Washington, Sept. 10--U.S. Department of Agriculture researchers are finding clues to why some farm practices and soil properties help reduce the chance that herbicides--chemicals used for weed control--make their way into ground water.

"We've found that conservation tillage reduces the likelihood of some herbicides like atrazine reaching ground water, particularly in sandy, coastal plain soils," said I. Miley Gonzalez, USDA's Under Secretary for Research, Education and Economics. "Leaving plant residue on the surface increases the organic matter in the top soil layer. That increases the soil's ability to retain herbicides like atrazine and reduces the potential for leaching into ground water."

Jeffrey M. Novakof USDA's Agricultural Research Serviceis currently studying the effects of various tillage practices on pesticide leaching at the ARS Coastal Plains Soil, Water, and Plant Research Center in Florence, S.C. He presents his findings today at a meeting of the American Chemical Society in Las Vegas, Nev.

"Once we better characterize all the processes that affect pesticide degradation and leaching, we can design more environmentally friendly management practices for farmers," said Novak. "Conservation tillage encourages the growth of microbes living in the carbon-enriched topsoil. These microbes degrade pesticides, greatly reducing the likelihood they'll end up where we don't want them--in ground and surface waters."

Novak studied the fate of the herbicide atrazine, applied to Iowa glacial and Carolina sandy coastal soils. The scientists selected atrazine because of its use on millions of acres of U.S. cropland and its frequent detection in ground and surface water. In Illinois and Nebraska, millions of pounds of the chemical are used each year to control weeds, mainly in corn fields.

"Our studies in Iowa showed that soil features such as landscape position and organic carbon content greatly influence the amount of atrazine retained or absorbed," he said. "Soil aggregate size had little if any effect on this process." Novak added that atrazine leaching may actually be reduced in low-lying areas of fields because those spots are poorly drained and have greater carbon buildup. With increased carbon more atrazine is absorbed so less gets leached into ground water.

Herbicide leaching and runoff is being studied at several ARS laboratories including Beltsville, Md.; Tifton, Ga.; Ames, Iowa; and Morris, Minn.

Scientific contact: Jeffrey M. Novak, soil scientist, ARS Coastal Plains Soil, Water, and Plant Research Center, Florence, S.C. 29501-1241, phone (803) 669-5203, Novak@florence.ars.usda.gov.