Optimum nitrogen supply is important for onion size. Nitrogen supports the 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.
Trials with calcium nitrate confirm this is the most effective form of nitrogen for high yields. It is also the most effective formulation in increasing onion bulb size
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 the 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.
Onions show a good response to potassium which will restrict yield if limiting provided and allows better utilisation of nitrogen.
Together with nitrogen sulphur is an essential component of proteins and without sufficient sulphur, the plant is unable to utilise nitrogen efficiently. However, care has to be taken as high rates, particularly in S-rich soils, can have a detrimental effect on yield. It has also a marked effect on the pungency of the onion through increasing the pyruvic acid content of the bulb – a key quality characteristic.