The three major nutrients -- nitrogen, phosphorus and potassium -- bestow different benefits on plants. Nitrogen aids growth, phosphorus is needed for root growth and vigor, and potassium helps increase plant metabolism and disease resistance.
Nitrogen is the king of the hill. The world needs roughly 110 million tons of it annually, along with 33 million tons of phosphorus and 30 million tons of potassium, said Christine Gillespie, a spokeswoman for one of the world's largest manufacturers, Calgary-based Agrium Inc.
The nitrogen production is based on a process developed almost a century ago by German scientist Fritz Haber. The world was facing a shortage of nitrate fertilizer with the industrialization of agriculture in the late 19th century. Scientists knew the air we breathe is a sea of nitrogen, but it took Haber to invent an effective way of extracting it. Into a heated, pressurized canister containing a catalyst, he injected hydrogen and nitrogen at one end and extracted ammonia at the other.
The same principle is employed today, using natural gas both as a fuel and for its component hydrogen, mixed with air over a catalyst in a chamber that is pressurized and heated. The resulting ammonia gas is chilled into a liquid. This, in turn, is processed further into a number of different nitrogen fertilizers, including urea, nitric acid, ammonium nitrate, and a combination of urea and ammonium nitrate, which is blended with water to produce liquid nitrogen fertilizer. One of the most common forms of nitrogen in agriculture, especially in the Great Plains, is anhydrous ammonia, which is a liquid under pressure and injected into the soil. Agrium Inc. and some other manufacturers have stopped selling ammonium nitrate as a fertilizer because of the security issues and regulations now controlling its distribution. It can be made to explode, and was used by Timothy McVeigh in the 1995 Oklahoma City bombing.
The air is free, but extracting nitrogen from it is not. The process uses large amounts of natural gas, which rose sharply in price in 2005. It spiked to more than $14 per million British thermal units in December 2005.
Haber's invention changed the world. First, it prolonged World War I by giving Germany the explosives it needed to wage war; but it also allowed food to be grown on a massive scale and is directly linked to another explosion, that of the population of the planet.
The continued demand for chemical fertilizer is fueled not just by a net increase of 75 million people per year, but the fact that the inhabitants of large developing countries such as China and India are switching to diets higher in meats. This takes more grain to feed poultry, swine and cattle, and more fertilizer to grow grain.
Gillespie said few people realize that synthetic fertilizers are "chemically identical" to organic fertilizers. "I think people think because it is so involved a process it must be harmful, and that's not the case." Well, not quite. Even fertilizer companies acknowledge that the careless use of fertilizer can bring environmental damage by causing algae to grow, depleting rivers and estuaries of life-giving oxygen. Animal manure, too, contributes to the pollution.
Organic growers argue that chemical fertilizers do not build the soil, unlike organic fertilizers and nutrient-rich compost that improve the soil's structure and spawn microbial life. The bacteria and fungi that thrive in such soils enhance the ability of a plant to extract nutrients from the soil and at an even rate, said Patricia Millner, a soil scientist with the Agricultural Research Service in Beltsville. "Studies here and elsewhere show you can reduce the input of chemically synthetic fertilizers by up to one-third in some cases because of the synergies obtained when you increase the organic matter and natural nutrient cycling in soils," she said.
Still, Gillespie said, chemical fertilizers are loaded with nutrients and can be blended to provide precise levels of nitrogen, phosphorus and potassium in a way that organic fertilizers cannot. Ammonia contains 82 percent nitrogen, she said, and some animal manures just 3 percent.
To sustain the expanding human race without it, she said, would mean a world where much more forest would be given over to crop cultivation, and a planet awash in animal manure.