How to choose a lawn fertilizer: numbers, release type and timing

Read the three numbers on the bag

Every fertilizer bag shows a grade of three numbers, such as 16-4-8. Iowa State Extension explains that they give the percent of the bag’s weight that is nitrogen, phosphate (P2O5) and potash (K2O). In 16-4-8, that is about 16% nitrogen, 4% phosphate and 8% potash. The numbers do not add up to 100 because the rest is other nutrients and a filler that helps spread it evenly.

For lawns, nitrogen is the number that matters most. Lawn fertilizers carry higher nitrogen than other nutrients, and recommendations are usually written as pounds of nitrogen per 1,000 square feet.

Test the soil before you buy

The University of Maryland Extension says not to guess what nutrients a lawn needs, but to measure them with a laboratory soil test. It says testing about every 3 years is enough, and that over-applying and under-applying can both cause problems.

A test shows whether you need phosphorus or potassium at all. In Maryland, formulations must be phosphorus-free unless a test shows a deficiency or the lawn is new. Rules differ by state, so check your own, and use the lab’s recommendation to choose a ratio close to what the soil needs.

Quick-release or slow-release nitrogen

Nitrogen comes in water-soluble (quick-release) and water-insoluble (slow-release or controlled-release) forms. According to Maryland Extension, water-soluble nitrogen costs less and greens a lawn up fast, but it has only short-term effects and can burn the grass. Water-insoluble nitrogen costs more and releases evenly as soil microbes break it down.

Slow-release nitrogen also lowers the risk of leaching into water runoff, so lawns on sandy soil or near a high water table should favor it. The label lists the water-insoluble nitrogen (WIN), and you can work out the share: divide the percent WIN by the total percent nitrogen and multiply by 100.

Match the timing to your grass

Maryland Extension says cool-season lawns, such as fescues and bluegrass, are best fertilized from late summer into autumn. Warm-season lawns, such as zoysia and bermudagrass, are fertilized from late spring into summer.

Rates are given in pounds of nitrogen per 1,000 square feet. The Maryland chart for tall fescue lists 0.7 to 0.9 pounds in September and again from October to mid-November, with a yearly maximum of 2.7. Those figures are for Maryland, so use your own state’s extension schedule.

How much to apply

Work from the nitrogen rate you want. Iowa State’s method: divide the pounds of nitrogen you want per 1,000 square feet by the nitrogen percentage on the bag, then scale to your lawn.

Example from Iowa State: to apply 1 pound of nitrogen per 1,000 square feet with a 32-0-4 fertilizer, divide 1 by 0.32 to get 3.125 pounds per 1,000 square feet. For a 5,625 square foot lawn, multiply by 5,625 and divide by 1,000 for about 17.6 pounds. Measure your lawn first, and do not exceed the rate on the label.

• Measure the lawn: length times width, or a map tool for odd shapes.
• Pick the pounds of nitrogen per 1,000 square feet from your extension schedule.
• Divide by the bag’s nitrogen percentage, then multiply by your lawn area over 1,000.

Other choices that change the result

Do not over-fertilize. Maryland Extension says to skip applications marked optional in its chart unless the lawn is new, recovering from heavy use or has another listed problem, and that excess nitrogen can make a lawn more vulnerable to disease and less resistant to stress. If you mulch clippings back into the lawn, they return some nutrients, so the lawn may need less.

Check your state’s rules before you buy, because some limit nitrogen and phosphorus and set no-application dates. Calculate the amount before you buy, and follow the label for rain and temperature limits. If you top-dress with compost, count it: Maryland Extension says about 5 to 10% of its nitrogen is available to plants in the first year, and that compost alone will not meet a lawn’s needs. Organic and synthetic sources both count toward legal nitrogen limits where those limits exist.