Plastic mulch's dirty secrets.

In many areas of the country and for many crops, mulching fields with sheets of plastic has become de rigueur. Not only do these impermeable blankets help retain moisture, but they also warm the soil. As a result, mulched crops tend to ripen sooner (SN: 12/13/97, p. 376), a boon to many farmers.

These benefits, however, come at some expense to the environment, a new Agriculture Department study finds. The practice increases both erosion and runoff of toxic pesticides.

Many farmers had noticed that more rain flows from plastic-mulched fields, via dirt furrows between the covered rows, than from fields covered with plant-stubble mulch. However, "nobody had bothered to ask how much more [water runs off]," notes Cathleen J. Hapeman, an inorganic chemist at the Beltsville (Md.) Agricultural Research Center (BARC). So, during 2 years, she and her colleagues collected all the rainwater flowing from tomato fields and measured the amount, as well as any contaminants in it.

In 1998, the year with the more dramatic results, they found that an average of 63 liters of water ran off each square meter of the plastic-mulched soil. That's four times the runoff from a field mulched with material from a plant known as hairy vetch. Even "more disturbing," Hapeman reported at the American Chemical Society meeting last month in New Orleans, is that the plastic-covered field lost 4,950 kilograms of dirt per hectare that year--almost 15 times as much as the vetch-mulched field. Clearly, she observes, with such a slowly renewing resource as soil, "you cannot sustain such losses for very long."

Then, she looked at chemical runoff. Each of the two test fields had been sprayed with the same amount of the fungicide chlorthalonil and the insecticide endosulfan. Because the plastic-mulched field has less exposed soil that can bind the pesticides, rain washed away 19 times as much of the chemicals from it as from vetch-covered rows. The researchers then added this runoff to containers holding local aquatic inhabitants, including hard clams and diatoms. The plastic mulch's runoff was usually much more toxic than the vetch's, says Hapeman.

Using plastic mulch enables farmers to harvest crops 3 or 4 weeks early. Such vegetables can command high market prices. "However, I'm having a hard time justifying that 3-week-earlier harvest in exchange for this loss of soil and pesticides," Hapeman says.