According to Jeff Turnbull the president of the LCS Lawn Service located in the Twin Cities, the best time. To apply fertilizer to your lawn is in the spring when the temperature of the soil reads 55 degrees Fahrenheit. You will know that the soil reaches 55 degrees Fahrenheit when you see the lilacs start to blossom and grass begins to grow. In some parts of the country, that connotes initiating the first application in the middle of April. The second application should take place four weeks after your first application or after a month. That will fall in the middle of May. After that, try to fertilize your lawn every six to eight weeks until October. During the third feeding, try to use organic matters like cow dung instead of using synthetic fertilizers.
Turnball also said to take note of the feeding during the fall because it is very crucial. According to him, this is the time when the grass is still growing and the roots are creeping down. This is the time that it needs fertilizer the most and it is the most important time to use one.