Optimum nitrogen supply is important for onion weight. Nitrogen supports production of leaves and hence, increases the number of bulb scales and the size and weight of the harvested bulb whether garlic, onion or even leeks.
Where crops are grown on soils with very low natural phosphorus levels, higher rates are required to boost yield, thereby increasing bulb weight and size.
Potassium is particularly important where high rates of nitrogen have been applied, as potassium helps to maintain yield with reduced risk of storage problems that can be caused by excess nitrogen.
Onions are very prone to salt stress, so it is important to use sources of potassium that are less likely to cause damage. Toxic levels of chloride in the plant can lead to loss of yield and onion bulb weight in storage. Sulfate and nitrate forms of potassium usually result in lower bulb weight loss in storage.
Sulphur is important for onion bulb weight and yield, and helps to improve the crop’s utilisation of nitrogen. 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.
Trials confirm that boron has a positive effect on onion yield by increasing bulb weight. It is important to maintain the correct balance of calcium, nitrogen and boron in the soil as high Ca and high N levels can reduce boron uptake.