Nitrogen fixation

Fixation involves converting atmospheric nitrogen into a nitrogen compound. In agriculture, this involves fixing it into a form that is usable by plants.

In the air nitrogen generally exists in combination with itself. One atom of nitrogen is bonded to another atom of nitrogen. Nitrogen makes up nearly eighty percent of the Earth's atmosphere.

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Nitrogen from the atmosphere is free and useful to all plants. But plants cannot meet their need for it from the air, they must get it from the soil.

There is no shortage of nitrogen, however many plants grow in situations that are nitrogen deficient, because they are not getting enough from the soil.

Other plants get a heap of nitrogen because they have the capacity to fix it. These include the legumes, the group of plants that are very important in agriculture and are absolutely essential in any form of sustainable agriculture.

Nitrogen fixing is the common term and nitrogen fixation is the more formal term for the process.

When an element is fixed it is combined with another element to form a new chemical compound. Nitrogen is fixed by being combined with oxygen to form nitrates.

A small amount of nitrogen fixation occurs in thunderstorms where the lightning fixes nitrogen.

Much more of it is done through some free-living bacteria in the soil and through bacteria in the roots of legumes and similar plants. Some of these harmless beneficial bacteria (Rhizobium - plural rhizobia) form nodules (a swelling) on the roots of legumes.

Rhizobia live in the roots of plants, particularly legumes such as

alfalfa (lucerne)
clovers including the remarkable subterranean clover
beans
peas and
a host of trees such as
- acacia species (wattles in Australia)
- honey locust (Gleditsia triacanthos)
- robinia (Robinia pseudoacacia)
- the tree lucernes, including
  - tagasaste, which grows in temperate and up to sub-tropical conditions Chamaecytisus and
  - Leucaena which is tropical and sub-tropical.

In Australasia a similar process happens on the roots of Casuarinas and their relatives through an organism called Frankia.

Some forms of rhizobium (which means bacterium that lives in the root) are able to "fix" or convert the nitrogen in the soil atmosphere into a form that is available to the legume it lives in. When the legume dies or sheds roots, the nitrogen becomes available for other plants and other soil organisms.

The bacterium can survive only because the legume provides sugars and a home in return for nitrogen. Between them, these two improve the soil for the legume first and then for all other organisms that need nitrogen and cannot fix it themselves.

This is a simple example of how some plants change their environment.

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This page was updated on December 27, 2007