I've been reading the Alchemy of Air by Thomas Hager. It is one of the most interesting books I've read in a long time. It chronicles the history of nitrogen fertilizer, and Hager does a got job describing the fact that the availability of nitrogen has been the key limiting factor to food and population growth throughout much of man's agricultural history. He writes:
As a species we long ago passed the natural ability of the planet to support us with food. Even using the best organic farming practices available, even cutting back our diets to minimal, vegetarian levels, only about four billion of us could live on what the earth and traditional farming supply. yet we now number more than six billion, and growing, and around the world we are eating more calories on average than people did in Crookes's day [late 1800s].
He describes the problem as follows:
These three elements, carbon, oxygen, and hydrogen, constitute more than 90 percent of our bodies by weight . . . But, the most important element in many ways for humans is the fourth most common in our bodies - and the hardest to find in nature (at least in forms we can use): nitrogen. It is stitched into every gene in your DNA and is built into every protein. If you don't get enough nitrogen, you die. . . . the absolute necessity of nitrogen or life leads to a paradox: We are swimming in nitrogen, but we can never get enough.
The problem is that all the nitrogen in the air is unavailable to humans and to most plants. It must be "fixed" before it becomes chemically available to us.
Hager documents the fact that cover crops and manures in the middle ages simply weren't enough to replenish nitrogen in the soil, so there were constant yield declines. The only answer was to find new land. No wonder colonization was so attractive to European powers. By accident, farmers found that various forms of nitrogen found in nature could enhance yields, but after depleting guano deposit in Peru and mining the Chilean desert, there still wasn't enough to go around.
It is hard to imagine a greater discovery for the prospects of mankind than the process created by Haber, Bosch, and other German scientists in the early 1900s to "fix" nitrogen from the air. And yet, it is probably one of the the most under-rated scientific advancements of all times (perhaps because it cause a great deal of damage too by allowing Germany to prolong WWI by converting ammonia into gun powder).
Indeed, I had a college student try to tell me in a Q&A after a lecture I gave at another University a few weeks ago that nitrogen was not a limiting ingredient for agriculture. It might be possible for a small farm feeding a small number of people to be somewhat self sufficient in nitrogen (yet even they must get their nitrogen from somewhere). But, it is simply not possible to support urban populations like those in NYC, Boston, London, or Hong Kong by using only the nitrogen available in manure and fixed by currently available cover crops.
If you're bored over the Christmas break, you could probably find worse things to do than read a little about the history of nitrogen.