My son came home the other day complaining about his math assignment. “Why do I have to solve all these word problems?” he protested. “I’ll never need this long division when I grow up. I’ll use a calculator!”

I thought about my son and his math problems when I received a question recently from a gardener in Cobb County. He had a very basic question: “What is the best fertilizer form for bermudagrass: liquid or granular? What are the advantages or disadvantages associated with either?”

I briefly advised the gardener that either fertilizer type could be used successfully but that issues of cost and features also play a role. My son’s gripes to the contrary, basic math is essential when you are a grown-up doing lawn work, especially when you are comparing the prices of different products.

THE BIG PICTURE FOR LAWN FEEDING We have to keep in mind the big picture when fertilizing turfgrass. All lawns need extra nutrients (fertilizer) during their growing season in order to look their best. Centipedegrass needs very little fertilizer to look good while fescue and bermudagrass need comparatively more. Whether the fertilizer is in granular or liquid form is immaterial to the grass. So how much fertilizer does grass need?

Scientists recommend that one pound of “actual nitrogen” be applied to a thousand square feet of bermudagrass lawn each month during the growing season. What is “actual nitrogen”? It’s the amount of concentrated nitrogen-containing chemical (ammonium nitrate, urea, ammonium phosphate, etc.) that you spread or spray in one application. Remember, a bag of 10-10-10 granular plant food isn’t all fertilizer. Thirty percent is chemical nutrients and the rest is filler material. The percentage of actual nitrogen is what the first of the three “fertilizer numbers” on a bag refers to.

DETERMINING ACTUAL NITROGEN Most gardeners realize that it takes more pounds of 10-10-10 than 29-4-6 fertilizer to feed a lawn. The later has a higher percentage of actual nitrogen. But how much of either would it take to apply the magical “one pound of actual nitrogen per one thousand square feet”?

The calculation is simple: just divide the first fertilizer number into 100. The result is the numbers of pounds of that fertilizer that equal one pound of “actual nitrogen” you’ll apply each month.

Example: Your bag of turf fertilizer has a 29-3-4 analysis. Divide 29 into 100. The result is just about 3.5 pounds. In plain English, you need to spread 3.5 pounds of the 29-6-6 onto one thousand square feet of lawn to equal the pound of actual nitrogen your grass wants.

Second example: You have a bag of granular 16-4-8 fertilizer. How much of it do you need in order to equal a pound of actual nitrogen? Answer: Divide 16 into 100. The answer is 6.25 pounds; that’s how much of the 16-4-8 equals a pound of actual nitrogen.

Third example: You look at the fertilizer numbers on a liquid feed product: 15-30-15. Divide 15 into100 and you find that it takes 6.66 pounds to yield one pound of actual nitrogen.

THE POCKETBOOK QUESTION I recently visited a garden center to find the current price of the three products above. The 29-3-4 costs $1.67 for a pound of actual nitrogen. The 16-4-8 costs only $.93 for the same amount of nitrogen. The 15-30-15 costs $17.76 for a pound. Why in the world is there such a huge discrepancy?!

It all boils down to convenience. The 29-3-4 product is manufactured to release its nutrients over a two month period while the 16-4-8 fertilizer must be applied every month. Some consumers value the eight week interval between feedings that a slow-release product affords. They pay more in order to save time. The liquid feed product recommends application every two to four weeks. However, the applicator is only a small hand-held sprayer: no lawn spreader or bags of fertilizer need be stored. In addition, the liquid feed product will be absorbed by the grass much faster than the other two. It will only take a few days before starving bermudagrass turns green again. Any of the three products can be used effectively as long as you follow the directions.

We all enjoy restaurant dining. The food might cost more than it would if prepared in our own kitchen but the convenience of no cooking and no clean-up make the experience worthwhile. The different forms of lawn fertilizer could be described as “convenience food” too. Whether deciding where to eat or what to feed a lawn, the “convenience versus cost” argument is one my son will encounter throughout his life.