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: –
Fixation from the air by bacteria
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.
Nitrogen fertilizer production
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.
Why natural fixation is not enough!
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.
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.
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.
- Green light harvesting pigment, chlorophyll.
It is also the nutrient, which normally produces the greatest yield response in crop plants.