As a contribution to the United Nation’s “Development Decade” of the 1960s, the UN FAO and UNESCO collaborated to produce a Soil Map of the World. Because of soil’s privileged place in mid-twentieth century conservationist thought and its material characteristics, which were extraordinarily resistant to standardized classification, analysis of this project reveals with particular clarity how scientists made knowledge about the global environment in the international community. Producing credible global environmental knowledge required a worldwide network of disciplined observers, but soil scientists understood the Soil Map of the World as a means to produce this transnational community of experts. At a scale of 1:5 million, the units of the map applied to no place in particular; it was a heuristic device. The legend, which presented a new international classification system, was the critical accomplishment because it promised to unify diverse national soil science communities in a single discipline. The rigorously empirical descriptions of soil categories reveal the interplay of the cosmopolitan values of scientific internationalism with the nationalist tensions of the Cold War and decolonization.