Nitrogen (N)
Nitrogen is a major nutrient essential to
life. It is vital for building DNA and
proteins in plants and animals in the
food chain 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, peas and beans - where they create growths called
"nodules". These then capture nitrogen gas from the air
found between lumps of soil and, using plant carbohydrates
for energy, they transform (fix) the nitrogen gas into
ammonium or nitrate.
Click here for larger image(40k)
Lightning and combustion in cars.
These processes fix nitrogen which is then deposited back
to the earth as oxides.
Nitrogen fertiliser 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
Conversion of nitrogen forms in the soil
Nitrogen Fertiliser
are limited.
Nitrogen Fertiliser.
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.
Fertiliser
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 is solidified into granules or
made into bead-like prills which can be
evenly applied to the land using a fertiliser
spreader.
The ammonia can also be used to
produce other solid nitrogen fertilisers
such as urea or ammonium sulphate.
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.
FACT!
When taken up by plants, nitrate
and other nutrients are identical
in form whether they come from
organic matter, soil reserves or
applied fertilisers.
Solid fertiliser spreader
Nitrogen - the concerns and dilemmas
Human influences, especially the need to feed the world's increasing population, have led to
an increased need for nitrogen in agriculture. This has raised some concerns and dilemmas.
Nitrate Loss
Nitrogen is the most soluble of the three major soil nutrients. As nitrate it moves easily down through
the soil when it rains. It can then be lost to rivers (surface waters) or more slowly to underground
water stores (ground waters). This can take over 40 years depending on the type and structure of
the rock formations through which it passes.
Nitrate loss from agricultural land would occur even if no manures or fertiliser were applied, and the
extent of this loss is increased by cultivation. The disturbance and aeration of the soil increases the
activity of the soil organisms, this in turn increases the breakdown of the large stores of organic
matter to produce nitrate which is then easily washed out, or "leached". That is why the wide-spread
ploughing of old grassland during the 1940s contributed to the nitrate levels in much of the groundwater we use today. The other sources of additional nitrate have been from the use of mineral
nitrogen fertilisers and manure.
Leached nitrate may raise concerns if there is insufficient rainfall to dilute it to acceptable levels.
That is why on the drier, eastern side of the UK where most arable crops are grown, water tends to
have a higher nitrate content. In the west, which recieves more rain and where there is more
grassland, nitrate levels in water are lower.
Health Concerns
Nitrate is not a poison, but the level of consumption is important. In the European Union there are
laws which limit how much nitrate is allowed in drinking water.
People are sometimes worried because of what they are told about the effects of nitrate on health.
Apart from the exceptional cases in the 1970s when a number of babies fell ill after being bottle-fed
with well water, which had both a high bacteria content and high nitrate, there have been no further
problems. Certainly in the UK, with its high quality public water supplies similar conditions have not
occurred but as a precaution, a nitrate limit is sensible.
At one time it was thought that high nitrate levels might be linked to stomach cancer. Few scientists
now believe this and people with a lifelong exposure to high levels of nitrate through their work show
no increased vulnerability to cancer or any other disease. Recent scientific results seem to indicate
that nitrate is important to health. Again the danger may be in the dose, so a limit is sensible - but no
threat to health results if this is occasionally exceeded.
Environmental Concerns
There are also environmental concerns about nitrate in water because increasing levels can cause
certain plants and algae to grow excessively or "bloom". This deprives other plants and fish of
oxygen, light and space. However, because plants and algae need phosphorus as well as nitrogen,
a lack of phosphorus can stop blooms from occurring. This is generally the case in UK lakes and
rivers but in sea water phosphorus levels are higher and there have been some problems associated
with nitrate.
The effect of increasing nutrient levels in water above the natural balance is called eutrophication.
Tidal movement on the UK coastline generally disperses nitrate flowing into the sea from our rivers.
However, in the eastern North Sea, and in the Baltic, when shallow water is calm and warm, high
nitrate levels can result in the formation of toxic algal blooms.
Limiting nitrate loss from farmland
The UK Government has identified areas where nitrate levels in water are above, or are likely to rise
above, the level set by the European Union. In these areas, action plans have been drawn up which
farmers will be required to follow. Also, to encourage responsible use of fertiliser generally, guidelines are made available to farmers by the Ministry of Agriculture, Fisheries and Food, the fertiliser
industry and other organisations. These contain information on the proper storing, handling and use
of manure and fertilisers which help limit nutrient loss to acceptable levels.
Fluid fertiliser applicator