Lawn fertilization is the application of plant nutrients to turfgrass. Fertilizer bags list three "N-P-K" numbers giving the percentages of nitrogen, phosphorus (as P2O5) and potassium (as K2O).[1] When and how much to fertilize depends on the grass: warm-season lawns are fed during the summer growing season and cool-season lawns mainly in fall.[2][3]
Fertilizer labels
A 100-pound bag of 18-4-10 fertilizer, for example, contains 18 pounds of nitrogen, 4 pounds of phosphate and 10 pounds of potash; the rest is inert material or a carrier that helps spread the nutrients evenly.[4]
Soil testing and pH
A soil test reports pH, phosphorus, potassium, calcium and magnesium, but not nitrogen, which is assumed to be needed.[2] The University of Georgia gives target pH ranges of 5.0–6.0 for centipedegrass, 5.5–6.5 for St. Augustinegrass, bermudagrass and tall fescue, and 6.0–7.0 for zoysiagrass.[2] Most Georgia soils are too acidic, so lime is often needed to raise pH, while sulfur lowers it; a soil test is the best guide, and centipedegrass should not be limed unless a test calls for it.[5] In Illinois, University of Illinois Extension refers residents to certified commercial soil testing laboratories.[6]
Timing
Warm-season lawns should be fertilized only during the growing season, once the soil four inches deep stays at or above 65 °F; nitrogen applied earlier is more likely to leach away. Tall fescue, a cool-season grass, should be fed in fall and late winter rather than late spring or summer.[2] For bermudagrass, NC State recommends feeding a few weeks after full green-up and then about one pound of nitrogen per 1,000 square feet every four to six weeks through August, with no fertilizer from December to February.[7]
For cool-season lawns in the Midwest, Purdue and Illinois Extension call September "the most important time" to fertilize, recommending about one pound of nitrogen per 1,000 square feet in mid-September and another 1.0–1.25 pounds after the last mowing while the grass is still green.[3] In northern Illinois the first fall feeding is applied in early August.[8]
Nitrogen rates
The University of Georgia recommends the following annual nitrogen rates, split into several applications:[2]
| Grass | Pounds of nitrogen per 1,000 sq ft per year |
|---|---|
| Centipedegrass | 1–2 |
| Zoysiagrass | 2–3 |
| Bermudagrass | 2–4 |
| St. Augustinegrass | 2–4 |
| Tall fescue | 2–4 |
NC State's tall fescue calendar calls for one pound of nitrogen in mid-September, another in November and another in February, with any spring or summer feedings limited to one-fifth to one-half pound.[9]
Slow- and quick-release nitrogen
Quick-release nitrogen sources such as ammonium, nitrate and urea are inexpensive but carry a high risk of burning grass. The University of Georgia advises that a slow-release lawn fertilizer should have at least 30% of its nitrogen in slow-release form, measured as water-insoluble nitrogen divided by total nitrogen; at 50% or more, twice the amount can be applied half as often.[1] Clemson gives a legal threshold of 33% for a fertilizer to be labeled slow-release.[4]
Runoff and water quality
The EPA advises fertilizing only when needed and at the recommended rate, avoiding application before windy or rainy weather, keeping fertilizer away from waterways, and filling spreaders on a hard surface where spills can be swept up.[10]
References
- ^ ^ Waltz, Clint; Griffin (July 18, 2016). "Turfgrass Fertility: Understanding Fertilizer Labels, Macronutrients, and Micronutrients" (C 1058-2). University of Georgia Extension. fieldreport.caes.uga.edu
- ^ ^ ^ ^ ^ Garton, Stephen (March 26, 2012). "Spring fertilization of lawns should be guided by soil testing, temperatures". University of Georgia CAES. fieldreport.caes.uga.edu
- ^ ^ "Lawn Improvement Programs" (AY-13). Purdue Extension and University of Illinois Extension. ag.purdue.edu
- ^ ^ Polomski, Robert (May 27, 2022). "Reading A Fertilizer Label" (HGIC 1228). Clemson Cooperative Extension Home & Garden Information Center. hgic.clemson.edu
- ^ "Turfgrass Fertility: Soil Texture, Organic Matter, Aeration, and pH" (C 1058-1). University of Georgia Extension, revised May 10, 2021. fieldreport.caes.uga.edu
- ^ "Soil Test Labs". University of Illinois Extension. extension.illinois.edu
- ^ "Bermudagrass Lawn Maintenance Calendar" (AG-431). NC State Extension, July 30, 2021. content.ces.ncsu.edu
- ^ Enroth, Christopher (September 5, 2024). "Five fall tips for cool-season lawn care". University of Illinois Extension. extension.illinois.edu
- ^ "Tall Fescue Lawn Maintenance Calendar" (AG-367). NC State Extension, July 8, 2021. content.ces.ncsu.edu
- ^ "What You Can Do: In Your Yard". U.S. Environmental Protection Agency. epa.gov