Lawns usually need to be fed to look their best, but the diet must be strict. Overfertilizing results in grass that is weak and top-heavy, a perfect target for disease and insects. Excess nutrients can run off lawns and into the water table. Always follow the package directions, and fertilize only during growth spurts?in spring and fall for cool-season grasses, in early summer for warm-season grasses. Most commercial fertilizers are a combination of nitrogen, phosphorus, and potassium. The most impor-tant ingredient for grass is nitrogen, which promotes leaf growth and good color. You already have an excellent source of nitrogen: grass clippings. Leaving them on the lawn is an easy and inexpensive way to fertilize. The fertilizer you choose depends on the type of grass you have and the nitrogen content of the fertilizer. If you are unsure about how much fertilizer to apply, do a soil test; then you can adjust the amount and type of fertilizer to correct any deficiencies.