Potassium is the third major nutrient required by turfgrass, often designated by the letter ‘K’ in fertilizer analysis, following nitrogen and phosphorus. While nitrogen promotes leaf blade growth, potassium supports the internal machinery of the grass plant. It plays a significant role in regulating processes that govern the plant’s survival and overall health. Applying potassium is a strategy focused on fortifying the turf against environmental challenges and biological stressors.
The Role of Potassium in Turf Health
Potassium’s primary function in turfgrass is regulatory, acting as a conductor for numerous internal processes. It helps control the opening and closing of the stomata, the pores on the leaf surface responsible for gas exchange and water vapor release. This mechanism, known as osmoregulation, allows the grass to manage water retention more effectively, significantly improving drought tolerance.
The presence of adequate potassium also contributes to strengthening the plant’s cell walls. Stronger cell walls provide physical resistance, making the grass blades more resilient to foot traffic and environmental damage. This structural integrity is linked to improved resistance against common turf diseases, such as dollar spot and brown patch. Furthermore, potassium facilitates the transport of nutrients and sugars (carbohydrates) throughout the plant, which is crucial for energy storage and root development.
Diagnosing the Need for Potassium
The only reliable method for determining if a lawn requires supplemental potassium is through professional soil testing. A soil test provides an accurate measurement of nutrient levels available to the grass roots. Laboratories typically categorize results as Low, Medium, or High, providing recommendations for application rates tied directly to the “Low” and “Medium” categories.
Visual signs of deficiency should be treated only as supporting evidence, as they are often ambiguous and can mimic symptoms of other nutrient deficiencies or environmental stress. A severe potassium deficiency may present as chlorosis, or yellowing, that begins along the margins of the older, lower leaf blades. This yellowing may progress into a scorched or brown appearance, known as necrosis, as the deficiency worsens.
Because potassium is mobile within the plant, the grass moves the limited supply from older leaves to support newer growth, causing symptoms to appear first on the lower foliage. If a soil test is not performed, applying a potassium supplement risks over-application, which can be wasteful or lead to nutrient imbalances. Soil testing should be performed every few years to monitor potassium levels.
Selecting Potassium Sources
Potassium is sold in several forms, with the choice depending on the application method and soil conditions. Granular fertilizers are labeled with an NPK ratio, where the third number represents the percentage of potash, or potassium oxide (\(K_2O\)). To calculate the actual amount of elemental potassium (K) applied, the \(K_2O\) percentage must be multiplied by a conversion factor of 0.83.
Muriate of Potash (Potassium Chloride or KCl) is the most common and least expensive source, typically containing around 60% \(K_2O\). However, its high salt index increases the risk of dehydrating plant roots or causing “salt burn” if over-applied. Sulfate of Potash (Potassium Sulfate or \(K_2SO_4\)) is a popular alternative, offering about 50% \(K_2O\) with a significantly lower salt index.
Sulfate of Potash is often preferred for turfgrass because the lower salt content reduces burn risk and provides the added benefit of sulfur, a secondary nutrient required for protein synthesis. Water-soluble liquid potassium supplements are also available, formulated for foliar feeding or spoon-feeding the turf. These liquid products deliver the nutrient rapidly and are suitable for frequent, light applications.
Application Techniques and Timing
The application rate must be precisely calculated using the soil test recommendation and the \(K_2O\) percentage of the chosen product. To determine the pounds of fertilizer product needed per 1,000 square feet, divide the target pounds of \(K_2O\) required by the percentage of \(K_2O\) in the bag (expressed as a decimal). This calculation ensures the correct dosage is delivered.
For granular products, a rotary spreader must be calibrated to ensure uniform application and prevent streaks of over- or under-fertilization. Calibration involves a test run over a measured area to verify the spreader setting dispenses the calculated amount of product. Walking at a consistent, brisk pace is essential for achieving an even spread pattern.
The most beneficial time to apply potassium is in the late summer or early fall, especially for cool-season grasses. Fall application supports root growth and carbohydrate storage, which improves the turf’s winter hardiness and its ability to recover in the spring. Regardless of the timing, all granular potassium products should be lightly watered into the soil immediately after application to dissolve the granules and wash corrosive salts off the grass blades. Applying too much potassium at one time can be counterproductive, potentially leading to a salt burn or creating an imbalance that interferes with the uptake of other nutrients like magnesium.