Onion Ferilizer & Crop Guide: Nutrition of Onion Crop

The roles of plant nutrients

Nitrogen (N) 

Dry bulb production typically requires 200–300 kg N/ha. It is advisable to incorporate 25–30% of the total nitrogen prior to planting. The remaining 70–75% should be applied in split doses every 10–14 days over a period of 2–2.5 months, starting about two months after sowing.

• Deficiency: Severely limits growth and reduces yield.
• Excess: Encourages overly succulent plants, making them more vulnerable to frost, diseases, and premature flowering. High nitrogen levels late in the season may delay maturity, promote double centers, and reduce storage quality.

It is recommended to avoid nitrogen applications once the bulb has reached 2/3 of its final size.

Phosphorus (P) 

Phosphorus is essential for early root development and plays a vital role throughout the growth cycle. Because it is relatively immobile in the soil, most phosphorus should be applied prior to planting or up to the bulb development stage.
Onions rely heavily on arbuscular mycorrhizal fungi to absorb phosphorus in low- to medium-P soils. These fungi also enhance uptake of zinc and other micronutrients in calcareous, high-pH soils.

• Deficiency symptoms: Slow growth, delayed maturity, dull-green leaves, tip dieback, and thick-necked bulbs (without the yellowing seen in N or K deficiency).
• Soil requirement: Maintain available P at 25 ppm. For every 1 ppm below target, apply 1 kg P/ha.

Potassium (K)

Potassium is crucial for water regulation, cell wall integrity, and energy transfer in plants.

• Recommended practice: Apply 30–50% of the total K before planting, and the remainder during bulb development to enhance bulb size and quality.
• Deficiency symptoms: Dark-green foliage, wilting tips (especially on the upper leaf surface), drooping leaves, and a transition to papery texture with chlorosis resembling nitrogen deficiency.
• Soil requirement: Target 12 meq/100g (CaCl₂ extraction method).
• Apply 200–300 kg/ha of potassium nitrate as a top dressing before the onset of bulb swelling.

Magnesium (Mg)

Magnesium is essential for chlorophyll synthesis and photosynthesis.

• If soil Mg is low, apply 30 kg Mg/ha before planting.
• Deficiency symptoms: Slow growth and uniform yellowing of older leaves.

Sulfur (S) 

Sulfur supports protein synthesis and enzyme function.

• Early application of sulfur is advisable in direct seeded and transplanted onions.
• Apply sulfur in two stages:
  - Half at planting or transplanting,
  - Half before bulb development.

• Application rate: 45–70 kg S/ha.

• Do not exclude sulfur from the fertility program, even in sulfur-rich soils.

Boron (B) 

Boron is important for cell wall strength and tissue development.

• Apply 1 kg B/ha before transplanting or sowing if soil tests show deficiency.
• Caution: Boron can be toxic—do not exceed the recommended rate.
• Deficiency symptoms: Distorted, stunted growth; brittle leaves; grey- or blue-green discoloration; chlorosis, cracking near the leaf base.

Zinc (Zn) 

Zinc is required for enzyme activity and growth hormone production. Onions are very sensitive to zinc deficiencies.

• Apply 6 kg Zn/ha only if a deficiency is confirmed by soil testing.
• Apply only if needed, as Zn excess might be toxic
• Deficiency symptoms: Stunted growth, interveinal chlorosis, leaf twisting.
• Zinc is typically applied as base fertilization.

Fertilizer Supply Strategy

Due to their shallow and sparse root system, onions require precise nutrient and water management. 
Immobile nutrients such as P, K, and micronutrients are not easily reached by roots, while mobile nutrients like nitrogen are prone to leaching.

To maximize nutrient use efficiency:

• Apply fertilizers close to the root zone.
• Use Nutrigation™ (fertigation) or controlled-release fertilizers to provide a steady, balanced supply throughout the season.

These methods improve nutrient uptake, reduce losses and enhance grower’s returns while minimizing environmental impact.
 

Onion Leaf Analysis

Plant part to sample is the tallest leaf blade.

Source: UC Davis, 2016