Temperature or drought stress can significantly harm growth if occurring at critical periods. Plant density and spacing are essential to determine onion and garlic bulb size and number. Yields are higher where early leaf cover is produced and then maintained for a long period prior to bulb formation.
Onions and garlic have a relatively low nutrient uptake efficiency, due to their shallow root system characterized by thick roots with very few and short hairs. Therefore it is critical to implement a fertilizer program adapted to those conditions in order to help to achieve optimum yields. It is essential to time the applications of nutrients as close as possible to the rooting zone in frequently and readily available forms.
Major nutrient removal figures show that nitrogen and potassium are the nutrients needed in relative greatest quantities in onion and garlic crops. They are fundamental for achieving high marketable yields.
High rates of nitrogen are needed to satisfy crop demand. However it is important not to over-apply nitrogen, particularly in bulb onions, as this can delay maturity, soften bulbs and lead to storage rots.
While much lower levels of micronutrients are needed to satisfy yield and quality onion crop production, the correct balance of these trace elements is essential.
The key micronutrients needed in greatest quantities are boron, and iron. These have an influence on yield and quality. Zinc also pays a role in seed germination.