Inputs and outputs of nitrogen and phosphorus in three corn cropping systems with similar yield potential: a low-input corn-based system in western Kenya; a highly fertilized wheat-corn double-cropping system in north China; and a corn–soya bean rotation in IL, USA. Actual yields of corn were 2000, 8500 and 8200 kg ha−1 yr−1 per crop in the Kenya, China and USA systems, respectively; the Chinese and USA systems also yielded wheat and soya bean, respectively, in a separate cropping season. From Vitousek et al. (2009).