S, ;-, NORTH FLORIDA EXPERIMENT STATION
September 8, 1959
NFES Mimeo Report 60-2
SOIL TREATMENTS FOR WEED CONTROL IN SHADE TOBACCO PLANT BEDS
by R. R. Kincaid
Weed control is one of the critical factors in production of shade tobacco
seedlings. Methods of soil treatment for weed seed control have progressed
during the last 40 years from burning to steaming to chemical treatments. In
recent years, the urea-cyanamide and methyl bromide treatment have been widely
Comparative tests of chemical treatments have been conducted at the North
Florida Experiment Station for 15 years. Results of replicated experiments for
the last four seasons are summarized in the accompanying table, which gives
materials, rates, methods of application, number of weeds (including-grasses)
per square yard, and cumulative yields of seedlings per square yard. Stand
counts include all seedlings remaining after the last regular pulling.
Several treatments gave about equally good weed control, but varied
considerably in seedling yields during the various seasons and in adaptability
to various soils. Five treatments are discussed in the following sections.
All Treatments.- In preparation for soil treatment with chemicals, drop
residues are allowed to become well decayed; soil amendments, such as peanut
hulls or stable manure, are added; moisture is adjusted to a fairly high level
for a few days to allow weed seeds to swell; and the soil is well cultivated.
Before or immediately after treatment, ditches are provided to control
erosion and prevent recontamination of the soil by surface water.
Urea-Cvanamid-Ethylene Dibromide.--One pound of urea (NuGreen) and J pound
of cyanamide (Granular Cyanamid) per square yard are applied 90 days before
sowing. The materials are broadcast separately, and evenly on the soil. They
are mixed thoroughly with the soil 4 to 5 inches deep, once over with a rotary
tiller, or four to five times with a disc harrow. The soil is then fumigated
with EDB-40, 15 gallons per acre, immediately (same day), or, if preferred, 7 to
10 days before the urea-cyanamide treatment; the purpose of the EDB fumigation
is to control nematodes and to control nitrification is such a way as to promote
growth of seedlings during the latter part of the plant bed season.
The treated soil is left undisturbed for a month. Afterward, the soil is
irrigated every week or two, if the weather is dry, to promote decomposition of
the chemicals. Fertilizer analyzing approximately 4-8-3 is applied at the rate
of li pounds per square yard, and mixed to a depth of 3 to 4 inches. After
sowing, the soil is kept damp to prevent soluble fertilizer materials from
becoming too concentrated and causing seedlings to slough off at the sol 1, -;.
surface. Urea and cyanamid contain large amounts of available nitrog9 '
therefore additional nitrogen may not be needed unless the plant bed 'son
is prolonged and wet. S
Sl 29 1959S
The urea-cyanamide-ethylene dibromide treatment has given consistently
good weed control. It gave best seedling yields in 1958 and 1959, when the
plant bed seasons were cold and wet. This treatment is recommended for light
to medium soils, but not for very heavy soils.
Cyanamide-Ethylene Dibromide.-Cyanamide alone, 1 to li pounds per square
yard, used in conjunction with EDB fumigation as described above, is not quite
as consistently satisfactory as the combination treatment. It has been recommended
for flue-cured tobacco beds in Georgia, and is considerably cheaper than the other
Methyl Bromide.-This material, preferably with 2% chloropicrin (tear gas)
added as a warning odor, is applied at the rate of 1 to l1 pounds per 100 square
feet, under plastic covers sealed by covering the edges with soil. The cover
is removed after 24 hours. A waiting period of at least a week is recommended.
Fertilizer is applied at the rate of 1F to 2 pounds per square yard, and addi-
tional nitrogen as top-dressing may be required. The methyl bromide treatment
has been found more satisfactory for very heavy soil than the urea-cyanamid
Weed control has been generally satisfactory, but seedling growth has been
somewhat erratic. Roots of seedlings may become brown, and growth may be retarded.
Where this condition is associated with excess ammonia in the soil, a light
top-dressing of nitrate is suggested. A manufacturer of methyl bromide has suggested
that poor results may be associated with soil which is too wet or too cold
(below 500F.), or with covers left too long on the soil.
Based on general observations, it is suggested that the treatment be applied
in November or early December, when the soil is warm, and more time will be
available for normal biological activity to be restored in the soil before sowing.
Vapam (or VPM) is a liquid material which has given promising results for
four seasons in the Station tests. It is used at the rate of 24 gallons per
100 square yards. The usual method of application is to dilute the concentrated
material with enough water to moisten the soil to a depth of 4 inches and apply
as a drench.
Another method of application which has been satisfactory in Station tests
is to sprinkle the material, diluted as necessary, on the soil, mix it immediately
4 to 5 inches deep with a rotary tiller, and irrigate to moisten the soil to the
same depth. The method of injection included in Station tests is not recommended
at present. Other methods are under investigation at this Station and elsewhere.
Mylone is a dry material, chemically related to Vapam or VPM, which has
given promising results for two seasons in Station tests. It is used at the rate
of 11 pounds of 50% material per 100 square yards. It has been spread evenly on
the soil, mixed 4 to 5 inches deep with a rotary tiller, and then watered to
moisten the soil to the same depth.
Vapam (VPM) and Mylone in Station tests have been applied 90 days before
sowing. A minimum waiting period of at least one week (two weeks if the soil is
cold and wet) is suggested; the soil should be cultivated after five days to promote
Since neither of these materials has any appreciable fertilizer value,
fertilization at the rate of 2 pounds per square yard is suggested. Top-dressing
may be needed if pulling is delayed by cold, wet weather. Experiments are in progress
to find a source of nitrogen which will supply sufficient nitrogen-throughout the
season and eliminate the need for top-dressing.
Results of Soil Treatments for Weed Control in Shade
Crop Rate per
year Material(s) per acre Methodsa Sn. yd.
Tobacco Plant Beds.
Yield of seedlings
per square yard
3/26 3/30 & 3 Stand
1956 Urea b I
1957 Urea b
1958 Urea b
1959 Urea b
33 138 238
aMethods: R, rotary tillage; W, water to moisten 4 to 5 inches deep; T,
b I, injection;
Treatment includes fumigation with EDB-40, 15 gallons per acre.