Her family, including six brothers and sisters, struggled to survive on rations from the commune where they lived, eating little protein. The occasional pigs they raised on rice stalks and mush “fattened very slowly,” Ms. Nha recalled.

But with market reforms, better seeds and increased fertilizer use, Vietnam’s rice yields per acre have doubled and corn yields have tripled, allowing farmers to fatten a growing herd of livestock.

Several times a season, Ms. Nha and her neighbors walk down their rows of corn with battered metal buckets full of chemical fertilizer, which looks like coarse gray sand, sprinkling a bit at the base of each plant. Ms. Nha’s husband, Le Van Son, remembers villagers’ amazement in the 1990s when they learned that a pound of chemical fertilizer contained more of the major nutrients than 100 pounds of manure.

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.

“Markets are asking farmers to step on the accelerator,” said Michael R. Rahm, vice president for market analysis and strategic planning at Mosaic, a fertilizer producer in Plymouth, Minn. “They’ve pressed on it, but the market has told them to step on it harder.”

Fertilizer is plant food, a combination of nutrients added to soil to help plants grow. The three most important are nitrogen, phosphorus and potassium. The latter two have long been available. But nitrogen in a form that plants can absorb is scarce, and the lack of it led to low crop yields for centuries.

That limitation ended in the early 20th century with the invention of a procedure, now primarily fueled by natural gas, that draws chemically inert nitrogen from the air and converts it into a usable form.