Nitrogen notes MPX STEM 10-28-13

Nitrogen is important to all life. Nitrogen in the atmosphere or in the soil can go through many complex chemical and biological changes, be combined into living and non-living material, and return back to the soil or air in a continuing cycle. This is called the nitrogen cycle.

lants need nitrogen to grow, develop and produce seed. The main source of nitrogen in soils is from organic matter. Soils in Missouri commonly contain one to four percent organic matter. Organic matter largely arises from plant and animal residues. The nitrogen in organic matter is largely in organic forms that plants cannot use. Bacteria found in soils convert organic forms of nitrogen to inorganic forms that the plant can use. Nitrogen is taken up by plant roots and combined into organic substances in the plant, such as enzymes, proteins and chlorophyll. Chlorophyll gives the plant its green color. When the plant dies, it decays and becomes part of the organic matter pool in the soil.


Plant and animal wastes decompose, adding nitrogen to the soil.
Bacteria in the soil convert those forms of nitrogen into forms plants can use.
Plants use the nitrogen in the soil to grow.
People and animals eat the plants; then animal and plant residues return nitrogen to the soil again, completing the cycle.



The largest single source of nitrogen is the atmosphere. It is made up of 78 percent of this colorless, odorless, nontoxic gas. However, plants are unable to use nitrogen as it exists in the atmosphere. Nitrogen from the air enters the nitrogen cycle through several  types of microorganisms that can convert N2 gas to inorganic forms usable by plants. Some of these microorganisms live in the soil, while others live in nodules of roots of certain plants.

Nitrogen also can enter the cycle from other sources besides the air, manure and decaying plant materials. Nitrogen also can enter the cycle from the application of commercial nitrogen fertilizers.

Nitrogen can be lost from the cycle. It can be lost to the atmosphere, removed by harvesting crops or lost to surface water or groundwater. However it is lost, nitrogen can enter the cycle again through one of the processes discussed above or through other processes. These additional pathways of gains and losses to the nitrogen cycle are illustrated in Figure 2.