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Nitrogen is the most commonly deficient nutrient in many soils
Nitrogen is a key element in digestibility of pastures
Nitrogen is a key element in the synthesis of proteins necessary for animal growth and development
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 about 80% of the Earth’s atmosphere.
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 cannot grow well because they are in soil that is low in nitrogen.
Other plants get a heap of nitrogen because they have the capacity to fix it. These include the legumes, plants that are very important in agriculture and are absolutely essential in any form of sustainable agriculture.
When an element is fixed it is combined with another element to form a new chemical compound. Nitrogen is fixed into a plant-usable form, ammonium.
A small amount of nitrogen fixation occurs in thunderstorms where the lightning fixes it.
More is done through some free-living bacteria in the soil and water 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.
Rhizobium means bacterium that lives in the root. So rhizobia live in the roots of plants, particularly legumes such as:
- alfalfa (lucerne)
- clovers including the remarkable pasture legume that is the main profit driver in the wheat-sheep belt of Australia, subterranean clover
- 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.
A similar process happens on the roots of casuarinas and their relatives through an organism called frankia that also works in the roots of other species including alder and bayberry.
Once the rhizobium fix the nitrogen in the soil atmosphere into a form that is available to the legume it lives in, the plant can get ahead of other plants around it.
When the legume dies or sheds roots (such as from Grazing shock , the nitrogen becomes available for other plants and other soil organisms.
The bacterium can survive only because the legume provides food 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.
As nitrogen is the most commonly deficient nutrient in many soils around the world, this is a key process for most farmers.
This is a simple example of how some plants change their environment.