Nitrogen nutrition and fertilization of onions (Allium cepa L.)–A literature review

•Onion bulbs contain on average 1.7 kg N Mg−1 of total bulb yield at harvest.•N in bulbs accounted for 65% of the N in the aboveground biomass.•There is no consensus on the best form of N fertilizer for onions.•N use efficiency can be improved with slow- or controlled-release fertilizers.•Split and band applications can improve N use efficiency.