Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
Explain how nitrates are made available for plant uptake, including the…
Full specification wording
Topic 9 – Ecosystems and material cycles: 9.15 Explain how nitrates are made available for plant uptake, including the use of fertilisers, crop rotation and the role of bacteria in the nitrogen cycle
Plants need nitrogen to build amino acids and proteins, but they absorb it only as nitrate ions (NO₃⁻) from soil water.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 14 of 18
Study notes
From the checked teaching notes for this specification point.
The idea
Plants need nitrogen to build amino acids and proteins, but they absorb it only as nitrate ions (NO₃⁻) from soil water. Nitrogen gas (N₂) in air is unreactive, so the cycle converts it. Nitrogen-fixing bacteria in root nodules of legumes (peas, beans, clover) reduce N₂ to nitrogen-containing compounds the plant uses. Decomposers break down dead organisms and waste, releasing ammonium compounds; nitrifying bacteria oxidise these to nitrites then nitrates. Farmers add nitrate fertilisers to boost yield, but excess can leach into rivers, causing eutrophication. Crop rotation alternates legumes with cereals: legumes fix nitrogen, then the next crop uses the enriched soil, reducing fertiliser need. Learners must explain each route by which nitrate reaches roots.
What earns the marks
- Nitrogen-fixing bacteria in root nodules of legumes convert atmospheric nitrogen (N₂) into nitrogen compounds usable by the plant.
- Decomposers break down dead organisms and waste, releasing ammonium compounds that nitrifying bacteria convert to nitrates.
- Fertilisers add soluble nitrates directly to soil, which roots absorb as nitrate ions (NO₃⁻) for making amino acids and proteins.
- Crop rotation places nitrogen-fixing legumes in a sequence so the following crop benefits from enriched soil, reducing reliance on fertilisers.
- Excess nitrate fertiliser can leach into watercourses and cause eutrophication, so its use must be managed.
Common mistakes and how to put them right
Thinking plants absorb nitrogen gas directly from the air.
Put it right: Plants take up nitrogen mainly as nitrate ions from soil water.
Believing fertilisers are the only source of nitrates.
Put it right: Bacterial fixation, decomposition and nitrification also supply nitrates.
Confusing decomposers with nitrifying bacteria.
Put it right: Decomposers release ammonium compounds, while nitrifying bacteria oxidise them to nitrates.
Exam tips
- Name the specific bacterial role (nitrogen-fixing, nitrifying, decomposing) rather than saying 'bacteria help'.
- Link each route to the plant's use of nitrate for amino acids and proteins.
- When discussing fertilisers, mention both the benefit to yield and the risk of leaching and eutrophication.
Try the lesson
The start of the Topic 9 – Ecosystems and material cycles: 9.15 Explain how nitrates are made available for plant uptake, including the use of fertilisers, crop rotation and the role of bacteria in the nitrogen cycle lesson, made from checked teaching notes for this specification point.
Have a go first
Which group of soil micro-organisms converts ammonium compounds in dead matter and waste into nitrites and then nitrates?
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