Adequate potassium (K) boosts yields and increases plant resistance to drought, frost, and disease. It is also essential for root system development, nitrogen fixation in legumes, enzyme activity, and fruit quality.  Potassium is a natural plant nutrient and K salts and minerals are widely distributed throughout the world, although plant deficiencies are widespread. In soils and plants, K salts are very soluble and generally originate from underground deposits or from salt lakes. In selecting a K source for organic farming, primary consideration should be given to the short- and long-term solubility. Two mineral sources of K are commonly used for organic production; potassium sulfate and potassium-magnesium sulfate. Chloride-based K sources are sometimes discouraged due to a desire to minimize chloride in the soil. Due to their very low solubility, rock K sources such as granite dust, greensand, and feldspar are generally not suitable nutrient sources and frequently require years or centuries to solubilize. Seaweed will contain some K, but its high cost generally limits wide-scale use. Potassium salts in composts and manures are very soluble and easily leached from the solid phase. In manures and composts, K behaves in an identical manner to fertilizer K. Potassium is quite abundant in the earth's crust and the world K supply is very large. Sustainable crop production requires replacement of the large amounts of K removed in harvested crops. Case studies will be presented where soil K levels are declining on organically managed farms and also where K levels are rapidly increasing. Excessive K is not associated with any adverse environmental or health concerns. Sustainable soil fertility management involves using K in proper relationship with all plant nutrients to maintain productivity and soil health.