The nutrients included in fertilizer -- generally nitrogen, phosphorus and potassium, but sometimes including added benefits like calcium -- are sometimes the key to getting your lawn to sprout healthy and bright green as the spring turns into summer. However, overfertilizing grass can lead to serious damage that will ruin the look and feel of your lawn instead of promoting optimal health.
Lawn burn is the most common unpleasant result of applying too much fertilizer to your grass. This occurs when the soil and grass blades themselves become overwhelmed with nitrogen, and results in dry, brown patches or streaks spread out all over your lawn. Lawn burn can be as minor as a few brown blades that can be helped by significant watering, or as major as whole areas of your grass dying off because they have absorbed too much nitrogen and cannot properly absorb water.
Having excess fertilizer in a lawn can also lead to environmental dangers. Extra fertilizer not absorbed into the soil or the blades of grass will dissolve and run off with water from rainfall or your sprinklers. This can lead to chemicals from the fertilizer ending up in streams, lakes or sources of groundwater. These chemicals and high-concentration nutrients are not meant for human or animal consumption, and can lead to illness or even death if a large enough amount is spread around.
In general, grass needs 1 lb. of nitrogen per 1,000 square feet of coverage area; the other nutrients cannot cause as much damage and are not as vital to the health of the lawn. Look at your chosen fertilizer product; it will list a percentage of nitrogen (i.e., 10 percent nitrogen content). Divide 100 by the percentage to find out how many pounds per 1,000 square feet you need to achieve optimal nitrogen coverage (an example: 100 divided by 10 equals 10 pounds of fertilizer per 1,000 square feet). Then measure the length and width of your lawn and multiply them together to get the square footage of your area (example: 20 feet long by 50 feet wide equals 1,000 square feet). Divide the square footage of your house by 1,000 (1,000 divided by 1,000 equals one). Multiply that number by the number of pounds you determined to find out how much fertilizer to apply (one times 10 lbs. equals 10 lbs. of fertilizer to cover the whole area). Remember that you must distribute the fertilizer evenly over the whole area; too much fertilizer in a single place can cause the same problems.
If in doubt about the amount of fertilizer to apply or the nutrients your lawn needs, contact a professional at a garden center, nursery or extension service for help. These pros can conduct a soil test (or provide one for you to complete) that will identify your lawn's nutrient needs, as well as any soil amendments the grass might need in terms of acidity or water absorption.