Every fertilizer bag lists three prominent numbers. Learn how to calculate actual pounds of nitrogen, phosphate, and potash for your yard.
1. The Guaranteed Analysis Ratio
The three bold numbers on a fertilizer bag represent percentage by weight of Nitrogen (N), Phosphate (P2O5), and Potash (K2O).
A 50 lb bag of 10-10-10 fertilizer contains 5 lbs of actual Nitrogen, 5 lbs of available Phosphate, and 5 lbs of soluble Potash.
The remaining 35 lbs consists of inert filler materials like limestone dust or sand that help distribute granules evenly.
2. Calculating Nitrogen Application Rates
Extension agronomists recommend applying 1 lb of actual nitrogen per 1,000 sq ft for established lawn turf.
To calculate how much fertilizer you need, divide 1 by the nitrogen percentage (0.10 for a 10-10-10 bag). You need 10 lbs of 10-10-10 per 1,000 sq ft.
If using a high-nitrogen blend like 29-0-4, divide 1 by 0.29 to get 3.4 lbs of fertilizer per 1,000 sq ft.
| Fertilizer Bag Ratio | Pounds Needed for 1 lb N per 1,000 sq ft |
|---|---|
| 10-10-10 Balanced | 10.0 lbs of fertilizer |
| 15-5-10 Garden Special | 6.6 lbs of fertilizer |
| 29-0-4 Lawn Starter | 3.4 lbs of fertilizer |
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Find Your County Extension Office →3. Selecting Zero-Phosphorus Blends
Many municipal watersheds restrict phosphorus lawn applications to protect lakes from algae blooms.
If your soil report shows optimum phosphorus levels, choose a zero-middle-number fertilizer like 15-0-15 or 24-0-11.
- Check local watershed phosphorus regulations.
- Calculate fertilizer rates based on nitrogen targets.
- Use slow-release nitrogen for summer lawn feeding.
- Store opened fertilizer bags in sealed plastic tubs.