It's early autumn in Harlan, Indiana, and beneath the bright blue sky the flat geometry of corn and soybean fields spreads into the distance, interrupted only by the cloud of dust that trails a combine harvester through rows of tawny bean plants. The corn harvest is still a couple weeks out, and the green stalks tower over brothers Jon and Adam Roemke as they inspect an experimental plot just off the state road. In between rows of corn grow radishes, clover, rye and grasses—a motley mix that looks like weeds. After the corn and radishes are harvested, the rest of the cover crops will remain over the winter. Their purpose is to grow, wilt and then contribute organic material to a field that would otherwise have lain fallow.
The family began experimenting with cover crops after a seed salesman showed them soil samples: one from a farm that had hosted corn and soy in continuous rotation, and another from a farm that used cover crops between the seasons.
“It was obvious which was better,” says Jon. “The cover-cropped soil was loamier, it took in more water, it took longer to break down.” Dark, moist and teeming with beneficial microorganisms, it was exactly what a farmer wants soil to be.
Agriculture is a business with tight margins, and it takes a leap of faith to plant crops that will never be harvested. But the Roemkes are part of a growing movement of farmers who, with support from government agencies, conservation groups and even some industry leaders, are working to improve their soil. These practices may prove vital for the future of individual farms—but they are even more important as a tool to mitigate agriculture’s global impact on water, natural habitats and the climate. And it all hinges on a largely invisible biological powerhouse: soil microbes.
“Agriculture often treats soil like a dead substrate: Dump in some chemicals and grow the plants,” says Deborah Bossio, who joined The Nature Conservancy as lead soil scientist in 2016, after working on soil health in the developing world for 20 years.
In fact, healthy soil teems with life. Microscopic fungi, bacteria and protozoa constantly consume dead plant and animal material in the soil, collectively known as soil organic matter. Microbes recycle nutrients, breaking them into forms that plants can absorb. Microbes also extract inorganic minerals from rocks and soil, making these available to plants as well. Plants themselves encourage this help by releasing additional proteins and sugars that feed microbes at their roots. All this activity attracts predators and bigger recyclers, such as nematodes and earthworms and insects, which further enrich and aerate the soil. The sheer magnitude of this system tests the imagination: There are more microbes in a single teaspoon of healthy soil than there are people on Earth.
Healthy soils also provide a host of benefits to the surrounding environment. They soak up precipitation like a sponge and they filter water gradually (unlike soils with less organic matter, which tend to form surface crusts that shed water and send pollution into streams, rivers and lakes). Soil can even help mitigate climate change: Microbes break carbon molecules into a form that latches onto soil particles for decades or even centuries.
Conventional agriculture—especially the excessive use of chemical fertilizers—tends to undermine these beneficial systems, Bossio says. “It erodes soil’s ability to grow our crops,filter and hold water, and even store carbon.” Adding chemical fertilizers gives plants a few important nutrients, but not the diverse diet that microbes deliver. Failing to replenish organic material starves microbes below the surface. “The cycle only goes in one direction,” Bossio says. Microbes eat up what little they can find underground, their populations dwindle and soil fertility steadily declines. Meanwhile, the conventional practice of leaving croplands fallow for parts of the year makes soil vulnerable to erosion by wind and rain.
In fact, healthy soils are becoming increasingly rare, says TNC’s Michael Doane, who directs the organization’s Working Lands program. The organic matter in soil has fallen by between 30 and 50 percent across croplands worldwide, largely because of farming practices that degrade soils. That’s a conservative estimate, Doane says. In the American Midwest, “a lot of soils are at less than 2 percent organic matter, which is shocking,” he says. Organic matter in healthy U.S. soils usually reaches 5 percent or even higher. “Depleted soils have very little life and they function poorly.”
Poor soil health may be a looming crisis, but the news is not all bad. Farmers can restore organic matter without abandoning agriculture—in fact, it’s possible to maintain and even increase crop yields while healing the soil.
That’s especially important in Africa, Latin America and tropical Asia, where soil naturally tends to contain less organic matter. “We are dealing with soils that have very low nutrients, especially nitrogen and phosphorus,” says Job Kihara, an agronomist in Kenya with the International Center for Tropical Agriculture, which partners with TNC. In East Africa, poor management practices have damaged soil across the region. One way to reverse the damage is intercropping: planting corn and bush beans, the region’s two most popular crops, side by side in the same field, sometimes with peanuts or pigeon peas as well. “In an intercropping system, because of the different organic materials available, you tend to have greater diversity of soil microorganisms,” Kihara explains, which can rebuild fertility.
Here, however, planting seeds that won’t be harvested makes little sense to farmers who may worry about having enough food for their families. Even leaving dry cornstalks and bean plants to decompose is a hard sell, because often they are needed for fuel or animal fodder. But Kihara is convincing farmers to plant legumes among their main cash crops a few weeks before harvest. These remain in the field as cover crops during the dry season, and farmers can harvest the green beans as supplemental nutrition while they’re waiting to grow their main crops again.
“Farmers are very, very enthusiastic,” Kihara says, “but we need to ensure that they have support. Working together we can achieve much more.”
Applying soil health practices at a meaningful scale will have to involve partners from all sectors that touch agriculture. In the United States, TNC aims to make soil health the leading indicator by which most farms measure their own economic and environmental success. Companies like General Mills are getting behind such efforts because they want to maintain a responsible and reliable supply chain, says TNC’s Doane. “Many people are starting to recognize the impact of current methods of farming—and what we’re losing.”
One of the most promising avenues for change, Doane says, is the Soil Health Partnership. This joint effort of the National Corn Growers’ Association, industry and nonprofits, including TNC, uses a network of demonstration fields on farms across the American Midwest to gather long-term data on how reduced tillage, cover crops and diversified rotations can improve profitability and environmental performance. In the United States, Doane says,“there’s no good alternative for having a local farmer, that other farmers respect, who starts to manage for soil health, to show what’s working and not working, to collect data, share data, open up their farm for discussion.”
Benoit and Sarah Delbecq run one such demonstration plot outside Auburn, Indiana.
“This is a 15-species cocktail mix,” Sarah says, standing at the edge of a field that looks more like a meadow than cropland. “You’ve got sorghum, you’ve got buckwheat, sunflowers, several varieties of clover …”
“Here’s a turnip,” adds her husband, yanking up a purpletinted root and taking a crunchy bite out of it.
The cover crops will grow through the fall and die in the winter, adding nutrients to the soil that will feed the corn the couple will plant in the spring.
“There’s more and more evidence that going beyond the two-year, corn-soybean rotation creates more opportunities to enhance soil health,” says Sarah. “We believe in this, and we’re fairly confident that we can get it right.”