How to increase onion bulb size

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

Nitrogen increases leaf area and so 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.

Effect of nitrogen rate and placement on onion size

Effect of nitrogen rate on onion size

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.


Calcium nitrate is the most effective source of nitrogen

Effect of calcium nitrate on onion size

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

Phosphorus allows increased 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 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.

Effect of phosphate rate on onion size and weight

Potassium deficiency will reduce onion yeild

Onions show a good response to potassium which will restict yeild if limiting provided and allows better utilisation of nitrogen.

Effect of potassium and nitrogen on onion yield

Sulphur allows optimum nitrogen efficiency

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.

Effect of sulphur on onion bulb size