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 Materials and methods
 Results and discussion
 Summary and conclusions
 Literature cited














Title: Persistence of simazine in Florida mineral and organic soils
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Title: Persistence of simazine in Florida mineral and organic soils
Physical Description: Book
Creator: Scudder, W. T.
Publisher: University of Florida Agricultural Experiment Station
Publication Date: 1963
Copyright Date: 1963
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Bibliographic ID: UF00027103
Volume ID: VID00001
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Table of Contents
    Title Page
        Page 1
    Table of Contents
        Page 2
    Introduction
        Page 3
    Materials and methods
        Page 4
        Page 5
        Page 6
        Page 7
    Results and discussion
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
    Summary and conclusions
        Page 21
    Literature cited
        Page 22
        Page 23
Full Text
June, 1963



Persistence of Simazine

in Florida Mineral

and Organic Soils

W. T. Scudder





















J. R Director

"". Ie
, 1 I II "!-

















CONTENTS

Page

INTRODUCTION ..................... .... . ....-....- .......... 3

REVIEW OF LITERATURE .. ........ ....--- ----- ..- ----- -----------..- ..-- .... ... 3

MATERIALS AND METHODS -.............. ..----..-......------ --......-----....... 4

RESULTS AND DISCUSSION .....-- ..-- ------.....---- -....- ........----. ----... .... .. 8

SUMMARY AND CONCLUSIONS -.....-------.. --------......--..----- ..---........ 21

LITERATURE CITED .....-- --.... ........-- ...-------.. -------............... 22

























ACKNOWLEDGMENTS

This study was supported in part by a grant from Geigy Agricultural
Chemicals, Yonkers, New York.
The writer is indebted to T. D. Lemon and J. B. Morris, field assistants,
for supervision of much of the field work, and to C. F. Carlisle for aid with
the statistical analyses.










PERSISTENCE OF SIMAZINE IN FLORIDA
MINERAL AND ORGANIC SOILS

WALTER T. SCUDDER1


INTRODUCTION
Simazine, 2-chloro-4,6-bis(ethylamino)-s-triazine, has proved
to be an efficient pre-emergence herbicide for use in corn and cer-
tain other crops. Recommended rates for corn vary from 1 to 2
pounds per acre on mineral soil and from 2 to 4 pounds per acre
on organic soil. Under normal conditions, applications within
these ranges will control most species of annual broadleaf weeds
and grasses for six weeks or more. Although corn is tolerant
to this chemical, most other annual farm crops are highly sensi-
tive to it.
In Florida, simazine may be used advantageously in the pro-
duction of sweet corn. However, most growers plant other veg-
etable crops, sensitive to simazine, on their sweet corn fields
during succeeding crop seasons. One or two additional crops
a year are common. Knowledge of the persistence of simazine
activity in these soils is essential before this herbicide can be
considered safe. Where small residues remain, the possibility
of an accumulation or build-up to phytotoxic levels following re-
peated use presents a further hazard.
Simazine is almost non-volatile and has a water solubility
of only about 5 parts per million. As a result, its herbicidal
action persists much longer than with most other pre-emergence
weed killers. This prolonged residual activity makes it efficient
as a soil sterilant when used at higher rates. This led to early
concern over the rate of dissipation of simazine, lest it be too
slow to permit normal growth of crops planted 6 to 12 months
after its application to corn.

REVIEW OF LITERATURE
Using C14-labeled simazine, several workers have studied the
mechanism responsible for the variable responses of plant spe-
cies to this chemical. Montgomery and Freed (9)2 observed
that C1402 was released to the atmosphere by corn leaves. Only
1Associate Horticulturist, Central Florida Experiment Station, Sanford.
SNumbers in parentheses refer to Literature Cited.








4 Florida Agricultural Experiment Stations

traces of simazine, if any, remained in the plant at harvest time.
Both tolerant and susceptible species absorb the chemical and
break it down, but, according to Davis, Funderburk, and Sansing
(5), high levels of degradation products remain in the sensitive
crops. With oats, cucumbers, cotton, and corn, the amount of
non-metabolized simazine or its C1'-containing degradation prod-
ucts accumulating in the plants was correlated with the relative
susceptibility of the four species (5,17).
Many factors appear to be related to the rate of simazine
dissipation from soil. Even though it resists leaching, Sheets
(15) observed that simazine moves downward faster in sandy
soil than in clay loam. In spite of this, several workers have
found that this herbicide disappears much faster in the finer tex-
tured soils such as clays. Soils high in organic matter responded
similarly to clays in tests conducted by Burnside and Behrens
(1). This reduced phytotoxicity of simazine in high organic
matter soils appears analogous to that of several other herbi-
cides, such as 2,4-dichlorophenoxyacetic acid (2,4-D) reported
early by Meadows and Smith (8) and 3-(p-chlorophenyl) -1-1-di-
methyl urea (monuron) reported by Rahn and Baynard (13).
Ogle and Warren (11) attributed this effect to the high absorp-
tive capacity of these soils. On the other hand, Chandra, et al.
(4) suggested that these effects were due to greater chemical
and microbial activity. Additional factors supporting this, in-
cluding high soil temperature, high soil moisture, and cultiva-
tion, have been cited for their influence on the depletion of herbi-
cide residues (1, 6, 13, and 14).
There appear to have been few studies relative to the ac-
cumulation of toxic residues in soils from repeated herbicide
applications. In one study, Rahn and Baynard (13), working
with monuron applied to a loamy sand in Delaware, observed
that semiannual applications of 1.6 pounds per acre did not per-
sist from one year to the next over a three-year period. When
3.2 pounds were used, a residue remained from year to year, but
there was no accumulation in the soil from the repeated treat-
ments.
MATERIALS AND METHODS
Two long-range experiments-one on Leon fine sand and the
other on Everglades mucky peat-were set up during 1957 to
study the activity of simazine residues remaining after the use
of this herbicide on corn crops. This activity, evaluated by meas-
uring the growth response of sensitive crops and weeds subse-







