The circulation of nitrogen in nature is called the nitrogen cycle. Nitrogen is continuously recycled between free gaseous nitrogen in the atmosphere and the combined nitrogen in the living organisms.

Agricultural activities have depleted (reduced) the nitrogen level in the soil, so fertilizers are added to restore its level.

In the preparation of ammonia, the flask is mounted with its neck stopping downward so that the water formed during the reaction is prevented from trickling back into the hot part of the flask and cracking it.

 
During lightning flashes, some nitrogen and oxygen in the atmosphere combine to form oxides of nitrogen (NO and NO₂). When these oxides react with water vapor or rain, they form dilute solutions of nitrous acid (HNO₂) and nitric acid (HNO₃). When nitric acid gets into the soil and nitrates are produced; atmospheric nitrogen is also converted into nitrates bacteria. Some of these bacteria exist freely in the soil other bacteria live in the root nodules of leguminous plants (Eg: beans) where converted to atmospheric nitrogen into organic nitrogenous compounds in plants used for producing proteins. The process of converting atmospheric nitrogen into a form used by plants is called nitrogen fixation.

When plants and animals die, their protein decomposes to form ammonium compounds which are converted into ammonia by putrefying bacteria (putrefaction) in the soil. Ammonia is converted into nitrates by nitrifying bacteria (nitrification). Nitrates in the soil are converted into free atmosphere nitrogen by denifying bacteria (denification).