Fitting Farming Practices to Minimize Water Pollution

How much herbicide or fertilizer runs off farm fields to pollute streams and rivers may depend less on the amount of the chemicals applied and more on other factors such as soil characteristics, farming systems, and how soon it rains after the chemicals are applied, according to studies by Agricultural Research Service scientists.

For example, ARS scientists at Columbia, Mo., found that heavy rains that often fall on Midwest claypan soil soon after fertilizer application may pose the greatest risk for nitrogen losses in the forms of nitrate and ammonium. That's why, in the five-year study on the claypan soil that is representative of 10 million acres, 75 percent of such losses occurred within six weeks of application.

Fertilizer was also more susceptible to runoff when it was spread evenly and then incorporated into the soil by tilling than when it was knifed into the soil surface in narrow bands. And the herbicides atrazine and alachlor were more prone to runoff in a no-till farming system than when they were incorporated into the soil in a minimum-tillage system.

In another watershed study with different soil characteristics and amounts of row cropping, the influences of agricultural chemicals on water quality were nearly the opposite. The scientists found that herbicide concentrations were much lower in streamwater from watersheds with soils having good structure and pore space. However, nitrate concentrations were higher because farmers in such watersheds typically rotate soybeans and corn and apply more nitrogen fertilizer. Nitrate moves easily with percolating water heading toward the stream.

The information from these studies could lead to development of better management practices for specific farming regions to maximize the potential for water quality improvements.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.

Scientific contact: Robert N. Lerch, ARS Cropping Systems and Water Quality Research Unit, Columbia, Mo., phone (573) 882-9489, fax (573) 882-1115, LerchR@missouri.edu.