Combine air and natural gas over an iron oxide catalyst under high pressure and intense heat and what do you get?

The answer, surprisingly, is plant food: ammonia, the chemical precursor to nitrogen fertilizers.

Ammonia gets converted into nitrites and nitrates, which when sprinkled onto plants, allow them to grow larger. This is the basic idea behind the huge increases in agricultural yields, doubling between 1950 and 1990, seen in the 20th century. (Caveats about the "quality" of this growth and the environmental impacts of nitrogen are noted, but left aside for a later post in this continuing series).

Back around 1915, the world produced almost no nitrogen fertilizer, largely because there was no usable nitrogen supply. Now, the world produces about 87 million tons of N-based fertilizers. This increase is primarily due to the Haber-Bosch process for pulling nitrogen out of the air. (The development of new plant varieties that are able to soak in excess nitrogen will also be the subject of a separate post).

Clearly, the Haber-Bosch process has been successful. As we've noted before, at least one professor has estimated that 40 percent of the world's food can be traced back to the process. But the process is encountering major problems in the increasingly resource-constrained world.

Here's why: the main reaction in the process is cooking N 2 and H 2 together at 500 degrees Celsius and 200 atmospheres of pressure. You need all that heat and pressure because breaking apart an N 2

molecule turns out to be incredibly difficult. A nitrogen atom has five electrons in its outer shell (valence electrons), so it has a tendency to share three electrons with another nitrogen atom to reach its stable (octet rule) state. That's what generates dinitrogen's triple covalent bond, one of the strongest in nature. The energy required to break the bond is 946 kilojoules of energy per mole of nitrogen, or twice the energy required to bust an O 2 molecule.

Luckily, or so we thought, fossil fuels were cheap, widely available, and incredibly energy dense: 1 cubic foot of natural gas contains 1.055 gigajoules of energy.

That's enough energy to convert a lot of moles of nitrogen into ammonia. So, once the Haber-Bosch process established it could be done, chemists across the world began to burn a lot of natural gas to get dinitrogen to react with hydrogen. And where do we get the hydrogen? Why, we use the natural gas for that too, naturally: it is CH 4 after all.

Taken together, there's a lot of natural gas going into the production of nitrogen fertilizer. So much so that when I tweeted about my fertilizer investigation, my friend Celeste LeCompte, managing editor at the

Sustainable Industries Journal, tweeted back, "Think: natural gas."

In effect, we've been pumping fossil energy into our food supply, and eating it. While diminishing fossil fuel supplies and climate concerns have given us perfect hindsight into why this could be a dubious path for the future, at the time, it must have seemed like an excellent idea, given that the alternative–not producing enough food–was both real and horrific.

Until relatively recently, the price of natural gas, which tracks the price of oil very closely, was relatively low. Now, with oil over

$120 a barrel and natural gas prices having doubled since the mid-90s to over $11 per thousand cubic feet of the stuff, the cost of ammonia has tripled. As in biofuels or alternative energy, the rising cost of oil is driving innovation.

As we've noted before, legumes developed symbiotic relationships with bacteria who can pull nitrogen out of the air at room temperature and standard atmospheric pressure. They use a specialized enzyme known as a nitrogenase that consists of iron and the metal molybdenum. In fact, scientists estimate that 200 million tons of nitrogen are fixed via natural processes, or more than twice human production.

Now teams of scientists across the world from Richard Schrock at MIT to

David Tyler at the University of Oregon are racing to find just the right catalyst to recreate the natural nitrogen fixation process. While they wouldn't eliminate the use of natural gas as a feedstock, they would reduce the amount of energy used in the creation of ammonia.

How much? Eliminating the Haber-Bosch process, which uses an estimated one percent of the world's total 15 terawatts of energy consumption (xls) would mean 150 gigawatts of energy savings for the world. That's about as much coal generating capacity as the US is planning to add between now and 2030.

In the next post in this continuing series dedicated to exploring new fertilizer technologies that could reduce their environmental impact and energy usage while increasing food security*, we'll explore these scientists' biomimetic work. *

Image: A fertilizer factory in the UK. flickr/Addictive Picasso

See Also:

In Search of New Fertilizer Tech (No, Really)

What Makes Plants Grow