Garden Views: Fall is for fertilizer

By Kim Sullivan
Contributing Writer

Fall is the time for almost everything for your lawn – fertilizing, broadleaf weed control, aerating, dethatching. Fertilizer calculations take the spotlight today.

As soil cools to 50-65 degrees, grass roots grow best. Nitrogen improves shoot growth and food production. Potassium and phosphorus promote root growth and overall health to store food over the winter.

Don’t use weed and feed. Fertilize as early as the middle of August, with Labor Day being ideal. Use a hose or irrigation system to wet down the fertilizer. But only do this if it does not rain soon after you apply it. Treat broadleaf weeds with a spray directly on the plant between the beginning of September and middle of October. The spray must be left on the plant – no watering. Always read and follow directions on the label.

For many people, three pounds of nitrogen per 1000 square feet per year is acceptable. Typically, you’ll put down two-thirds of your annual nitrogen application in the fall – so two pounds of nitrogen now. Fertilizers have ratios and grades. You’ll recognize “24-0-12”, that’s a grade. The numbers indicate that your bag has 24 percent by weight of nitrogen, 0 percent by weight of phosphorus, and 12 percent by weight of potassium. The ration of that grade is 4-0-3.

Here’s the math to figure out how much fertilizer you need to put down on a 1,000 square foot lawn if you have a 24-0-12 grade fertilizer: Take the weight of the bag, e.g. 20 pounds. Multiply 20 pounds by the percentage of nitrogen in the bag of fertilizer – 24 percent. That’s 4.8 pounds of nitrogen in the bag. Divide the weight of the bag (20 pounds) by the percentage of fertilizer (4.8 pounds). In this case, you’ll need to apply four pounds of fertilizer to get one pound of nitrogen per 1,000 square feet. If your lawn is 3,000 square feet, you’ll need three pounds of nitrogen, which means you’ll use 12 pounds of fertilizer.

Use a fertilizer with close to a 4-0-2 ratio or a winterizer with a 5-0-7 ratio. Independent hardware stores, garden centers and feed mills are places you may find fertilizer with appropriate ratios. Many products miss the mark on ratios needed here in Minnesota.

Last, nitrogen that’s quick release can cause serious burn injury on the lawn. Look for a fertilizer that has up to 50 percent slow release nitrogen. The names of slow release nitrogen are Methylene Urea, Sulfur, polymer-coated, “insoluble” and corn gluten meal. Quick release nitrogens are urea and ammonical nitrogen.

To calculate the percentage of slow release nitrogen, take the total nitrogen percentage (24 percent if you have 24-0-12). If the label tells you that 11 percent of the total nitrogen is “insoluble”, divide 11 by 24 and that will tell you that your product is 46 percent slow release. Slow release will feed continuously into the fall, encouraging a healthy lawn.

Fertilizing now will color and thicken grass into the early spring, shading out spring weeds. Ready, set, go fertilize!

The Anoka County Master Gardeners invite you to visit our web page for important plant information.

Kim Sullivan is an Anoka County Master Gardener.