Synthetic nitrogen (N) fertilizer has enabled modern agriculture to greatly improve human nutrition during the 20th century, but it has also created unintended human health and environmental pollution challenges for the 21st century. Averaged globally, about half of the fertilizer N applied to farms is removed with the crops, while the other half remains in the soil or is lost from farmers’ fields, resulting in water and air pollution. As human population continues to grow and food security improves in the developing world, the dual development goals of producing more nutritious food with low pollution will require both technological and socio-economic innovations in agriculture.