Rick Cruse directs the Iowa Water Center, is Professor of Agronomy at Iowa State University, and is part of the Iowa Daily Erosion Project.
Cruse has made it his mission to share some startling information about soil erosion in Iowa.
His message is two-fold: 1) We’re losing much more soil than we think because not all types of erosion are part of the models that estimate soil loss. 2) If you want to keep soil in place, you’re not going to do it by growing only corn and soybeans, no matter how many conservation practices you use.
Cruse was asked to address a question posed by The Leopold Center for Sustainable Agriculture. What has been the impact -- in terms of dollars of lost yield – of soil erosion in Iowa?
To answer this question, Cruse set up a research project that included seven farms, each with a six-year cropping history of corn and soybean rotation and similar management practices.
Next, Cruse looked at soil erosion loss estimates from the National Resources Inventory, a survey conducted every 5-7 years since 1977 by the USDA’s Natural Resources Conservation Service. The calculations use a conservative estimate of 5.5 tons per year per acre for cropland in Iowa, resulting in a total average loss of 6.8 inches of topsoil in Iowa since 1850.
So what’s the impact of that loss on actual yields? Cruse went back to the data collected from those seven Iowa farms, calculating yield based on a topsoil thinning of 6.8 inches since about 1850. Results varied from no change in selected fields up to 29 bushels lost yield per acre for corn. He says that a modest estimate would be an average of 10 bushels per acre yield loss.
“If you look at those figures and the amount of corn acres in Iowa, you quickly surpass a billion dollars of annual lost revenue,” Cruse says. “And this is a very conservative estimate.” He estimates that about 30 percent of topsoil is lost in ephemeral gullies, those channels created in fields after heavy rainfall, especially before and shortly after annual crops are planted in spring. Soil loss equation models used by the Iowa Daily Erosion Project and the Natural Resources Inventory do not include loss from ephemeral gullies.
The surprise for most audiences, he adds, is the very slow process required to develop new soils.
“The best science indicates that we can redevelop soil at a rate of only a half-ton per acre per year,” Cruse explains. “You’re converting raw parent materials into biologically active materials. Erosion also can remove minerals that are the product of centuries of weathering and other processes.”
Cruse points out that most agencies use 5 tons per acre per year – 10 times the replacement rate – as an acceptable level of soil loss. Adding organic matter helps, but it does not match the productive capacity of the original, uneroded soil.
He points to cover crops, no-till, terraces, grassed waterways and any number of practices that contribute to soil conservation in row-cropping enterprises.
He emphasizes that some landscapes are sustainable only under an agronomic system that includes perennial crops – trees, prairie and pasture.
This article was abridged. The entire article can be seen at http://www.kmaland.com/ag/cruse. Leopold Center For Sustainable Agriculture, Oct 1, 2015.
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