N-P-K: fertilizer numbers explained
What the three numbers on the label mean, and how to choose
The three numbers
Every fertilizer label lists three numbers, such as 10-10-10 or 20-10-5. They are the percentage by weight of nitrogen (N), phosphorus (as phosphate) and potassium (as potash), always in that order.
So a 50-lb (22.7 kg) bag of 10-10-10 holds 5 pounds (2.3 kg) each of nitrogen, phosphate and potash. The rest is filler that helps you spread it evenly.
What each one does
Each nutrient has a main job.
- N, nitrogen: fast leafy growth and green color. Usually the one plants need most.
- P, phosphorus: root growth, and good flowers, fruit and seeds.
- K, potassium: helps plants handle drought and disease, and builds stronger stems.
Bigger numbers aren't better
Higher numbers just mean a more concentrated product, so you use less of it. A 20-20-20 and a 10-10-10 can feed the same plant; you simply follow each label's rate, which is based on the nitrogen.
Too much of any fertilizer can burn roots, and too much nitrogen gives soft growth that invites insects and disease.
Reading the ratio
A complete fertilizer contains all three. A balanced one has equal amounts, like 10-10-10 or 20-20-20. Most plants need more nitrogen and potassium than phosphorus.
Many garden soils already hold enough phosphorus and potassium, and adding more does nothing. A soil test tells you what your garden actually needs.
Fast or slow
The label also tells you how much of the nitrogen is slow-release. A product sold as slow-release must have at least a third of its nitrogen in slow form. One with half or more feeds for longer, and less of it washes away.
Beyond N, P and K
Plants also need calcium, magnesium and sulfur, plus tiny amounts of micronutrients such as iron, manganese and zinc. Many fertilizers list these on the label too.
Sweep up any fertilizer that lands on paths or driveways, where rain washes it into lakes and streams. Store the bag closed and out of reach of kids and pets.
Related plant problems
Sources
- Clemson HGIC: Reading a Fertilizer Label
- NC State Extension: Extension Gardener Handbook, 1: Soils and Plant Nutrients
- Colorado State University Extension: Understanding Fertilizers
- Clemson HGIC: Choosing a Fertilizer
General guidance, not a diagnosis. Written by PlantWiz in our own words from the sources above.