Can coastal agriculture withstand rising seas, migrating marshlands, and frequent storms?

Saltwater intrusion on a corn field in Hyde County, NC. Lighter green areas are fallow/abandoned sections of the fields with salt concentrations too high to plant. Photo © Dr. Matthew Ricker / NCSU

By Hannah Richter, Circle of Blue – August 22, 2022

  • Saltwater intrusion threatens coastal agriculture on the Delmarva Peninsula and in the Carolinas. Thousands of acres are already unable to be farmed.
  • Conservation easements can facilitate a transition of cropland to salt marsh, providing numerous ecosystem services and up to 90% of the market value for farms.
  • Despite promising adaptation strategies, sea level rise is projected to drown tens of thousands of acres of farmland within the century.

At age 61, wading through swampy rows of submerged plants and trying not to step on any cottonmouth snakes, Rollen Chalmers farms his family’s legacy in rice. He makes his living on the Turnbridge Plantation in his hometown of Hardeeville, South Carolina, 30 minutes from the Atlantic Ocean. For 16 years he’s grown Carolina Gold Rice, a sweet species brought in the slave trade from West Africa’s “Rice Coast.” Today, Carolina Gold is cultivated by only a handful of small-scale farmers. 

Chalmers, a Gullah Geechee farmer with 30 acres, is among the largest of them. How much longer he’s able to farm remains a question. Saltwater intrusion on Chalmers’ rice paddies is forcing him to look for land elsewhere.

All along the southeastern Atlantic coast, similar stories are playing out for farmers. 

Storm events are getting steadily more intense. Atlantic sea levels are rising three to four times faster than the global ocean average. Land is subsiding at rates greater than 1 millimeter per year. All of it converges around a leading threat to coastal farmers: salt water is ruining their land. 

A salinizing coastal rice paddy in South Carolina. Photo: © Dr. Raghupathy Karthikeyan / Clemson University

Chalmers first began to worry seriously about salt when Hurricane Matthew struck the southeastern U.S. in September 2016. “That kind of devastated most of the fields because we got that storm surge,” he recounted. “It came right over the roads and embankment, and all of these fields were probably under, say, three, four feet of salt water.”

Of his 30 acres, only seven could be cultivated after the hurricane. Now, six years later, he’s managed to get back to 22 acres. It’s still a sizable expanse, but less than he once had. 

“What I’m trying to do now is just get more rice fields inland, because it’s always threatening. You got erosion, real bad erosion, high tide, higher tides than normal,” Chalmers said. He is set to purchase 25 more acres inland next year to keep farming Carolina Gold Rice. 

“I think the farming community is starting to recognize this as a problem, and that’s a good thing,” said Dr. Keryn Gedan, associate professor of biology at George Washington University. 

Salt, a crafty contaminant, infiltrates fields from above, during storm surges, and below, due to sea level rise. It has a partner in crime: ditches, once installed to help drain soggy fields of excess water but now serving as a conduit connecting nearby salty surface waters to those same fields. During storm events, heavy winds coming off the sea push salt water through the ditches in the opposite direction of field drainage, providing an easy pathway from surf to turf. 

Climate change is increasing the frequency and severity of storm events, causing fields to become inundated repeatedly. When the water drains or evaporates, it leaves salts behind in the first few inches of soil, also called the root zone. It can take as few as 20 days or as long as months for a storm surge to drain out, researchers say.

A salt-affected corn plant on a field in Somerset County, MD. Browning, yellowing, and wilting leaves are a sign of high soil salinity. Photo © Manan Sarupria / UD

Plants regulate their salt concentrations much like how our bodies regulate blood pressure. Naturally, plants have higher concentrations of salt in their cells than in the surrounding soil. This draws water into the plant through osmosis, which creates Turgor pressure, the force of water against cell walls enabling the plant to stand upright. Conversely, when the salt concentration is higher in the surrounding soil than in the plant cells, water moves out of the cells into the soil, causing dehydration and wilting. If the sodium concentration is exceptionally high, it is toxic to plants.

Keenly attuned to their environments, plants notice salt at far lower concentrations than the five senses that enable people to detect salt in drinking water. Most plants can only withstand 1 to 2 parts per thousand of salt. Any more than that and they begin to yellow, brown, and wilt. Today, fields are measuring as high as 20 parts per thousand. By comparison, seawater is 35. 

If the salt concentration is high during the germination stage of a plant, the most sensitive time in its life, it may not grow at all. In the fields of the American southeast, that is indeed what’s been occurring: patchy or barren fields, some with white rings of salt, unable to support growth of some of our nation’s most important crops, like soy and corn.

Crop damage radiates inward gradually from the edges of a field, where the salt hits first.

“You can look at a field, and you can see a corn plant that looks like it’s gonna make it, and then two inches away, the corn plants are all dead, and it’s because of like three, four inches of soil,” said Dr. Kate Tully, associate professor of agroecology at the University of Maryland. “I mean, we’re fighting for inches.” 

