Vaclav Smil worked out the specific numbers and they can be found here: http://www.geo.cornell.edu/hawaii/bgc/Smil_SciAm_N2cycle.pdf http://www.geo.cornell.edu

Vaclav Smil worked out the specific numbers and they can be found here: http://www.geo.cornell.edu/hawaii/bgc/Smil_SciAm_N2cycle.pdf http://www.geo.cornell.edu/hawaii/bgc/Smil_SciAm_N2cycle.pdf The tipping point was around 1915 when the world had slightly less than 2 billion people, with the introduction of the Haber-Bosch process. After that the population growth is virtually dependent upon artificial nitrogen fixing fertilizers. Naturally-produced fertilizers can provide approximately 200 kg. of nitrogen per hectare annually; this allows for the production of between 200 and 250 kg. of plant proteins. This places a theoretical limit on the number of people that each hectare of land could sustain. Under ideal conditions this would amount to around 15 people per hectare; in practice, the historical limit has been about 5 people per hectare. In this paper he shows - dramatically - that there is a hard limit to the number of people that can be supported by natural nitrogen fixation. The population could have continued to grow through more drastic conversion of land to agriculture and better use of natural mechanisms, but at the end of the day, as he puts it: "... at least two billion people are alive because the proteins in their bodies are built with nitrogen that came from a factory using [the Haber-Bosch] process."