Role of Calcium in Onion Production
Calcium is a key component of cells maintaining the structure of cell walls and stabilizing cell membranes.
It also has a direct influence on the salt balance within plant cells and activates potassium to regulate the opening and closing of stomata to allow water movement from the plant. Calcium enhances pollen germination; regulates some enzyme systems; and influences the growth and health of cells and conductive tissues.
Calcium has a direct effect on root establishment and elongation as well as early crop growth, increasing plant height and vigor (USA studies). Calcium nitrate is particularly useful applied early to improve establishment, ensuring crops are quick to reach the desired (5.9in) height before transplanting.
Calcium helps improve tolerance to diseases. Trials confirm crops with high levels of calcium in the bulbs have reduced levels of black rot due to Aspergillus niger (trials in India) and, in association with magnesium, less storage rots. Calcium has also been found to minimize neck rots such as Botrytis allii.
Calcium helps protect against chilling injury, which can lead to cell breakdown, as studies with Red Onions in USA show. This can be valuable in stores where there is poor temperature control.
USA trials (Vidalia Sweet Onion, Granex 33) with calcium nitrate confirm this is one of the most effective forms for high yields.
The same trials show that the greatest bulb size is obtained when using calcium nitrate.
Calcium nitrate is also the most effective formulation in reducing the number of bolters and ensuring minimal rots in store (USA studies).
Calcium effect at growth stages
| Stage | Calcium effect |
| Vegetative Growth | Maintain vigorous, healthy leaf growth and to build plant supplies prior to bulbing |
| Bulb Formation | Ensure good supply to the bulb to maximize eventual storage quality |
| Bulb Fill | Maintain good bulb firmness and quality with reduced storage problems |
See more on Onion Growth Stages.
General Guidelines for Calcium Application
Calcium is probably the third most important nutrient needed by onions and has a key role to play in maintaining crop quality. Calcium nitrate is particularly useful applied early to improve establishment, ensuring crops are quick to reach a desired height before transplanting. Although relatively low, levels of calcium in the bulbs are critical for storage and quality, improving the strenght of the cells.
Calcium Deficiencies in Onion
Leaf tips – or short lengths of the leaf - die back, without any previous yellowing of the leaf, causing the top of the leaf to fall over and die. Onion bulbs are of low density and reduced quality, and have softer skins.
