Nitrogen Cycle, the series of natural processes by which certain nitrogen-containing substances from air and soil are made useful to living things, are used by them, and are returned to the air and soil. All living things must have nitrogen to build proteins. Because of the chemical nature of nitrogen gas, however, they cannot obtain that element directly from the air. Instead, food-making organisms such as plants obtain it from the soil by absorbing nitrates (various nitrogen compounds containing oxygen) and ammonium compounds (various nitrogen compounds containing hydrogen). The nitrogen cycle is essential to plants in unfertilized soils because in such soils the nitrogen compounds are not available to the plants in any other way.

Animals, and other living things that do not make their food, depend on the nitrogen cycle indirectly. Most animals, for example, eat plants or eat plant-eating animals.

The nitrogen cycle consists of four natural processes: nitrogen fixation, nitrification, denitrification, and decay.

Nitrogen Fixation

is the process in which nitrogen gas from the air is continuously made into nitrogen compounds. These compounds (primarily nitrates and ammonium compounds) are made by nitrogen-fixing microorganisms in the soil and by lightning.


is the process in which ammonia in the soil is converted to nitrates. Nitrification is performed by nitrifying bacteria. Plants absorb the nitrates and use them to make proteins.


is the reverse of the combined processes of nitrogen fixation and nitrification. It is the process by which nitrogen compounds, through the action of certain bacteria, give up nitrogen gas that then becomes part of the atmosphere. The amount of gas released by this process is relatively small.

Decay Processes

are those by which the organic nitrogen compounds of dead organisms and waste material are returned to the soil. These compounds are chiefly proteins and urea. The many bacteria and fungi causing decay convert them to ammonia and ammonium compounds in the soil.

Thus, through the nitrogen cycle, food-making organisms obtain the necessary nitrogen through nitrogen fixation and (to a greater extent) through nitrification. At the same time, nitrogen compounds are returned to the soil through decay and nitrogen is returned to the air through denitrification.

In soils in which many plants are raised and few are left to decay (as in farm soils), the nitrogen cycle does not supply enough nitrogen to support plant growth. In these soils natural or artificial fertilizers, containing nitrates or ammonium compounds, are needed.