What Do the NPK Numbers Mean?
The three numbers on a fertilizer label (e.g., 10-10-10) are the guaranteed percentage by weight of Nitrogen (N), Phosphorus as P2O5 (P), and Potassium as K2O (K) — always in that fixed N-P-K order. Nitrogen drives leafy growth, phosphorus supports roots/flowers/fruit, and potassium supports overall vigor and stress tolerance.
The one thing to know: The three numbers on a fertilizer label (e.g., 10-10-10) are the guaranteed percentage, by weight, of Nitrogen (N), Phosphorus as P2O5 (P), and Potassium as K2O (K) in that bag — in that fixed N-P-K order, always. Nitrogen drives leafy green growth, phosphorus supports root and flower/fruit development, and potassium supports overall plant vigor and stress tolerance. The numbers say nothing about micronutrients (calcium, magnesium, iron, etc.) or the soil's existing nutrient levels — a soil test, not the fertilizer bag, tells you what your soil actually needs.
The label order never changes
Every US fertilizer label lists the three numbers in N-P-K order by regulation, so a 10-10-10 bag is always 10% nitrogen, 10% phosphorus (as P2O5), 10% potassium (as K2O) — never a different nutrient order. See the full reference in The Garden Gear Numbers.
Soil pH decides whether those nutrients are even available
Soil pH ranges are typically read on a 0-14 scale where 7 is neutral; most common vegetables and lawn grasses prefer a slightly acidic-to-neutral range of roughly pH 6.0-7.0, while acid-loving plants like blueberries and azaleas want roughly pH 4.5-5.5. A soil test's pH reading tells you whether existing nutrients (including N-P-K already in the soil) are even chemically available for plant roots to absorb — very acidic or very alkaline soil can lock out nutrients that are technically present.
This page is general reference information about label conventions, not pesticide, chemical-application, or plant-health advice.
Frequently Asked Questions
They're the guaranteed percentage, by weight, of Nitrogen (N), Phosphorus as P2O5 (P), and Potassium as K2O (K) — always listed in that fixed N-P-K order. A bag labeled 10-10-10 is 10% nitrogen, 10% phosphorus (as P2O5), and 10% potassium (as K2O) by weight, with the remainder being filler and other minor ingredients.
Nitrogen primarily drives leafy, green vegetative growth. Lawn fertilizers, which prioritize green blade growth, often carry a notably higher first number relative to the other two compared to a balanced vegetable-garden fertilizer.
Phosphorus (measured as P2O5) supports root development and flower/fruit formation. Fertilizers marketed for blooming plants, bulbs, or root vegetables often carry a relatively higher middle number.
Potassium (measured as K2O) supports overall plant vigor, disease resistance, and stress tolerance (drought, cold). It's often the number gardeners pay least attention to, though it plays a role across nearly every growth stage.
The label tells you what's IN the bag, not what your soil is missing. A soil test is the only way to know your soil's existing nutrient levels and pH (which affects nutrient availability) — matching fertilizer to an actual test result is more reliable than picking a ratio based on a plant-type generalization alone.