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Farmers Now Part of the Global Warming Solution as U.S. Agriculture Becomes Net Carbon Sink

By Don Comis
May 17, 1999

WASHINGTON, May 17, 1999--Sometime in the past 15 years, American farmers turned an environmental corner, Agricultural Research Service Administrator Floyd Horn announced today.

“A dramatic change in tillage techniques shifted U.S. farm soils from net carbon dioxide producers to net accumulators of carbon--in the form of valuable soil organic matter. This makes their soils more productive and part of the potential global warming solution, rather than part of the problem,” Horn said. “American farmers have virtually abandoned the moldboard plow used to break open the American West.”

Horn said that Raymond R. Allmaras, a soil scientist with the U.S. Department of Agriculture’s Agricultural Research Service in St. Paul, Minn., made these findings after a thorough search of published reports and surveys for several major crops, comparing 1940 to 1990 conditions.

“These reports showed that, in 1980, 75 to 85 percent of American farmers were still using the plow. By 1993, a USDA survey showed that farmers used the moldboard plow on only 6 to 9 percent of corn, soybean, and wheat fields,” Horn said.

Horn said Allmaras used yield records to estimate amounts of crop parts that would be left after harvest. “He also used long-term tillage experiments conducted by himself and ARS colleagues nationwide. Dale E. Wilkins, an ARS agricultural engineer in Pendleton, Oregon, assisted him in the study,” Horn said.

Allmaras said that one of his tillage experiments showed there was no carbon accumulation in soils during a 10-year period when corn and soybeans were planted after annual plowing; another showed that abandoning the moldboard plow produced distinct increases in soil carbon in as little as 10 years.

“The soil is storing more carbon that otherwise might be in the atmosphere as carbon dioxide, which is one of the greenhouse gases thought to cause global warming,” Allmaras said. “The plow lifts and inverts an 8 to 12-inch slice of soil, and also buries stubble and other unharvested crop residue that was once on or near the surface. That places the residue deep in the plow layer where different microbes live. These microbes convert the residue to a form of carbon that readily converts to CO2, which can escape to the atmosphere,” he said.

As farmers put aside the plow, they leave more residue on the soil or within a depth of 4 inches, Allmaras said. “For example, corn and grain sorghum farmers are returning about twice as much residue than in 1940, and they are keeping it on or near the surface. Here, the residue readily decays to valuable organic matter, a more stable carbon compound and a key component of the black, fertile prairie soil originally broken open by the plow. The moldboard plow robbed the soils of the increased organic matter offered by the yield increases since 1940,” Allmaras said.

Allmaras said the dramatic shift away from the moldboard plow has altered the farm landscape in another equally broad way: the reduced tillage and increased organic matter in the soil has led to a looser, less erodible soil that holds more water for crops. “This noticeable shift in the soil is the main reason farmers have abandoned the moldboard plow,” he said.

Horn added that farmers too often are criticized for causing environmental problems. “Here’s a case where they’re definitely part of the solution, and for that, they deserve a pat on the back,” he said.

Scientific contact: Raymond R. Allmaras, ARS Soil and Water Management Research Unit, St. Paul, MN, phone (617) 625-1742,

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