The grade on a fertilizer bag, such as 32-0-4, gives the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Lawn recommendations are usually given in pounds of nitrogen per 1,000 sq ft, so divide that rate by the nitrogen fraction: to apply 1 lb of nitrogen with 32-0-4, spread 1 ÷ 0.32 = 3.125 lb of product per 1,000 sq ft. That is the method Iowa State University Extension teaches.
The lawn fertilizer calculator does the conversion for your lawn size and bag.
Reading the label
- First number, N: nitrogen, which drives leaf growth and color. This is the number the rate is based on.
- Second number, P: phosphate. Many established lawns do not need it, and some states restrict it on lawns.
- Third number, K: potash, for stress tolerance.
- The bag also lists the spreader settings and the area one bag covers at its recommended rate.
Pounds of product per 1,000 sq ft
| Nitrogen rate | 10% N (10-10-10) | 20% N | 24% N | 32% N (32-0-4) |
|---|---|---|---|---|
| 0.25 lb | 2.5 lb | 1.25 lb | 1.04 lb | 0.78 lb |
| 0.5 lb | 5 lb | 2.5 lb | 2.08 lb | 1.56 lb |
| 0.75 lb | 7.5 lb | 3.75 lb | 3.13 lb | 2.34 lb |
| 1 lb | 10 lb | 5 lb | 4.17 lb | 3.13 lb |
Worked example
A lawn is 75 × 75 ft, or 5,625 sq ft. To apply 1 lb of nitrogen per 1,000 sq ft with 32-0-4: 3.125 lb per 1,000 sq ft × 5.625 = 17.6 lb of product. With 12.5 lb bags, that is two bags, with some left over.
The same lawn at half the rate, 0.5 lb of nitrogen, needs 8.8 lb; with a 24% product at 1 lb of nitrogen it needs 4.17 × 5.625 = 23.4 lb.
How much nitrogen, and when?
The right rate depends on your grass, climate, soil and whether clippings are left on the lawn. A soil test and your local extension office are the best guide, and some states limit how much nitrogen can be applied at once and when. Do not spread before heavy rain or on frozen ground, and sweep granules off paving back onto the lawn.
Seeding as well? See grass seed rates.
Product for common lawn sizes
| Lawn | 1 lb N, 32% product | 1 lb N, 24% product | 0.5 lb N, 32% product | 0.5 lb N, 24% product |
|---|---|---|---|---|
| 2,000 sq ft | 6.2 lb | 8.3 lb | 3.1 lb | 4.2 lb |
| 5,000 sq ft | 15.6 lb | 20.8 lb | 7.8 lb | 10.4 lb |
| 7,500 sq ft | 23.4 lb | 31.2 lb | 11.7 lb | 15.6 lb |
| 10,000 sq ft | 31.2 lb | 41.7 lb | 15.6 lb | 20.8 lb |
Checking the bag coverage
Many bags state how many square feet they cover. You can work out the nitrogen rate the bag assumes: bag weight × N fraction ÷ coverage × 1,000. A 12.5 lb bag of 32% fertilizer that covers 5,000 sq ft applies 12.5 × 0.32 ÷ 5 = 0.8 lb of nitrogen per 1,000 sq ft. Compare that with the rate your extension office suggests.
Frequently asked questions
What do the three numbers mean?
Nitrogen (N), phosphate (P₂O₅) and potash (K₂O), each as a percentage of the bag weight. A 32-0-4 bag is 32% nitrogen, no phosphate and 4% potash.
Why calculate by nitrogen?
Nitrogen is the nutrient lawns use most and the one most recommendations are written in, so converting the product to nitrogen lets you compare bags with different grades.
What if I spill granules on the driveway?
Sweep them back onto the lawn. Fertilizer left on paving can wash into storm drains.
Sources
- Iowa State University Extension: How to Calculate Fertilizer Rates for the Lawn (converting a nitrogen rate into pounds of product)
