Nitrogen
Nitrogen is the most-used fertilizer nutrient, and often gives large yield increases when applied to non-legume crops. Smil (Enriching the Earth, 2001, MIT Press) estimates that 40 percent of the human population would not be alive today without nitrogen fertilizer.
Nitrogen is also more prone to loss than the other primary nutrients. Nitrogen lost from crop fields contributes to low-oxygen coastal waters and to high nitrate concentrations in drinking water.