soil
NPK
Last reviewed 2026-10-04
NPK is shorthand for nitrogen, phosphorus, and potassium. The three numbers on a fertilizer label state the guaranteed analysis, by weight, of those nutrients in that order. A 5-3-4 fertilizer contains 5 percent nitrogen, 3 percent available phosphate expressed as P2O5, and 4 percent soluble potash expressed as K2O.
What it means
Nitrogen supports leafy growth and is part of chlorophyll. Phosphorus participates in energy transfer and reproductive development. Potassium supports water regulation and many plant processes. Plants need all three, but the correct rate depends on the crop, soil, product, and existing nutrient levels.
The label numbers are not a simple score. A higher number is not automatically better, and the second and third numbers use conventional fertilizer-label expressions rather than the elemental mass of phosphorus and potassium. Follow the product label and local recommendations.
How it affects the garden
A soil test is a better starting point than guessing from leaf color. Excess fertilizer can damage roots, pollute water, or produce lush foliage with poor fruiting. Use the fertilizer calculator to convert a stated nitrogen rate into product weight, then verify the product label and application instructions.
Example
A 10 pound bag of 5-3-4 fertilizer contains 0.5 pounds of labeled nitrogen, 0.3 pounds of phosphate, and 0.4 pounds of potash. The calculation does not tell you whether that amount is appropriate for a bed without knowing the crop and recommendation.
Common confusion
NPK does not describe every nutrient a plant needs. Calcium, magnesium, sulfur, and micronutrients may also matter. It also does not describe soil texture or organic matter.
Sources
University of Minnesota Extension explains soil testing and fertilizer decisions for gardens.
