By John Otis, National Public Radio

To explain why he grows coca, the raw material for cocaine, rather than food crops on his 5-acre farm in southern Colombia, Luis Tapia does the math.

Every three months, Tapia, 60, harvests the bright green coca leaves with his bare hands, then mixes them with gasoline, sulfuric acid and other chemicals to make coca paste. He then sells the paste to drug traffickers who turn it into powder cocaine. A pound of paste, he says, sells for more than one ton of corn.

"That's why everyone grows coca," Tapia says.

Last year, Colombian farmers like Tapia produced 422,550 acres of coca, the largest coca crop in Colombia's history, according to a United Nations survey  released last month. That's enough to make about 1,500 tons of cocaine, the report said.