In Brazil, the world's biggest soybean producer, a 20% cut in potash use could bring a 14% drop in yields, according to industry consultancy MB Agro. In Costa Rica, a coffee cooperative representing 1,200 small producers sees output falling as much as 15% next year if the farmers miss even one-third of normal application. In West Africa, falling fertilizer use will shrink this year's rice and corn harvest by a third, according to the International Fertilizer Development Center, a food security non-profit group.

"Probably farmers will grow enough to feed themselves. But the question is what they will have to feed the cities," said Patrice Annequin, a senior fertilizer market specialist for IFDC based in Ivory Coast. When you add increased hunger across West Africa on top of existing risks like terrorism, "this is absolutely dangerous for many governments in our region."