Potassium occurs in the soil in three forms: as exchangeable (available) potassium (K+) adsorbed onto the soil CEC; fixed by certain minerals from which it is released very slowly to available form; and in unavailable mineral forms (most of the potassium in soils). Plants take up potassium as the K+ ion.
The common source of fertilizer potassium is muriate of potash (0–0–60), which chemically is potassium chloride (KCl) (Table: "Description of fertilizer materials"). Potassium chloride is highly water soluble. At excessive rates, muriate of potash can cause salt damage to plants. Potassium is between N and P in mobility. It is not lost as readily as N, but it will move into the soil to the roots more quickly than P. Potassium fertilizers are most effective if applied ahead of planting or as a topdressing on perennial forage crops. Because of the salt concern, the amount of K that can be safely applied in a starter band is limited. The rule of thumb for salt injury is that no more than 70 lb of N + K2O should be applied in a band within 2 inches of the seed or no more than 10 lb of N + K2O should be applied directly with the seed. If a potassium deficiency is observed after planting, a topdressing of K fertilizer can somewhat overcome the deficiency. Even if the impact on the current crop is limited, if there is a deficiency, the K will need to be applied at some point, so it is best to go ahead and topdress some K fertilizer to get what immediate benefit you can and get a head start on building the K in the soil back up into the optimum range. One additional consideration with K deficiency is that compaction can often result in K deficiency, even in a soil testing optimum or higher. A soil test should be taken to make sure the problem is low soil K before topdressing in these situations.
Other potassium materials used in specialty fertilizers also are listed in Table: "Description of fertilizer materials".
Reference: The Agronomy Guide - Section 2: Soil Fertility Management