Free nitrogen – the nitrogen found in the atmosphere, animal wastes, and dead and decaying organisms – is all around. However, only a few organisms can use it just as it is. These organisms “fix” the nitrogen for all other organisms to use. They are called nitrogen fixing bacteria.

You might find it helpful to study the nitrogen cycle in two parts!

In the nitrogen fixation part of the cycle, nitrogen-fixing bacteria found in the soils and in the roots of certain plants, change (or convert) free nitrogen into substances that other organisms can use. When the fixing process is finished, free nitrogen is converted into nitrates, nitrites, and ammonia. These substances can be used by plants. As the plants become food, the nitrogen can be used by animals.

Just as there are nitrogen-fixing bacteria, some bacteria have the job of denitrifying the soil to keep the nitrogen in balance. These bacteria take the nitrogen compounds and return them to nitrogen gas that is released back into the atmosphere.

In another part of the cycle, animals eat plants containing usable nitrogen. That nitrogen returns to the soil as organic material (animal waste, decaying plants and animals) is decomposed by bacteria and other decomposers.

As in the first part of the cycle, denitrifying bacteria keep the nitrogen level in the soil balanced.