Potassium is the mineral element found in the second-highest concentration in turfgrass leaves; only nitrogen is at a higher concentration. But how much potassium fertilizer is enough? Dr. Doug Soldat from the University of Wisconsin wrote a post for the Turf Diseases website entitled How I'd Manage Potassium on Cool-season Turf, and he has expanded on that in an excellent article in the May/June issue of The Grass Roots.
Dr. Soldat makes a quick review of when and how potassium is likely to provide improved cold, heat, drought, and wear stress. You might be surprised to find that the research results aren't very strong or conclusive in supporting the idea that high levels of potassium fertilization confer these increased stress tolerances to cool-season grasses. There is no doubt that potassium is an important mineral element, but if there is already enough potassium available to the grass, adding more doesn't seem to have much of an effect. For example, we can read in textbooks that more potassium will lead to a more extensive root system, but is that really the case?
Does potassium fertilizer really increase roots?, I asked, in this article from TurfNet Monthly. My conclusion was that potassium fertilizer will do nothing to increase roots, and in fact may actually cause a decrease in rooting, unless the potassium was being applied to correct a potassium deficiency. Potassium application to turfgrass is likely to increase rooting only when it eleminates a potassium deficiency. I would classify a turfgrass soil as requiring additional potassium at about the 50 ppm level using the Mehlich 3 soil test extractant. If the soil is less than 50 ppm potassium, we might expect some response from a potassium fertilizer application. If the soil has more than 50 ppm potassium, the grass can almost certainly get all the potassium fertilizer it requires from the potassium that is already in the soil, and potassium fertilizer would be unlikely to influence turfgrass performance.