PREDICTING HOW herbicides move in soil requires accurate estimates of how these chemicals bind to soils and geologic materials — vital information that's often lacking for materials below the soil's surface.

Now, Agricultural Research Service microbiologist Thomas B. Moorman at the National Soil Tilth Laboratory in Ames, Iowa, working with researchers at Florida International University-Miami and Iowa State University-Ames, has measured how one important herbicide, atrazine, binds to and lets go of particles in different soil types. Unlike previous research, this project measured atrazine's binding deep into Iowa soil.

Atrazine is an organic compound, widely used as a herbicide for control of broadleaf and grassy weeds. During the 1980s, atrazine was estimated to be the most widely used herbicide in the United States. Today, because of its low cost, it is still applied to millions of acres of U.S. croplands, especially corn and sorghum fields.

The scientists used a variety of simulation models to predict the risk of this herbicide's movement into groundwater. For accurate prediction, these models integrated information about rainfall, waterflow, soil types and atrazine use.

The team found that the soils were low in organic carbon. But they retained more herbicide than would have been predicted, based on past research. The researchers also found that certain glacial till materials — geologic sediment of sand, silt and clay in the saturated zone beneath the groundwater surface — were able to retain atrazine quite strongly, greatly limiting its leaching. This geologic sediment was deposited as glaciers retreated from Iowa about 15,000 years ago.

The researchers believe this knowledge should increase scientists' and farmers' ability to predict herbicide contamination of groundwater and aid in developing practices that protect water resources from contamination. This will help producers manage herbicides more carefully and assure better water quality for the general public.