Onion crops thrive in a variety of soils. The soils considered ideal are those of medium and fine texture, sandy loam, warm, with good drainage and rich in organic matter. High yields are achieved in such soils, and bulbs with high quality characteristics are produced. Light sandy soils boost early ripeness, yet require more frequent irrigation and increased fertilizing.

Firm clay and saline soils prevent oxygenation and normal function of the root system, create diseases, and lower the yields and the quality of the produce.

Regarding acidity, onions favor slightly acidic to neutral soils (pH: 6.5 - 7). In soils with a pH lower than 5.5 or higher than 8, the plants exhibit reduced growth and yields, and correction of the soil acidity is therefore required.

Onion crops produce high yields and have high requirements in nutrients. Due to the root system being thin and near the surface, nutrient supply needs to be targeted and adequately cover all stages of the life cycle. In order to get satisfactory produce in terms of both quantity and quality, the requirements in fertilizer units depend on the type of crop.

Nutritional requirements for production of six tons of dry onion (kg/dec.)
Type Nitrogen
(N)
Phosphorus
(P2O5)
Potassium
(K2O)
Calcium
(CaO)
Sulfur
(S)
from seed 20 - 26 14 - 18 22 - 26 6 - 8 4 - 6
from onion sets 16 - 22 12 - 16 18 - 22 4 - 6 3 - 6
spring onions 18 - 22 8 - 12 16 - 18 4 - 6 2 - 4

From sprouting to the onset of bulb formation (1st - 4th leaf)

From sprouting to the onset of bulb formation (1st - 4th leaf)

After sprouting and until the 3rd leaf emerges, the onion grows slowly, both above the ground and in the soil.

Once the 3rd leaf has emerged, the growth of the crop is accelerated. With the contribution of higher temperatures, the plants produce more and larger leaves in order to increase their photosynthetic activity and produce reserves of substances in order to form the bulbs.

The nutritional status of the plants at this stage is critical for the onset of bulb formation, the development of the crop, and the final yields.

Nitrogen (Ν) and Sulphur (S) promote the formation of the plant’s organs (leaves - root) during the early stages, and they boost an early and robust sprouting, which is required for bulb formation. An excessive Nitrogen supply should be avoided, because it leads to great losses and delays bulb formation.

Phosphorus (Ρ) is the most important element for the development of the root system, the strong establishment of the crop, and the onset of bulb formation. Due to the root system being thin and near the surface, onions do not absorb Phosphorus easily, which is why it should be emphasized in its application, both in basic and top fertilization.

Potassium (Κ) promotes the growth of the roots and the leaves, increases foliar activity so that bulb formation can begin, improves water absorption, and protects the crop against diseases and adverse weather conditions.

During the early stages, Calcium (Ca), Boron (Β), Zinc (Ζn), and Iron (Fe) are key elements for the formation and development of plant tissue, and they also increase the production of storage substances and boost bulb formation.

Bulb formation (5th - 7thh leaf)

Bulb formation (5th - 7th leaf)

The leaves accumulate the products of photosynthesis and become enlarged at their base. Their diameter doubles in the soil compared to the collar, and the bulb is formed. New, larger leaves are produced, and the first three leaves gradually dry up. Length of daylight, temperature, and the nutritional condition of the plants are the main factors affecting bulb formation.

A good nutrient and water supply to the plants contributes to smooth development and completion of bulb formation.

Nitrogen (Ν) sustains the photosynthetic activity of the leaves required for bulb formation. The supply of the plants needs to be regular, without any exaggerations that would delay bulb formation.

Phosphorus (Ρ) and Potassium (Κ) play a significant part, because they are involved in the production and assimilation of the storage substances accumulated in the bulbs. The plants need to be adequately supplied with both of these nutrients through both basic and top fertilization.

Calcium (Ca) promotes healthy bulb formation, improves their qualitative characteristics, and increases their storability after the harvest.

