Role of Nitrogen in Onion Production
Nitrogen is a key component of enzymes, vitamins, chlorophyll and other cell constituents, all of which are essential for crop growth and development.
High application rates of 134 lb/ac or above are usually required in onions for top yields. This is a function of the usually light, hungry soils on which they are grown and also the crop’s poor root structure and N-uptake capability.
High rates of nitrogen are also important for onion size. The more N-applied the more leaves and hence the number of bulb scales produced, and the bigger the harvested bulb (trials with Valenciana Onions, Chile).
Nitrogen has a direct effect on date of maturity, as shown in studies in Denmark. The more N-used, the faster the crop reaches maturity as judged by the gradual senescence and turn-over of the crop leaves.
Although nitrogen can also help to improve the bulb’s resistance to pressure by increasing the specific weight of the outer skin, reducing damage handling and storage, overall bulb firmness may decrease with high rates of nitrogen due to a softening of internal tissue (trials with Granex 33 Onions, USA).
Care has to be taken since excessive nitrogen can result in bulb decay and storage rots, causing yield losses in store. Excessive N can also weaken plant tissues increasing susceptibility to cold damage (USA studies).
Trials confirm that a balance between nitrate and ammonium-N is most effective at promoting leaf and bulb growth. However, during later stages of growth, nitrate forms boost yield by providing higher dry matters in the bulb (trials with Granex 33 Onions, USA).
Nitrogen effect at growth stages
| Stage | Nitrogen effect |
| Pre-Planting | Promote strong early growth |
| Vegetative Growth | Ensure continued growth and development |
| Bulb Formation | In nitrate forms to maintain bulb development and early growth |
| Bulb Fill | In reduced amounts so as to maintain dry matter production but not compromise bulb quality |
See more on Onion Growth Stages.
General Guidelines for Nitrogen Application
Nitrogen is the most important nutrient required for high yields and bigger bulb size. High application rates of 134lb/ac or above are usually required. Placement is more effective than broadcasting. Because the crop is shallow rooted with poor root branching, it is common practice to use split N-applications to maximize uptake efficiency. It is also important to apply nitrogen in the right form to maximise uptake during specific periods of crop growth.
Nitrogen Deficiencies in Onion
Deficient crops are slow growing and have pale yellow-green colored leaves. New leaves are thin, erect and smaller in diameter. They are also more prone to breakage.