Simazine in Florida Mineral and Organic Soils 5

quent to several single and repeated simazine applications, was
used to determine the relative rates of simazine dissipation, per-
sistence, and accumulation in the soils. The mineral soil study,
designated Experiment 1, was located on the Central Florida
Experiment Station Farm at Sanford. Experiment 2 on the
organic soil was located on the Station's Zellwood Farm near
Lake Apopka. The latter experiment was initiated in April 1957
and continued through eight seasons over a four-year period.
The mineral soil experiment, started in October 1957, had a
duration of three and one-half years before both trials were ter-
minated in March 1961.
The two experiments were planned similarly, except that
higher chemical levels were used on the organic soil of Experi-
ment 2. Table 1 gives a general outline of the experiments,
listing the 12 treatments on each soil type. The application
rates and total amounts of simazine applied throughout the
course of both experiments are given for each treatment. All
simazine rates are given in pounds per acre of active ingredient.
The plots of Treatment 1 received no simazine. They were hoed
and cultivated and served as checks for comparison with the
simazine-treated plots. The plots of Treatments 2 through 6
were treated only once by mixing dry simazine into the top 4
inches of soil prior to the first planting. Weighed amounts of
the 50 percent wettable powder formulation were mixed with
dry sand and distributed evenly over each plot. The chemical
was then incorporated thoroughly into the surface soil. This
was done by hand spading at Zellwood and by using a Seaman
rotary tiller at Sanford.
Each season after the crops were planted, the pre-emergent
sprays of Treatments 7 through 12 were applied according to
the planned schedule. (Table 1). All applications were made
using broadcast equipment, employing low-pressure flat-fan noz-
zles calibrated to deliver approximately 30 gallons per acre. The
simazine sprays for Treatments 7, 8, and 9 were repeated once
yearly after each spring planting. Similar applications were
made twice yearly, both spring and fall, on treatment plots 10,
11, and 12. These semiannual applications resulted in more
simazine being applied to the Treatment 12 plots than to any
of the others during the course of the experiments. During
seedbed preparation at the beginning of each spring and fall
crop season, care was used to minimize the intermixture of soil
between adjacent plots. This was done with a rotary-tiller
whenever feasible.















TABLE 1.-TREATMENTS USED IN STUDIES ON THE ACTIVITY OF SIMAZINE RESIDUES IN TWO FLORIDA SOILS.

Pounds Simazine Applied per Acre
Mineral soil* Organic soil **
Treatment Each Total for Each Total for Application Method of
Number application experiment application experiment Schedule Application
1 0 0 0 0 (Check) Cultivated, hoed
2 1 1 2 2 Dry, incorporated in top
3 2 2 4 4 Single initial 4 inches of soil
4 4 4 8 8 treatment only
5 8 8 16 16
6 16 16 32 32
7 1 3 2 8 Once yearly Sprayed on soil surface
8 2 6 4 16 every spring pre-emergent to crops
9 4 12 6 24
10 1 6 2 14 Twice yearly Sprayed on soil surface
11 2 12 4 28 spring and fall pre-emergent to crops c
12 4 24 6 42


Experiment 1 on mineral soil was begun in October 1957. All of the simazine for Treatments 2 through 6 was applied at that time. Treatments
7, 8, and 9 received three annual applications and Treatments 10, 11, and 12 received six semiannual applications.
** Experiment 2 on organic soil was begun in April 1957. As in Experiment 1, all of the simazine for Treatments 2 through 6 was applied at that
time. Treatments 7, 8, and 9 received four annual applications and Treatments 10, 11, and 12 received seven semiannual applications.







