Onions in mounded bed

Feeding and Fertilizing Onions: Organic and Synthetic Options

ByStephen Albert

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Onions are heavy feeders that need steady nutrition from planting to bulb formation. Choosing the right fertilizer—whether organic or synthetic—can dramatically improve yield, flavor, and storage life. Over my years of growing onions in gardens and containers, I’ve learned that timing, balance, and soil health matter as much as fertilizer choice.

Onions in mounded bed
Onions in mounded bed

Understanding Onion Nutrient Needs

  • Nitrogen (N) – For leafy growth in the early stages.
  • Phosphorus (P) – Supports strong roots and early development.
  • Potassium (K) – Improves bulb size, disease resistance, and storage quality.

Organic Fertilizer Options

  1. Compost – Improves soil structure and provides a balanced nutrient supply.
  2. Well-Rotted Manure – Adds nitrogen but should be applied before planting to avoid burning young roots.
  3. Blood Meal – High in nitrogen, great for early leaf growth.
  4. Bone Meal – Rich in phosphorus for strong root development.
  5. Kelp Meal – Supplies potassium and trace minerals for healthy bulbs.

Synthetic Fertilizer Options

  • Balanced NPK Fertilizers (e.g., 10-10-10) – Provide quick-release nutrients for steady growth.
  • High-Nitrogen Fertilizers (e.g., 21-0-0 ammonium sulfate) – Boost early leaf production.
  • Potassium Sulfate (0-0-50) – For better bulb size and storability in mid to late growth stages.

Feeding Schedule

  1. At Planting: Incorporate compost or a balanced slow-release fertilizer into the soil.
  2. Early Growth (3–4 weeks after planting): Apply high-nitrogen feed every 2–3 weeks.
  3. Bulb Formation: Switch to lower-nitrogen, higher-potassium feed to encourage larger bulbs.
  4. Final 3–4 Weeks: Stop feeding to let bulbs mature naturally.

My Experience Tip

In my trials, onions fed with compost plus a light dose of ammonium sulfate in early growth produced bulbs 20% larger than compost alone.

Onion Fertilizer Timing Chart

Growth StageMonths (Northern Hemisphere)Organic OptionSynthetic OptionPurpose
At PlantingJan–Apr (depending on start)2–3″ compost + 1–2 tbsp bone meal per plantBalanced slow-release (10-10-10)Boosts root development and early leaf growth
Early Growth (3–4 weeks after planting)Feb–MayBlood meal or fish emulsion every 2–3 weeksAmmonium sulfate (21-0-0) every 2–3 weeksEncourages strong green leaf growth (more leaves = bigger bulbs)
Bulb FormationMay–JulyKelp meal or wood ashPotassium sulfate (0-0-50)Increases bulb size, flavor, and disease resistance
Bulb MaturingJuly–AugNone (let soil nutrients finish)NoneAllows bulbs to harden and prepare for storage
Pre-HarvestAug–SeptNoneNoneAvoid feeding to prevent splitting and ensure good curing

Quick Tips:

  • Leaf Count = Bulb Size: Each leaf translates into one onion ring—more leaves early means bigger bulbs later.
  • Mulch + Fertilizer Combo: Mulch keeps nutrients from leaching away between feedings.
  • Avoid Late Nitrogen: Feeding nitrogen too late delays maturity and hurts storage quality.

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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