To say that my thoughts on turfgrass management have changed over the years may be an understatement. “Radical change” may even be the more appropriate phrase for a couple of ideas. My management ideas have not changed for every single turfgrass topic, but on several. This does not mean that what I learned during my undergraduate studies in the early 2000’s was ‘wrong’, but rather how much useful information turfgrass scientists have discovered since then or even how much useful information was waiting to be rediscovered from science of the past. Best management practices (BMP’s) generally have a shelf life and many of the turfgrass management BMP’s I learned in undergrad have either expired or have been replaced with an improved BMP through the process of scientific inquiry. Science is ever-changing and as professionals, we too should change our thinking when the supporting evidence is sufficient.
The purpose of the ensuing essays are to highlight how, over the last 25 years, my ideas have changed on a plethora of turfgrass management topics. Some of these ideas may not be mainstream or widely adopted, but I feel that they should be given much more attention and consideration both by teaching academics and practitioners. The reader should understand that for the lack of time, I will not cite every single source for every single idea presented here. I will insert citations where I see fit, but when evidence is overwhelming for a particular topic, it is a waste of my time to line up citation after citation. After all, this is my blog and I will do whatever I please. Nevertheless, if you email or message me directly I will gladly send you any citations or textbooks I have used over the years (and in these writings) to support my stance. I do not possess a single viewpoint that relies 100% on anecdotal evidence. Every “take” I have or will write about has merit from the scientific literature. If you have a counterargument or refutation, I encourage you to reach out. I am willing to change my mind and I will literally believe anything as long as there is rational supporting evidence.
Soil pH
Week 1 of any soil nutrient management course we are shown the pH nutrient availability diagram (Fig. 1). The diagram is presented in such a way that it almost feels like in order to be a successful turfgrass manager one should always strive to maintain a near-neutral pH. It feels a bit like fear-mongering where the probability of a nutrient deficiency is likely if your soil pH is too far outside of the recommended range. While that may be true for blueberries, or corn, or soybeans, it doesn’t seem to be so for turfgrasses. I have very little faith in the usefulness of this diagram for turfgrass systems for several reasons. First, I have seen countless swards of turf performing well in what is traditionally considered a “sub-optimal” pH range on both the lower and upper end of the pH scale. It just goes to show that turfgrasses can perform well in a wide range of pH. Turfgrasses have evolved ingenious physiological mechanisms (root exudates) to solubilize and acquire nutrients. Even in acidic or alkaline soils, oftentimes, if there is enough nutrients present the turfgrasses will find a way to access those pools by manipulating the pH in the immediate area surrounding the roots (rhizosphere) with exudates. This pH change can solubilize the soil nutrients for plant uptake. Can we get too extreme of a pH where microbial activity is severely inhibited, nutrients are truly unavailable (insoluble), or toxicities occur? Of course! But, what I am saying is that pH should not be a number that is constantly chased unless TQ is not up to standard and every other soil analysis parameter (i.g. sufficient nutrient concentrations, salinity or sodicity issues etc.) has been ruled out.
I need to highlight a couple caveats from the above paragraph to account for the differences in the root zones of sands (i.g. USGA or “California” putting greens) and native soils.
At some point in the semester of soil nutrient management 101 we see a diagram similar to Figure 2. This diagram shows the relationship between concentrations of soluble aluminum (Al) and soil pH. The general rule of thumb is that when the soil pH is above 5.5, Al toxicity is essentially rendered null and the probability of toxicity decreases with every incremental increase in pH. Soil organic matter (SOM) can play a role in Al toxicity too. As SOM increases, a lower pH will be required to increase exchangeable Al (Al on cation exchange sites). So to some extent, increased SOM provides a buffer to Al toxicity as pH becomes more acidic. Ok, we got all of the difficult ideas out of the way, how does this practically apply to turfgrass management in sand and native soil root zones? I will attempt to bring all of these ideas together in the following paragraphs.
Sand root zones vs. native soils
The main source of Al in soil is found in clay particles. What is often lacking in a modified sand root zone? If you shouted “clay particles!”, you would be correct. There is very little clay in sand root zones, such as a USGA spec putting green. If clay is the source of Al and sand root zones lack clay, a lower soil pH will be required to induce Al toxicity in sand. These factors allow for turfgrass managers who grow grass in sand root zones to maintain a lower soil pH when compared to a native soil that has significant clay content. The pH of a native soil should obviously be monitored a little more closely as one approaches 5.5 or less, but a sand root zone may require a further decrease in pH before Al toxicity hazard is worrisome.
Tangential benefits of maintaining an acidic soil pH
From golf course superintendents to lawn care operators to sports field managers, annual bluegrass (Poa annua) can be a pain in the ass to turfgrass managers everywhere. Although, for many greenkeepers it is highly coveted and is the primary grass grown on putting green surfaces. It has been demonstrated in a multitude of research projects, both historical and contemporary, that as soil pH becomes more and more acidic, annual bluegrass struggles to persist. Within the same body of work, creeping bentgrass (Agrostis stolonifera) has shown the ability to have a higher tolerance to Al than annual bluegrass and will persist in a soil pH where annual bluegrass has died. In sum, if you want to keep your surface free of annual bluegrass with cultural practices, you should slowly drive the pH as low as possible. On the other hand, if annual bluegrass is desirable and you are actively selecting for its survival and persistence, you should be looking to maintain a more alkaline (>5.5) pH.
A similar story can be told for native soils. The Park Grass experiment in Rothamsted, U.K. is the longest running native soil fertilizer trial in history. Micah Woods, Frank Rossi, and others have written several pieces about this project and I encourage you to search out those articles. Park Grass plots have been receiving various ratios and rates of fertilizer materials since before the American Civil War!! These trials tell an amazing story of how certain combinations of fertilizers and soil pH can influence species composition over time. (Click here for an interactive experience with photos of how fertilizers and lime impact species composition). What seems to be the most useful piece of information for turfgrass management is the results from the N only (ammonium sulfate) and lime treatments. In short, as soil pH has decreased over the years from only applying ammonium sulfate, grasses are the only plants that can be found in those plots. Broadleaf plants are absent and Festuca and Agrostis are the two dominant species in the plots. Annual bluegrass isn’t to be found. Wherever lime has been added the pendulum shifts to favor broadleaf plants and non-desirable grasses. The take home message, in my opinion, is this…If you desire a stand of only grasses, lowering soil pH over time is a way to actively select for fescues or bents and more or less eliminate annual bluegrass and broadleaf competition.
How would I use this information if I was still a practitioner?
- I would never worry about pH unless I had a problem or a decline in TQ
- I would try to keep the pH as low as possible for grasses grown in sand or native soil
- I would never apply lime unless Al toxicity was diagnosed and even then I would only apply enough to raise the pH just enough so that the toxicity is eliminated.
Be on the lookout for the next topic I will discuss….phosphorus!
Figure 1.
Figure 2. (Spectrumanalytic.com)
