Nitrogen encourages top growth, interfering with the transition to dormancy when warm season grasses typically shift from top growth to energy storage in the roots, rhizomes and stalons. Phosphorus promotes root growth, however, helping turfgrass prepare to survive winter months.
On the other hand, cool season turfgrasses, like tall fescue and bluegrass varieties, are just gearing up for their peak growing season, so fall is prime time for nitrogen-rich fertility application. Again, soil testing will give you the exact formula for amending the soil to achieve optimum growth and overall turfgrass health.