Human influence on soil nutrient capital and cycling depends on the chemical element, soil system, and history and current impacts of human management. We examined human effects on the soil N cycle across agricultural landscapes in the Southern Piedmont of South Carolina. In each case, whether Mississippian maize, antebellum and post-bellum cotton, or old-field pine, progressive N limitation appears to limit net primary productivity and harvests within a few decades. Nitrogen amendments combined with management practices that limit N loss, forestall N limitation and make the ecosystems more sustainable.