acre, to the raised bed surface at plastic laying. (Note: A minimum 30-day
an herbicide tank mix of napropamide (2 pounds) and s-metolachlor (0.71
pounds, 3rd party label obtained through FFVA) per treated acre is applied
after the Telone II Pre-Bed and chloropicrin injection to the raised bed at
plastic laying for weed control. Recent research on soil application technologies
in Florida and Georgia have demonstrated improved nutsedge control
with metam sodium or potassium applied through a series of minicoulters to
the established plant bed just before installation of the plastic mulch.
AVOIDING PLANT PHYTOTOXICITY
All of the fumigants, including the gas and water phases of these
fumigants, are toxic to plants. Soil dissipation and the persistence of toxic
in soil condition which promotes a cooling and/or wetter soil condition will
typically delay dissipation of a fumigant compound from soil. As a result, soil
applications must always be made well in advance of a seeding or planting
The problem is usually most severe with fumigants of low vapor pressure,
where longer-term planning horizons must be adopted to avoid problems
of phytotoxicity or to avoid long unscheduled delays in planting after the
plants have arrived from the nursery. Growers who use fumigants, typically
soil application two to three weeks earlier than normal to avoid problems of
potential phytotoxicity that may result from use of impermeable mulches, or
unexpected cold fronts or storms producing abundant rainfall. Recent studies
have also demonstrated the addition of chloropicrin in the fumigant mix
can destroy microbial populations in soil that digest fumigants like DMDS,
and thereby contribute to longer persistence in soil and potential delays in
plant back into the bed than would have occurred with DMDS alone.
High Barrier / Gas Impermeable
Plastic Mulch Films (VIF,TIF)
FUMIGANT CONTAINMENT
fumigation has repeatedly demonstrated effective soilborne pest and disease
control when applications were made under optimal soil conditions, uniformly
applied at the appropriate dosage and depth, and using containment systems
that prevented rapid escape of gases from soil. Any system designed to provide
Left undisturbed, the chisel traces act as chimney vents for volatilizing fumigant
gases. This can be accomplished with press wheels behind the chisel or chisel
may also be rolled to compact surface soil, so as to increase soil density and
reduce air passage size and volume. Irrigation is also often applied over the top
of the treated area and or rolled surface to form a surface water seal to further
inhibit fumigant out-gassing from soil. In some instances, repeated irrigations
may be necessary to manage fumigant containment and offsite movement of
fumigant gases.
After a fumigant is applied, the treated soil may also be tarped with a
plastic polyethylene mulch to provide an additional measure of fumigant
containment to soil. The plastic mulches, often a prerequisite for fumigant
use, are not only a barrier in themselves but act as a condensation surface
for the formation of water droplets that redeposit to soil as a water layer,
adding another barrier to soil volatilization loss. There are a range of
different mulches which can be used to reduce fumigant emissions from
soil. Low density polyethylene (LDPE) or high density polyethylene (HDPE)
tarps have been extensively used in combination with soil fumigation as
a soil covering to reduce fumigant emissions from soil. Unfortunately, the
barrier properties of LDPE and HDPE to fumigant gases is quite poor,
and, depending on the fumigant, much of it may quickly escape the soil.
In general, the permeability of a plastic mulch to a fumigant gas is directly
resistance to fumigant outgassing has been achieved with use of virtually
of barrier polymers such as ethylene vinyl alcohol (EVOH) or polyamide
(nylon) sandwiched between other polymer layers (typically LDPE) or have
metalized coatings to reduce the amount of fumigant that can move through
are generally manufactured the same as VIF except the center layer which
produces the barrier is made of EVOH instead of nylon.
the residence time of fumigants in soil. Because of their excellent barrier
properties, fumigant application rates can often be reduced as much as 30
to 50% without loss of pest control activity or crop yield. These mulches,
although not completely impervious to fumigant gases, reduce the dissi-
treatment by subjecting soil pests to greater cumulative dosage levels of the
-
quiring extended periods to allow for soil water and air concentrations to fall
below levels phytotoxic to seedlings and transplants. To use the high barrier
mulch technology, plantings may have to be delayed to insure soil residues
have dissipated and plant injury will not occur. A monitoring program using
colormetric detector tubes (GasTek, Kitagawa, Sensidyne) or MiniRae type
VOC meters to assess residual fumigant gases in soil should be considered
before a commitment to planting is made.
Due to increased environmental and regulatory scrutiny, VIF or TIF high-
to receive buffer-zone reducing credits (20-60%) to allow fumigant applications
within certain proximities of human-occupied structures. They are also
extensively used to allow for reduced-rate applications of the different fumigants
without compromising concentration and time dosages, thereby main-
use of TIF mulches installed immediately after its application. Today, over a
barrier, VIF, or TIF status with EPA currently approved for buffer zone credit.
Additional information regarding EPA approved tarps can be found at http://
www.epa.gov/soil-fumigants/tarps .
REDUCED RATE APPLICATION TECHNOLOGIES
Currently, soil injection equipment for many of the different fumigant
compounds is designed to dispense as much as 20 to 40 gallons of a liquid
meter and rear manifold and then through each of three chisels per bed.
The system is designed and calibrated to do this while moving at 3½ to 5
mph, uniformly dispensing multiple liquid streams of fumigant within 7,260
full, with liquids moving as continuous streams without in-line voids or bubbles.
At 30 to 50% reduced rates of application, such as those demanded
30 2019 Vegetable Production Handbook of Florida
/tarps