The most important thing you can do for a lawn is to provide it with proper nutrition. A well-fed, healthy lawn has a better root system to combat heat, cold, drought, mowing, foot traffic and other stresses. There are two fertilizing times in the year that are most important: early spring and late fall. If you only can fertilize your lawn just a few times, these are the two you need to have (two fertilizer applications are the minimum). Fall is the most important with early spring the next most important time.
Early spring (February to April) — Lawns wake up hungry from the winter months. A spring feeding strengthens roots and gets lawns off to a good start before the heavy growing season. In most regions, it is wise to apply a combination fertilizer with a pre-emergent to control crabgrass. If you're apply a pre-emergent, this should go down before the soil temperatures reaches above about 55 degrees to be most effective.
Late spring/early summer (May to June) — grass is actively growing at this time and using nutrients quickly, therefore it's important to give the lawn a proper feeding designed for this time of year. Since this also is a time when broadleaf weeds are actively growing, a combination fertilizer with broadleaf weed control is ideal (weed and feed).
Warm season grasses grow the most in late spring to early summer. This is the time that they need the additional nutrients supplied by fertilizers available. A first application in early spring (when grass starts to green up and frost danger is past) will help the grass to start growing.
Each variety needs varying amounts of fertilization and nitrogen levels during the growing season to continue healthy root development and increased foliage. The amount is dependant upon the specific variety, soil fertility, and the climatic conditions, watering schedules and the type of fertilizer that will be added.
Fescues, Bluegrasses and Ryegrasses do well with one to two light feedings in the early fall to promote root growth but not heavy enough to promote top growing. Mid spring feeding will help promote top growth and thicker foliage development. Never heavily fertilize in the summer or late spring and do not use a fast acting fertilizer on a cool season grass late in spring.