The Nitrogen Cycle

Nitrogen is a gas that makes up about 78 percent of the air you breathe. All living things need nitrogen to grow. But most organisms cannot use nitrogen gas directly from the air. The nitrogen cycle is the process that changes nitrogen into forms that plants and animals can use.

Fixing Nitrogen

Special bacteria in the soil and on plant roots can grab nitrogen from the air and change it into a form plants can absorb. This is called nitrogen fixation. Lightning can also fix nitrogen by adding energy that combines nitrogen with oxygen. Plants take up the fixed nitrogen through their roots.

Returning Nitrogen

Animals get nitrogen by eating plants or other animals. When plants and animals die, decomposer bacteria break them down and release nitrogen back into the soil. Other bacteria convert nitrogen in the soil back into nitrogen gas, which returns to the atmosphere. The cycle then starts over.

Fun Facts

• Bean and pea plants have special bacteria on their roots that fix nitrogen from the air.
• Farmers rotate crops with beans and peas to add nitrogen back into the soil naturally.
• About 78 percent of the air you breathe is nitrogen gas.

Did You Know?

Without the nitrogen cycle, plants could not grow and there would be no food for animals. The tiny bacteria that fix nitrogen are some of the most important organisms on Earth!