NPK Fertilizer Guide: Nitrogen, Phosphorus & Potassium Explained

The three numbers on every fertilizer bag (e.g., 10-10-10) represent the percentage by weight of three essential macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K).

Nitrogen (N) — Green Growth

• Drives leafy, vegetative growth.
• Core of chlorophyll → photosynthesis.
• Sources: composted manure, blood meal, fish emulsion, canola meal.
• Excess: Weak, leggy growth; dark green leaves; delayed flowering.
• Deficiency: Yellowing older leaves, stunted growth.

Phosphorus (P) — Roots, Flowers, Fruit

• Promotes root development, bloom set, fruiting, disease resistance.
• Sources: rock phosphate, bone meal, guano.
• Deficiency: Purplish stems, dull green leaves, poor flowering despite lush foliage.

Potassium (K) — Overall Vigor & Stress Tolerance

• Regulates water uptake, enzyme activation, cold hardiness, disease resistance.
• Sources: greensand, sulfate of potash magnesia (Sul-Po-Mag), kelp meal, wood ash.
• Deficiency: Leaf edge scorch (blue/yellow/purple), weak stems, small fruit, poor storage life.

Choosing the Right Ratio

• Lawns / leafy greens: Higher N (e.g., 12-4-8)
• Flowering / fruiting / transplants: Higher P (e.g., 5-10-5)
• Fall prep / stress recovery: Higher K (e.g., 0-0-22)
• Balanced maintenance: 10-10-10 or 5-5-5 (organic)

Always soil test first. Amend based on deficits—not habit.