Bulb growth (from the 7th leaf to the onset of ripening)

Bulb growth (from the 7th leaf to the onset of ripening)

At this stage, the plants speed up the rate of leaf production. After the 13th leaf has emerged, the rest cannot grow above the ground, so they remain inside the bulb and increase its diameter. At the same time, the storage substances produced in the grown leaves migrate to the formed bulbs and contribute to their further enlargement.

During the time of bulb growth, onions have the highest requirements in nutrients and water, more so than at any other developmental stage.

Nitrogen (Ν) has a determining effect on the size of the bulbs. Its application at the early stages of enlargement boosts leaf production and protein accumulation in the bulbs. Adding Nitrogen too late and in excess should be avoided, because doing so leads to softer bulbs, thus damaging their quality and reducing their storability.

Phosphorus (P) is necessary for the size increase, formation of the outer layers, and ripening of the bulbs. Onions absorb large quantities of Phosphorus during this period and therefore requires an adequate phosphorus supply. Phosphorus deficiency leads to smaller bulb size, delays ripeness, and degrade their qualitative characteristics.

As the bulbs grow, their requirements in Potassium (Κ) are maximized. The latter contributes to carbohydrate production in the leaves and their migration to the bulbs. An adequate supply of Potassium during this time has a determining effect on the size and weight of the onions, their “sweetness”, and their storability after the harvest.

Ripeness - Harvest (shedding - complete drying of the leaves)

Ripeness - Harvest (shedding - complete drying of the leaves)

As the bulb enlargement comes to an end, the older leaves dry up while there is dryness at the ends of the newer leaves as well. At the same time, the outer layers of the bulb are formed. Gradually, all leaves lose their turgidity, shrivel up and fall to the ground, while the collar become feebler and can no longer support them.

Despite the fact that the plants’ photosynthetic activity halts, the bulbs keep growing during ripening due to the migration of the substances which had been stored in the leaves towards the bulbs.

The goal is to fulfill the requirements in the nutrients necessary for the development of a robust and rich root system, and to create adequate and early sprouting which will promote bulb formation. Due to the root system of the onion being thin and near the surface, and adequate Phosphorus and Potassium supply to the crop should be emphasized for the strengthening of the root system and the proper establishment of the crop.

The supply at this stage is:

  • 1/3 of Nitrogen (Ν)
  • 1/2 of Phosphorus (P)
  • 1/2 of Potassium (Κ) and Sulphur (S)

Due to the root system being near the surface and the slow rate of absorption from the crop, Nitrogen in basic fertilization should be supplied through compound stabilized fertilizers rich in Ammoniac Nitrogen (NutrActive). Fertilizers with high Nitric Nitrogen (ΝΟ3-) content lead to great losses and are not considered suitable for this crop.

Note: In order to better utilize the nutrients, basic fertilization can be applied in two doses. The first through integration before sowing and the second immediately after sprouting.

Onion fertilizing
Type of Fertilizer Stage of implementation Dosage* (Kg/dec.)

NutrActive triple-S 15-15-15 (+25)
NutrActive special 12-12-17 (+30) +2MgO + TE
Complefert blue-star 12-12-17 (+30) + 2MgO + TE
Complefert extra 12-10-20 (+28) +2MgO + TE
Complefert premium 8-12-18 (+30) + 2MgO + TE
Nutrifert base 11-15-15 (+32)
Organofert 12-12-12 (+35) +10% O.Y.

Basic fertilizing

80 - 120 kg

*The dosages mentioned are indicative. Regarding the fertilizing of crops one should take into account the advice of local agronomists.

Sprouting - 1st application (3rd - 5th leaf)

The goal is the formation of a rich leaf area, which will contribute to the onset of bulb formation. Nitrogen can be supplied in doses with two applications: one early on (between the 2nd and 3rd leaf) together with Calcium, and one before bulb formation (between the 4th and 5th leaf) together with Phosphorus.

