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Soil Evolution par for the [golf] Course

2017; Wiley; Volume: 62; Issue: 6 Linguagem: Inglês

10.2134/csa2017.62.0616

2325-3584

Madeline Fisher,

Turfgrass Adaptation and Management

In 2008, Glen Obear was interning at a golf course in Hawaii when the superintendent asked him to help diagnose a mysterious problem. Some of the course's putting greens were developing bald patches, spots where the turfgrasses were dying and thinning out. The failures were troubling because the expensive, exquisitely crafted greens were just five years old. A new green is normally expected to last at least five times as long. The superintendent suspected the issue lay not with the turfgrass itself but with the constructed soils underneath, known in the industry as “modified root zones.” Carefully engineered to resist compaction and promote drainage—while also retaining enough water for plant life—these soils are usually composed of 30 cm of sand over a layer of gravel. But when Obear and the superintendent dug into one, they found something curious: a red layer of cemented material about a foot down that appeared to be impeding drainage. No one had seen anything quite like it before, so Obear—then a University of Wisconsin–Madison (UW-Madison) undergraduate—took a chunk home with him to Wisconsin. His colleagues at first were underwhelmed. “To be honest, I was not interested in this when Glen started,” says Doug Soldat, a UW-Madison extension specialist in turfgrass management and urban soils who was Obear's master's degree adviser at the time. “But I let him do it, and I'm really glad I did.” The pair eventually found similar layers beneath putting greens in nearly 30 U.S. golf courses, including one in Madison. Obear went on to investigate why the layers develop and is now spearheading research at University of Nebraska–Lincoln (where he's currently a Ph.D. student) that should one day help golf course managers prevent the strange layers from forming. Along the way, the team discovered something else: The layers weren't so strange after all, but merely evidence of what all soils do—age and evolve. “The big difference is that in [turf] soils, it happens quickly because you irrigate them, and you apply lots of iron and fertilizer,” says UW-Madison pedologist Alfred Hartemink, who chairs the UW-Madison Department of Soil Science. “But there is something happening that we can explain. It's soil formation.” A putting green may seem delicate, but it's actually one tough surface. Although greens make up just 2% or so of a golf course's area, they are the spots where all play converges. Plus, the grasses—mowed to heights of less than a centimeter—are under tremendous stress. “So, the soil basically needs to be perfect,” Soldat says. The U.S. Golf Association (USGA) recognized this way back in the 1950s and has been designing, researching, and tweaking its “sand-based root zones” to meet exacting standards ever since. To ensure these soils continue providing ample air space for plant roots as golfers walk around on top, their primary component is sand—preferably coarse to medium in size and angular in shape. “You really can't over-compact a sand to the point where the grass will decline,” Soldat explains. Sand also keeps the surface dry for golfing by facilitating rapid infiltration of water. However, it's also not good for grasses if the green dries out too quickly. This is where the gravel layer underneath comes in. The presence of fine-textured sand atop a coarse gravel creates a “textural discontinuity” that boosts the water-holding capacity of the sand. The design “is beautiful in this regard,” Soldat says, and many people outside the golf industry today agree. The USGA's basic putting green design is being adapted by athletic fields and heavily trafficked green spaces around the country, including those of the Green Bay Packers, the Milwaukee Brewers, and the National Mall in Washington, DC. Tough as they are, though, modified root zones aren't immune to the passage of time—something that golf course managers have realized from the start. Still, this issue of “root zone aging” wasn't formally studied until the 1990s. It began when Bob Carrow, a University of Georgia turfgrass scientist, was asked to investigate a seasonal thinning of turfgrasses in the Southeast, known then as summer bentgrass decline. Most people attributed the decline to disease. But Carrow disagreed. “Bob was really innovative,” says University of Nebraska–Lincoln turf science professor, Roch Gaussoin. “He said, ‘No, this is due to excessive organic matter accumulation at the surface of the green as it matures, and the aggressive way we manage bentgrass for golf greens.’” As Carrow went on to describe, the heavily irrigated and fertilized bentgrasses were growing roots at such a rate that microbial decomposition couldn't keep pace and organic matter (OM) was building up rapidly in the soil. Once the thatch reached a critical level, infiltration slowed and oxygen concentrations dropped. These changes, in turn, deprived the grasses of oxygen, fostered anaerobic microbial activity, and caused other issues. The problems that Carrow observed were somewhat extreme, Gaussoin says, because Carrow was studying a cool-season grass in the humid Southeast. So, shortly afterward, Gaussoin and several University of Nebraska colleagues launched their own USGA-funded study: a 10-year experiment to understand how soil physical properties change as golf greens age. To gain even broader insight, they followed up in 2006 to 2008 with a survey of 300 putting greens on more than 100 golf courses in 15 states. What they found is that regardless of location, annual rainfall, construction methods, and other factors, OM levels do increase in sand-based putting green soils as they get older. In the survey, the average OM concentration in the soils was 3%, with levels reaching 8% or more in some cases. The Nebraska experiment further showed that air-filled pore space was reduced by 40% over time, and infiltration rates decreased by 70 to 75%. But another key finding was that OM can be kept at manageable levels through routine golf course practices—especially “topdressing,” where sand is brushed weekly or biweekly onto greens. Topdressing was nothing new at the time; it had been used for years to keep thatch at bay, Gaussoin says, along with cultivation. What the research did, however, was validate the wisdom and efficacy of this general recommendation. A profile showing the organic matter accumulation on the surface in the topdressing layer. Source: Roch Gaussoin. “The pioneers of greens management—the generation before me—always said, ‘You need to dilute that organic matter at the surface [with sand] or you're going to have problems,’” Gaussoin says. “And what we found with multiple years of data and multiple studies is that topdressing is the most important component in managing organic matter in golf course greens.” The effect of age on water infiltration for two USGA-recommended putting green root zones at the University of Nebraska. Root zone effect was not significantly different after eight years. Source: USGA. It's just one example of how the turf industry's 50 years of experience with constructed root zones helps today's managers tackle specific problems. But there is also a larger lesson here for people just getting started in soil engineering. “What happens as the soil ages is the big take-home message,” says Bill Kreuser, a University of Nebraska–Lincoln turfgrass extension specialist and Obear's Ph.D. adviser. “I think we get so caught up in the specs for construction that we forget—or underappreciate—just how dynamic the system is.” Iron- and manganese-cemented layer at the interface of sand and gravel (30-cm depth) on a golf course in Wisconsin. Source: Glen Obear. The situation was quite different when Soldat and Obear began studying the unusual chunk of cemented material from the Hawaiian course. There was no vast turf science literature or management expertise to tap into because no one had described such a thing in a putting green before. But it turned out the material was known, and it took UW-Madison pedologists Hartemink and Jim Bockheim just a short time to identify it. “The layer looked like rust, and that's what it was,” says Obear, who invited the UW soil scientists to inspect it. “A red, crusty, impermeable pan layer of iron oxide.” Iron layers often develop in iron-rich Spodosols, typically in places where a waterlogged, oxygen-depleted soil layer sits above a drier, more oxygenated one. The difference in redox potential across the boundary causes soluble, reduced iron in water to precipitate out as iron oxide—cementing clays and soil organic matter together in the process. Over decades or centuries, the accumulating iron forms an impermeable pan, called a placic horizon in soil taxonomy. Bockheim, in fact, had published a paper on placic horizons. Why similar layers were developing beneath putting greens in a fraction of the time remained unclear, but the literature on natural soils offered an excellent place to start. “I hadn't worked in these soils before,” Hartemink says. “But you take what you know about the processes in other soils and apply them to the constructed soil. By doing that, we found an effective way to explain the formation of these iron layers in the turfgrass soils.” The model that Obear, Soldat, and Hartemink ended up proposing focuses on the interface of sand and gravel in constructed root zones. Their hypothesis is that this textural discontinuity—so useful for holding moisture in the sand for turfgrasses—inadvertently sets up the conditions for iron layer formation: a saturated, sand layer sitting above a drier, more oxygenated gravel. When reduced iron reaches this boundary, it precipitates as iron oxide, just as in Spodosols. The team also found iron layers beneath the topsoil in some greens although these layers were less strongly cemented than those at the sand-gravel interface. Light and frequent sand topdressing creates smooth, firm putting surfaces. Source: USGA. Obear is quick to point out, however, that while iron layers have now been identified in turfgrass soils in more than 30 sites, they don't occur everywhere. “Right now, we're still trying to answer the basic question of why they form in some soils and not others,” he says. One factor may be the iron fertilizer that managers often add to greens. Applied to make turfgrasses greener without stimulating their growth, the extra iron may increase the risk of a cemented pan forming below. Experiments by Obear and Kreuser at University of Nebraska also suggest pH is important. “In a natural soil, the pH is defined by thousands of years of rainfall and the mineralogy,” Obear says. “But in an engineered soil, you might bring in sand from Florida and limestone [gravel] from the Southwest. So, you can have some really odd combinations of pH.” When the difference is large—for example, when an acidic sand sits atop an alkaline gravel—a pH boundary develops that also encourages iron oxidation, Kreuser says. “So, it's early, but we're pretty confident at this point that matching the pH of the sand and gravel will help.” That said, he, Obear, and Soldat are reluctant to give formal recommendations to golf course managers until they fully understand the contributing factors—especially when the advice may add expense. “One of the issues we're running into is, how do we keep the cost down?” Soldat says. “Because if we start specifying the pH of the sand and the gravel, then we also have to start going further away to get those source materials.” Slides, audio, and video from a symposium at last year's ASA, CSSA, and SSSA Annual Meeting titled, “Manufactured, Blended, and Engineered Soils for Urban Applications” are available in the ACSESS Digital Library at https://dl.sciencesocieties.org/publications/meetings/2016am/16011. The scientists will say this, though: Anyone who works with engineered soils today needs to consider not just the physical specifications, such as particle sizes, porosity, and drainage rates, but redox potential, pH, and other chemical properties, as well. Largely overlooked in soil engineering specs to date, “the chemistry, we're discovering, is really important and where we need more guidance,” Kreuser says. Then, as Gaussoin and his colleagues demonstrated a decade ago with soil physical properties, people should be prepared for the ground to shift, quite possibly in unpredictable and confusing ways. “The engineering specs take you to Day 1, and then evolution happens and the soil starts changing,” Obear says. “That doesn't give people a practical action to take, but it's a frameshift in thinking about these soils.” Change—and the problems it often brings—are indeed par for the course; the trick is to persevere and stay curious, Soldat adds. “You're going to see failures all the time in engineered media—rain gardens, rooftop mixes, putting greens, athletic fields,” he says. “The easy thing to do is say, ‘Let's tear it up and rebuild it.’ The harder thing to do is to Glen Obear conducting a pedological investigation of a putting green in Mississippi. These greens were being removed and replaced due to drainage failure from layers that had formed between the interface of sand and gravel. ask,‘Why? Can we figure out why?’ So, when something fails, really pay attention to it. And if you don't understand why it failed, that's an opportunity.”