Is your lawn riddled with bare patches due to dog spots, heavy traffic or neglect? If so, you may need to apply grass seed to fill in those bare patches. This solution is known as "overseeding lawns." Apply a slow-release nitrogen fertilizer when you overseed. Five weeks after the grass germinates, apply a quick-release nitrogen fertilizer.
However, spring isn't the very best time for overseeding lawns. Fall is the preferred time, when the new grass won't have to compete with crabgrass, which is killed off by autumn frosts. So postpone overseeding until fall, unless your situation is dire.