Two answers here.
First, your grass does need phosphorous. It seems to stimulate healthier root growth, among many other things. healthy roots will help the grass overwinter as well as withstand tough summer drought and simple heat stress.
Secondly, unless you are located on top of a sand ridge you likely have more than enough phosphorous to get your lawn through the years. Unlike nitrogen P is pretty tightly bonded to the soil, meaning it won't wash away with rain or irrigation water the way N will. The roots are "working" to get it stripped off of the soil particle, but they are getting it off of there.
Green lawns are striking, very pleasing to the eye. Green ponds, not so much. Do your watershed a service and cut the phosphorous out of your lawn. You will do your lawn a greater service by mowing one notch higher this summer.
