The bacteria which work to decompose organic matter, use nitrogen as a fuel. If the nitrogen is not adequate, the bacteria will fight the plants for the rights to the nitrogen. Phosphorous is deficient in many areas around the country. 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.