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PRAIRIE POTHOLES ARE USUALLY SMALL IN SIZE – but when farmed, these perennially wet spots on the landscape can have outsize implications for the environment and farm profitability.
The Prairie Pothole Region extends from Canada south and east, through parts of Montana, North Dakota, South Dakota, Minnesota and Iowa.
In Iowa, many of these areas are found in the Des Moines Lobe, an area that spans the north-central part of the state, ending around the Polk-Story County line – and the vast majority of them are farmed. (If you’re unsure exactly what a prairie pothole is or looks like, take a look at some of the photos below.)
These areas of crop fields habitually yield poorly and drag field yield averages down – and they are prone to nutrient loss and leaching, raising questions about the benefits of continuing to grow corn and soybeans in them.
On. Aug. 31, about 25 people gathered at the farm of Jerry Peckumn, near Jefferson, to explore these questions and learn what new research from Iowa State University is revealing about these unique remnants of Iowa’s glacial past.
Part I: Research on Pothole Profitability
JERRY FARMS IN THE RACCOON RIVER WATERSHED with his sons, Tom and Ben, raising corn and soybeans, along with some hay and beef cattle, on about 1,900 acres. Part of the land is in native prairie, providing wildlife habitat and a refuge for native plants.
Jerry continues to farm the potholes on his land – but is starting to consider if he should manage these problematic areas on his farm differently.
In partnership with researchers at Iowa State University, Jerry has started to collect data from the in-field potholes on his farm, which he hopes will offer guidance about the best way to manage those areas in the future.
“Prairie potholes need to be evaluated to find the best use of the land,” Jerry said. “Our goal is to make a good return on labor and management while trying to balance conservation practices.
Insights From Team Pothole
The first half of the field day, guests heard from two members of Team Pothole, a group of ISU researchers studying different approaches to managing farmed potholes, as well as the impacts of farming these areas.
Amy Kaleita, an agricultural engineer at ISU, is studying the water quality side of things. She gave some background on the project, including some of the main questions researchers want to answer:
- What’s the hydrology of prairie potholes? (For instance, how long do they stay flooded, and how or when do neighboring potholes connect to each other and to downstream locales?)
- What’s their impact on the crop?
- What’s their impact on profitability?
- How would alternative management practices affect their environmental and economic impacts??
“Potholes are so tiny – from less than an acre to several acres in size,” Amy said. “We’re among the first to be asking these questions.”
In addition to partnering with farmers like Jerry, Team Pothole has research sites at several of Iowa State’s research and demonstration farms.
“The [research farm] sites are all very conventionally managed,” Amy said. “One site is managed using aggressive, deep tillage. Another is using tile lines, and a third has been in the Conservation Reserve Program for 10 years.”
Amy invited guests to share their favorite approaches to managing the potholes in their fields. Responses ranged from adjusting the timing of spring planting to doing nothing and accepting the risk of a wet year.
Jim Sayers, a long-time PFI member who farms near Humboldt, was among those who tries to take a conservative approach. “I wait until later in the spring, or I [plant] in the mud,” he said.
“We till them in the fall,” Jerry said, noting that he’s observed differences in how well this method works on different potholes. “Not all are the same. There are some that, if I till that small area when they dry out in the fall, they seem to disappear. Other potholes, that doesn’t work at all.”
Amy replied that compaction is a possible culprit for the variable success Jerry has had with that method. Because tillage can alleviate compaction, she said Jerry’s observation “doesn’t surprise me at all.”
Key Research Findings
Amy then shared some of the key observations researchers have made so far:
- All the potholes in the project flood from time to time, sometimes for less than a day – but sometimes for as long as a few weeks. “A couple were flooded knee-deep, for two weeks, on a graduate student,” Amy said.
- Corn and beans are unlikely to survive more than 1 cm of flooding for more than two days. “Therefore, there is crop loss some years at all our research sites,” Amy commented.
- Potholes in the study flood even when they have tile drains, and when there are surface inlets. “Extra drainage tile could help somewhat, but it won’t eliminate the problem,” Amy said.
- More water ultimately drains through soils in potholes than in upslope positions. “This was a counterintuitive finding,” Amy said, because of the natural tendency of prairie pothole soils to drain poorly.
- Prairie potholes are hotspots for nitrous oxide production – a major contributor to climate change.
Ag and Climate Change: A Possible Missing Link
Steven Hall, a biogeochemist at ISU, picked up the presentation at this point. His work with Team Pothole focuses on processes that produce greenhouse gases – and he said it turns out, farmed potholes may have been flying under the radar.
