Nitrogen cycle

Most of the air is nitrogen gas. Living things need nitrogen to make proteins, but plants and animals cannot use the gas directly. It first has to be changed into compounds such as nitrates.

The main steps

• Fixing: some bacteria turn nitrogen gas into compounds plants can use. Lightning does this too.
• Nitrifying: other soil bacteria change these compounds into nitrates.
• Uptake: plants take in nitrates through their roots and build proteins. Animals get nitrogen by eating plants.
• Decay: when living things die, decomposers return their nitrogen to the soil.
• Denitrifying: some bacteria turn nitrates back into nitrogen gas, which returns to the air.

A small example

A farmer grows peas or beans, then wheat in the same field. Why might the wheat do well? Pea and bean plants carry nitrogen-fixing bacteria in small lumps on their roots, called root nodules. The bacteria add nitrogen compounds to the soil. The next crop can use them.

Trace the nitrogen in the wheat: air, to bacteria in the nodules, to the soil, to the wheat's roots, to its proteins. It then passes along the food chain to whoever eats the grain.

Where it fits

This goes beyond the Class 10 board syllabus. HBCSE's Junior Science syllabus lists it under biogeochemical cycles, in the unit on our environment. That syllabus is for NSEJS, which the Indian Association of Physics Teachers (IAPT) conducts.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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