by Lisa Johnson, horticulture educator for Dane County UW-Extension
Fertilize lawns every year to maintain grass density and ability to shade out weed seedlings and prevent runoff. Nitrogen fertilizers should be applied at least twice a year — around Memorial Day and Labor Day. An application can be made around the Fourth of July, if conditions are moist enough. Apply no more than 1 pound of actual nitrogen per 1,000 square feet per application, with a total of 3 pounds of nitrogen per season.
Fall is the most important time of year to fertilize, especially in early September. This promotes faster green-up and growth next spring. For conventional lawn fertilizer, choose one with at least 25 to 50 percent slow-release (insoluble) nitrogen by formulation. This information is on the bag.
Avoid blends with high rates of quick-release (soluble) nitrogen. Turf grown in sun needs 1 pound of nitrogen per 1,000 square feet. Turf in shade needs 1/2 pound of nitrogen per 1,000 square feet. You can reduce the amount of fertilizer you need by up to one application per year by leaving clippings on your lawn. They release nitrogen as they decay.
You can use organic lawn fertilizer products versus conventional ones, but realize they contain less soluble nitrogen and grass won’t green up as fast. Soil microorganisms must break them down to release the nitrogen, unlike conventional soluble products. In general, you need to apply about twice as much of the organic products to get the same result as conventional products, but this depends on the form of the organic product and how much nitrogen it contains. A benefit to organic products is that after roughly 10 to 20 years of use, organic nitrogen accumulates in the soil and grass begins to need less fertilization.