An annual top dressing with a screened compost is one of the best things you can do for a home lawn. Compost is an excellent source of important soil micro-organisms and of slowly available nitrogen and other essential elements, which will feed the lawn for several months. It also increases soil aeration and water-holding capacity. A layer no more than half-an-inch thick spread evenly over the entire lawn should be worked down through the grass to the soil surface. Green grass should be visible above the compost. The main difficulty in applying compost is the quantity required. One cubic yard (one pickup truckload) of compost will cover an area of about 650 square feet at a cost of about $35, if you buy in bulk.

This can make it impractical to apply compost more than once a year, so commercial lawn fertilizers can be used as necessary in addition to this annual treatment. Fertilizer requirements of the various turf grasses in Hawai'i differ greatly. Hybrid Bermuda grasses have the highest fertilizer requirements, zoysia grasses, seashore paspalum and St. Augustine need less, and centipede grass has the lowest requirement. The main objective of a good turf fertilizer program is to promote good color and vigor without producing excessive growth. Over-fertilizing with nitrogen promotes lush growth, which can lead to a number of problems, including rapid thatch buildup, increased susceptibility to disease and insect infestation and, of course, mowing more often.

A good general lawn fertilizer should provide all three of the major essential elements: nitrogen, phosphorus and potassium (abbreviated NPK). The three numbers on the front of every fertilizer bag represent the percentage by weight of these three elements. The first number represents the percentage of nitrogen. Nitrogen is required in the greatest amount by turfgrass and should have the highest value by a ratio of at least three to four times the other two. The nitrogen can be present as a mixture of several different materials. Some of these are very water-soluble and are rapidly released in a short period. Others are formulated to be released slowly over a longer period.

All commercial fertilizers are chemical compounds that must first be dissolved in water and allowed to soak into the soil before they can be absorbed by the roots of plants. Highly soluble forms of nitrogen, such as urea or ammonium, dissolve quickly and are a readily available source of nitrogen. The grass greens up and growth rate increases sharply about two days after application, reaches a peak in about two weeks and tapers off to the original condition after four or five weeks. These peaks and valleys in growth rate are typically produced by readily soluble fertilizers and require more frequent application to keep the lawn looking uniform. There also is a greater danger of burning the grass by applying too much at one time.

The newer slow-release formulations for nitrogen overcome several shortcomings of soluble sources. These are synthetics that have a much slower, longer residual nitrogen release pattern and a greatly reduced burn potential. They don't produce peaks and valleys in growth rate. Several forms of slow-release nitrogen are available. The most common found in typical home lawn fertilizers is called sulfur-coated urea, or SCU. This is usually mixed with more soluble nitrogen forms.

The more slow release the formulation, the better. To determine what, if any, amount of slow-release nitrogen is present, look on the back of the bag in the label section marked "guaranteed analysis." The total nitrogen percentage will be broken down into the various forms of nitrogen in the mixture. The soluble forms are listed as ammoniacal nitrogen or urea. If there is any slow-release nitrogen, it will be listed with an asterisk on the urea as "slowly available nitrogen" usually in the form of SCU. Look for the fertilizer containing the greatest amount of slow-release nitrogen.

The final considerations when using commercial fertilizers are how much to apply at one time, and how to spread it. The pounds of fertilizer you apply will depend on the formulation you buy. The recommended amount for grass is one pound of nitrogen for every 1,000 square feet of lawn. To determine how many pounds of fertilizer this will require, look at the first number on the front of the bag — the percentage of total nitrogen. Divide that number into 100. This gives the pounds of that particular fertilizer to use for every 1,000 square feet of lawn. For example: a 2,000-square-foot lawn using a 30-5-5 formulation requires 3.33 pounds of fertilizer (100 divided by 30) per 1,000 square feet, or 6.6 pounds for 2,000 square feet.

Always use a rotary spreader to make the application, and water well immediately afterward. Hand-spreading always results in uneven application that can cause some areas to be burned and others to remain unfertilized. And by the way, grass clippings do not add to thatch buildup and are best left on the lawn if the grass is mowed often enough. The clippings contain large amounts of nitrogen and other essential nutrients that will be recycled back into the soil. Thatch accumulates more rapidly as a result of too much water and nitrogen fertilizer. Higher mowing heights also allow for thicker thatch.

Jay Deputy is an education specialist in landscape horticulture and turf at the University of Hawai'i-Manoa, Department of Tropical Plant and Soil Sciences, and state administrator for the Certified Landscape Technician Program sponsored by the Landscape Industry Council of Hawaii. Got a lawncare or turf question? Write to deputy@hawaii.edu.

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