

Common Pitfall: When writing chemical equations, make sure they are balanced and include the correct state symbols. When describing eutrophication, remember to explain the entire process, from nitrate leaching to oxygen depletion and its impact on aquatic life. Don't just mention algal blooms; explain why they are harmful.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A fertiliser company produces NPK fertilisers by mixing different raw materials. A particular batch of fertiliser is labelled 15-10-20, meaning it contains 15% nitrogen, 10% phosphorus (as $P_2O_5$), and 20% potassium (as $K_2O$) by mass.

(a) Explain why the percentage of phosphorus and potassium is expressed as $P_2O_5$ and $K_2O$ respectively, rather than as elemental phosphorus and potassium. [2]

(b) A farmer buys 50 kg of this 15-10-20 fertiliser. Calculate the mass of nitrogen, phosphorus (as $P_2O_5$), and potassium (as $K_2O$) in the bag. [3]

(c) The farmer spreads this fertiliser over a field with an area of 2 hectares (1 hectare = 10,000 $m^2$). Calculate the mass of nitrogen applied per square meter of field. Give your answer in grams per square meter ($g/m^2$). [4]

Worked Solution:

(a)

1. It is convention to express the amount of phosphorus as $P_2O_5$ and potassium as $K_2O$. This highlights the conventional representation.

2. This is because the phosphorus and potassium are present in the fertiliser in the form of phosphate and potassium oxide compounds, respectively. This explains the forms in which they are present.

How to earn full marks:

• State that it is convention to express phosphorus as $P_2O_5$ and potassium as $K_2O$.
• Explain that they are present as phosphate and potassium oxide compounds.

(b)

1. Mass of nitrogen = 15% of 50 kg = 0.15 x 50 kg = 7.5 kg Calculates the mass of nitrogen.

2. Mass of phosphorus (as $P_2O_5$) = 10% of 50 kg = 0.10 x 50 kg = 5.0 kg Calculates the mass of phosphorus as $P_2O_5$.

3. Mass of potassium (as $K_2O$) = 20% of 50 kg = 0.20 x 50 kg = 10.0 kg Calculates the mass of potassium as $K_2O$.

How to earn full marks:

• Calculate the mass of nitrogen (7.5 kg). $\boxed{7.5 , kg}$
• Calculate the mass of phosphorus (as $P_2O_5$) (5.0 kg). $\boxed{5.0 , kg}$
• Calculate the mass of potassium (as $K_2O$) (10.0 kg). $\boxed{10.0 , kg}$

(c)

1. Total area of the field = 2 hectares x 10,000 $m^2$/hectare = 20,000 $m^2$ Calculates the total area in square meters.

2. Mass of nitrogen applied = 7.5 kg = 7500 g Converts the mass of nitrogen to grams.

3. Mass of nitrogen per square meter = 7500 g / 20,000 $m^2$ = 0.375 $g/m^2$ Calculates the mass of nitrogen per square meter.

How to earn full marks:

• Calculate the total area of the field in $m^2$ (20,000 $m^2$).
• Convert the mass of nitrogen to grams (7500 g).
• Calculate the mass of nitrogen per square meter (0.375 $g/m^2$). $\boxed{0.375 , g/m^2}$

Common Pitfall: Remember to convert units when necessary, especially when calculating the mass per area. Pay close attention to the percentages given in the fertiliser label and use them correctly in your calculations. Also, understand why phosphorus and potassium are expressed as oxides – it's a conventional way to represent their content in fertilisers.