The Nitrogen CycleVaclav Smil, in his July, 1997 Scientific American article “Global Population and the Nitrogen Cycle”, describes the remarkable relationship between the rise in human population over the past century and the availability of synthetic nitrogen fertilizers.
These fertilizers where made possible by the Haber-Bosch process developed in 1899.
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.
The use of synthetic nitrogen fertilizer has allowed these limits to be broken. At current levels, world-wide, these fertilizers account for 40 percent of all nitrogen taken up by cultivated crops. And since these crops contribute 75 percent of all nitrogen in consumed proteins, this means that one third (i.e., 75% of 40%) of humanity’s diet depends on synthetically produced nitrogen.
Smil puts it into these very concrete terms:
“… 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.”
He has a new book, The Earth’s Biosphere: Evolution, Dynamics, and Change that I would highly recommend. It’s an interdisciplinary survey of the biosphere and the systems and processes that provide for its self-regulation. It touches on a wide variety of subjects including chemistry, biology, genetics, geology and astronomy; and while it can be a bit jargon-heavy at times, anyone with a reasonable exposure to these areas would not have any trouble following the material. I came away from the book with an even greater appreciation for the workings of our planet than I already had.