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Vegetative Mulch Cuts Pesticide Losses in Runoff

By Sarah Tarshis
August 25, 1999

Growing vegetables with an organic mulch, hairy vetch--a legume--instead of plastic mulch cut pesticide losses by as much as 90 percent in experiments in Maryland. The vetch also greatly reduced water runoff and sediment losses.

Today, Agricultural Research Service scientists presented the findings--the first showing the tactic's effect on pesticide loss--at a meeting of the American Chemical Society in New Orleans. ARS is the USDA's chief scientific wing.

The Chesapeake Bay and its aquatic organisms have long been threatened by polluted runoff from farms, lawns, golf courses, streets, septic tanks and other sources. And on the Bay's Delmarva peninsula, runoff from vegetable fields may contribute to endangering shellfish such as shrimp, clams and oysters. The new findings point to ways to reduce this threat.

Polyethylene mulch is used to conserve soil moisture and control weeds on about 4,600 acres of tomato and floral crops in Northhampton and Accomack counties, Virginia, that make up Virginia’s eastern shore. Runoff increases, however, because plastic covers 50 to 75 percent of the fields. Pesticides that aren't absorbed into the soil can be removed easily from the plastic surface by rain.

In a three-year collaborative study, ARS chemist Cathleen Hapeman and her co-workers grew tomato plants in both plastic-covered and hairy vetch mulch plots at the agency's Beltsville, Md., Agricultural Research Center. They measured the runoff and its sediment and pesticide concentrations. Compared to the plastic-covered plots, the vetch-mulch plots lost about two to 10 times less water, 10 times less soil and on average, ten times less pesticide.

Scientists tested some runoff from the plots on clams and other bottom-feeding organisms that serve as indicators of water quality. Preliminary results of the tests, conducted at the University of Maryland's Chesapeake Biological Laboratory in Solomons, Md., showed that runoff from polyethylene plots can harm the aquatic organisms. The organisms were less affected by runoff from plots mulched with hairy vetch.

Scientific contact: Cathleen Hapeman, ARS Environmental Chemistry Laboratory, Beltsville, Md., phone (301) 504-6511, fax (301) 504-5048,