Phosphorus does not move easily through the soil, so the plant roots must extend themselves searching for adequate supplies. Because phosphorus tends to remain in the soil rather than easily leach away, it is possible that too much phosphate gathers in the soil. So be carefull. Potassium is needed in fairly large quantities to regulate metabolic reactions within the plant. Photosynthesis being one of these important activities.
Potassium behaves much like nitrogen, in that it dissolves quickly.
When applied to the surface of the soil, potassium will move quickly unto the root areas. Some plants require lots of potassium, more than any other nutrient. A plant deficient in potassium usually is weak and the leaf edges appear scorched. There isn't much potassium in most organic material. If your garden soil is deficient, you may add granite dust or other rock powders.