Quick answer
The three numbers on a bag, such as 29-0-4, are the percentages by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). To apply a target amount of nitrogen, divide the pounds of nitrogen you want by the nitrogen percentage as a decimal. For 1 lb of nitrogen per 1,000 sq ft from a 29-0-4 product: 1 / 0.29 = about 3.4 lb of product per 1,000 sq ft. Most lawn guides suggest no more than about 1 lb of nitrogen per 1,000 sq ft in a single application.
What N-P-K means
| Number | Nutrient | What it does |
|---|---|---|
| First (N) | Nitrogen | Leaf growth and green color; the nutrient lawns need most |
| Second (P) | Phosphorus, listed as phosphate (P2O5) | Root development; most important for new lawns and low-P soils |
| Third (K) | Potassium, listed as potash (K2O) | Stress, drought and cold tolerance |
A 50 lb bag of 10-10-10 contains 5 lb of nitrogen, 5 lb of phosphate and 5 lb of potash. The rest is other nutrients, filler and carrier that help it spread evenly.
Many lawn fertilizers have a 0 in the middle. Many established lawns already have enough phosphorus, and several states limit phosphorus in lawn fertilizer to protect lakes and streams, often allowing it only for new lawns or when a soil test shows a need.
How much nitrogen does a lawn need?
Extension guidance commonly suggests about 2 to 4 lb of nitrogen per 1,000 sq ft per year for many home lawns, and more for high-maintenance or warm-season grasses such as Bermuda. That yearly amount is split into several applications of about 0.5 to 1 lb each. Shaded lawns usually need about half as much as lawns in full sun. A soil test and your local extension office give the best target for your grass and region.
The calculation
Pounds of product per 1,000 sq ft = pounds of N wanted / (N percent / 100)
Pounds for your lawn = pounds per 1,000 sq ft x lawn area / 1,000
| Fertilizer | Product for 1 lb N per 1,000 sq ft | Product for 0.75 lb N | For a 5,000 sq ft lawn at 1 lb N |
|---|---|---|---|
| 32-0-4 | 3.1 lb | 2.3 lb | 15.6 lb |
| 29-0-4 | 3.4 lb | 2.6 lb | 17.2 lb |
| 24-0-6 | 4.2 lb | 3.1 lb | 20.8 lb |
| 10-10-10 | 10 lb | 7.5 lb | 50 lb |
| 5-3-2 (organic) | 20 lb | 15 lb | 100 lb |
Low-analysis organic products deliver the same nitrogen but require much more weight per application. The fertilizer calculator does this math for any N-P-K and lawn size.
Check the bag's coverage claim
You can reverse the math to see what a bag actually delivers. A 12.5 lb bag of 32-0-4 holds 4 lb of nitrogen. If it says it covers 5,000 sq ft, it applies 4 / 5 = 0.8 lb of nitrogen per 1,000 sq ft. That is a reasonable single feeding. If a bag covers far less area for its nitrogen content, it is applying more than you might expect.
Slow release vs quick release
The guaranteed analysis section breaks nitrogen into forms. Look for terms like these:
- Quick release (water soluble): urea, ammoniacal nitrogen, nitrate nitrogen. Greens fast, lasts about 4 to 6 weeks, and burns grass more easily if overapplied.
- Slow release: listed as "slowly available," "water insoluble nitrogen (WIN)," or as coated or reacted products such as polymer-coated urea, sulfur-coated urea, or methylene urea. Feeds over about 6 to 10 weeks with less burn risk and less runoff.
- Natural organic: from materials such as feather meal, poultry manure or biosolids. Mostly slow release and depends on soil microbes, so it works slower in cool soil.
The label usually states the percent of total nitrogen that is slowly available. A product with 30 to 50 percent or more slow release is a good general choice for lawns, especially on sandy soil or before heavy rain.
Other parts of the label
- Secondary and micronutrients: sulfur, iron, magnesium and others. Iron greens grass quickly without pushing growth.
- Derived from: lists the raw materials, which tells you the nitrogen sources.
- Combination products: "weed and feed" or crabgrass preventer products include pesticides with their own rules on timing, rates and reseeding. Read the use directions carefully before overseeding.
- Spreader settings: a starting point for common spreader models. Calibrate if your spreader is not listed.
Application tips
- Apply to a dry lawn and water in lightly afterward unless the label says otherwise.
- Sweep granules off sidewalks and driveways back onto the lawn so they do not wash into storm drains.
- Do not fertilize dormant or drought-stressed grass, or right before a heavy storm.
- Feed cool-season grass mainly in fall and lightly in spring. Feed warm-season grass from late spring through summer while it is growing.
Seeding at the same time? The grass seed calculator covers seed rates.