This article assesses the ongoing South American soy expansion from a world-historical perspective, comparing the case of Brazil with the cases of China and the USA. For this purpose, it applies the concept of commodity frontier, involving both external and internal modes of capitalist incorporation. The Chinese soy expansion (1900s–1930s) shows a predominant shift of the external frontier, associated with the peasant mode of farming. The US soy expansion (1930s–1970s) represents a predominant shift of the internal frontier, connected to the entrepreneurial mode of farming. The Brazilian soy expansion (1970s–2010s) reveals a flexible combination of extensive and intensive frontier shifts, corresponding with the capitalist mode of farming. These commodity booms were driven not only by nation states, capitalist enterprises and social movements, but also by the potentials and limitations of the soybean plant itself. Shifts of commodity frontiers often disrupted society and nature and, hence, were contested among diverse actors, both human and non-human.