KOULIKORO, Mali — When Suleiman Diarra Banani’s brother said that the poisonous black seeds dropping from the seemingly worthless weed that had grown around his family farm for decades could be used to run a generator, or even a car, Mr. Banani did not believe him. When he suggested that they intersperse the plant, until now used as a natural fence between rows of their regular crops — edible millet, peanuts, corn and beans — he thought his older brother, Dadjo, was crazy.

“I thought it was a plant for old ladies to make soap,” he said.

But now that a plant called jatropha is being hailed by scientists and policy makers as a potentially ideal source of biofuel, a plant that can grow in marginal soil or beside food crops, that does not require a lot of fertilizer and yields many times as much biofuel per acre planted as corn and many other potential biofuels. By planting a row of jatropha for every seven rows of regular crops, Mr. Banani could double his income on the field in the first year and lose none of his usual yield from his field.

Poor farmers living on a wide band of land on both sides of the equator are planting it on millions of acres, hoping to turn their rockiest, most unproductive fields into a biofuel boom. They are spurred on by big oil companies like BP and the British biofuel giant D1 Oils, which are investing millions of dollars in jatropha cultivation.

Countries like India, China, the Philippines and Malaysia are starting huge plantations, betting that jatropha will help them to become more energy independent and even export biofuel. It is too soon to say whether jatropha will be viable as a commercial biofuel, scientists say, and farmers in India are already expressing frustration that after being encouraged to plant huge swaths of the bush they have found no buyers for the seeds.