Increasing Onion Bulb Size

Planting density is a very important management factor in determining bulb size in onion crops. In addition, good crop nutrition can help growers to achieve a desired diameter. Nitrogen, potassium and phosphorus are essential to influence onion bulb size.

Crop Nutrition and Onion Bulb Size


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.

Calcium nitrate

Trials with calcium nitrate confirm this is the most effective form of nitrogen for high yields and larger bulbs.