Optimum nitrogen supply is important for onion size. Nitrogen supports production of leaves and hence, increases the number of onion bulb scales and the size of the harvested bulb. Because these crops are shallow rooted with poor root branching, it is common practice to use split N applications to maximize uptake efficiency. Placement has been proven to be more effective than broadcasting.
It is important that sufficient phosphorus is available to balance potential storage losses resulting from high N application rates. Phosphorus can be used in such a way to ensure top yields through use of high rates of nitrogen, yet at the same time providing little deterioration in crop quality.
Trials with calcium nitrate confirm this is the most effective form of nitrogen for high yields and larger bulbs.