The function of phosphorus in plants is very important. It helps a plant convert other nutrients into usable building blocks with which to grow. Phosphorus is one of the main three nutrients most commonly found in fertilizers and is the “P” in the NPK balance that is listed on fertilizers.here is no understating the importance of phosphorus in plant growth. Without it, a plant simply cannot be healthy. The basic function of phosphorus makes it possible to have beautiful and abundant plants in our gardens.