Best Fertilizer for Large Onion Bulbs: Shift from 20-20-20 to High P-K Formulas

Balanced fertilizers like 20-20-20 grow great tops but disappoint on bulb size. Onions need phosphorus and potassium dominance.

Recommended Ratios

• Ideal: 5-10-10 or 10-20-20 (1:2:2 ratio).
• Alternative: 5-15-5 + light dusting of 0-0-50 (sulfate of potash).

Where to Find Them

Check lawn fertilizer sections in spring/fall—some brands offer 10-20-20 in 40-lb bags ($18), far cheaper than small-box "tomato fertilizers" (often 5-10-10).

Why It Works

Bulb development consumes heavy P and K. Excess nitrogen (first number) fuels leaves at the expense of storage tissue.

Zone 5 Tip (Trenton, ON)

Apply at planting and again at bulb initiation. Avoid late-season nitrogen to ensure proper curing.