The nitrogen cycle and water for drinking

Nitrogen and water both cycle through living things and the environment. Bacteria do most of the work in the nitrogen cycle.

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The nitrogen cycle

Air is about 78% nitrogen, but plants cannot use nitrogen gas. They absorb nitrate ions from the soil and use them to make amino acids and proteins.
Nitrogen-fixing bacteria (in the soil and in root nodules of legumes such as peas and clover) turn nitrogen gas into nitrogen compounds. Lightning also fixes some nitrogen.
Decomposers break down proteins in dead organisms and waste into ammonium compounds.
Nitrifying bacteria turn ammonium compounds into nitrates.
Denitrifying bacteria, in waterlogged soil with little oxygen, turn nitrates back into nitrogen gas.

How farmers help

Crop rotation: growing a legume crop every few years adds nitrogen compounds to the soil.
Fertilisers add nitrates (or ammonium compounds) directly.
Ploughing and drainage let air into the soil, helping nitrifying bacteria and reducing denitrification.

The water cycle and potable water

Water evaporates from seas and land, transpires from plants, condenses into clouds and falls as precipitation. It provides fresh water for living things.
In areas of drought, drinkable (potable) water can be made from sea water by desalination: distillation (boiling and condensing) or reverse osmosis (pushing water through a membrane that holds back salts). Both use a lot of energy, so they are expensive.
Worked example

A field is waterlogged for weeks. Explain why the crop growing there may become short of nitrates.

  1. Waterlogged soil has very little oxygen.
  2. Denitrifying bacteria thrive in these conditions.
  3. They turn nitrates into nitrogen gas, which plants cannot use.

Answer: Low oxygen in waterlogged soil favours denitrifying bacteria, which convert nitrates into nitrogen gas, so fewer nitrates are left for the crop.

Key idea

Nitrogen-fixing bacteria turn nitrogen gas into nitrogen compounds; decomposers make ammonium; nitrifying bacteria make nitrates; denitrifying bacteria turn nitrates back into nitrogen gas. Legumes, fertilisers and ploughing help farmers. Desalination makes potable water from sea water.

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Extra practice

2 more questions

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  1. Which bacteria convert ammonium compounds in the soil into nitrates?

    1. Nitrifying bacteria
    2. Denitrifying bacteria
    3. Nitrogen-fixing bacteria
    4. Decomposers
    Show the answer

    Answer: Nitrifying bacteria

    Nitrifying bacteria make nitrates that plants can absorb.

  2. Why do legumes grow well in nitrogen-poor soil?

    1. They do not need nitrogen
    2. Bacteria in their root nodules fix nitrogen
    3. They absorb nitrogen gas through leaves
    4. They make nitrogen from glucose
    Show the answer

    Answer: Bacteria in their root nodules fix nitrogen

    Mutualism with bacteria.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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