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

Nitrogen (N) is an essential nutrient, which often limits yield in crop production. Due to their sparse and shallow root system, the N fertilizer use efficiency in onion (Allium cepa L.) production is often low and the risk of nitrate leaching losses high. This review provides an overview of the scientific literature on efficient N management practices in onion and the effects of N availability on onion growth and bulb quality. Nitrogen demand of onions strongly depends on yield. On average 65% of the total N in the aboveground biomass is in the bulbs at harvest while 35% is in the leaves. The N concentration in the biomass decreases during the growing season. At harvest, bulbs contain on average 1.7 kg N Mg−1 total bulb yield. In addition to plant demand, the optimal fertilizer application rate depends on the availability of N from non-fertilizer sources, such as residual soil nitrate, nitrate in the irrigation water and N mineralized from organic material during the growing season. All of these sources are site-specific and can vary considerably. Therefore, recommended optimal N application rates need to be developed locally. There is no consensus on the best form of N fertilizer for onions. Differences in growth and yield found in individual studies appear to be due to the other ions included in the fertilizer materials and the effects of the fertilizers on the uptake of other nutrients and on soil properties. However, the N use efficiency can be improved with slow- or controlled-release fertilizers. Split applications can be timed to match the N available with crop N demand. This reduces the residence time of fertilizer N in soil and thus the risk of N being lost. Band application of N is another practice that can significantly increase yield and N uptake compared to broadcasting. Nitrogen availability can also affect bulb quality. Increasing N availability generally increases bulb size and decreases incidence of bolting. In contrast, increasing N availability, especially when it exceeds N demand, can contribute to a higher occurrence of thick neck, sprouting and bulb decay during storage. The overview of onion N use and N fertilizer management practices presented in this article can help choose optimal N fertilization practices based on site-specific conditions to achieve high N use efficiency and minimize losses.