gants are also used in a variety of other lower-value fruit, vegetable, and
agronomic crops such as potatoes, sweet corn, Brassicae, melons and
other curcurbit crops, to manage nematodes and other soilborne pests
and diseases. In some cases, fumigants have been adopted almost to the
exclusion of all other soil pest management strategies because of their
of crop growth, uniform development, fruit quality, and yield.
METHODS OF APPLICATION
Fumigants are formulated and applied to soil in a number of different
ways. Liquid fumigants can be either applied by directly injecting them into
the soil using shanks or chisels, or via direct injection into the irrigation
is treated, and this represents an overall or broadcast treatment. In other
situations, fumigants are only applied in the plant row or raised plant bed. In
band, or in-row treatment. Most liquid fumigants with high vapor pressure
are shanked or knifed into the soil. As liquids with relatively high vapor
pressure, they are usually stored and applied as liquids (under pressure)
and begin to vaporize shortly after injection in the soil. Narrow knifelike
shanks are tractor-drawn through the soil at the required depth to inject the
fumigant into soil. Metal delivery tubes attached to the trailing edges of the
shanks provide the conduit for injection to soil. Delivery tubes release the
fumigant in the bottom of the furrow made when pulling the shank through
rates through properly sized delivery lines.
formulation is required for drip application. Metam sodium, metam
and applied through a low-volume trickle or drip irrigation system. Drip fumigation
(chemigation) is the most common method of application for some
fumigants like metam sodium, metam potassium, or allyl isothiocyanate.
Once these fumigants are applied to soil in the presence of water, they
decompose to form MITC or AITC. Because of the slow and limited diffusion
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livery in irrigation water following premixing has generally resulted in more
uniform soil distribution with enhanced nematode control and crop yield for
these products when compared with conventional chisel injection methods.
MITC distribution in soil initially follows the water distribution patterns, but
unlike other fumigants, the distribution patterns do not change over time.
Moreover, MITC and AITC disappears from the soil gaseous phase at faster
rates than most of other fumigants.
The proximity of the plant to the drip tube has also been demonstrated to
response with a drip chemigated fumigant. Given the sandy nature of most
Florida soils, narrower bed widths, drip tubes with closer drip emitter spacing
(mostly in the range of 8-12 inches), and planting practices that place
plants closer to the drip tube are needed to more effectively utilize the drip
tape for chemigational purposes.
Soils and grower production practices differ markedly, and these
differences in soil type, compaction, and depth to restrictive layers can all
restricts downward penetration of drip water and fumigants contained within
it. In general, the average depth, width, and cross-sectional area wetted by
drip irrigation water increases with total water volume applied. For a given
water volume, the use of two tapes per bed increases spatial distribution of
irrigation water simply because of the spacing between drip tubes and the
increased number of emission points along the bed. Drip fumigant users are
encouraged to consult with county agricultural extension personnel regard-
application rates, product concentrations in irrigation water, water volumes,
and optimal injection periods to maximize movement and proportions of
treated bed volumes for their farm locations.
Other formulations, like those marketed for shank applications of chloro-
compressed gas cylinders. In most cases, these fumigants are shank-injected
into the soil 8 to 15 inches deep using a positive pressure-closed system
in which the fumigant cylinders are pressurized with nitrogen. Nitrogen is an
systems and through armored lines to the rear shanks or chisels, exiting
through a steel delivery tube welded to the back side of the shank or chisel.
Deep placement is not only a requirement of the fumigant label but is essential
for prolonged fumigant retention in soil. In general, the closer to the
soil surface a fumigant is applied, the faster the out-gassing or escape from
soil, and in general the poorer the pest control response. A plastic, polyethylene
tarp may be laid down over the soil immediately behind the injection
equipment to provide a diffusion barrier for containment of the fumigant.
that high population densities of plant-parasitic nematodes can be broadly
distributed to depths 3 feet below the plant bed surface. Subsequent studies
have demonstrated the importance of deep shank fumigant applications
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zons where upwardly mobile nematodes reside. The physical and practical
challenge that remains to be resolved is how to achieve fumigant movement
in lethal concentration to the lowest soil depths where nematodes reside.
THE IMPORTANCE OF FUMIGANT PLACEMENT
Proper fumigant placement is another critical element that must be considered
when using any of the different soil fumigant compounds, because of
the large difference in vapor pressure (Table 1). In some instances, application
depth and placement varies with the formulation of the fumigant product, giving
the grower the opportunity to choose between a drip or shank application. For
the 3-Way system approach that utilizes 1,3-D, chloropicrin, and metam, there
are essentially three different locations to consider for fumigant placement.
depth that places the fumigant 12–15 inches from the top of the bed or nearest
soil interface. If fumigants are placed at this depth and used in conjunction with
a seepage irrigation system, growers should ensure that the soil is not wet at
this depth. Otherwise, poor nematode control and longer aeration times and
crop plant-back intervals may be observed. Chloropicrin, applied over the top
of the Telone treatment, is then applied 8 to 10 inches deep into soil as the
beds are being formed and compressed. Metam sodium (Vapam) or metam
potassium (K-Pam) should be applied 4 inches from the top of the bed and 4
inches apart using multiple coulters or shanks. After application, the bed must
be recompressed and mulched immediately to prevent rapid escape of volatilizing
gases. The primary objective for such shallow placement is to achieve
maximum control of weeds. Most other fumigants should be placed 8–12
inches deep with shanks no further than 9–12 in. apart.
While methyl bromide will rapidly diffuse as a gas across a bed, many
of the currently proposed alternatives lack the vapor pressure or chemical
properties to move as fast and effectively throughout the bed. Their movement
in the bed is clearly limited and must be accounted for in fumigant
placement. For example, if the shanks are not close enough together for
26 2019 Vegetable Production Handbook of Florida