Getting Larger Onion Bulbs: Potassium vs. Nitrogen Balance

Diagnosis: Lush Tops, Small Bulbs

Vibrant green foliage with tiny bulbs means excess nitrogen and insufficient potassium. Nitrogen fuels leaf growth; potassium (potash) drives root and bulb development.

Soil Preparation

Plant in soil rich in organic matter: compost, peat moss, shredded leaves, dried grass clippings, aged manure. This provides balanced, slow-release nutrition.

Fertilizer Timing and Type

• When foliage reaches 8 inches, apply a granular fertilizer high in potassium (third number in N-P-K, e.g., 5-10-15).
• Avoid high-nitrogen fertilizers after this stage.

Key Nutrient Roles

• Nitrogen (N): Leaf growth
• Phosphorus (P): Roots, flowers, fruit
• Potassium (K): Bulb size, disease resistance, overall vigor

Balance your soil nutrition, and your red onions will bulb up properly.