Phosphorus promotes strong root growth, makes stems strong and improves disease resistance. Most soils are deficient in phosphorus, and when the soil pH is too high or too low, phosphorus becomes even less available. Phosphorus is not very water-soluble, so it doesn't move through the soil. For best results, it should be incorporated into the root zone where it can be utilized. Microorganisms convert phosphorus into a form that is available to plants, which is why phosphorus deficiencies are often apparent in the spring when soil microbes are not very active.
Potassium helps plant grow strong root systems and promotes disease resistance.
It also regulates the uptake of other nutrients. Most soils have an adequate reserve of potassium, but in a form that is unavailable to plants. Organic sources include seaweed extract or kelp meal, granite dust, greensand, leaf mold and wood ashes. Secondary elements and trace minerals. You can avoid most trace mineral deficiencies by adding organic matter and by using organic fertilizers that contain trace elements.