Reading the N-P-K numbers
Every bag of fertilizer shows three numbers, such as 24-0-6. They are the percentage by weight of:
- N: nitrogen, for green leaf growth
- P: phosphate (P₂O₅), for roots and seedlings
- K: potash (K₂O), for stress and disease tolerance
A 50 lb bag of 24-0-6 holds 12 lb of nitrogen, no phosphate and 3 lb of potash. The rest is filler and carrier that help you spread it evenly.
How much product to apply
Lawn recommendations are given in pounds of nitrogen per 1,000 square feet, so the calculation starts from nitrogen:
- Product per 1,000 sq ft = nitrogen target ÷ nitrogen percentage
- Total product = product per 1,000 sq ft × lawn area ÷ 1,000
Worked example
You want to apply 1 lb of nitrogen per 1,000 sq ft on a 5,000 sq ft lawn with a 24-0-6 fertilizer sold in 50 lb bags at $45.
- Product per 1,000 sq ft: 1 ÷ 0.24 = 4.2 lb.
- Total product: 4.2 × 5 = 20.8 lb, less than one bag.
- Cost of the product used: 20.8 ÷ 50 × $45 = $18.75.
- Potash applied: 4.2 × 6% = 0.25 lb per 1,000 sq ft.
Set the spreader to the setting printed on the bag for that rate, or calibrate it on a measured area.
Yearly nitrogen needs
| Lawn type | Nitrogen per year (lb per 1,000 sq ft) |
|---|---|
| Kentucky bluegrass, perennial ryegrass | 3 to 4 |
| Tall fescue | 2 to 3 |
| Fine fescue | 1 to 2 |
| Bermudagrass | 3 to 5 |
| Zoysiagrass, centipedegrass | 1 to 3 |
Split the total into several applications of 0.5 to 1 lb. Cool-season lawns benefit most from fall feeding; warm-season lawns are fed during their summer growth.
Fast and slow release
Quick-release nitrogen greens a lawn within days but can burn grass and leach away in heavy rain. Slow-release forms, listed on the label as water-insoluble or controlled-release nitrogen, feed for 6 to 10 weeks. Keep quick-release applications at 0.5 lb of nitrogen or less.
Protecting water
- Do not fertilize before heavy rain or on frozen ground.
- Sweep granules off driveways and sidewalks back onto the lawn.
- Keep a buffer of 10 to 25 feet from lakes and streams.
- Skip phosphorus unless a soil test shows a need or you are starting a new lawn. Several states restrict it on lawns.
A soil test from your local extension service is the most reliable way to know what your lawn actually needs.