By KEITH BRADSHER and ANDREW MARTIN

XUAN CANH, Vietnam - Truong Thi Nha stands just 1.4 meters tall. Her three grown children tower over her, just as many young people in this village outside Hanoi dwarf their parents.
The biggest reason the children are so robust: fertilizer.
Ms. Nha, her face weathered beyond its 51 years, said her growth was stunted by a childhood of hunger and malnutrition. Just a few decades ago, crop yields here were far lower and diets much worse.
Then the widespread use of inexpensive chemical fertilizer, coupled with market reforms, helped power an agricultural explosion here that had already occurred in other parts of the world. Yields of rice and corn rose, and diets grew richer.
Now those gains are threatened in many countries by soaring prices for fertilizer, the most essential ingredient of modern agriculture.
Some kinds of fertilizer have nearly tripled in price in the last year, keeping farmers from buying all they need. That is one of many factors contributing to a rise in food prices that, according to the United Nations’ World Food Program, threatens to push tens of millions of poor people into malnutrition.
In the United States, farmers in Iowa eager to replenish nutrients in the soil have increased the age-old practice of spreading hog manure on fields. In India, the cost of subsidizing fertilizer for farmers has soared, leading to political dispute. And in Africa, plans to stave off hunger by increasing crop yields are suddenly in jeopardy.
The squeeze on the supply of fertilizer has been building for roughly five years. Rising demand for food and biofuels prompted farmers everywhere to plant more crops. As demand grew, the fertilizer mines and factories of the world proved unable to keep up.
Some dealers in the Midwestern United States ran out of fertilizer last fall, and they continue to restrict sales this spring because of a limited supply.
“If you want 10,000 tons, they’ll sell you 5,000 today, maybe 3,000, said W. Scott Tinsman Jr., a fertilizer dealer in Davenport, Iowa. “The rubber band is stretched really far.
Agriculture and development experts say the world has few alternatives to its growing dependence on fertilizer. As population increases and a rising global middle class demands more food, fertiliz- er is among the most effective strategies to increase crop yields.
“Putting fertilizer on the ground on a one-acre plot can, in typical cases, raise an extra ton of output, said Jeffrey D. Sachs, the Columbia University economist who has focused on eradicating poverty. “That’s the difference between life and death.
The demand for fertilizer has been driven by a confluence of events, including population growth, shrinking world grain stocks and the appetite for corn and palm oil to make biofuel. But experts say the biggest factor has been the growing demand for food, especially meat, in the developing world.
Overall global consumption of fertilizer increased by an estimated 31 percent from 1996 to 2008, driven by a 56 percent increase in developing countries, according to the International Fertilizer Industry Association.
On a tour of his rolling farm in Oxford Junction in eastern Iowa, Jayson Willimack pointed to the future sites of two buildings that will hold 2,400 hogs. Their manure will eventually replace commercial fertilizer on 162 hectares, about 10 percent of his farm, and save him perhaps $50,000 annually. “Every little bit helps, he said.
Such a strategy has severe limits - manure contains so little nitrogen that several metric tons are required on each hectare. That means farmers in Iowa and abroad have little choice but to pay the higher prices for commercial fertilizer.
In many countries, those cost increases have so far been offset by record high prices for crops. But fertilizer inflation has created a crisis in countries that subsidize fertilizer use for farmers. In India, for instance, the government’s subsidy bill could be as high as $22 billion in the coming year, up from $4 billion in 2004-5.
Once new supplies become available, the rising use of fertilizer will still pose difficulties.
Environmental groups fear increased use, particularly of nitrogen fertilizer made using fossil fuels. Because plants do not absorb all the nitrogen, much of it leaches into streams and groundwater.
“This is a basic problem, to feed 6.6 billion people, said Norman Borlaug, an American scientist who was awarded a Nobel Peace Prize in 1970 for his role in spreading intensive agricultural practices to poor countries. “Without chemical fertilizer, forget it. The game is over.