Lawn Fertilizers have a limited time effect, and regular supplement is due to keep your turf lawn well fed.
By Mid Summer, lawn growth slows down, and temperatures rise. A subtle fertilizer is appropriate, so look for fertilizers with low nutritional balance. 10-6-4 balance is ideal.
The goal of the mid-summer fertilizer is to provide the turf lawn root system with the strength it needs to cope with higher temperatures, and retain its green color throughout the Summer months. We feel there is nothing worse than a yellow hibernating lawn in the middle of the Summer, especially if all you have is 6-7 months of healthy growth anyway (coming from a Canadian perspective).
A simple Fertilizer application, followed by thorough watering of course, goes a long way in maintaining the strength and vigor of your lawn throughout the Summer.