What Number Fertilizer for Lawns?

What You Need to Know

Understanding which fertilizer numbers to use for your lawn is essential for achieving a healthy, green turf without wasting money or causing environmental harm. The three numbers on every fertilizer bag represent the NPK ratio: nitrogen (N), phosphorus (P), and potassium (K), always in that order.

For established lawn maintenance, the ideal fertilizer emphasizes nitrogen with moderate potassium and little to no phosphorus. Recommended ratios include 32-0-4, 20-0-10, 24-0-12, 30-0-4, 16-4-8, and similar formulations where nitrogen dominates.

Nitrogen is the first number and the most important for lawns. It drives the green color, blade density, and vigorous growth that characterize a healthy lawn. Most lawn care programs deliver 2 to 4 pounds of actual nitrogen per 1,000 square feet per year, split across 3 to 5 applications during the growing season.

To calculate actual nitrogen from a fertilizer bag, multiply the bag weight by the first NPK number expressed as a decimal. For example, a 50-pound bag of 32-0-4 contains 50 times 0.32 equals 16 pounds of actual nitrogen. At an application rate of 1 pound of nitrogen per 1,000 square feet, that bag covers 16,000 square feet.

Phosphorus is the middle number. Most established lawns have adequate to excess phosphorus in the soil and do not need additional phosphorus unless a soil test specifically indicates a deficiency. Many states have enacted phosphorus fertilizer restrictions that prohibit applying phosphorus to established lawns. The exception is newly seeded or sodded lawns, which benefit from a starter fertilizer with high phosphorus (such as 10-20-10) for root establishment.

Potassium is the third number and is important for stress tolerance, disease resistance, and winter hardiness. While not directly visible like nitrogen's green-up effect, potassium quietly strengthens the lawn against drought, cold, traffic, and disease pressure. A moderate potassium level in your lawn fertilizer, combined with a potassium-focused winterizer application in fall, supports year-round lawn health.

Beyond the NPK ratio, look at the nitrogen source on the guaranteed analysis label. The percentage of slow-release or water-insoluble nitrogen (WIN) affects how the fertilizer performs. Premium lawn fertilizers typically contain 30 to 50 percent or more slow-release nitrogen, which provides gradual feeding over 6 to 12 weeks, reduces burn risk, and requires fewer applications.

Here are specific recommendations by situation. For spring green-up, use a nitrogen-dominant formula like 30-0-4 or 20-0-10 applied when the lawn begins active growth, typically when soil temperatures reach 55 degrees Fahrenheit at the 4-inch depth.

For summer maintenance of cool-season grasses, use a slow-release formula at a reduced rate of 0.5 pounds of nitrogen per 1,000 square feet to sustain the lawn without pushing excessive growth during heat stress.

For fall feeding, which is the most productive fertilization period for cool-season lawns, apply 1 pound of nitrogen per 1,000 square feet of a balanced formula. This supports root growth, carbohydrate storage, and thickening of the turf canopy.

For winterizer, use a high-potassium formula like 24-0-12 or 10-0-20 applied in late fall after the last mowing. This builds reserves for winter survival and spring recovery.

For warm-season grasses like bermudagrass and zoysiagrass, fertilization timing and rates differ. These lawns are fertilized during summer when they are actively growing, with nitrogen applied at 0.5 to 1 pound per 1,000 square feet every 4 to 8 weeks during the growing season.

Regardless of grass type, the most reliable approach is to base your fertilizer selection on soil test results. A soil test removes the guesswork and tells you exactly which nutrients and how much of each your lawn needs. Contact your local cooperative extension service for affordable testing and personalized recommendations.