Fertilisers and Eutrophication
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of the nitrogen cycle and farming practices. You can test your knowledge on these below.
Why do farmers use fertilisers on crops?
To replace lost mineral ions in the soil and increase crop growth and yield.
What is the role of nitrates in plants?
Nitrates are needed for making amino acids and proteins, which are essential for growth. (As well as DNA, RNA and ATP)
Topic Explainer Video
Check out this @LauraDoesBiology video that explains fertilisers and eutrophication or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Types of Fertilisers
- Natural (Organic) Fertilisers
- Made from dead/decaying remains of plants and animals or manure.
- Nutrients are released slowly as organic matter is decomposed by saprobionts.
2. Artificial (Inorganic) Fertilisers
- Manufactured chemical compounds (e.g., ammonium nitrate, NPK fertilisers).
- Contains known, targeted ratios of nitrogen (N), phosphorus (P), and potassium (K).
Both types increase productivity by raising the mineral ion content in the soil, allowing for faster growth (↑ NPP).
Environmental Issues of Fertilisers
1. Leaching
- Occurs when water-soluble nutrients (especially nitrates) are washed out of the soil into waterways, leading to eutrophication.
- More common with inorganic fertilisers, as ions are immediately soluble. But also occurs with excess livestock waste, particularly chicken faeces.
- Leads to contamination of drinking water, which can cause health issues.
2. Eutrophication
A chain of events caused by excess mineral ions entering aquatic systems:
Step-by-Step Process of Eutrophication:
- Excess nitrate ions from fertilisers are leached into rivers/lakes.
- Rapid growth of algae and aquatic plants (algal bloom).
- Algae blocks sunlight → aquatic plants below die (no photosynthesis).
- Dead matter is decomposed by saprobionts which increase in number.
- Saprobionts use up oxygen in aerobic respiration.
- Oxygen levels fall, leading to death of fish and other animals. Ultimately, biodiversity is reduced.
Key Terms
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Fertiliser: Substance added to soil to improve fertility by increasing mineral ions.
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Leaching: The removal of dissolved nutrients from soil by water.
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Eutrophication: Excessive mineral ions in water causing algal blooms and oxygen loss.
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Exam Tips
Learn the step-by-step sequence of eutrophication – this is often worth 3–5 marks.
Be clear that oxygen depletion is a result of increased decomposition by saprobionts.
No answer provided.
Describe how the use of fertilisers can lead to the death of fish in freshwater lakes. (5 marks)
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Fertilisers contain nitrates which are leached into waterways.
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Nitrate ions cause rapid algal growth (algal bloom).
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Algae block sunlight, preventing photosynthesis in submerged plants.
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Plants die and are decomposed by saprobionts.
- Saprobionts use up oxygen, reducing its availability, causing fish to die due to a lack of oxygen for aerobic respiration.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!