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Chloropicrin as a Soil Fumigant
Stephen N. Wilhelm, Niklor Chemical Company, Inc., Chairman,
Chloropicrin Manufacturers Task Force, Long Beach, CA
90810-1695.
One soil fumigant being tested as a possible alternative for methyl
bromide (MeBr) is already EPA registered, available, and practical to use.
That compound is chloropicrin (CCl3NO2). For over 75
years it has effectively controlled soil pests and pathogens utilizing
proven cultivation methods.
Chloropicrin was first tested as a preplant soil fumigant in 1920. In
1957, fruit and vegetable production took a giant leap forward when a
mixture of chloropicrin and methyl bromide demonstrated remarkable
synergism. Although straight chloropicrin is still applied today in severe
soil-borne disease situations, it is typically formulated with MeBr
(20-33% chloropicrin) or with 1,3-dichloropropene
(1,3-D).
Chloropicrin (molecular weight 164.4) is a small, single-carbon
organic molecule that possesses the properties of rapid diffusion through
agricultural soil and selective toxicity to common root destroying fungi.
It is a clear, colorless, nonflammable liquid with a moderate vapor
pressure (18.3 mmHg at 68oF) and boiling point
(234oF). Chloropicrin is unique because it is a strong
lacrimator (tear producer); therefore, it warns against
potentially harmful exposure.
Chloropicrin is injected as a liquid into the soil approximately
6-10 inches below the surface, 14 days or more before crop planting. It
kills target fungi within 48 hours of application. Chloropicrin also
controls some root-destroying nematodes, soil insects, and other
plant-limiting pests.
The importance of soil fumigation in the control of plant
pathogens cannot be overstated. Even in agricultural soil with adequate
nutrients, water and oxygen, plant growth and crop yields can decline over
time due to increasing levels of pathogenic fungi and other pests. In the
1950s, before soil fumigation with chloropicrin, California strawberry
growers resorted to applying 500 pounds/acre or more of nitrogen because
of plummeting crop yields. The problem was not lack of soil nutrients--it
was lack of healthy roots. Strawberry root diseases were widespread at the
time and the partially rotted roots were not capable of absorbing the
abundant nitrogen that was available. By making high crop yields
predictable and at the same time reducing the use of fertilizers,
chloropicrin/MeBr combinations have made it possible to replant the same
fruit and vegetable land year after year. Predictable crop yields have
allowed breeders to concentrate their efforts on fruit quality,
appearance, and shipability.
Environmentally, chloropicrin does not have a significant ozone
depletion potential because it undergoes rapid breakdown in sunlight. It
is metabolized in soil to carbon dioxide. Under anaerobic/aquatic
conditions, chloropicrin is converted to nitromethane within hours. In a
plant metabolism study utilizing soil treated with radiolabelled
chloropicrin, no chloropicrin or nitromethane was detected in any plant
tissue or harvested produce.
The breakdown products of chloropicrin in soil (carbon dioxide,
nitrate, chloride) are basic nutrients not only for the plants but also
for the microorganisms that inhabit crop soils. The extra salutary effects
over and above what would be anticipated from the control of target fungi
alone on infested soils are believed to result from the biological
activity of root-friendly microorganisms that recolonize the fumigated
soil.
Since chloropicrin is only slightly soluble in water (1.6 g/liter at
77oF) it will not move rapidly in aquatic environments. The
half-life of chloropicrin in water exposed to simulated sunlight was 31.1
hours with the final products being carbon dioxide, bicarbonate, chloride,
nitrate and nitrite. Chloropicrin does not undergo hydrolysis in the
absence of sunlight.
The octanol/water partition coefficient
(log10KOW) for chloropicrin is 2.5, indicating that
it will not significantly bioaccumulate in animal cells. Chloropicrin did
not induce cancer in six long-term animal bioassays performed by
inhalation, oral, and gavage dosing. Chronic toxicity was limited to
inflammatory and other degenerative changes associated with chronic wound
healing at the site of dosing (stomach, mouth, lungs). In some in
vitro ('test tube') mutagenicity studies, chloropicrin induced both
negative and positive responses. In animal teratology studies via
inhalation, there were no treatment-related fetal malformations.
Reproductive fitness was not adversely affected in a two-generation
inhalation rat study.
Like most fumigants, chloropicrin is a Restricted Use Pesticide
so its distribution and use are highly controlled. Since it does not have
the excellent herbicidal properties of MeBr or the broad nematocidal
properties of 1,3-D, chloropicrin's use as an alternative will be in
conjunction with 1,3-D and compounds with broader herbicidal properties
such as metam sodium, dazomet, and pebulate. In the meantime, existing
USEPA registrations that contain higher formulation ratios of chloropicrin
to MeBr (i.e., 1:1, 1:1.3) than what is typically applied today (1:2, 1:3)
can be used. These formulations will provide excellent soil pathogen and
weed control without the need to alter current proven cultivation
methods.
[July 1996 Table of Contents]
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Last Updated: October 7, 1996
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