“When it comes to the impact of corn and soybeans on climate change, the biggest issue is the production of nitrous oxide,” he said. “We’ve found that [farmed] potholes are hotspots for nitrous oxide (N2O) production.”
Steven said farmed potholes in the study produce 10 times more nitrous oxide than other areas of crop fields.
“When flooded, they provide the perfect conditions for denitrification.” For this process to occur in soils, he explained that three things are required: nitrate, organic matter and anaerobic conditions.
“We’ve found that [farmed] potholes are hotspots for nitrous oxide production . . . . When flooded, they provide the perfect conditions for denitrification.”
— Steven Hall
Saturated prairie potholes provide all three. “Potholes are draining the rest of the landscape, where nitrogen then sits,” Steven said. “The darker soils in potholes? That’s organic matter.”
Microbes in the soil then feed on the glut of nitrogen, producing nitrous oxide – a highly potent greenhouse gas (it is 300 times more effective at absorbing radiation than carbon dioxide).
N02 is also more destructive than CO2: In addition to heating the atmosphere, it breaks down the earth’s protective ozone layer, Steven explained, making people and the planet more vulnerable to the effects of solar radiation.
The cumulative environmental effect of farming millions of acres of prairie potholes could thus be significant, he added.
“An average of 9 percent of [Iowa’s] landscape is in potholes. It depends on the county you’re in, but ranges from 5 to 10 percent,” Steven said. “We now think prairie potholes could account for a missing chunk of the nitrous oxide tally not accounted for elsewhere.”
With most climate models predicting more intense precipitation, Amy said that means prairie potholes are likely to flood more often in the future.
The good news is farmers have a range of options for managing these areas:
- One option is to retire them. “Enroll those acres formally in CRP or stop farming them,” Amy said. “That spares nutrients from being dumped in them, and reduces phosphorus, potassium and nitrogen going into tile lines.”
- Switch crops: A second option is to plant more resilient perennial crops, such as miscanthus – an energy crop which has the added benefit of potentially providing some income from those acres.
- Keep farming them, but reduce water flow into potholes: Another option is to adjust some other farm management practices so less water is delivered to pothole areas in the first place. “Dial back on tillage, or switch to some type of conservation tillage,” Amy said. “The pothole is still farmed, but management practices encourage better water infiltration upland.”
Team Pothole’s research has found this approach decreased the number of days of standing water in farmed potholes by almost half, from around four to five days to just two or three days. The chances of crop yield loss were also reduced from 30 percent in any given year to 15 percent, Amy said.
Questions From Attendees
- Constraints to different management? George Pollack, a PFI member from Guthrie Center, asked what’s preventing farmers from putting prairie potholes into set-asides.
“Some are so small, it’s the hassle factor,” replied Jim Sayers. “It’s easier to just go through it. Having profitability numbers might help.”
Amy said some farmers she’s spoken to have shared that they continue to farm potholes so they can have all their farm acres producing. “The motivation isn’t so much profitability, but getting yield off everything.”
- What about planting prairie? Jerry Peckumn wondered about the value of putting prairie in or near some of those wet spots. “There must be some value to wildlife habitat from an economic standpoint,” he said.
Amy replied that there are indeed wildlife benefits to having water remain in the potholes a little longer. “At around four to five days is when freshwater shrimps might appear,” she said, commenting how they can stay dormant in the soil until conditions are right. “That would draw birds. We also see ducks swimming in them.”
- Pothole behavior in CRP vs. row crops? Stefan Gailans, Practical Farmers’ field crops and research director, asked if there are any differences in how potholes behave in CRP versus row cropped areas.
“That’s next on the list of questions to answer, but we expect [nitrous oxide production] to be less in CRP,” Steven said. “A big issue is you might have killed off the crop [in the pothole], but probably put nitrogen on in full earlier in the season. Microbes will take advantage of all that free nitrogen. If you cut down on available nitrogen, there’s less for microbes to work on.”
- What about trees? Jim Sayers asked about planting trees in prairie potholes. Amy said that’s a question her team would love to answer, but “trees are hard to simulate. We would love to partner with someone in the real world to test this.”
Steven shared a final thought with the group: “Find something that will grow in there, that’s compatible with the rest of your farm – and maybe even produces some income.”