The supply at this stage is:

  • 1/3 of Nitrogen (Ν)
  • 1/4 of Phosphorus (P)
  • The full amount of Calcium (Ca)

The use of Ammonia Nitrosulphate (Fertammon) or Nitrogenous stabilized fertilizers (NutrActive), which perfectly cover the needs of the crop and boost bulb formation, is recommended. Part of the Nitrogen and the Potassium (if necessary) can be supplied through the irrigation system, in the form of Calcium Nitrate or Potassium Sulphate, thus covering the needs of the crop in Calcium and Potassium.

Onset of bulb enlargement - 2nd application (7th - 8th leaf)

The goal of the second top fertilization is to fulfill the high nutritional requirements of bulb growth. The fertilizer should be applied early on, before bulb formation has concluded, and it should include all three macronutrients (Ν, Ρ, Κ) required for the size and weight increase of the bulbs.

The supply at this stage is:

  • 1/3 of Nitrogen (Ν)
  • 1/4 of Phosphorus (P)
  • 1/2 of Potassium (Κ)

The use of the compound multinutrient fertilizers (Complefert) is recommended, as they are highly soluble and supply the crop with fully assimilable forms of Ν, P, K, S, Mg, Trace Elements, as well as ensure high yields and qualitative characteristics of the produce.

Onion fertilizing
Type of Fertilizer Stage of implementation Dosage* (Kg/dec.)

NutrActive 27N 27-0-0 (+28)
Fertammon special 25-0-0 (+29) + 0,5FeSO4 + 0,5Zn
Nitrocan special 27-0-0 + 5MgO + 0,2B
Nutrammon 34,5-0-0

Sprouting period
(3rd - 5th leaf)

30 - 50 kg

Complefert master 15-5-20 (+20) + 2MgO + TE
Complefert extra 12-10-20 (+20) + 2MgO + TE
Complefert blue-star 12-12-17 (+30) + 2MgO + TE
Complefert special 12-8-17 (+27) + 2MgO + TE
Complefert magni-plus 14-7-14 (+25) + 4MgO + 0,1Fe + 0,1Zn

Onset of bulb enlargement
(7th - 8th leaf)

40 - 50 kg

* The dosages mentioned are indicative. Regarding the fertilizing of crops one should take into account the advice of local agronomists.

Basic fertilizing Sprouting stage
(3rd - 4th leaf)
Onset of bulb enlargement
Onion NutrActive®
Complefert®
Nutrifert®
NutrActive®
Fertammon®
Nutrammon®
Complefert®

(Basic - Top fertilization)

  • Extended crop nutrition with slow-release Nitrogen
  • Complete nutrition, according to the requirements of the crop at each developmental stage
  • Increased bulb formation and high yields due to the supply of both forms of Nitrogen to the plants
  • Ideal for the thin and close to the surface onion root system, minimizing losses due to leaching and vaporization
  • Improved water and nutrient uptake
  • High Phosphorus solubility, up to 90%, for the nutrition of the crop in all soil types
  • Potassium in the form of Potassium Sulphate, for higher yields and better quality
  • Additional enrichment with Sulphur for better utilization of Nitrogen and Phosphorus.
  • They do not include Urea, which is toxic and inhibits the sprouting of the seeds
  • Guaranteed nutrition of the crop in all soil types and all weather conditions

(Basic - Top fertilization)

  • Full in primary (Ν, Ρ, Κ,) and secondary (S, Mg) elements, as well as trace elements (Β, Ζn), they provide a complete nutrition to the crop, with all of the necessary nutrients
  • Fully assimilable forms of the nutrients and in a specialized ratio, adapted to the needs of the onion
  • High Ammoniac Nitrogen content, for prolonged crop nutrition
  • Optimal utilization of the nutrients by the crop
  • Potassium in the form of Potassium Sulphate, for higher yields and better quality
  • High Phosphorus solubility, up to 90%, for the nutrition of the crop in all soil types
  • Additional nutrition with Sulphur for better utilization of Nitrogen, Phosphorus, and Trace Elements
  • They do not include Urea, which is toxic and inhibits the sprouting of the seeds
  • Guaranteed nutrition of the crop in all soil types and all weather conditions