Potassium (K) is one of the three primary macronutrients necessary for robust plant development, alongside Nitrogen (N) and Phosphorus (P). This element is universally represented by the third number in the three-digit N-P-K ratio found on all fertilizer labels. A fertilizer with a high third number indicates a substantial concentration of potassium, or potash, which is the term commonly used for potassium in a fertilizer compound. Understanding the specific function of this nutrient and the available sources allows growers to efficiently address their plants’ needs.
The Function of Potassium in Plants
Potassium functions as a regulator within plant systems, managing several internal processes without becoming a structural part of the plant tissue itself. A major role involves regulating water uptake and retention by controlling the opening and closing of stomata (tiny pores on leaf surfaces). This mechanism helps plants manage drought stress and maintain appropriate turgor pressure, keeping the plant firm and upright.
The element is instrumental in moving sugars and starches produced during photosynthesis from the leaves to other parts of the plant, such as developing fruit, flowers, and roots. This supports the accumulation of sugars and starches, improving the quality, flavor, and storage life of produce. Adequate potassium levels strengthen cell walls, enhancing resistance to certain diseases and pests.
Identifying and Choosing High-Potassium Fertilizers
High-potassium fertilizers can be categorized into synthetic chemical sources and natural organic sources, each with distinct characteristics concerning concentration and release rate. Reading the N-P-K label is the simplest way to identify a high-potassium product, as the third number will be significantly higher than the first two.
Synthetic Sources
The most common synthetic source is Muriate of Potash (MOP), typically labeled as 0-0-60 or 0-0-62. While MOP is the most cost-effective option, it contains a high percentage of chloride, which can be detrimental to chloride-sensitive crops like potatoes, tobacco, and certain fruits. The high salt index of MOP (around 116) also increases the risk of “salt burn” to delicate plants or seedlings if applied incorrectly.
Sulfate of Potash (SOP), or Potassium Sulfate (K₂SO₄), is a premium alternative, typically labeled 0-0-50. SOP is preferred for chloride-sensitive plants because it contains minimal chloride. Its lower salt index (around 46) makes it a safer option for applications placed close to the seed or root zone. SOP also provides sulfur, a secondary macronutrient essential for protein synthesis in plants.
Organic Sources
Organic options generally have a lower concentration of potassium but offer a slower, sustained release and often provide other micronutrients. Wood ash, derived from burned wood, is a readily available source with a highly variable K content, often ranging from 1% to 10% potash (K₂O). Because wood ash has a high alkaline pH, it should only be used on acidic soils to avoid raising the soil pH.
Kelp meal, made from dried seaweed, is a gentle, slow-release source that generally has an N-P-K ratio around 1-0-2 or 2-0-4. It is valued for its potassium and its array of trace minerals. Greensand, a naturally occurring mineral sediment, is an extremely slow-releasing source, typically providing a potassium concentration of about 0-0-3.
Practical Application: Dosage and Timing
Before applying any high-potassium fertilizer, a soil test provides an accurate measure of existing nutrient levels and pH, preventing unnecessary application. Without a soil test, over-application of high-salt fertilizers can lead to “salt burn,” where excess salts draw water out of the plant roots.
Potassium is relatively immobile in the soil, meaning it does not move far from where it is placed, unlike nitrogen. For this reason, broadcast application—where the fertilizer is spread evenly across the soil surface and incorporated before planting—is effective. This helps ensure the nutrient is available across the entire root zone.
For established plants, side dressing involves placing the fertilizer in a band or ring near the plant or row after growth has begun. While this method can be used for a mid-season boost, pre-plant broadcasting is often more effective for potassium. The optimal timing for potassium application is in the spring before planting or in the fall, allowing the nutrient time to move into the root zone and support the plant before fruiting and flowering begin.