Observations from Wisconsin and How Dr. Soldat Would Manage Potassium on Cool-Season Turf

Hello again from the Midwest. Its be a while since I have posted and its because old man winter will not relinquish his grip on us. We have only had the opportunity to rate one of our 5 snow mold sites. Last week three of our sites received another 5 to 12 inches of snow. Where the snow has melted, I am assuming winter kill and breakthrough were minimal. My only metric to assess this is my phone has been eerily quiet. So since I do not have much to report on with respect diseases, I am privileged to post comments from Dr. Doug Soldat on managing potassium on cool-season turf. I think this fits well into the discuss John started concerning Poa management.

Turf Diseases Blog: How I’d Manage Potassium on Cool-Season Turf

Doug Soldat

Dept. of Soil Science

University of Wisconsin-Madison

Over the last several years as the turfgrass nutrient and water specialist at the University of Wisconsin-Madison, I’ve slowly been developing my philosophy of potassium management based on my research, the scientific literature, theory and observation. I teach my view point every fall to a group of students who are too green to be shocked, but when I revealed my philosophy to a group of workshop attendees at the Canadian International Turfgrass Conference in Vancouver in March, it was clear that my recommendations were a major reversal from what they’ve been taught. Similarly, you may have heard Dr. Rossi (Cornell) and Dr. Gaussoin (Nebraska) debate the importance of potassium at GIS and elsewhere. So, I thought I’d throw my hat into the ring; and because disease plays a major role in my management philosophy, I’d thought I share my thoughts with this community. A much more detailed defense of the ideas below will be published in the Grass Roots in June (official publication of the Wisconsin GCSA). I’d be happy to email you a copy of that when it’s available, just let me know djsoldat@wisc.edu. So, without further ado, here’s how I’d manage potassium if I were a Golf Course Superintendent:

Fairways:
I’d apply potassium only according to soil test levels. I’d use modern interpretations of optimum (i.e. 50-100 ppm, depending on soil test) instead of the more common values of 200-400 ppm. I expect even the 50-100 ppm levels will be adjusted downward as more soil test calibration studies are conducted, but for now 50-100 is fine.

If/when potassium dips below that soil test threshold, I’d make a 1 lb/M application in May. I’d re-test the soil in fall to determine if another application is warranted the following spring. Why May? The evidence for potassium improving drought tolerance is much more convincing that the evidence that potassium increases winter hardiness (although, I believe the opposite is true for warm season grasses). Also, Dave Moody and Frank Rossi’s work at Cornell University has clearly linked increased potassium in the leaf to increased severity of gray snow mold (and pink to a lesser extent). Similarly, Dr. Ebdon et al. (2006 – Mass.) reported more severe gray snow mold at higher K application rates on ryegrass. In summary: there is only weak evidence that K increases cold tolerance for cool season-grasses, strong evidence that it increases snow mold, decent evidence that it improves drought tolerance. I’d go with May.

If my fairways were something other than sand, I’d apply muriate of potash (0-0-60), if they were sandy, I’d consider using a polymer coated K source or spoon feeding in 0.25 lb/M increments if practical. Most non-sand soils have a high enough cation exchange capacity to retain a 1 lb/M application of potassium. Sandy soils may need some help provided by the polymer coating or spoon feeding approach.

Greens:
Assuming most greens are sand-based, either from construction or years of topdressing, I’d spoon feed potassium along with nitrogen in the ratio of 2 parts nitrogen to 1 part potassium beginning in May and ending in August (but continuing with N after that). I like to fertilize my research greens about every other week with 0.2 lbs N/M as urea, so that’d put me at 0.1 lbs K₂O/M per application or about 1.0 lbs/M for the season. I feel this is a very conservative approach which replaces the potassium removed by clippings. But as the research continues to unfold, I can imagine that my management philosophy for greens may evolve to look more like my fairway program. That said, if I were an actual superintendent, I’d leave the research to the researchers and use this conservative but research-based approach.

Why stop in August? Same philosophy as above, the benefits of potassium as a drought stress nutrient are much more convincing that the cold tolerance argument and we have seen that high tissue K increases snow mold pressure. Also, research by Woods et al. (2006 – New York) and Johnson et al. (2003 - Utah) has shown clearly that high soil potassium levels in sand based greens are always substantially reduced by spring, presumably by the snow melt leaching the potassium out of the root zone. Therefore, a large application to a sand root zone in fall will do two things: increase your susceptibility to gray snow mold, and 2) leach out of the root zone, becoming unavailable in spring resulting in a complete waste of time and resources.

One Final Note:
All this talk recently about fostering an environment that favors bent over Poa makes me wonder why a superintendent with such a mindset would want to apply any potassium at all. Poa is obviously more susceptible to heat, drought, and cold stress than bentgrass, so why apply a nutrient that supposedly increases tolerance to those things? Yet, I’ve not heard mention of potassium management as a key strategy. I’m not saying it should be -- I tend to think we’ve overestimated the role of potassium in stress tolerance —it’s just interesting that potassium has been left out of the discussion so far.