Nitrogen Fixation

Converting the inert nitrogen gas that makes up most of the air into ammonia and nitrates a plant can actually use, either biologically, by bacteria such as rhizobia living in the root nodules of legumes like clover, soybean and alfalfa, or industrially, by forcing nitrogen and hydrogen together under roughly 200 atmospheres of pressure and 400 degrees Celsius, the Haber-Bosch process. The industrial route is credited with helping the human population grow from around two billion in the early twentieth century to more than eight billion today, by putting a fertilizer input behind harvests that biological fixation and manure alone could never have supported at that scale.