Onion Container Soil Mix
Allium cepa
Shallow-rooted heavy feeder early, then tapering N — wants loose, organic, evenly moist soil.
- Container:
- 5 gal wide pot, multiple bulbs
- pH:
- 6–7
- EC:
- 1.4–2 mS/cm
- Sun:
- 8–14 hr
- Light:
- Full sun; long-day varieties need 14+ hr daylight to bulb
Components
Percentages by volume. Quantities scaled for a 5-gallon container (US units).
| Component | % | Amount |
|---|---|---|
| Quality potting mix Base structure, initial nutrients | 45% | 9 qt |
| Aged compost Microbial diversity | 20% | 4 qt |
| Coco coir (low-EC, buffered) CEC, moisture buffering | 15% | 3 qt |
| Perlite Drainage, aeration | 10% | 2 qt |
| Vermicompost / Worm castings Nutrients, microbes, growth hormones | 10% | 2 qt |
Per-container amendments
Scaled linearly to your container size. Apply at transplant or as side-dress per the notes on each line.
- Balanced organic fertilizer (4-4-4 / 5-5-5) Mix in at planting; side-dress twice mid-season, stop once tops yellow.2–4 tbsp
- Bone meal Slow phosphorus + calcium for bulb development.2–3 tbsp
- Kelp meal Potassium + trace minerals; do not over-apply.1–2 tbsp
Growing notes
- Match variety to latitude: long-day for north (14+ hr), short-day for south (~10-12 hr).
- Shallow-rooted — stay on top of moisture, especially as bulbs swell.
- Side-dress urea or fish emulsion at 0.25-0.5 lb / 25 ft of row once or twice mid-season.
- Stop feeding nitrogen once tops fall over; excess N gives soft, poor-storing bulbs.
- Do not hill soil over bulbs — encourages stem rot.
- Cure for 2-4 weeks in warm (75-90 F), ventilated shade before storage.
Want to scale this to a different container size, save it to a plot plant, or track applied dates? Open it in the interactive calculator.
References
- Growing onions in home gardens — University of Minnesota Extension . University of Minnesota Extension
- Growing Onions in a Home Garden — University of Maryland Extension . UMD Extension Home & Garden Information Center