Nitrogen will almost always have to be added because it is regularly flushed from the soil by rain or irrigation. N promotes growth and gives the turf a healthy color. Without sufficient N, growth stops and the lawn becomes yellowish. By contrast, too much N can over stimulate the turf, favoring shoot growth at the expense of good root growth. The turf then becomes less drought and stress resistant.
P is essential to produce strong root growth.
It is particularly important for root development of new plantings. It is not readily flushed from the soil by watering and is needed in smaller quantities. The exception would be starter type fertilizers. K is needed for overall hardiness and stress resistance in turf. K is flushed from the soil by watering, although at a much slower rate than nitrogen.