Today half of the world population would not be alive without the 68 million tons of nitrogen (N) fertiliser applied to agricultural soils yearly. However, synthetic N has become too much of a good thing because most applied N is not used by crop plants and escapes to pollute groundwater as nitrates and atmosphere as nitrogen oxides. An alternative and cheap solution is to return plant residues to the soil, thus producing ‘natural’ N by plant residue decomposition. Chen et al. reviewed the effects of crop residues on soil N behaviour to better understand the mechanisms of transformation, and to predict farming practices that optimise natural fertilisation.