by Lisa Johnson, horticulture educator for Dane County UW-Extension
Fall is the most important time to fertilize your lawn. If you only do it once a year, do it now! Research, based on the growth cycle of turfgrass, shows that Labor Day is the best time to fertilize. Memorial Day is the second-best. Fertilizer is most effective when the highest amounts of root and shoot biomass are present to absorb and use it. Nitrogen, phosphorus and potassium are the macronutrients that lawns need most. Test your soil before applying fertilizer blends so you know whether fertilizer is needed and what type to buy.
Apply no more than 1 pound of nitrogen per 1,000 square feet per feeding, and a maximum of 4 pounds per year. Nitrogen is usually the limiting factor in turf growth, because it doesn’t accumulate in soils as phosphorus and potassium do. Lawn maintenance fertilizers with 25 to 50 percent of the nitrogen content in a slow-release, or water-insoluble form are best. They release nitrogen slowly rather than all at once, minimizing nitrogen loss due to leaching. Leaving grass clippings on the lawn and applying 1/4 of compost reduces nitrogen fertilizer needs, too.
There is a statewide ban on phosphorus in turf fertilizers, with an exception for some organic products in some counties of Wisconsin. Phosphorus, which helps build strong roots, may be recommended in soil tests if you are starting a new lawn from seed or maintaining a lawn in low-phosphorus soils. If testing recommends application, bring your soil test results to the store in order to purchase turf fertilizers containing phosphorus. In south-central and southeastern Wisconsin, we often have high to excessive amounts of phosphorus in our soils (especially in urban soils), so more is not recommended. However, this isn’t the case statewide.
Potassium helps turf increase its disease and drought resistance and prepare for winter dormancy. Potassium is present in higher amounts in winterizer turf fertilizers, but this doesn’t mean you can only apply it in fall. In many areas of Wisconsin, we have high potassium levels in soils, so it may not be needed.