Fertilizer for Onions: NPK Guide and Application Rates
Onions are heavy nitrogen feeders during the leaf-building phase because each green leaf corresponds to one ring on the bulb. More leaves mean bigger onions. Nitrogen feeding must stop once bulbing begins, or the bulbs will not cure and store properly.
Quick Reference
What NPK Ratio is Best for Onions?
The recommended NPK ratio for onions is 10-10-10. A balanced 10-10-10 feeds all three macronutrients equally, which works well when soil tests show no single deficiency.
Onions NPK needs shift at each growth stage. Balanced feed for sets, transplants, or seedlings during the Seedling phase, then reduce nitrogen once bulbing begins once flowering begins.
NPK by Growth Stage for Onions
| Growth Stage | NPK Ratio | Notes |
|---|---|---|
| Seedling | 10N 10P 10K N 10% Nitrogen: leaf and stem growth P 10% Phosphorus: roots and flowers K 10% Potassium: health and disease resistance Learn more about NPK | Balanced feed for sets, transplants, or seedlings |
| Vegetative | 21N 0P 0K N 21% Nitrogen: leaf and stem growth P 0% Phosphorus: roots and flowers K 0% Potassium: health and disease resistance Learn more about NPK | Ammonium sulfate for top growth and leaf count |
| Flowering | 5N 10P 10K N 5% Nitrogen: leaf and stem growth P 10% Phosphorus: roots and flowers K 10% Potassium: health and disease resistance Learn more about NPK | Reduce nitrogen once bulbing begins |
| Fruiting | 0N 0P 0K N 0% Nitrogen: leaf and stem growth P 0% Phosphorus: roots and flowers K 0% Potassium: health and disease resistance Learn more about NPK | Stop fertilizing when tops begin to fall over |
When Should You Fertilize Onions?
In zone 3, start fertilizing onions in May. In zone 10, begin as early as January. Apply every 2 weeks through the growing season.
Which Zones Can Grow Onions?
Onions grows in USDA zones 3 through 10, spanning a 8-zone range.
Fertilization Start by Zone
| Zone | Start Month |
|---|---|
| Zone 3 | May |
| Zone 4 | April |
| Zone 5 | April |
| Zone 6 | March |
| Zone 7 | March |
| Zone 8 | February |
| Zone 9 | January |
| Zone 10 | January |
What Are the Best Fertilizer Products for Onions?
Look for products with an NPK close to 10-10-10. Espoma Garden-tone is the most popular choice among home gardeners for onions.
- Espoma Garden-tone
- Miracle-Gro All Purpose
- Down to Earth Blood Meal
Prices vary by retailer and region. Check current pricing before purchasing.
Container and Raised Bed Adjustments for Onions
Feed container onions every 10 days with liquid nitrogen fertilizer until bulbing, then stop
Common Fertilizing Mistakes with Onions
- Continuing nitrogen after bulbing starts causes thick necks that rot in storage
- Using the wrong day-length variety for your latitude means onions never bulb
- Fertilizing after tops fall over delays curing and shortens storage life
Tips for Fertilizing Onions
- Feed nitrogen every 2-3 weeks until bulbing begins (when the neck softens)
- Each leaf above ground equals one ring on the bulb, so more leaves mean bigger onions
- Stop all nitrogen once the tops begin to fall over and let bulbs cure in the ground
Organic Fertilizer Alternatives for Onions
Sources
Source:Clemson University ExtensionLast updated: