FARIBAULT, Minn. – When the earthworms started coming back, Tim Little knew he was doing something right.

Little, who farms north of Faribault, saw more worms in his fields, gathering piles of dead plants above their holes, after he made a drastic departure from farming tradition. He planted cover crops in them over winter.

"It was like it put the earthworms on steroids," Little said.

The worms are a visible manifestation of healthier soil. The cover crops — cereal rye and tillage radishes, for example — absorb water, knit the soil together, feed microbes, prevent erosion and help regulate a field's temperature.

They also provide food for the worms, which burrow deep into the soil, adding carbon-rich organic matter to the dirt and punching holes that help fields hold more air and water.

In a year when flooding and erosion smacked farmers in Minnesota and much of the country, a small but growing number of farmers are leaving fields unplowed and planting cover crops to protect their soil in the winter.

"To me it's the only bright spot in ag right now because it's new and we're trying to learn more about it and it's got us serious and excited," said Little, the unofficial leader of a group of late-career converts to cover cropping whose farms straddle Interstate 35.

The problems caused by the far more common practices of tilling fields in the fall and leaving them bare in the winter are highly visible and extend beyond farmland. Farmers were forced to delay planting or keep acres fallow this year when fields couldn't absorb heavy rains. The dead zone in the Gulf of Mexico is the result of decades of nitrate runoff from Midwestern farms. The yellowish snow that fell in the Twin Cities in April was caused by soil erosion swept into the sky in Texas.

Even a small jump in soil conservation offers hope that some of this damage can be reversed.

"If we could get everybody to do this, we could be part of the solution instead of part of the problem," Little said.

Skipping fall tillage and planting cover crops goes against hundreds of years of farming tradition in the United States.

As far back as the 1700s and the influential Englishman Jethro Tull — who argued farmers should till their soil until it was pulverized into powder — fall tillage has been standard practice. Just 20 years ago, a field that wasn't black by winter was considered an eyesore by all but the rarest of farmers.

"You were considered a poor farmer if when you were done plowing you could see any cornstalks," said John Becker, who farms a mile west of Dundas. "It needed to be totally flipped over and black."

That's still the view held by most farmers. Less than 1% of ground in the state showed evidence of cover crops in satellite photos last winter, according to the Conservation Technology Information Center.

"We can't quite get over the hurdle of getting a large proportion of producers interested," said Troy Daniell, the state conservationist for the USDA's Natural Resources Conservation Service (NRCS). "It's a cultural change that has to happen, and some of it is going to be generational."

Most farmers believe a bare-dirt field will absorb more sunlight in the spring, causing it to warm and dry faster for planting.

And even if they get past that obstacle, cover crops are no simple silver bullet. A 2018 study by economists at Iowa State University suggested the financial returns are mixed. Farmers who used a cover crop on a field they then planted with corn lost about $20 per acre on average in the study. For soybeans, they gained about $25 per acre on average.

In Minnesota, the shorter growing season is an obstacle, leaving a small window to plant cover crops. Missing it could mean the plants don't germinate before the first frost, a costly outcome since seed costs $20 to $60 an acre.

"It's all about management," said Larry Conrad, another one of the farmers planting cover crops near Dundas, Minn.

The group of farmers have counterpoints to the arguments against cover crops. They say a crop like cereal rye absorbs moisture and heat in the spring, warming and drying perhaps even faster than a bare-dirt field and giving tractors something to drive on instead of bare mud. And a loss in corn yields will happen only if a farmer waits too long to knock down the cover crop in the spring, they say.

But the farmers acknowledge that cover crops add complexity with little near-term financial benefit beyond fuel and labor savings.

"It's transparently beneficial for society. It's not transparently beneficial for an individual farm enterprise," said Anna Cates, state soil health specialist at the University of Minnesota's Office of Soil Health.

"There's not a return on it in the first year," she said. "There's a return on it as you improve your land for the long-term, but when you think about how many farmers are farming rented land, that's not as motivating of a factor, and it can make your farming more complex."

Despite the obstacles, the number of Minnesota acres that showed cover growth in the winter increased ninefold from 2015 to 2017.

The group of farmers near Dundas all grew up on dairy farms in the 20th century. Their parents raised alfalfa, oats and corn and sent their cows to graze — and fertilize — fields they would plant the next spring.

"Our dads were cover croppers, they just didn't know it," Little said. "But as the cows left, we got into this corn and soybean rotation."

The seven farmers all started planting cover crops regularly in the mid-2010s, making use of county Soil and Water Conservation District grants or NRCS grants that help pay for seed for up to three years.

Now a few are paying out of pocket. They share ideas and equipment — a crimp roller built by Conrad to knock down cereal rye, for instance. They hire a plane together each fall to seed their fields all at once with the same mix of cereal rye and radishes and other plants, to reduce cost. The seed costs about $30 per acre, they said.

The real payoff, and what gets them excited, is to see the way the soil is replenished after just a year or two. The radishes push down deep and rot in the ground, leaving behind air, food and sluiceways for water. The cereal rye weaves the surface together. Microbes feed on the sugars produced by the roots and form the basis of a lively ecosystem.

Last week four of them walked into one of Little's fields with shovels to show off the green growth under the snow and lovingly pull apart dark clods of rich soil threaded with roots. Conrad held up a piece of soil with a clear earthworm hole through it. Little tore a chunk apart, comparing the sound to a rip of Velcro.

"See those roots? That's what starts the soil biology. Everything feeds off that," said Little. "It just starts the cycle of life."

The healthier soil is something akin to what existed on the prairie when the buffalo roamed, managing the grassland in their own way, said Chris Schmidt, a farmer near Garvin, Minn.

Schmidt was manning a booth at a soil and water conference in Bloomington last week where he touted the benefits of healthy soil with an unusual illustration: four pairs of underwear pinned to a bulletin board.

Each pair had been fresh and white when it was buried in a different field. But after six weeks in varied dirt, the differences were striking.

The skivvies buried in farmland treated with conventional means, tilled and left bare the previous winter, was a little dirty but mostly intact, evidence that little life existed in the soil.

A pair buried in ground that hadn't been tilled for two years and had been planted with cover crops was in shreds. It had been eaten by microbes and worms, and torn apart by roots.

"A more active, healthy community," Schmidt said. "That soil is alive."

Adam Belz • 612-673-4405 Twitter: @adambelz