Potassium is an essential element for plant growth, but it is often limited in garden soils. Potassium is known as the “health element”, and a shortage of potassium effects both quality and yield. Even before visible signs of a potassium deficiency appear, the plant can suffer from “hidden hunger”. Potassium is an important cofactor for producing ATP, the energy molecule of the plant. Since ATP drives many important chemical reactions in the plant, adequate potassium is required at every stage of plant growth. With insufficient potassium, plant growth stalls, resistance to pests and disease weakens, fruit quality suffers, and the plant leaves eventually yellow and die. Therefore, supplemental potassium is beneficial for all vegetable gardens, unless a soil analysis indicates otherwise.
Potassium sulfate is a great source of potassium for gardens. It provides potassium in a stable, pH-neutral form, and some brands are even OMRI listed as certified organic. Organic gardeners are often limited in their choices for potassium fertilizers. For example, carefully formulated composts are a good source of potassium, but the potassium is easily leached from the composts by heavy rainfall. Hardwood ash is another organic source. In fact, the word “potash” originally referred to the practice of dissolving ash in a pot of water to use as fertilizer. Unfortunately, wood ash also has a very high pH, and it can cause serious problems in alkaline soils! Greensand and granite dusts are good potassium sources, but they are sparingly soluble and may take years to become fully available to the plant. If only a small amount of potassium is required, naturally-derived potassium chloride can be used. Potassium chloride is highly soluble, but the chloride content can kill beneficial microorganisms! Potassium sulfate eliminates all of these potential drawbacks. So a good potassium-sulfate fertilizer is usually the best choice for both conventional and organic gardeners.
Not all potassium-sulfate fertilizers are alike. Synthetic potassium sulfate is not OMRI approved, nor is any potassium-sulfate fertilizer produced by acidulation or chemical reaction. To be organically certified, potassium sulfate must come from natural, mined sources with little or no processing. Unfortunately, some unprocessed raw materials are barely water soluble, or are loaded with unwanted minerals. Only a few sources of organic potassium sulfate meet the requirements of purity, water solubility and low-chloride content desirable in a garden fertilizer. If in doubt, ask for more details. Well-informed sales staff should know about the source of their fertilizers, whether the product has an OMRI label on it or not!
The “grade” of the potassium sulfate fertilizer is also important. Large chunks of potassium sulfate are scarcely soluble and generally not a good choice. The best grades are called soluble-fines, with particle sizes below 0.015 mm. Fine grades dissolve more rapidly and are better suited for irrigation or foliar sprays. Foliar feeding is an efficient way to provide fast-acting potassium to plants, especially if they are beginning to show signs of potassium deficiency. Potassium deficiency usually shows up first on the lower leaves, with the margins of the leaves turning yellow first, followed by yellow or brown spots on the leaves. A fine mist with clean, water-soluble potassium will prevent the problem from spreading. Keep in mind, though, that highly-concentrated fertilizer salts can “burn” the leaves. So use mild concentrations of potassium foliar sprays, with multiple applications if necessary.
Abbreviations and notes: K = potassium; Mg = magnesium; S = sulfur, Cl-= chloride; Na = sodium.