Role of Potassium in Onion Production
Potassium is involved in the production and transport of carbohydrates and sugars to the bulb, enzyme activation and synthesis of proteins.
It also maintains the ionic balance and water status within the plant, thereby improving resistance to cold injury and drought.
Potassium rates of around 178 lb/ac produce yield responses on high yielding sites where soil K levels are low. However, over-application can lead to yield reductions.
Potassium is particularly important where high N rates have been applied, as trials in Australia show. In these situations it helps maintain yield by increasing dry matter and sugar accumulation in the bulb.
Use of potassium allows growers to maintain high rates of nitrogen for yield with reduced risk of storage problems such as rots or sprouting, as studies with White Onions in India show.
Onions are very prone to salt stress, so it is important to use sources of potassium less likely to cause damage, particularly in drier regions and where irrigation is used. Widespread use of muriate of potash can result in high levels of chloride in the plant, leading to loss of yield and bulb weight in store compared to other forms of potassium fertilizer (trials in South Africa).
Sulfate and nitrate forms create less salt stress than muriate (trials in India and UK). Sulfate will also provide valuable sulfur in a crop which has a high S requirement and at the same time ensure other nutrient uptake is less restricted when compared to muriate of potash.
Potassium effect at growth stages
| Stage | Potassium effect |
| Pre-Planting | Promote strong early growth |
| Vegetative Growth | Ensure continued growth and development |
| Bulb Formation | Peak requirements are during bulbing – for yield and quality |
| Bulb Fill | Maximize dry matter & sugar accumulation as well as quality |
See more on Onion Growth Stages.
General Guidelines for Potassium Application
Onions have a relatively high K-requirement. Maximum uptake is later than that of nitrogen and peaks during bulb formation and enlargement. However, over-application can lead to yield reductions.
Potassium Deficiencies in Onion
Leaf tips turn brown and there is some slight yellowing of older leaves. Growth slows and leaves become erect. The entire leaf may also droop, and appear thin and papery. Bulbs are soft with thin skins.
