Table 4.1. Physical and chemical properties of various soil fumigants.
Fumigant Compound & Physical State Boiling Point (°C ) Vapor Pressure (mm/Hg) Soil Degradation Half-life (d) Solubility (mg L-1)
1,3–Dichloropropene 120 28 3-5 2250
Chloropicrin 112.4 18 1-2 2270
Methyl Bromide 3.6 1420 12-20 13400
110 28.7 3000
Metam Sodium 112 0.4 4-5 578290
Metam Potassium 114 24 4-5 complete
Allyl Isothiocyanate (AITC) 112 20 89400
some fumigants, an overlap in the area between shanks may not occur,
leaving streaks of pests down the length of the bed. Many products are
application, proper placement of the fumigant generally mandates two drip
tapes per bed to improve bed coverage.
The vertical and horizontal distribution of diseases, weeds, and nematodes
in correlation with spatial movement of soil fumigants from their points
of injection, are now being considered as some of the primary causes of
fumigant treatment inconsistency and origins of bed-recolonizing popula-
additional streams of fumigant placed under the tuck areas of the plastic
wilt incidence and severity within the tomato planted row compared with
the standard in-the-row grower application practice. Additional streams of
-
todes and improvements to crop yield.
dense, highly compacted soil layer), was shown to unavoidably cause
changes in the downward percolation of water, permeability and diffusion to
fumigant gases, and root penetration into soil. In practical terms, the compaction
zone occurs just below the depth of the deepest tillage operation or
allowing fumigant gases to make contact with deep-dwelling nematodes
diffusion in soil of a fumigant when it is applied above the restrictive layer,
application below the layer reduces damaging populations of nematodes
which would have otherwise survived the fumigant bed treatment.
PHYSICAL AND CHEMICAL PROPERTIES
After application, the fumigants penetrate the soil and quickly become
partitioned in liquid, gas, and adsorbed soil phases. Immediately after soil
injection, fumigant movement in soil is driven by density and pressure
gradients from a narrow, linear band of concentrated product deposited
directly below the chisel path. As the fumigant’s partial pressure falls, soil
by many factors, including chemical properties, soil properties, and environmental
conditions. A partial listing of some of the physical and chemical
properties of the primary soil fumigants available in Florida is provided in
Table 1.
Volatilization continues as long as the fumigant remains in soil and
upward movement of the fumigant occurs as long as a concentration difference
exists between the soil surface and soil atmosphere. Vapor pressure
is a measure of the tendency of a fumigant compound to change into the
gaseous or vapor state. The temperature at which the vapor pressure at the
surface of a liquid becomes equal to the pressure exerted by the surroundings
is called the boiling point of the liquid. The lower the boiling point, the
more volatile the fumigant compound. Note the differences between methyl
bromide and all the other fumigants listed. Hot and dry soil conditions favor
more rapid escape of fumigants like methyl bromide and chloropicrin, particularly
within surface soil horizons. Fumigant products with high solubility
and low vapor pressures are typically better suited for drip application rather
than shank, which is why metam sodium, potassium and allyl isothiocyanate
are described as being better suited for drip fumigation.
MODE OF ACTION AND LETHAL DOSE
Fumigant mode of action refers to the lethal action of a chemical on vital
life processes of an organism. For example, a broad-spectrum fumigant
can penetrate the body wall of a nematode directly and does not have to be
eaten to be effective. Once inside the body cavity of the nematode, different
the fumigant compound. Metam sodium and potassium are very soluble
compounds which become activated in water. The fumigants and their
products interfere with many different vital processes, including enzymatic,
nervous, and respiratory systems. Death of the nematode is rapid under
these conditions, and development of tolerant or resistant populations of
nematodes to the chemical is unlikely because so many integral bodily
functions are simultaneously affected. This is not to say that all organisms
metam sodium can disappear rapidly and fail to provide a lethal dose following
application, indicating the key role of microorganisms in accelerated
degradation of the product. In general, however, the fumigant concentra-
with soil concentrations within about 30 minutes to 4 hours. In some cases
fumigant concentration may accumulate to much greater levels within the
nematode than in soil.
In general, the lethal effect of a fumigant is determined by two compo-
usually expressed as parts per million (PPM). The second is the length of
time (T) the pest organism is exposed, expressed in minutes, hours, or
days. The level of pest control is then related to dosage, the amount of
fumigant pesticide placed in the environment of the soilborne pest for a
known length of exposure time (concentration x time). Total exposure is
the sum of CT products. Computed in this way, the cumulative dosage or
concentration-time index (CT), is often used as a criterion with which to
evaluate the effectiveness of soil fumigation. For most organisms, there is a
concentration level below which kill is not obtained regardless of the length
of exposure. For most nematodes, long exposures to low concentrations of
fumigant nematicides above the minimum concentration seem to be more
effective than short exposures to higher concentrations.
Fumigants move through soil air, dissolve in the soil water and kill in
the soil water. Fumigants are volatile substances and change into gases
upon injection into the soil as liquids. The vapors can only move through
continuous soil air space. Pest and disease organisms normally exist within
2019 Vegetable Production Handbook of Florida 27