A struggling corn field in Somerset County, MD. A salty ditch runs through the field. Photo © Dr. Keryn Gedan / GWU

“By the time you can visually see it, it’s too late in some cases,” added Dr. Jarrod Miller, assistant professor and extension specialist in agronomy at the University of Delaware. With tight windows on which to grow, harvest, and sell crops, farmers often can’t afford the margin of error that salinized fields bring.

Not only are southeastern crops contending with salt, they are also experiencing repeated flooding from storm events. The combination of wet and salty is a distressing cocktail for most plants, especially if it occurs during their early growth stages. Chris Miller, manager and plant specialist at the U.S. Department of Agriculture’s Cape May Plant Materials Center, referred to it as a “double whammy.”

“There are some crops that actually tolerate salt, but they don’t tolerate wet, they don’t tolerate flooding. Conversely, there are some crops that tolerate some wet but they don’t tolerate salt. And there are not a lot of plants that will tolerate both,” he said.

Barren fields, visually striking if not disheartening, have enabled researchers to map the extent of saltwater intrusion damage using aerial images of affected regions. In Maryland’s Somerset County, an area facing both high rates of sea level rise and land subsidence, 2 percent of farmland was lost to salt between 2009 and 2017, totaling over 1400 acres. 

“When we first quantified the visible signatures, it seemed small,” said Dr. Pinki Mondal, an assistant professor at the University of Delaware and a leading researcher on this work. “But what we noticed is that it’s changing pretty rapidly. Still small, but it’s on a very steep curve.”

Salt crust on the soil surface (white) broken up by a planter in Somerset County, MD. Photo © Dr. Jarrod Miller / UD CANR

But nature rarely stays barren for long. While these fields may no longer support crops, they are conducive to an important coastal environment that itself is under threat: salt marshes. Salt marsh plants, including switchgrass, coastal panicgrass, and eastern gamagrass can withstand salt concentrations of 20 parts per thousand, according to Chris Miller. Corn, by comparison, has a salt threshold of 0.9, and soybeans’ threshold is 2.7 parts per thousand. 

Some marsh grasses, classified as halophytes (derived from the Greek words for “salt” and “plant”), can incorporate salt into their tissues and release the excess through pores in their leaves, called stomata. Researchers can identify these plants by white salt crusts on their leaves, mirroring the much larger salt crusts seen on barren croplands. Other marsh grasses can change their root structures to tolerate different wet and dry sites.

While hardy and fast-growing by nature, salt marshes are being drowned out by rising sea levels, with the most pessimistic studies estimating a 97 percent decrease in U.S. salt marsh area by 2100. Far from surrendering, though, salt marshes have been migrating inland at rapid rates, replacing 61 percent of the 1400 acres of farmland in the Somerset County study. 

For farmers whose land is no longer productive, government programs known as conservation or wetland easements encourage the transition from cropland into marsh, explained Mike Dryden, a restoration specialist with The Nature Conservancy. Such programs, funded by the USDA’s Natural Resources Conservation Service and different state agencies, pay farmers as much as $4,000 an acre to stop farming their land. In return, the farmer agrees to be a steward of their soon-to-be-marsh, helping to mow, keep invasive species like Phragmites at bay, and inventory wildlife for the remainder of their ownership. Farmers can continue to hunt and make recreational use of the land, even harvesting salt marsh grasses for hay or biomass burning. 

Salt marshes also provide a number of ecosystem services: they serve as a habitat for migratory birds, increase biodiversity, promote carbon sequestration, reduce erosion, serve as a buffer to storm surges, and even accrete land mass to combat land subsidence.

Though a conservation easement appears to benefit the farmer and the environment on paper, it’s not such an easy choice.

“I think that researchers often will take this kind of top down approach with farmers and say, well, farmers should be doing this, this and this,” said Tully. “But when you work with the farmers from the bottom up level, and they’re people, and they’re just trying to feed their families or the land has been in their family for 400 years, it’s a very different perspective.”

The Nature Conservancy is hopeful that just because farmers haven’t yet made the transition from crops to salt marsh doesn’t mean they’re not open to the idea. In a survey the Conservancy conducted of farm landowners, 40 percent weren’t aware conservation easements were an option; when they learned about the program, 60 percent were interested.

South Carolina farmer Rollen Chalmers. Picture: © Rollen Chalmers

For farmers like Rollen Chalmers, saltwater intrusion presents a threat not only to livelihoods, but to history. “My mother told me a lot about it, how her grandmother would farm, work at some of these plantations. Growing this rice, it means a lot to me just to keep it going.”

For those willing to keep up the battle, Chris Miller and his team at the USDA’s Cape May Plant Materials Center are working to find innovative solutions engineered by Mother Nature. “We operate under the auspices that there’s a plant in nature that will help us solve a problem. It’s just a matter of finding it, testing it, and increasing it,” said Miller.

