Like oxygen, nitrogen is essential for living things to survive on Earth. Animals and plants need nitrogen to build amino acids in proteins, which are the building blocks of life. Unlike oxygen, nitrogen cannot be absorbed directly from the air by animals and plants.
The nitrogen cycle, illustrated on this page, shows how nitrogen gets from the air into the soil then plants then animals, and back into the soil. Plants are a crucial part of the nitrogen cycle.
Nitrogen cycle diagram
1. About 78% of the air around us is nitrogen but this gas is unreactive and needs to be processed into nitrates before it can be used by plants:
- a) Nitrogen gas in the air is converted into nitrates by lightning. Rain and other precipitation then bring the nitrates down into the soil. Although not shown on the diagram, the man-made Haber process also uses nitrogen in the air to produce plant fertiliser on an industrial scale.
- b) Nitrogen gas in the soil is converted into nitrates by bacteria in the soil and in nodules in plant roots.
2. Plants absorb nitrates through their roots and then use them to build proteins and grow.
3. Animals (including humans) eat the plants and absorb the nitrates by converting them into animal proteins that enable them to grow and function healthily.
4. Nitrogen returns to the soil as ammonia through:
- a) animal droppings and urea being broken down by decomposers, such as bacteria and fungi
- b) decomposers, such as bacteria and fungi, breaking down the dead bodies of animals.
Learning about the nitrogen cycle
We hope that adults will find our diagrams useful for teaching children the crucial role of plants in the nitrogen cycle. Teachers of GCSE Science (Biology and Chemistry) pupils (aged 14-16 years) should find it useful.
Processes in the nitrogen cycle
The diagram below is another interpretation of the nitrogen cycle, showing each part of the process in more detail. You can download a larger PDF of this nitrogen cycle diagram here.