farm chemical toxicity
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It was thought that as farm
chemicals start degrading in contact with soil they become less
toxic, but new research from the University of Illinois, published
in the Aug. 15 issue of Environmental Science & Technology, suggests
that certain pesticides can actually become more toxic in contact
with reduced, iron-bearing clays commonly found in soils.
The long-held theory was that farm
chemicals became less harmful in the soil through a process of
microbial degradation and sequestration. No one expected, therefore,
to discover a chemical reaction with clay particles that increased
the toxicity of a compound.
"We expected to see some reaction because other studies we have done
showed that reduced clay is more reactive than oxidized clay," said
Joe Stucki, University of Illinois professor of soil chemistry. "But
we were surprised that the toxicity of one of the compounds
The researchers in this project mixed four different herbicides with
reduced or unoxidized ferruginous smectite, a specific group of clay
minerals. They compared the toxicity of the pesticide on its own to
the toxicity of the pesticide that had reacted with the smectite.
They used a widely accepted toxicity test employing mammalian cells
of a hamster.
Treatment with reduced smectite, or clay mineral with little or no
oxygen attached to it, substantially increased the toxicity of
dicamba by as much as 33 percent, decreased the toxicity of oxamyl
by 50 percent, slightly decreased the toxicity of alachlor, and for
2,4-D the toxicity remained about the same.
Dicamba is one of the most widely used products for controlling
broadleaf weeds in corn. Oxamyl is widely used for control of
insects, mites and nematodes on field crops, fruits and ornamentals.
The majority of oxamyl is applied to apples, potatoes and tomatoes.
Alachlor is a herbicide for control of annual grasses and broadleaf
weeds in crops, primarily on corn, sorghum and soybeans. Alachlor is
the second-most widely used herbicide in the United States, with
particularly heavy use on corn and soybeans in Illinois, Indiana,
Iowa, Minnesota, Nebraska, Ohio and Wisconsin.
The herbicide 2,4-D is used for control
of broadleaf weeds in agriculture and for control of woody plants
along roadsides, railways and utilities rights of way. It has been
most widely used on such crops as wheat and corn and on pasture and
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"Virtually every study
looking at the effect of soil and clay minerals on the fate of
pesticides has ignored oxidization state. That's an omission because
soils are commonly in a reduced condition, and now we have shown
that the oxidation state makes a big difference," said Stucki.
"What we're showing here, at least in the case of dicamba, is that
the compound became more toxic when it came into contact with the
reduced clay, so we can't always assume that when a compound comes
into contact with the soil itís a positive environmental outcome,"
The pesticide levels tested were comparable to pesticide
concentrations commonly found on farms. Also, smectite clays are
abundant in many soil profiles. Reduced smectite clay particles are
often found near the soil surface due to rainfall events and
Stucki says other degradation products were found in the research,
and the research team is currently attempting to identify those
compounds and measure their toxicity.
The research was part of a dissertation project of Kara Sorensen,
who now works for the Naval Research Laboratory in San Diego. Her
initial interest grew out of a desire to study cancer in wildlife
and what might cause it.
Funding came, in part, from the Department of Energy, National
Science Foundation, U.S. Israel Bi-National Agricultural Research
Fund, International Arid Lands Consortium and the U of I
Agricultural Experiment Station.
of Illinois news release]