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| Nitrogen is a major nutrient essential to life. It is vital for building
DNA and proteins in plants and animals in the feedchain and is sometimes likened to the
fuel in an engine. Air is 79% nitrogen. Nitrogen also occurs naturally in many compounds
in soil and water. Like all matter, nitrogen cannot be destroyed it can only change its
form. The movement between these forms is called the Nitrogen Cycle. |
| Fertile soils hold vast reserves of organic nitrogen in decaying plant
matter (humus) and in many soil organisms. To be usable by plants it has to be converted
into the inorganic ammonium or nitrate ions which are released when bacteria and fungi
breakdown organic matter. |
| Plants and animals cannot make direct use of nitrogen from the air so the
first step in the nitrogen cycle is to convert nitrogen in the air into ammonium ions.
This process is called fixation and can take place in various ways such as: - |
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| Some bacteria live freely in the soil, others become attached to the roots
of plants known as legumes – such as clover, pea's and beans – where they create
growths called ‘nodules’. These then capture nitrogen gas from air between
clumps of soil and, using plant carbohydrates for energy, they transform (fix) the
nitrogen gas into ammonium or nitrate. |
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| This also fixes nitrogen from the air. |
| In nature most fixing processes are slow and the amount of nitrogen that
is added to the soil can vary enormously. |
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| If the soil’s supply of ammonium and nitrate ions is not replenished
after each harvest, organic soil reserves of nitrogen will gradually be used up. Crop
growth will then fall and be limited by the nutrients released in one year. That is why
modern food production depends on supplementing the soil’s natural nitrogen content. |
| Organic material such as animal manure can and should be recycled, but
supplies are limited. |
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| In the mid 19th century a better knowledge of the principles of
plant nutrition led to nitrogen fertilisers being introduced. Initially these took the
forms of imported Guano (bird droppings) mineral nitrate from Chile and the use of
by-product ammonia from the production of gas from coal. This was followed in 1912 by the
development of the industrial process for the production of ammonia, known as the Haber
Bosch process. This is now the basis for the production of all mineral nitrogen
fertilisers. |
In the Haber Bosch process ammonia is made by combining nitrogen from the
air with hydrogen from natural gas (methane) and water, using the energy from the gas and
a catalyst. Nitric acid is then made by burning (oxidising) the ammonia over a catalyst.
The nitric acid is combined with more ammonia to produce ammonium nitrate which when
solidified into granules or made into bead-like prills which can be evenly applied to the
land using a fertilizer spreader.The ammonia can also be used to produce other solid
nitrogen fertilisers such as Urea or Calcium ammonium nitrate. Ammonium nitrate, urea and
ammonium sulphate may also be dissolved in water to give a fluid fertiliser, which can be
sprayed or dribbled onto the soil.
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| When taken up by plants, nitrate and other nutrients are identical in form
whether they come from organic matter, soil reserves or applied fertilisers. |
Nitrogen is an essential component of amino acids and therefore, of
proteins which include; -
- Nucleic acids.
- Enzymes
- Green light harvesting
pigment, chlorophyll.
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| It is also the nutrient, which normally produces the greatest yield
response in crop plants. |
