The application of iron to your lawn in the spring should occur when temperatures are between 60 and 70 degrees Fahrenheit. It is not advisable to spray or apply iron at a high temperature. You prevent taking too much of the medication, be sure to carefully follow the instructions on the packaging. You do not need to add iron to your grasses as long as the soil test does not indicate that there is an iron deficiency. Even though your grass is yellow, you may not need to give it iron supplements if you get a proper amount of water and nutrients.