| A math quiz we'd like to see: If Mary uses 20 pounds of 5-10-10 per 1,000 square feet of lawn, and Mike uses 20 pounds of 20-5-15, how much more nitrogen than Mary did Mike apply? Actually, a better question is: How much is it going to cost Mike to plant his new lawn now that he just killed his old one? And for extra credit: Why didn’t Mike read directions? Don’t be like Mike. Here’s why those numbers are important and why you need to follow instructions.
Three digits separated by hyphens tell you the percentage, by weight, of nitrogen (N),
phosphorus (P), and potassium (K) in the fertilizer. Lawns need N, P, and K in roughly a
3:1:2 proportion; that’s why you might notice seemingly odd numbers such as 20-6-11 on a bag of turf fertilizer. It’s also why you should avoid applying balanced fertilizers such as
10-10-10 to your lawn.
When you fertilize lawns, apply no more than 1 pound of nitrogen per 1,000 square feet per application. Any more than that poses a risk of burning your turf. Most lawn fertilizers range between 10 and 30 percent nitrogen, so there is no one-size-fits-all application rate. Fortunately, that doesn’t mean you have to do your own calculations. With few exceptions, product labels tell you how much product to apply. See? Math isn’t so hard — especially when you let someone do it for you. (Read the instructions.)