A lesson in lawn fertilizer math

By Marianne C. Ophardt, Special to the HeraldMarch 22, 2008 

It made me so mad!

It was a television commercial for a national company that sells lawn care products. They showed a family man proudly announcing that he was applying this company's fertilizer product to his lawn early in the spring. He wanted to be the first one on the street with a green lawn. WSU recommends not fertilizing until late spring. Commercials like this create confusion. When should fertilizer be applied? How much? What type of fertilizer is best? Let me try to clear things up a bit.

Complete fertilizers contain nitrogen, phosphorus and potassium (N, P and K) and are represented in that same order by the first three numbers on any fertilizer bag, such as 16-16-16 or 21-0-0. This is the percentage by weight that the bag of fertilizer contains of N, P and K. A bag of 34-4-6 contains 34 percent nitrogen, 4 percent phosphorus and 6 percent potassium.

Without a soil test there is no way to tell what nutrients are lacking in your soil, but area soils almost always lack adequate levels of nitrogen. Because of this, we're most concerned about fertilizing to provide enough nitrogen for healthy grass growth.

Turfgrass researchers have determined that lawn grass utilizes available N, P and K in an approximate ratio of 3:1:2. Because of this they recommend using fertilizers that have N, P and K with a ratio of 3:1:2 or 4:1:2. Unfortunately, fertilizer chemistry doesn't seem to work out that way so we must select fertilizers that have as close to these ratios as possible. The nitrogen should be the highest number and the phosphorous and potassium significantly lower. A good lawn fertilizer that's available from the store would be 34-4-6.

Turfgrass specialists also recommend getting a fertilizer that contains some nitrogen in a slow-release form. Fast-release or water soluble nitrogen is easily leached out of the soil past the grass roots, especially on sandy soils. It's important to select a fertilizer with some nitrogen in a slow-release form that's released gradually into the soil.

In our region, cool season grasses are recommended for the best looking and enduring lawns. WSU recommends providing cool season grass lawns with four applications of nitrogen per year, using one pound per 1,000 square feet each time. That's where it starts to get difficult for many. First you need to figure out how many square feet of lawn you have. This is a matter of measuring the different parts of your lawn and using geometry to determine the area of whatever you measured.

Once you know how many total square feet of lawn you will be fertilizing, you can easily figure out how much of a particular fertilizer you will need to apply. Since it is recommended that you apply only one pound of nitrogen in each application, divide 1 by the percentage of nitrogen in the bag. Because I'm using the 34-4-6 fertilizer, I'll divide 1 by .34. This comes out to 2.94 which I'll round off to 3 to make things easier. Then I multiply this by the number of square feet in my lawn and divide by 1,000. This tells me how much 34-4-6 fertilizer I'll need for each application during the year. If my lawn is 5,000 square feet, I'll need 15 pounds of this fertilizer per application. Simple right? (Well, maybe that geometry will be a little bit of a challenge if you've been out of high school as long as I have.)

You then need to calibrate your spreader to put the fertilizer on your lawn evenly. Many home garden lawn fertilizer products will recommend what setting to use on your spreader to put on the desired rate. Set it at half the amount you've determined that your lawn needs. After you apply half of the fertilizer to the entire lawn, apply the other half at a right angle to your first pass. This provides for more even distribution and helps avoid stripes and streaks. If you're unsure of how to calibrate your spreader, check the manual for instructions.

As already noted, WSU recommends that you fertilize cool season grass lawns four times a year. The very best times for these applications are early May, early to mid-June, early September and early November. (To help you remember this, think Mother's Day, Father's Day, Labor Day, and Halloween.) The most critical times to fertilize are September and November. Applying fertilizer earlier than May will encourage top growth to the detriment of root growth.

Yes, your lawn will be one of the first ones on your street to turn green, but once that happens you'll be mowing like crazy ... and not having much time for fun with your family like the guy in the commercial was anticipating!

* Marianne C. Ophardt is a horticulturist for the Washington State University Extension Office in Benton County.

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