Consider the grass type, season and temperature before you attempt to fix a yellow grass lawn. Some types of grass appear yellow during seasonal temperature changes when they enter a dormant or resting phase. For example, Bermuda grass is a warm season grass that goes dormant in winter. Kentucky bluegrass is a cool season grass that goes dormant in the summer. Besides dormancy, a grass lawn can turn yellow because of soil deficiencies or diseases. You need to first find the source of the problem to fix a yellow grass lawn, sometimes through trial and error.
Perform a soil test to determine the pH of the soil. A do-it-yourself kit will indicate whether the soil is acidic, neutral or alkaline. Some kits also provide the level of nitrogen, phosphorus and potassium. If you know the pH level, you may be able to find out how to treat the lawn grass .
Apply fertilizer if the soil test indicates that the lawn grass has an iron deficiency. Nitrogen facilitates chlorophyll formation, the stuff that keeps plant life green. When the soil lacks nitrogen, the entire grass lawn turns pale yellow, starting at the bottom of the blade. The nitrogen found in fertilizer provides the nutrients to turn grass green in a matter of days.
Spray an iron compound called ferrous sulfate on a yellow grass lawn to correct an iron deficiency. Like nitrogen, iron is involved in chlorophyll formation. When soil does not have enough iron, parts of the grass lawn turn yellow at the top of the blade. This type of yellowing occurs frequently in sandy soils with a high pH. According to the University of Florida Extension Service, it's possible to counter an iron deficiency with a mixture of 2 ounces of ferrous sulfate and 3 to 5 gallons of water applied to every 1000 feet of grass lawn space. You should see the color improve within 48 hours if an iron deficiency is the source of yellowing.
Use magnesium to fix a yellow grass lawn. Magnesium is another element that contributes to chlorophyll. The main symptom of a magnesium deficiency is yellow patches on the grass lawn. Add 1 teaspoon of Epsom salts to a spray bottle filled with 1 pint of water then spray the affected areas. Epsom salts are a form of magnesium sulfate that should turn the grass lawn green within 24 to 48 hours of application.
Treat patch diseases with a systematic fungicide. Fungus causes Necrotic Ring Spot in Kentucky bluegrass and fescue. The patches form large, irregular straw-colored rings. If left untreated, the rings turn bright yellow and sink into the soil. Yellow Patch is another fungus-induced disease that emerges in wet weather. The yellow patches are 6 to 12 inches in diameter and sometimes have red margins. Both diseases respond to fungicide application followed by a fertilizer program.
Mow the grass correctly to fix a yellow grass lawn. Scalping, or cutting the grass too short, leads to yellowing. Raise the lawnmower blade to the correct height for your type of grass.