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 brakes 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.