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Air is mostly nitrogen, yet crops run short of it. How does nitrogen get into plants, and where does it go?
Nitrogen Cycle lab · Last updated: 16 October 2026
Air is mostly nitrogen, yet crops still run short of it. In this 3D lab, you follow nitrogen as bacteria, lightning and fertilisers turn it into a form plants can use, and then trace how it returns to the air. It explains why farmers grow pulses and add manure.
What you'll discover
- Most living things cannot use nitrogen gas from the air directly.
- Bacteria, including those in the root nodules of pulses, fix nitrogen into compounds plants can absorb.
- Decomposers and other bacteria recycle nitrogen through the soil and eventually return it to the air.
The science in plain words
Nitrogen makes up about 78 per cent of the air, and every living thing needs it to build proteins and DNA. The trouble is that nitrogen gas is made of two atoms locked together by a very strong triple bond, so plants and animals cannot use it directly. Before it is useful, it must be fixed, meaning converted into compounds such as ammonia or nitrates.
Some soil bacteria can fix nitrogen. The best known, Rhizobium, lives in small swellings called root nodules on peas, beans, lentils and other legumes, giving the plant nitrogen in return for sugars. Lightning also fixes a little nitrogen, and factories make ammonia for fertilisers. Other soil bacteria then convert ammonia into nitrites and nitrates in a process called nitrification. Plants absorb nitrates through their roots and build them into proteins, and animals get nitrogen by eating plants or other animals.
When plants and animals die, or produce waste, decomposers break down their proteins and release ammonia back into the soil, ready to be used again. Denitrifying bacteria, especially in waterlogged soils, turn nitrates back into nitrogen gas, completing the cycle. Farmers keep soil fertile by growing legumes in rotation, adding manure and applying fertilisers. Too much fertiliser can wash into rivers and lakes, feeding algae that harm the water.
Where it fits
- India, NCERT: Class 9 science, Chapter 12: Improvement in Food Resources
- India, NCERT: The full nitrogen cycle goes beyond the NCERT syllabus for classes 6 to 10; the lab builds on Class 9 nutrients, manure and fertilisers.
- United States: Grades 6 to 8
- Next Generation Science Standards: MS-LS2-3
Common questions
Why can't plants use nitrogen from the air?
Nitrogen gas is made of pairs of atoms held together by an extremely strong triple bond. Plants have no way to break it. They rely on bacteria, lightning or fertilisers to convert nitrogen into compounds such as nitrates and ammonium, which they can absorb through their roots.
What is nitrogen fixation?
Nitrogen fixation is the conversion of nitrogen gas from the air into compounds that living things can use, such as ammonia. It is carried out mainly by certain bacteria, including those living in the root nodules of pulses, and also by lightning and industrial fertiliser factories.
Why do farmers grow pulses between crops?
Pulses such as peas, beans and lentils have bacteria in their root nodules that fix nitrogen from the air. When the pulse crop's roots and leftovers break down, that nitrogen enriches the soil, so the next crop needs less fertiliser. This practice is part of crop rotation.
Related labs
- Carbon Cycle: When we burn fuel, where does the carbon go? And how much of it stays in the air?
- Ecosystem Balance: Can plants and animals live for weeks in a sealed jar? Where does their oxygen come from?
- Food Web: Who eats whom in a meadow? Take one species away and see what happens to the rest.
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Try it yourself
The Nitrogen Cycle lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.