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Sharp Drop Seen in Soil Erosion Rates
19 August 1999 4:00 pm
A controversial paper published in tomorrow's issue of Science (20 August, p. 1244) calls into question the widespread notion that modern farming practices in the United States are causing massive erosion of precious topsoil. According to the study, erosion rates over the past decades have been only a tiny fraction of their historical peaks.
The conclusions are based on a large survey of the heavily farmed Coon Creek Basin, which drains into the Mississippi River in Wisconsin, carried out by Stanley Trimble of the Institute of the Environment at the University of California, Los Angeles. During the 1930s, the basin was studied intensively by the Soil Conservation Service of the U.S. Department of Agriculture (USDA). Trimble--who also happens to be a Tennessee farmer--tracked down the old monuments and markers and used them, just as USDA did, as benchmarks for measuring how much sediment has accumulated in the basin from erosion of the rolling fields around it. He then resurveyed the soil profiles in dozens of sections across the basin's valley in the 1970s and again in the 1990s to look for changes.
To trace erosion rates further back in time, he dug down to find other markers--old roads, railroad beds, concrete dams, and house foundations--that marked soil levels all the way back to the turn of the century. At greater depths, he found the dark, richly organic soil of the original prairie, a benchmark for the soil level when European farmers arrived in the 1850s.
The measured rates jumped in the late 19th century, skyrocketed in the 1920s and 1930s, and then dropped again as USDA pressed farmers there to stop using the traditional moldboard plow and adopt conservation practices like strip-cropping and leaving plant residue and stubble in the fields year-round to inhibit runoff and erosion. From the 1970s to the 1990s, the sedimentation rates dropped to just 6% of their peak.
Official USDA national averages for the last 2 decades have suggested a slight decline in soil loss, but the decline that Trimble reports is so precipitous that some experts find it hard to believe. Among them is Cornell entomologist David Pimentel, who says he distrusts routine soil-science methods such as locating and dating the original prairie surface. Trimble has "got a good imagination," he says, adding, in reference to the 19th century soil levels: "He wasn't there, and these are guesstimates at best."
Others accept the Coon Creek study but caution against extrapolating the numbers too far. "I think [this] is probably a really good study," says John Reganold, a soil scientist at Washington State University in Pullman. "But the problem is, it's just one area."