Your soil needs certain nutritional elements that need to be boosted occasionally for optimal productivity. You do this by adding those missing or lacking ingredients by fertilizing. Any lawn can benefit from fertilization, and all lawns should have it done a few times a year. The best way to know when to fertilize in Wisconsin is by using the holiday schedule
Memorial day – end of May
Labor day – early September
Thanksgiving – mid November
If you mulch your grass when you mow you can skip an application because your lawn gets an extra dose of nitrogen since the grass blade that is cut off is about 70% water and the rest being beneficial, it returns natural nutrients to the soil organically and does not create thatch like some believe.
If you use a slow release fertilize, the lawn can benefit in a healthier way. When the grass gets a dose of fast release synthetic fertilizer in the spring the blades and shoots grow faster than the roots and the grass plant can get stressed and have weak roots. When you use a slow release formula the grass plant takes what it needs when it actually needs it. In the end you will get a healthier plant with a more hardy structure.
Using the right balance of N-P-K is also important. These are the numbers that you find on the front of fertilizer bags and are always in the same order as I have listed. (N) Nitrogen should be the bulk of the mixture and the grass uses it to green up and thicken on top. (P) Phosphorus is good for root growth but is actually illegal to use in Wisconsin unless your soil test says to add some or you are establishing a new lawn. The reason for that is because Wisconsin soils in general have enough phosphorus already and adding more that the grass can’t use most likely will get washed away with rain into the storm drains. (K) Potassium is good for building a stronger grass plan that resists disease and drought better. This amount should be in between the (N) and (P), for example a ratio of 4-0-2.