Plants respond strongly to environmental heterogeneity, particularly below ground, where spectacular root proliferations in nutrient–rich patches may occur. Such ‘foraging’ responses apparently maximize nutrient uptake and are now prominent in plant ecological theory. Proliferations in nitrogen–rich patches are difficult to explain adaptively, however. The high mobility of soil nitrate should limit the contribution of proliferation to N capture. Many experiments on isolated plants show only a weak relation between proliferation and N uptake. We show that N capture is associated strongly with proliferation during interspecific competition for finite, locally available, mixed N sources, precisely the conditions under which N becomes available to plants on generally infertile soils. This explains why N–induced root proliferation is an important resource–capture mechanism in N–limited plant communities and suggests that increasing proliferation by crop breeding or genetic manipulation will have a limited impact on N capture by well–fertilized monocultures.