Question

state three methods by which free nitrogen can be converted into nitrates

Answer 1

Nitrogen gas is converted to nitrate compounds by nitrogen-fixing bacteria in soil or root nodules. 
Lightning also converts nitrogen gas to nitrate compounds.
The Haber process converts nitrogen gas into ammonia used in fertilizers. 

Answer 2

The process by which free nitrogen is converted into nitrates is called nitrogen fixation.

Explanation:

Nitrogen fixation uses several nitrogen-fixing bacteria to convert free nitrogen into nitrates. Three methods for nitrogen fixation are as follwoing:

1. Atmospheric nitrogen fixation: Nitrogen fixation is done atmospherically during lightning. Lightning produces high temperatures and pressures that allows the conversionof nitrogen in air into oxides of nitrogen.
2. Biological nitrogen fixation: several nitrogen fixation bacteria are present in environment which converts nitrogen molecules into nitrates and nitrites which further utilised by plants. Example of biological bacteria for nitrogen fixation are  Azotobacter or symbiotic bacteria like Rhizobium.
3. Industrial fixation: in industrial fixation such as in fertilizer industries, nitrogen and hydrogen from atmosphere is used and it is converted into ammonia and further into urea, ammonium nitrate, etcetra.

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