His team mainly tests native conservation crops, which consist of deep-rooted salt marsh grasses that tolerate both flooding and salt. When they find a promising species, they plant it in trial fields. After years of data collection, they can recommend it to nurseries and farmers, even giving out some starter plants to encourage farming. But still, cost remains a big factor. “It’s a matter of working out the economics,” said Chris Miller. “A farmer is not going to grow something if they can’t make a profit.”

One promising market for salt-tolerant conservation crops is actually the same market that farmers along the Delmarva Peninsula sell to now: the poultry industry. While the current corn-soy-wheat crop rotation used for chicken feed may no longer be viable, salt marsh grasses that can be used as bedding – also called poultry litter – may be one path forward.

Another promising path comes from researchers at Clemson University, who are partnering with Rollen Chalmers to test salt-tolerant varieties of rice. The researchers are also testing salt-tolerant soy, mustard greens, broccoli, cucumber, and watermelon, among other popular crops, detailed Dr. Raghupathy Karthikeyan, a professor at Clemson and one of the leading researchers. Sorghum and barley are already more salt-tolerant than corn or soy and have been advised as alternatives for years.

A native saltmarsh grass (eastern gamagrass, Tripsacum dactyloides) adjacent salinized cropland in Somerset County, MD. Photo © Chris Miller / USDA-NRCS

Conservation easements and farming salt-tolerant crops are only two of the three pillars of addressing saltwater intrusion according to Daniel Brinn, water and flood control coordinator for Hyde County, NC, a hotspot with thousands of acres already affected. He calls marshland the “flight” strategy and salt-tolerant crops the “adapt” strategy. 

His favorite is the “fight” strategy. That strategy, he explained, ranges from installing tidal gates, simple check valves that go onto ditches to prevent water from flowing in but not out, to installing large levees, dikes, and pumps. 

In Hyde County, a 17.7 mile dike has protected 11,000 acres and $64 million of property from salt since its completion in 2010. Not once has the dike been breached. The price tag? $13 million.

“It really comes down to a lot of cases,” Brinn said. “Is it financially feasible? Is the benefit worth the cost? Will you be able to recoup that money? And, I don’t want to see any acreage lost to saltwater intrusion, but there are cases where that cost benefit isn’t there.” 

Aside from fleeing, fighting, or adapting, there are some tactics which can be described more aptly as stalling. Many farmers rely on basic soil chemistry to stall: they spread gypsum, a powdered form of calcium sulfate, on their fields. Soil grips the calcium in gypsum tighter than the sodium in salt. Gypsum shoulders sodium aside so rains can flush it away. 

Though inexpensive, Tully labeled gypsum a “Band-Aid approach.”

Another technique to reduce salt concentrations is swamping fields with water. But the practice, known as flushing, wastes freshwater. Research shows it can take 300 to 400 gallons of freshwater to flush just 1 gallon of saltwater out of the soil. Pumping enough freshwater to flush fields also contributes to land subsidence and saltwater intrusion, perpetuating the destructive cycle.

On the west coast, agricultural pumping is a major cause of saltwater intrusion in the first place. By contrast, southeastern coast fields aren’t usually irrigated.

Saltwater intrusion damage to a field in Somerset County, MD, with salt marsh and the Atlantic Ocean in the background. Photo: © Dr. Jarrod Miller / UD CANR

Not to be forgotten is that farms are unequally equipped to deal with the threat of saltwater intrusion. Marginal and small-scale farms are the most at risk due to their limited resources. Gullah Geechee farmers like Rollen Chalmers are also especially at risk.

Explained Karthikeyan, “The marginal farmers are highly affected because most of them, they’re in the piece of land for hundreds of years. And when they’re losing those, they don’t have any other option. Corporate farmers, they can always buy a piece of property elsewhere.”

Chalmers is preparing to make that difficult transition off of his marshy farmland. “It’s just a game. You got to keep working on it until you get it back to where you can start planting those fields again,” he said. 

With an array of possible management tactics, it can be easy to ignore the grim reality that sea level rise and saltwater intrusion pose to the future of coastal agriculture. Farmers may stand their ground for now, but the National Oceanic and Atmospheric Administration’s projections for sea level rise bode poorly for low lying cropland in the Delmarva Peninsula, Hyde County, NC, and Beaufort County, SC. NOAA forecasts a 1-foot rise in relative sea level by 2050 and a 3-foot rise by 2100. Most of these regions are no more than 5 feet above sea level.

“No matter what you do, a lot of these strategies, they’re buying time,” said Dr. Becky Epanchin-Niell, associate professor at the University of Maryland. “These places are going to be underwater.” 

A shallow marshland in Wicomico County, MD that was once a row-cropped field with beans, wheat, and corn. Photo © Mike Dryden / The Nature Conservancy

Part two of three. Read the series here.