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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.”