Excessive fertilizer results in "fertilizer burn" on lawn grass. In the worst-case scenario, this can kill the grass, and you'll have to plant new grass to repair the lawn. If you catch it soon enough, though, or if the extra amount applied was small, it's often possible to save the grass.
Recognizing Fertilizer Burn
If you applied too much fertilizer, damage to the grass will show up within the next few days unless you catch it at application. Burned grass either turns yellow because it is stressed or brown because the grass is dead.
Fertilizer burn results when mineral salts contained in chemical fertilizers build up in the soil and dry the plants out. Fertilizers that contain nitrogen in a fast-release form are the most likely to burn lawns. If you want to minimize the risk of fertilizer burn, use fertilizers with nitrogen in a slow-release form and apply according to label directions. Applying to dry grass and then watering after application also minimizes risk of burn.
Reacting to Excess Fertilizer
If you know immediately that too much fertilizer was applied to the lawn or if you spilled extra fertilizer in one spot, use a broom or wet/dry vacuum to remove as much as possible from the lawn. This is easier if you applied fertilizer to a dry lawn, so the fertilizer doesn't dissolve as quickly.
Whether you are able to remove fertilizer or not, water the lawn as soon as you notice a problem. You can start immediately after fertilizing, or at the first sign of burn on the lawn. On the first day, water until the ground is saturated, but stop when you start to notice runoff. You want to dilute the excess mineral salts and move them deeper into the soil, not wash salt off into the local waterways. Continue to water in the mornings every day for a week to help minimize burn damage.
Repairing Damaged Lawns
Where excess fertilizer killed the grass, you'll have to replant in order to repair the lawn. First, remove the dead portions of the lawn by digging out the grass and 1 inch of the soil below. Loosen the soil with a rototiller if it's a large area or with a hand cultivator if it's a small area.
If you're repairing the lawn with seed, add enough good topsoil to make the surface of the bare spots in the lawn even with surrounding soil. For cool-season grasses, the best time to plant seeds is in the spring or fall when soil temperatures are at 50 to 65 degrees Fahrenheit. Warm-season grasses need soil temperatures over 70 degrees, and they grow best when planted in the late spring or early summer.
You can plant sod in a lawn any time during the growing season, as long as the grass is not dormant and you can supply enough water to keep the soil from drying out. If you're using sod to repair a lawn, remove or add as much soil as needed to make sure that when you lay sod, it is growing at the same level as the surrounding grass. Water thoroughly after laying sod.