The facts on lawn fertilizing

Pale, yellow-green grass is a tip-off that your lawn needs more nitrogen, the key ingredient in fertilizer. Using the right fertilizer at the right time is the quickest and easiest way to provide that nitrogen so that you lawn can better withstand pests and extreme heat and cold.

Where to begin? A good first step is to obtaining the proper soil pH through a soil test. Getting the right pH level (usually by adding lime) increases the effectiveness of any fertilizer. It is better to invest in lime in the spring rather than fertilizer.

Use a fertilizer formulated specifically for lawns and follow the directions on the label. That includes using the spreader the label stipulates so that you can use the recommended setting. But you'll still be faced with a plethora of choices. Using the wrong kind in the wrong way can hurt more than help. Indeed, too much fertilizer can pollute the ground and encourage lawn pests. Here's what's available and how to apply it:

Kinds of lawn fertilizers

There are three main types. The major difference is in how quickly their nitrogen gets to the grass roots.

Natural organic fertilizers
include manures, composts, and agricultural byproducts that might otherwise be wasted (see our Web site, www.GreenerChoices.org, for composting tips). Natural organics contain relatively low amounts of nutrients that are released slowly, so using too much probably won't damage the lawn. But you'll need to apply more of them. What's more, some may include weed seeds. Those that don't include alfalfa, blood meal, and soybean meal.

Slow-release chemical fertilizers
are more concentrated than natural organics and easier to apply. They're also unlikely to damage lawns if applied too liberally. Half or more of the nitrogen in brand-name lawn fertilizers is typically in this form, called water-insoluble nitrogen (WIN). These fertilizers don't produce an immediate effect, but that's usually better for the grass.

Fast-release fertilizers
are one way to green-up your lawn quickly. They're relatively concentrated, inexpensive, and easy to apply. But putting down too much or spreading it over a damp lawn in warm weather can burn the grass. Because their nutrients are used quickly, you'll have to apply them more often.

When to fertilize

If you fertilize once a year, do it in September for cool-season, Northern grasses, and early June for warm-season, Southern grasses. Otherwise, make two to three applications in fall, one month apart, and one in spring for cool-season grasses; three applications are needed during the summer for warm-season grasses.

How much

Lawn fertilizers contain nitrogen and, usually, phosphorus and potassium in that order. You'll know how much a fertilizer contains by checking its label. A 100-pound bag labeled 20-0-0 has 20 pounds of nitrogen, and no phosphorus or potassium, for instance.

Lawns typically need only 25 percent as much phosphorus and 50 percent as much potassium as they do nitrogen. So don't apply phosphorus or potassium unless it's needed. A soil test is the only way to tell (search the Web using the words "soil testing [your state]").

Recommendations for lawn fertilizers are usually given in actual nitrogen over a given area. Experts recommend no more than 1 pound of actual nitrogen per 1,000 square feet for each application. Once you know a fertilizer's nitrogen concentration, calculate how many times that first, nitrogen-percentage number in the fertilizer mix goes into 100, then apply that many pounds of fertilizer per 1,000 square feet.

For example, figure on using 5 pounds of 20-5-10 fertilizer, 10 pounds of 10-2 1/2-5 and so on. The total actual nitrogen applied per year should be 3 to 5 pounds. To double-check your calculations, use Purdue University's Turf Fertilizer Calculator (www.agry.purdue.edu/turf/fertcalc/Fertilization%20calc.html).

Hint: Returning mulched clippings to your lawn rather than bagging and disposing of them reduces the need for lawn fertilizer by 30 to 50 percent. That equals roughly 2 to 2.5 pounds of fertilizer per year to put down 4 pounds of actual nitrogen.

Calibrating a drop spreader

Fill the spreader with fertilizer, note its setting, and operate it over 50 feet with a collection pan or strip of plastic sheeting beneath it. Weigh the amount of fertilizer that fell on the pan or strip. Calculate the square footage you covered (50 feet times the spreader's width in feet). Then use that as a guide to how much the spreader will deliver over the 1,000 square feet specified on fertilizer labels. For example, if the spreader dropped 1 pound of fertilizer per 100 square feet, it will drop 10 pounds per 1,000 square feet at that setting. Increase or reduce the amount delivered as needed.