Read More About Potholes
- “Farming Prairie Potholes and Other Wet Areas” — Wallaces Farmer article
- “Stop Farming Potholes to Save Money” — Corn and Soybean Digest article
- “Wetlands: By the Numbers” — Iowa Learning Farms blog post
Part II – Creating Habitat for Native Bees: Prairie and Bare Ground
THE SECOND HALF OF THE FIELD DAY focused on in-field prairie plantings and bee habitat plots. Mary Harris, an assistant professor and pollinator specialist at Iowa State University, discussed Iowa’s native bees and the situation facing these vital pollinators as their habitat has disappeared to development and agriculture.
Mary, who works within ISU’s departments of Entomology and Natural Resource Ecology and Management, explained that Iowa has more than 300 native bee species, and the vast majority are solitary bees, where the female builds her own nest.
“Bees need flowering resources — but they also need undisturbed soil to nest, ground that is somewhat bare, as it might have been in prairie ecosystems due to badger diggings, for instance.”
— Mary Harris
Solitary bees don’t create communal hives, like honeybees, and they don’t get nearly the kind of media attention their European cousins do — but these and other native pollinators are hardworking underdogs of the pollinator world, helping pollinate everything from fruits and vegetables to a plethora of native plants.
“Bees need nectar and pollen — and they need access to pollen to feed their young,” Mary said. “But 75 to 90 percent of the landscape is planted to corn and beans. Corn is wind-pollinated, and soybeans — most bees do not prefer that pollen. Soybeans only produce a little pollen in the mornings and don’t need bees to pollinate. We need areas with a diversity of flowering resources.”
Mary has been conducting research with ISU’s STRIPs project, which looks at various effects and benefits of integrating prairie strips into row crop fields. Her research with the project has been focused on the impact these in-field prairie plantings have on native pollinator habitat and health.
Bare Ground = Vital Native Bee Habitat
Jerry is one of 10 farmers throughout Iowa cooperating with STRIPs (which stands for Science-based Trials of Row-crops Integrated with Prairie Strips). In addition to having prairie plantings at various spots on his farm, he is participating in a different sort of bee habitat experiment: leaving some ground intentionally bare.
“It’s counter-intuitive,” Mary said. “Bees need flowering resources — but they also need undisturbed soil to nest, ground that is somewhat bare, as it might have been in prairie ecosystems due to badger diggings, for instance.”
This step might still be too radical for many farmers, Mary admitted. Prairie strips offer a range of other benefits, from erosion control and habitat to significant water quality improvements — not to mention the possibility of cutting prairie strips for hay (one farmer Mary mentioned uses his cut prairie strips for animal bedding).
But intentionally removing plants and leaving the ground bare? Mary said it will probably take time to educate farmers on the vital role these strategically situated bald spots play in the lives of Iowa’s native bees.
Diversity Abounds on Jerry’s Farm
In Jerry’s prairie strip areas, Mary’s research team identified more than 48 species of bees — an impressive tally, Mary said, especially because not all pollinator plantings are equal when it comes to diversity of flowering plants.
“You have a very diverse bee community here,” she told Jerry. “And that’s in the middle of the peak [row crop] production season. We’re finding that if you have five flower types blooming at any one time, up to 10 types, you can support a pretty high diversity of pollinators,” Mary said. “There’s a pretty high correlation.”
Field day attendees had a chance to see both the prairie plantings and the special bare patch — situated among some prairie plants. The bald spot wasn’t huge, perhaps 10 feet in diameter — but evidence of pollinator activity was clearly visible. Holes pocked the ground — some probably too big for bee nests, but others potential nest entrances.
Guests were also treated to the sight of a native predator wasp (species not identified at the field day) attempting to dig into a bee nest it had discovered, and evidence of wildlife activity was abundant: bird feathers near the bee habitat plot, and as if on cue, a deer bounded through the prairie planting at the foot of a corn field.
The cumulative impression — for this author, at least — was that Jerry must be doing something right: It is possible for production agriculture fields to simultaneously nurture spaces where nature and wildlife can thrive.
Before the group left to head back to Jerry’s barn for lunch, Mary left the group with more philosophical food to ponder: “We may very well lose honeybees. It’s very hard to keep them over the winter in Iowa, and now there are all the stresses affecting them,” she said, referring to the wave of illnesses, die-offs — including from colony collapse disorder — that have been decimating honeybee populations across the U.S. over the past several years.
“Native bees can pollinate as well as honeybees. It’s important that we protect them.”
Some Extra Photos From the Field Day