The rich, black topsoil is 3 ft. deep on Karl Retzlaff’s farm in western Minnesota — but only on the lowlands. The buff-colored hilltops have been rubbed almost bare. A century of cultivation has pushed 18 in. of fertile topsoil down the hillsides, depleting the upper slopes and exposing the clay subsoil.

For the past four years, Retzlaff has tested a simple approach for restoring productivity to these eroded knobs: he moved the good earth back where it came from.

Using his own scraper, Retzlaff removed 6 in. of accumulated topsoil from the basins and added it to the hills above. Corn yields on the rehabilitated ground have jumped as much as 70%.

This method for reversing erosion, called soil-landscape rehabilitation, is being tested in Minnesota and other northern regions by the USDA Agricultural Research Service and South Dakota State University. It’s the latest precision-agriculture technique for tailoring management to soil variations within fields.

All tillage sends soil downslope, says soil scientist Sharon Papiernik, research leader of the North Central Agricultural Research Laboratory in Brookings, S.D. “Reducing the number and intensity of tillage operations decreases tillage erosion. However, some recent research suggests that secondary tillage and seeding can result in surprisingly high soil movement.”

Current and past erosion takes a high toll on crop production

“People think if you just put on enough fertilizer, you can compensate for erosion and loss of topsoil,” says Thomas Schumacher, professor of soil biophysics at South Dakota State University. “But that’s not true. Soil is more than a nutrient warehouse.” It also holds moisture, oxygen, organic matter and beneficial microbes essential for plant growth.

Research across the north-central states in the 1980s and 1990s quantified yield losses from erosion, Schumacher says. “On average, we saw 15-20% yield loss, and as high as 30-40%, depending on the soil.” This is “long-term loss that you can’t compensate for.”

The USDA estimates that erosion costs Midwest growers more than $100/sq. mile annual yield losses, Papiernik says. Finding an affordable way to restore that lost productivity would be a great benefit to farmers, she says.

About a third of Karl Retzlaff’s acreage near Cyrus, MN, is highly erodible, characterized by long ridges that slope up to 25%. For years, he raised strips of alfalfa along the contours to curb soil movement from chisel plowing. “Then I got too old to throw bales!”

Retzlaff’s farm is well tiled, so a few years ago, he eliminated tillage for soybeans; his corn ground gets a fall application of anhydrous ammonia and a light field cultivation ahead of the planter. “With my erodible land, no-till has been a real boon.”

These measures have nearly halted erosion, he says, but sharp yield differences still persist from the top to the bottom of slopes as a result of past management practices. Papiernik saw “a good opportunity to bump up productivity on his worst-performing land” by replacing the topsoil.

She set up replicated plot trials on Retzlaff’s farm and another hilly site near Sisseton, S.D., to compare corn and soybean yields on eroded and rehabilitated land. The trials are being repeated in Manitoba, Canada on wheat ground.

Benefits of rehabbed land are evident

On an overcast morning in mid-August, Retzlaff stands on a ridge overlooking a hillside where strips of rehabilitated land alternate with strips of eroded land. The benefits of soil replacement are clearly visible here. Lush, dark green soybeans grow in the restored strips. In the neighboring eroded areas, the plants are small and pale.

Four years of yield monitoring show that “Karl is getting 25-50% more yield in areas where soil was added,” Papiernik says. Corn yields on rehabilitated slopes rose an average of 36 bu./acre in 2007 and 48 bu./acre in 2009. Soybean yields rose an average of 6.5 bu./acre in 2006 and 8 bu./acre in 2008. “The increase in yield is highest in the most eroded areas, where adding 6 in. of soil produced up to 70% more corn grain than neighboring eroded land,” she says.

Yield increases are mainly “the result of improved infiltration and higher water-holding capacity of soil in the rehabilitated areas.”

At the base of the hills, where soil was removed, yields did suffer, dropping 12-63%, Papiernik says. But that loss is probably an anomaly caused by the experiment design, she says. Removing the soil in strips created artificial depressions where water ponded. If you were to restore topsoil on your own farm, she says, you would remove a thinner, more uniform layer of soil from a larger area.

What does soil restoration cost? About $800/acre, Papiernik estimates, including labor, machinery rent and opportunity cost. Retzlaff’s yield increases would provide a payback in roughly eight years, she says.

But “not all eroded soils will respond in the same way,” Schumacher notes. “Moving soil back is likely to produce a variable response.” GPS-GIS tools, including topographic maps, erosion models, and yield maps, will help growers pinpoint areas that would benefit most from adding topsoil, Schumacher says. The software to combine all that data into a soil-rehabilitation prescription map is not yet available in a convenient package, he says — but it’s coming.

If you have your own earth-moving equipment, the economics look better, Retzlaff says. “I have a scraper and backhoe, so I can do my own dirt work. Fuel costs would run $10 to $20/hour. If you have the time, and your equipment is older, so it doesn’t have to be working on paying jobs all the time, it might make sense. If you have smaller eroded hills like mine — two- to five-acre knolls — you could spread a few inches of soil over small areas.”

Uphill climb

Moving soil from non-cropped waterways to fields boosts yields.

Water drains off Mike Rolstad’s farmland along 25 acres of grass waterways. These buffers do more than curb erosion and runoff. Rolstad is also using them as a kind of “soil bank” to reverse tillage erosion.

Rolstad raises corn, soybeans and wheat on 650 acres of rolling terrain near the Little Minnesota River in Roberts County, S.D. Decades of tillage have carried more than 2 ft. of topsoil to low areas, while the uplands have almost no topsoil. Along old fence lines, there’s as much as 4 ft. of accumulated soil.

In 1997, Rolstad switched to minimum tillage, which has reduced soil movement. About the same time, he started regularly maintaining his grass waterways and also built some new ones. He pushed topsoil in the drainage ways back up the eroded slopes. “I cleaned one waterway three years in a row, because there was so much soil coming off my neighbor’s quarter.”

Restoring soil to cropped areas costs about three times as much as leaving the spoils near the waterway, Rolstad says. But the benefits are worth it, he says, for “improved soil fertility, soil moisture retention and water management.” In one field, for example, “I took a half acre of poor ground and turned it into great ground by moving soil and building a maintained grass waterway.”

Rolstad is working with soil scientists from South Dakota State University and the USDA-ARS

to measure the productivity effects of restoring topsoil. In areas where 6 in. of topsoil was added to eroded slopes, Rolstad’s soybean yields increased an average of 11%, and his wheat yields jumped 17%, says USDA-ARS soil scientist Sharon Papiernik.

In addition to farming, Rolstad also operates a gravel-mining and earth-work business, so he owns his own earth-moving equipment. He takes his Terex scraper to the fields for several days each fall, moving a couple of inches of topsoil at a time from non-cropped to cropped areas.

Owning his own scraper makes soil-landscape rehabilitation economically feasible for Rolstad, says Lorne Aadland of the Roberts County Natural Resources Conservation Service. There are no cost-share programs for soil restoration, he adds, so growers will need “some studies to show the cost payback.”

Thanks to cooperating farmers like Mike Rolstad, that data is coming, Papiernik says. Meanwhile, “If you are cleaning a drainage way or restoring a wetland — if you’re moving soil anyway — instead of just dumping it nearby, move it back to an eroded area.”