The nitrogen fertilizer conundrum: why is yield a poor determinant of crops’ nitrogen fertilizer requirements?

Sustainable agriculture requires optimizing nitrogen fertilizer applications to crops. It is commonly assumed that higher yielding crops need more nitrogen, and smaller less, despite some evidence that optimum nitrogen applications are poorly related to yields. In contrasting experiments, we show that year-to-year climatic variations cause the poor correlations. New approaches to nitrogen recommendations are needed to account for this uncertainty.