Garden plants and lawns need potassium for strong stems and disease resistance. Potassium-deprived plants can be hard to diagnose because their symptoms mimic other soil or plant care problems. Soil tests performed by your local extension service determine if your soil suffers from a potassium deficiency, and by how much. Several readily-available sources contain potassium, some of which you may already have or can haul away for free.
Considered one of the three major soil nutrients (along with nitrogen and phosphorous), potassium is the ''K'' in the N-P-K formulas in artificial fertilizers such as 10-10-10 or 5-20-10. Most extension services recommend the N-P-K ratio you should purchase if you go the chemical fertilizer route.
Wood ash makes an excellent fertilizer for soils low in potassium and low in pH, according to the University of Oregon Extension Service. That's because wood ash helps neutralize acid (low pH level) soils. If you own a wood-burning stove or fireplace, consider using those ashes directly as fertilizer or as a layer in the compost pile. Avoid using wood ashes if your soil is already on the alkaline side (with a pH level above 7.0), or if you are planting acid-loving botanicals, such as blueberries or azaleas. Wood ash may also promote potato scab if applied to the potato patch. Generally speaking, hardwood ashes yield higher amounts of potassium and other soil nutrients than softwoods.
The mineral sulfate of potash magnesia is usually sold as "sul-po-mag." It is used specifically to address low-potassium rates in the soil. If replacing a 20 lb. bag of artificial 5-10-15 to cover 1,000 square feet, for example, you'd want to use about 13 lbs. of sul-po-mag (along with 11 lbs. of blood meal for the nitrogen and 18 lbs. bone meal for the phosphorous). Sul-po-mag also contains significant amounts of magnesium and sulfur. If a soil test reveals a lack of these minerals along with potassium, sul-po-mag makes an excellent organic choice.
Of the two forms of the mineral potassium sulfate available to gardeners, only one is considered organic. This natural variety, from Great Salt Lake, Utah, is derived from natural evaporation, as opposed to the kind created by combining sulfuric acid with potassium chloride. Either form, however, adds potassium to the garden soil.
Also sold as granite dust or rock potash, rock dust is a slow-release organic source for potassium made by crushing granite. The National Sustainable Agricultural Information Service (NSAIS), however, considers granite dust to be a somewhat inferior potassium source because the minerals tend to not be as water-soluble as other organic sources, meaning they do not readily leach from the granite into the soil.
Gathered from the ocean floor, the clay greensand, also known as glauconite, adds potassium to garden soils, albeit slowly. NSAIS points out that greensand costs more than other natural fertilizers, making it prohibitive for large-scale farming. Home gardeners, however, may find greensand affordable.