Unlike nitrogen which is very mobile through the soil, phosphorous is not. Unless phosphorous is incorporated into the soil, it will take years of surface applications to get adequate amounts of phosphorous into the root zone. Most folks get enough phosphorous with their choice of a complete fertilizer. This raw form of phosphorous is very slowly available in the soil. If rock phosphate is finely ground, and used in combination with a highly organic soil, it becomes a very valuable and long lasting phosphorous source.
Potassium is the final element in the puzzle.
Potassium is also available in most commercial fertilizers. Like phosphorous, it moves slowly through the soil. Unless a soil test reveals that your soil is particularly low in potassium, the amounts provided in a standard fertilizer will be sufficient. In sandy soils, supplemental amounts of potassium are often needed. Consult your local extension service for specific advice.