Fertilizer that is too rich in nitrogen will burn your grass. Brown patches will begin appearing as roots are burned by nitrogen. Fertilizer-burned grass is only lost if not treated immediately. Fertilizers are made of mineral salts that become acidic when mixed with water. To avoid problems with fertilizer-burned grass in the future, take a soil sample and use a store-bought soil tester or send it to your state cooperative extension office for analysis. The test results will let you know exactly how much fertilizer and other soil amendments to use.
Take small soil samples from several places in your yard. Use a hand trowel to dig a small hole 6 to 8 inches deep.
Mix together the various soil samples. Use a pH soil test kit from your local garden center. Optimal pH reading should be 6.5 to 7.5. The lower the pH, the more acidic the soil.
Apply powdered gypsum to your yard at a rate of 10 pounds per 100 square feet. Water the grass heavily. The gypsum will bind with the mineral salts from the fertilizer raising the pH.
Test again after 2 days. Re-apply gypsum powder if the pH is still below 6.5.
Water daily, continuously soaking the soil, until the brown patches in the grass begin to show signs of new growth.
Fill a seed spreader with grass seed. Walk slowly to spread the grass seed evenly. Cover new seed with a 1-inch layer of straw. Water thoroughly. This will replace any grass that has been lost completely to fertilizer burn