Divide the class into two groups to debate the following topic:
Increasing the use of artificial fertilisers is the best solution to meet the growing food needs of the world's human population.
One group should take the position of
agreeing with the statement, and the other should
disagree . In your groups, discuss reasons why you have the opinion that you do, and record some notes of your discussion. Your teacher will then explain to you how to proceed with the debate.
The following presentation shows other ways to represent the nitrogen cycle.
Summary
Nitrogen is essential for life on earth, since it forms part of
amino acids ,
proteins and
nucleic acids .
The
atmosphere is composed mostly of nitrogen gas, but the gas is
inert , meaning that it is not available to living organisms in its gaseous form.
The
nitrogen cycle describes how nitrogen and nitrogen-containing compounds are changed into different forms in nature.
The nitrogen cycle consists of three major processes:
nitrogen fixation ,
nitrification and
denitrification .
Nitrogen fixation is the conversion of atmospheric nitrogen into compounds such as ammonia that are more easily used.
Nitrogen can be fixed
biologically through the actions of
bacteria ,
industrially through the
Haber-Bosch process or by
lightning .
Nitrification converts ammonia into
nitrites and
nitrates , which can be easily
assimilated by plants.
Denitrification converts nitrites and nitrates back into gaseous nitrogen to complete the nitrogen cycle.
Humans have had a number of
impacts on the nitrogen cycle. The production of
artificial fertilisers for example, means that there is a greater chance of runoff into water systems. In some cases,
eutrophication may occur.
Eutrophication is the contamination of a water system with excess nurtrients, which stimulates excessive algae growth at the expense of other parts of the ecosystem. This occurs as eutrophication reduces oxygen levels in the water.
Many nitrogen gases such as NO, N
2 O and NO
2 are released by agricultural soils and artificial fertilisers. These gases may combine with water vapour in the atmosphere and result in
acid rain . Some of these gases are also greenhouse gases and may contribute towards
global warming .
A number of
industrial processes are used to produce
articifical fertilisers .
The
Haber-Bosch process converts atmsopheric nitrogen into
ammonia .
The
Ostwald process reacts ammonia with oxygen to produce
nitric acid , which is used in the preparation of fertilisers and explosives.
If ammonia and nitric acid react, the product is
ammonium nitrate , which is used as a fertiliser and as an explosive.
Summary exercise
Look at the diagram and the descriptions of the nitrogen cycle earlier in the chapter:
Would you describe the changes that take place in the nitrogen cycle as
chemical or
physical changes? Explain your answer.
Are the changes that take place in the water cycle
physical or
chemical changes? Explain your answer.
Explain what is meant by each of the following terms:
nitrogen fixing
fertiliser
eutrophication
Explain why the fixing of atmospheric nitrogen is so important for the survival of life on earth.
Refer to the diagram below and then answer the questions that follow:
Explain the role of
decomposers in the nitrogen cycle.
If the process taking place at (3) is
nitrification , then label the processes at (1) and (5).
Identify the nitrogen products at (2) and (4).
On the diagram, indicate the type of
bacteria that are involved in each stage of the nitrogen cycle.
In industry, what process is used to produce the compound at 2?
Does the diagram above show a 'cycle'? Explain your answer.
NO and NO
2 are both nitrogen compounds:
Explain how each of these compounds is formed?
What effect does each of these compounds have in the environment?
There are a number of arguments both 'for' and 'against' the use of artificial fertilisers. Draw a table to summarise the advantages and disadvantages of their use.
