However, this supply of potassium is very important in the soil's ability to supply potassium over longer periods of time. The readily available potassium is that potassium held by the negative charged clay or organic matter particle. It is known as exchangeable potassium. Small quantities of potassium ions are also in soil water available for uptake. Exchangeable potassium and soil solution potassium are most important for growing healthy, high yielding crops.
Most soil tests attempt to measure the readily available potassium.
Unlike nitrogen and phosphorus, the organic matter in soils contains very little potassium but can hold potassium by cation exchange. Therefore, plant available potassium comes mainly from readily available forms or fertilizers. Smaller amounts are released from the slowly available forms. Major forms of soil postassium. For these soils, recommended rates of potassium fertilizer are often greater than crop removal rates expected.