For many years, there have been several products sold to greenkeepers for ‘hardening’ their turf as we approach peak disease season. Traditionally, chelated liquid iron and soluble ferrous sulphate have been used alongside nutritional products with lower nitrogen and higher potassium levels.
Lower nitrogen I think we can all understand, getting nitrogen applications correct is one of the most important ways of preparing your turf for winter. Too much N leads to soft growth that is susceptible to disease and too little N leads to weakened plants that are again weaker and more likely to become attacked by Microdochium Patch – the most common autumn disease of turf in the UK.
Not many would disagree that iron is beneficial in disease-reduction, ideally in as low an amount as possible and in a chelated form to limit damage to soil biology and blackening of leaves.
When I looked into research on preventing Microdochium several years ago, there were plenty of studies showing the beneficial effects of iron in reducing disease. You can also find good research on acidifying nutritional sources such as ammonium sulphate in keeping disease levels lower. More modern research shows the benefit of phosphite in reducing Microdochium, but when I looked at the use of potassium, I came across a bit of a dead end. There does not seem to be any research suggesting high K applications in autumn reduce Microdochium. The research I did find on potassium use in autumn was quite concerning to me as a Course Manager at the time. Below are a few examples of the research I found:
Now one research trial does not ‘prove’ any theory. Science builds up evidence brick by brick until reasonable conclusions can be drawn. The evidence for using potassium to prevent Microdochium in autumn seems to be pretty non-existent as far as I can see (Please send me anything to the contrary, I’d be very interested to see it). The evidence it can increase disease is still limited at the moment, but what we have seems fairly consistent in US-based studies. As a result of this I virtually eliminated potassium applications to my soil-based greens from September until March and found, in a very unscientific way, no difference at all. I still got the odd bit of disease, but certainly found no ill-effects from eliminating the high potassium granulars and ‘turf hardeners’.
It’s fair to say we need more independent UK-based studies to learn more of the effect of high K applications in autumn, but UK-based independent turfgrass studies are hard to find these days. All the sponsored trials I see don’t investigate the effect of potassium alone, it’s always nitrogen with potassium and often a bit of iron as well, so we don’t know the true effect of the high K levels.
You may feel that on your site there is a benefit to using high K products at the back end of the year. That’s fine, I’m not looking for an argument! What I would say is if you’re not 100% sure then why not have a little trial yourself. If you apply high K liquids at this time of year, why not spray one green without the liquid K and then add it in to your tank and spray the rest. You may or may not see any differences. If you apply a high K granular, for example something like a 4-0-30, then why not order 1 bag of something like a 4-0-2 and treat a putting green with that and see what you find. Ideally choose the same N source, as it’s highly likely that the N source is key to plant response.
Your site may not respond in the same way these trials did, but when you look at the research available, I would say that the ‘convention’ of applying high K in autumn is flawed and should be employed only when you have evidence from soil tests that K levels are low.
This brings up the question of how much K do we need in the soil? It seems likely that the recommendations for the requirements of potassium and some other elements in the soil have been much too high in the past. Recommended nutrient levels were based on agricultural models and did not reflect the specific needs of turfgrasses. This is why MLSN (Minimum Level of Sustainable Nutrition) was developed. A good basic guide to what MLSN is can be read here
These guidelines suggest that older guidelines such as SLAN (Sufficiency Level of Available Nutrients) recommendations of 117ppm of K were way over the top as MLSN now suggests keeping soil above 37ppm. There are also different methods of nutrient extraction for soil tests, but the basic message is: you do not need as much potassium in your soil as we previously thought!
I’m not totally against the use of potassium in turfgrass management. This is an interesting article on the benefits of potassium fertilisation through summer to reduce anthracnose and has links to some further research. Keeping potassium levels in the leaf higher through this period seems to provide a significant benefit in anthracnose management. Regular applications of a liquid fertiliser with elevated K levels seems a sensible approach in summer months based on these findings.
My approach now is to apply the highest levels of K through the summer to help reduce anthracnose severity and drought stress and then back off going into winter. Overall an N:K ratio of somewhere between 100:75 or 100:50 is a pretty good starting point until you have soil tests to confirm your potassium levels.
We still have much to learn about the use of potassium and its role in Microdochium incidence. It may well be possible that it can be beneficial in autumn, but it’s going to take some interesting new research to be convincing. One of the UK’s leading Turfgrass research scientists told me that they have never seen potassium deficiency on turf and I never have in over 25 years in this industry, so it’s fair to say that is unlikely to be an issue if you go down the road of reducing K inputs.
Micah Woods summarises it very nicely here . It’s all about getting the ‘Goldilocks’ level of applications. Like pretty much anything you apply, it’s not too little, not too much, but just right.
Geoff Fenn BSc (Hons) UK Technical Manager