<P> With average crop yields remaining at the 1900 level the crop harvest in the year 2000 would have required nearly four times more land and the cultivated area would have claimed nearly half of all ice - free continents, rather than under 15% of the total land area that is required today . </P> <P> Due to its dramatic impact on the human ability to grow food, the Haber process served as the "detonator of the population explosion", enabling the global population to increase from 1.6 billion in 1900 to today's 7 billion . Nearly 80% of the nitrogen found in human tissues originated from the Haber - Bosch process . Since nitrogen use efficiency is typically less than 50%, farm runoff from heavy use of fixed industrial nitrogen disrupts biological habitats . </P>

Where does the nitrogen for fertilizer come from