Imagine a car being driven at breakneck speed toward a brick wall with the brakes applied only seconds before impact.
This was the kind of calamity facing European farming more than a century ago as guano, a principal source of nitrogen crop fertilizer mined off the coasts of South American and South Africa, rapidly approached depletion.
As the shadow of famine extended across the continent, two German scientists, Fritz Haber and Carl Bosch, stepped forward with a way to fix nitrogen from the air, converting atmospheric nitrogen into ammonia gas.
The breaks were applied, disaster was averted and a technological revolution followed. Using this method, farmers were able to feed millions more using considerably less cropland.
Even so, one expert fears that humanity is drawing ever closer to a new nitrogen crisis, one even more complicated than the previous one. Whether science finds a way to apply the brakes this time remains an open question, he says.
Ironically, the problem stems from both a blessing and curse — having too much of one critical element and not enough of another, says Charles Mitchell, an Alabama Cooperative Extension System agronomist and Auburn University professor of agronomy and soils.
The blessing remains the immense and, up to now, cheap sources of nitrogen yielded through the Haber and Bosch method.
"We learned to take energy and do what Mother Nature does, only lots faster and in much larger quantities," Mitchell says. "Instead of getting 10 bushels of corn an acre, we now get 200."
This is part of where the curse factors into the equation: The more nitrogen is fixed from the atmosphere, the more the natural cycle becomes overloaded as more of this element washes into rivers and streams and, ultimately, into oceans.
"If you overload the soil with nitrogen, you're increasing the leaching of this element into groundwater — that's one example," Mitchell says. "That comes out into lakes and streams, runs into the Gulf of Mexico."
Excess nitrogen promotes algae growth, depriving marine species of oxygen, resulting in fish kills and, in some cases, massive oceanic dead zones.
"That's what nitrogen does — if it's in the soil, it produces green, leafy growth in the form of lush, green pastures," Mitchell says. "If it's in the Gulf of Mexico, it's producing algae and depriving water of oxygen."
A portion of this nitrogen also ends up in the air as nitrous oxide, a greenhouse gas some 300 times more potent than carbon dioxide as well as an ozone destroyer.
Some are promoting greater use of organic nitrogen substitutes as a way around this problem, but this will only take us so far, Mitchell says.
"Yes, we can do it organically, but organic fertilizer is simply using nitrogen that likely originated from some fixed nitrogen source," he says.
Legume crops, which are able to fix nitrogen on their own, are cited as another option. Yet, these crops present their own challenges.
For instance, research has revealed that kudzu, a legume, is one of the greatest nitrogen polluters, both to the atmosphere and groundwater.
"We could cover the world in legume crops, but we'd still have a nitrogen pollution problem," he says.
Yet another curse now factors in the equation: While atmospheric nitrogen is as readily available as ever, a key ingredient used in its manufacture — cheap energy — is not, even as farming struggles to feed an additional 2 to 2.5 billion people, Mitchell says.
Whether a brick wall awaits humanity in a few more decades remains an open question, Mitchell says. What is certain is that cheap nitrogen can no longer be taken for granted in farming.
"We've put on extra nitrogen to get that green leafy growth and assumed that this would make up for poor soils and bad management practices," he says. "But we can't do that anymore.
"Nitrogen is too expensive and we can't afford to mismanage it as we have in the past — we've got to switch to more sustainable approaches."