Understanding Fertilizer Numbers (NPK) Explained

title: "Understanding Fertilizer Numbers (NPK) Explained"

Every bag of fertilizer sold in the U.S. has three numbers on the label: something like 10-10-10 or 46-0-0 or 5-3-3. These numbers are the guaranteed analysis, expressed as the percentage of nitrogen (N), available phosphoric acid (P₂O₅), and soluble potash (K₂O) in the bag. They're always in the same order: N-P-K.

They don't tell you everything about a fertilizer. They don't reveal whether nitrogen is fast- or slow-release, organic or synthetic, or what else is in the bag. But understanding what the numbers mean — and how to use them to calculate actual nutrient amounts — is the foundation of sensible fertilization.

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Table of Contents

1. What Each Nutrient Does
2. How to Calculate Actual Nutrient Amounts
3. Reading a Fertilizer Label
4. Slow-Release vs. Fast-Release Nitrogen
5. Organic vs. Synthetic Fertilizers
6. Common Fertilizer Products by Use
7. Frequently Asked Questions

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