Kim Sullivan, Extension Master Gardener in Anoka County
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. Water in the fertilizer if it doesn’t 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, 3 pounds of nitrogen per 1000 square feet per year is acceptable. Typically, you’ll put down 2/3 of your annual nitrogen application in the fall – so 2 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% by weight of nitrogen, 0% by weight of phosphorus, and 12% by weight of potassium. 4-0-3 is the ratio of that grade.
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%. That’s 4.8 pounds of nitrogen in the bag. Divide the weight of the bag (20 lbs) by the percentage of fertilizer (4.8 lbs). In this case, you’ll need to apply 4 pounds of fertilizer to get 1 pound of nitrogen per 1,000 square feet. If your lawn is 3,000 square feet, you’ll need 3 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% 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% if you have 24-0-12). If the label tells you that 11% of the total nitrogen is “insoluble”, divide 11 by 24 and that will tell you that your product is 46% slow release. Slow release will feed continuously into the fall, encouraging healthy lawns..