Despite bacteria's ubiquity, their diversity in the world's soils is poorly understood. To get a handle on what makes the organisms thrive, Duke University researchers trekked far and wide to collect a few centimeters of dirt from 98 locations across North and South America, then analyzed each sample for genetic variation. To their surprise, the strongest predictor of high diversity was neutral pH. The acidic soil of the Peruvian Amazon, for example, harbored one half to one third as many species as did the neutral dirt of the arid American Southwest. “There are a lot of variables that didn't turn out to be very important,” says co-author Robert Jackson, who adds that a more exhaustive search of different habitats might turn up other stimulators of diversity, such as carbon abundance. The report was published online January 9 by the Proceedings of the National Academy of Sciences USA.