Grasses require at least 16 different essential elements in their diets, most of which are available from the plants' surrounding environment. But the growth demands of today's lawn owners usually mean that homeowners must help Mother Nature along.

Even if you are committed to having a low-maintenance lawn, you will need to fertilize it with nitrogen (N) to sustain thick, vigorous turf. In addition to bringing on deep green color, nitrogen is responsible for the sturdy growth and shoot density needed to fight off weeds and to stand up to disease, insects, and traffic.

All of these positive effects can easily turn into negative ones if you use too much fertilizer or apply it at the wrong time. The common practice of fertilizing in the early spring is actually not the best time in northern climes. It not only encourages excess blade growth which means more mowing, but it also gives your weeds a boost and increases thatch! Excessive spring growth also produces thin-walled grass blade cells that are more prone to injury and disease. Late summer to early fall is the preferred time for feeding northern lawns; mid-spring in the South.

In addition to needing nitrogen, your lawn may need phosphorus (P) and potassium(K). Depending on where you live, your soil may naturally contain adequate levels of these elements. Aiding in root growth and improving establishment rates, phosphorus is needed in small amounts and tends to remain in the soil. Potassium plays an important role in enhancing your grass's resistance to cold, disease, drought, and wear and is more prone to leaching from the soil. A soil test will help you determine which nutrients your soil needs.

A fertilizer with the designation "complete" contains all three of these nutritional elements. The percentage of the bag's contents made up respectively of nitrogen, phosphorus, and potassium, can be found by looking at the fertilizer grade. These three prominent numbers also tell you the percentage of nitrogen to phosphorus to potassium. For example, in a 50 pound bag of 20-10-10 grade, the ratio is 2:1:1, which means that 20 percent of the 50 pounds, or 10 pounds of the bag, is actual nitrogen; 10 percent (5 pounds) is phosphorous; and 10 percent (5 pounds) is potassium. The remaining 30 pounds of material in the bag may consist of additional elements such as iron and sulfur, as well as inert "filler" ingredients. Fillers are used to help ensure even distribution of the product and are frequently made from organic materials such as finely ground corn cobs.

Ratios are helpful in choosing which fertilizer to use for specific purposes. Those with a 1:2:2 ratio, such as a 6-12-12 fertilizer, are lower in nitrogen but higher in the nutrients desired when planting new grass or renovating old lawns. Fertilizers with high-nitrogen ratios of 2:1:1, 4:1:2, or 3:1:2 are frequently used for maintenance applications. They contain N, P, and K quantities closer to the plant's ongoing needs and are available in grades of 12-6-6, 16-8-8, 20-10-10, and so on.

While considering which bag of fertilizer is most appropriate for your yard, be sure to read the back label for the guaranteed analysis of the contents. If your soil test indicates that you don't need to add phosphorus or potassium, choose a bag with a low numeral or zero for that element. For example, a bag of 20-0-5 would have no phosphorus. In addition to checking the grade, you should also determine what type of nitrogen has been used, "water-soluble" or "water-insoluble."

Water-soluble nitrogen, once watered into the soil, can be immediately used by grass plants. Ammonium nitrate, ammonium sulfate, and urea are examples of this quick-release form of nitrogen. These provide a rapid green-up, but they also have drawbacks.

Water-insoluble nitrogen, found in slow-release fertilizers, must first be broken down by soil microbes into forms grass plants can use. These slow-release sources include synthetic organics, like ureaforms, or those derived from natural organic materials, such as composted manures. To spread the release of nitrogen over time, fertilizer companies can also manipulate the size of particles and sometimes coat them as well. Because these forms take longer to dissolve, they release nitrogen at varying rates. Common examples are isobutylidene diurea (IBDU) and sulfur-coated ureas.

When buying fertilizer, opt for the water-insoluble types or other slow-release forms. Using slow-release fertilizers will allow you to reduce the amount of time you spend behind your spreader. They last much longer and don't have to be applied as frequently as quick-release fertilizers, saving you money as well as time. Determine the type of fertilizer you have by reading the guaranteed analysis on the bag. Note: Many fertilizers have a combination of both fast-release and slow-release types of nitrogen. You should check carefully to find products that derive a majority of their nitrogen from slow-release sources.

The optimal time to apply fertilizers is when the grass roots and blades are actively growing. In the North this growth season occurs during the early to mid fall, when weed competition is minimal and fertilizing produces healthy root growth. This timing also allows plants to build up needed carbohydrate stores with just a moderate amount of topgrowth. For northern lawns, you should divide the annual amount of fertilizer and apply two-thirds in early fall and the remainder in mid to late spring, after the lawn's initial green-up. Because the grasses in southern lawns have a larger blade size and grow more vigorously, they will need at least two applications of fertilizer each year. Do the first about three weeks after the initial spring green-up; then fertilize again in late summer. You can add supplemental quick-release nitrogen between these times if weak growth and poor color indicate that it's needed.

For low-maintenance lawns, you should be applying 2 pounds of actual nitrogen per 1,000 square feet per year in the North and 2-4 pounds in the South. This may require an adjustment, given your specific growing environment, soil test results, the lawn's condition, and the type of fertilizer you use, whether slow- or fast-release. You can consult your Cooperative Extension Service for local recommendations. Quick-release fertilizers are usually applied at a rate of 1 pound of nitrogen per 1,000 square feet. Slow-release fertilizers usually require a higher rate of application to deliver their nitrogen. Follow the manufacturer's instructions and check the calibration of your spreader, as well as the square footage of your lawn, to ensure that you are applying the right amount. Remember, more is not necessarily better with fertilizers. Applying too much may "burn" your lawn and promote thatch formation and disease.

Keep in mind that lawns kept under irrigation throughout the summer or located in areas receiving heavy rainfall will require more nitrogen than their unwatered counterparts. Sandy soils are more prone to leach nutrients, but using water-insoluble fertilizers will help nutrients remain in the soil longer. Leaving grass clippings on the lawn over the course of a year will add about 1 pound of nitrogen per 1,000 square feet, so you can figure accordingly. The total amount of nitrogen that you'll need per year also varies with the type of grass you are growing. For example, Kentucky bluegrass and perennial ryegrasses require more fertilizer than the fescues, while in the South, Bermudagrass, zoysiagrass, and St. Augustinegrass need more than bahiagrass, centipedegrass, or carpetgrass.