Despite bacteria’s presence in all parts of the planet, their diversity in the world’s soils is poorly understood. To better understand what makes the organisms thrive, Duke University researchers trekked far and wide to collect a few centimetres of dirt as samples 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, harboured far fewer bacterial species than 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 the researcher Robert Jackson, who adds that a more complete search for different habitats might turn up other stimulators of diversity, such as carbon abundance.