Unit A

Module 1 ~ Lesson 4

The Nitrogen Cycle

Legumes are an important part of the nitrogen cycle. Legumes have nitrogen-fixing bacteria in the nodules.
You may have heard the term crop rotation in relation to farming. To maintain high levels of nitrogen in the soil, farmers will plant a legume in their soil once every few years to increase the nitrogen level in the soil without fertilizer. 

The atmosphere is composed of 78% nitrogen, yet most organisms do not obtain nitrogen from the air. Specialized nitrogen-fixing bacteria are the only organisms able to convert nitrogen gas into a form that can be used by plants and animals.

Nitrogen gas is very unreactive, so exists mostly in its elemental form, N2(g).  Animals obtain nitrogen from protein in plants, and plants uptake nitrogen as the nitrate ion (NO3- ) or the nitrite ion (NO2- ). For nitrogen gas to be used by plants, the gas has to be converted by certain types of bacteria into ammonia through a process called nitrogen fixation. The bacteria that perform nitrogen fixation are found in the soil and the nodules of roots of plants called legumes. Other types of bacteria in the soil, known as nitrifying bacteria, convert ammonia into nitrates and nitrites by a process called nitrification.


Read "The Nitrogen Cycle" on pages 48 to 49 of your textbook.

Exploring the Nitrogen Cycle

  1. Select the interactive to the right.
  2. The interactive is set at the water cycle. To view the nitrogen cycle, select Nitrogen Cycle.
  3. How are the nitrogen cycle and water cycle related?