Onion Nutritional Summary

Macronutrients

Nitrogen, followed by potassium, are the nutrients needed in greatest quantities to maximize growth and yield.

Nitrogen

Nitrogen is the nutrient needed in greatest quantities to maximize growth and yield. Peak need is during late leaf production and main uptake is earlier than for other macronutrients. 

Onions require around 3kg/t of bulb yield. In comparison, salad onions, which are harvested when green, have a larger nitrogen-requirement. Because onion crops are shallow rooting, high rates of nitrogen are needed to satisfy crop demand, and little and often applications during early growth are needed to maximize uptake and minimize potential soil losses. Most of the nitrogen taken up by onions is found in the bulb. 

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. 

In onions, over 70% of the phosphorus taken up is found in the bulb. Common practice is to apply phosphate pre-planting or as a starter fertilizer. Potassium is also needed in large quantities. Crop removal is around 2.0kg/t in bulb onions. Peak demand is later than that for nitrogen, during bulking. Base applications of potassium are usually followed by regular applications throughout the season. Most potassium taken up by the plant is utilized by the leaf. This is why removal is higher in salad onions.

Calcium

Calcium is also needed in relatively large quantities at around 0.8kg/t. Peak requirements mirror those of potassium and peak at bulb formation and early enlargement. While it is particularly important for leaf growth, the relatively small amounts that are found in the bulb maintain bulb density and crop storage quality. As with nitrogen, because calcium is important for good leaf growth, relatively more calcium will be removed in crops harvested green, e.g. salad crops and leeks.

Sulphur removal in dry bulb onions is low at around 10% of that of the calcium. Peak uptake is during the later stages of bulb maturity. Thus, earlier harvesting or restricting sulphur supply on low-sulphur supply soils will lead to production of sweeter onions. For the same reason, crops harvested earlier, e.g. salad onions have a reduced sulphur uptake and are less pungent. Only small levels of magnesium are utilized by the crop. Peak uptake is relatively late. All nutrient uptake figures above are given in elemental form.

Phosphorus

Most phosphorus is required early on in the plant’s development to ensure good root growth and to boost establishment. Over 70% of the phosphorus taken up is found in the bulb. Common practice is to apply phosphate pre-planting or as a starter fertilizer.

Potassium

Potassium is also needed in large quantities. Crop removal is around 4 lb/t in bulb onions. Peak demand is later than that for nitrogen, during bulking. Base applications of potassium are usually followed by regular applications throughout the season. Most of the potassium taken up by the plant is utilized by the leaf. This is why removal is higher in salad onions.

Micronutrients

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. All micronutrients play a role in seedling and leaf growth. Without good leaf productivity, growth slows and yield suffers. Leaf tissue analysis to assess micronutrient need, will enable deficiencies to be correctly diagnosed and treated, and ensure that onion production is maximized.

Boron

Boron is one of the key micronutrients needed in greatest quantity in onion production, following iron. It has an influence on yield and quality.

Copper

Though only needed in very small quantities, copper is important for skin finish and colour.

Iron

Iron is the micronutrient needed in greatest amount in onions production.

Zinc

Zinc plays an important role in seed germination.