The amount of fertilizer and herbicide applied isn't what determines how much chemical runoff pollutes rivers and streams. That's the main finding in a USDA-ARS study at Columbia, MO.

The scientists found that soil characteristics, farming systems and rainfall have more impact on runoff than does chemical application amounts.

The five-year study focused on Midwest claypan soil, found on 10 million acres of U.S. farmland.

The researchers say heavy rains that often fall soon after fertilizer applications may pose the greatest risk for nitrogen losses. Also, fertilizer is more likely to stay in claypan soil when knifed in narrow bands instead of spread evenly and incorporated by tilling. In addition, losses of atrazine and alachlor herbicides were lower under minimal tillage systems than under no-till.

This information could lead to development of better management practices for specific farming regions, the scientists say.