There are three things on which the mighty engine of U.S. agriculture depends: water, fuel, and synthetic nitrogen. Like water, nitrogen is elemental to life. It's the essential building block of the plants we eat. Farmers remove it from the soil when they harvest the year's crop, and they must replenish it for the following year's.
Compared with water and fuel, nitrogen is actually in one sense quite plentiful: it makes up about 80 percent of the air we breathe. Yet for all that ubiquity, it's also in a sense scarce: its extremely strong chemical bond -- it exists in the air in triple-bonded pairs of nitrogen known as N2 -- makes it difficult for plants to use.
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