When plants and animals die they are broken down by decomposers like bacteria and fungi. As a result, Ammonia is made.
The majority of plants get their nitrogen in the form of nitrates.
Nitrates are soluble so can move into plants roots by active transport.
Nitrification involves different bacteria in the soil which:
Ammonia ------> Nitrites (insoluble) ------> Nitrates (soluble)
Why do plants need nitrates?
They need it to make amino acids which can then make proteins for growth and repair.
Where nitrogen gas is fixed by bacteria found in root nodules e.g. leguminous plants (beans, pea's, clover)
Only happens in boggy or water logged areas.
Anaerobic bacteria convert excess nitrate back into atmospheric nitrogen.