Lawn Fertilizer Numbers


Soil rarely has sufficient nutrients to grow the lush lawn homeowners want. Even those soils that do have sufficient nutrients will be depleted after a season or two. Nutrients can be replenished in the soil by spreading fertilizer periodically. Professional lawn care companies provide fertilization service, but if you learn the nutrient requirements of your lawn and how to read a fertilizer label, you can economically and responsibly feed the lawn yourself.

Label Language

Every fertilizer product carries a three-number label. The three numbers represent the percentage by weight of nitrogen, phosphorus and potassium (N,P,K) in the product. The elements are always presented in the same order. For example, a fertilizer labeled 6-4-8 is 6 percent nitrogen, 4 percent phosphorus and 8 percent potassium. The remaining 88 percent is inert ingredients in synthetic fertilizer and micronutrients and inert ingredients in organic fertilizer.

Turf Grass Categories

Turf grass species fall into one of two categories. Cool-season grasses dominate the lawns of the Northeast, Mid-Atlantic, Midwest and Pacific Northwest. Bluegrass, tall fescue and perennial rye are examples of cool-season species. Warm-season grasses thrive in heat, but can't survive bitter winters. They're found predominantly in the South, Southwest and Pacific Coastal areas. Zoysia, St. Augustine and Bermuda are among the warm-season species. The University of Missouri Extension Service recommends three fertilizer applications per year for cool-season lawns and four per year for warm-season lawns.

Cool-Season Turf Schedule

Cool-season grasses naturally have a fast growth habit in the early spring. Adding fertilizer too early is like throwing gasoline on a raging bonfire--it's neither necessary nor advisable. University of Missouri turf experts suggest waiting until the spring growth spurt slows--about mid-May for the most of the U.S.--before applying fertilizer. Fall is the most important time to apply fertilizer to cool-season grasses. It gets them ready for winter and primed for spring. The University of Missouri Extension suggests an application in September and October. If the soil shows no significant deficiencies, use a fertilizer with a 4-1-1 or 4-1-2 ratio and apply 1 lb. of nitrogen per 1,000 square feet in each of the three applications.

Warm-Season Turf Schedule

Applying fertilizer to warm-season grasses before they have fully emerged from dormancy in the spring does little for the grass but boosts the vigor of weeds. Wait until the grass is growing and vibrant, and apply small amounts evenly throughout the growing season, stopping several weeks before the lawn goes dormant again. For the middle of the country, that equates to monthly applications from May through August. In the South, add one month to each side of that. The University of Missouri Extension suggests 1/2 lb. of nitrogen per 1,000 square feet, using 4-1-1 or 4-1-2 ratio fertilizer on lawns showing no significant deficiencies.

Application Calculation

To calculate the amount of fertilizer needed to apply the recommended amount of nitrogen per 1,000 square feet, divide the recommended amount by the nitrogen percentage. For example, a fertilizer labeled 16-4-4 has the recommended ratio of 4-1-1 and is 16 percent nitrogen. To find the amount needed for the 1 pound of nitrogen per 1,000 square feet recommended for cool-season turf, divide 1 by .16. The result tells you that you need 6.25 lbs. of fertilizer for each 1,000 square feet. Using the same fertilizer, you would need 3.13 lbs per 1,000 square feet for a warm-season lawn (1/2 lb. divided by .16).

Soil Testing

The recommendations given assume that the soil is relatively healthy and balanced. Turf experts unanimously urge homeowners to have their soil tested periodically to see if adjustments to the basic nutrient balance are needed. Most university extension services offer soil testing for reasonable fees and will return the results with recommendations targeted to your specific goals.

Keywords: lawn fertilizer, fertilizer analysis, fertilizer grade

About this Author

Jeff Farris has focused his career on instructional communication since 1980. He has written instruction manuals, promotional materials, instructional video scripts and website articles on a variety of hands-on topics. His work has appeared in "Scuba Diving" magazine as well as several websites. He graduated from the University of Missouri with a bachelor's degree in marketing.