Simazine in Florida Mineral and Organic Soils 7

All treatments were replicated four times, with the plots
arranged in randomized blocks. Plot size was slightly larger at
Zellwood than at Sanford due to the wider row spacing required
on the organic soil farm. All plots included four standard rows
or two multiple-row beds, 24 feet long. In Experiment 1 on the
sand, with rows spaced 30 inches apart, the plots were 10 feet
wide. They were 12 feet wide on the peat soil, where standard
row spacing is 36 inches.
The semiannual plantings included sweet corn and a wide
variety of simazine-sensitive crops. These were utilized to evalu-
ate the activity of the simazine present throughout the eight
crop seasons. Plant growth, green weight, and other factors
measurable in the field were selected in preference to plant and
soil chemical analyses because these led more directly to the
desired information. Growth of indicator plants as a measure
of herbicide residues remaining in soil also was used by Burschel
and Freed (3) and by Rahn and Baynard (13). The latter, work-
ing with monuron, reported that bioassays were superior to
chemical assays because the latter gave too high values when
related to plant toxicity. The fall plantings in each plot usually
consisted of four to six frost-hardy crops selected among beets,
carrots, endive, lettuce, peas, and several of the crucifers. The
latter included cabbage, collards, and turnips. Some of the
crucifers also were planted in the spring along with tender crops
such as beans, cucumbers, and squash. Oats were used occa-
sionally and were observed to be a good simazine-sensitive in-
dicator crop. This confirmed the work of Burnside and Behrens
(1), who listed oat susceptibility in the range of 0.25 ppmw in
contrast to 4 to 256 ppmw for corn. The chemical treatments
were discontinued after the spring 1960 applications. During
the fall of 1960, a uniform planting of oats was made over all
of both experimental areas to permit a final evaluation of results.
Data recorded during the course of these experiments include
the following:
1. Weed control ratings. Visual estimates of weed stunt-
ing and kill on the treated plots were made, comparing them with
the untreated check plots. These observations were recorded
approximately four weeks after each planting. The ratings were
based on a 0 to 10 scale with "0" indicating no control or no
effect on the weeds and "10" indicating complete control or
weed kill.
2. Crop tolerance ratings. A similar 0 to 10 scale was used
for rating crop tolerance, the reverse of crop injury. The "0"








8 Florida Agricultural Experiment Stations

rating was used for complete crop kill or no tolerance, while
"10" indicated normal development or full tolerance of the chem-
ical by the crop. Crop growth, as indicated by comparing the
development of the plants in the treated plots with those in the
checks, and also obvious simazine phytotoxicity symptoms were
considered in determining the tolerance ratings.
3. Crop yields. Fresh or green weights were taken from
12 feet of row for several of the crops. Frequently these were
whole plant weights instead of marketable yields since the crops
were not always carried to maturity.

RESULTS AND DISCUSSION
Since the plots of Treatments 10, 11, and 12 received addi-
tional simazine applications each season, the data for these
spray treatments reflect the crop responses to freshly applied
chemical as well as to incorporated residues in the soil from
earlier applications. This is also true with Treatments 7, 8,
and 9 during the spring trials. All crops except corn were ad-
versely affected by these new pre-emergent simazine applica-
tions.
Although corn was not a simazine-sensitive indicator crop,
it was planted at the start of the experiments and occasionally
thereafter to test the effects of the high simazine rates which
were generally lethal to the other crops. Rates of 8 or more
pounds per acre mixed into the soil just before planting pro-
duced some corn injury on both soil types the first season. In
addition to early stunting, the stalks were weakened and brace
root development was restricted by the higher simazine levels
on the peat soil. This resulted in severe lodging during a mid-
season storm on plots which had received 16 and 32 pounds per
acre. By maturity, however, the corn had recovered so that
there was little evidence of either stalk weakness or stunting.
Phytotoxicity to corn was apparent again six months later on
both soils, but one year after the original applications, corn
grown in all plots appeared normal.
Because of its high sensitivity to simazine, cabbage was se-
lected as a representative vegetable crop and used repeatedly
as an indicator during these studies. During the second crop
season, six months after the initial treatments were made, some
phytotoxic activity was evident in both soils. Tolerance ratings,
summarized in Table 2, show that in the spring of 1958 cabbage
was stunted on all mineral soil plots which had received 4 or








TABLE 2.-TOLERANCE OF CABBAGE TO TREATMENTS USED IN STUDIES ON THE ACTIVITY OF SIMAZINE RESIDUES IN TWO FLOR-
IDA SOILS.

Pounds Simazine Mineral Soil *Organic Soil **
per Acre per --
Application Spring Fall Fall Fall Fall Fall N
Treat- 1958 1958 1959 1957 1958 1959
ment Mineral Organic Application - ..- --. ..
No. soil soil Schedule Rating Rating Rating Rating Rating Yield Rating Yield
1 0 0 (Check) 10.0 10.0 10.0 10.0 10.0 31.9 10.0 13.0
2 1 2 10.0 10.0 10.0 10.0 10.0 34.8 10.0 13.2
3 2 4 Single initial 10.0 10.0 10.0 9.4 10.0 31.8 10.0 14.6
4 4 8 treatment 7.5 10.0 9.2 9.4 10.0 33.2 10.0 12.5
5 8 16 only 7.0 10.0 9.5 8.1 10.0 37.4 10.0 10.2
6 16 32 5.0 7.0 10.0 (.8 10.0 32.9 9.5 13.0
7 1 2 Once yearly 3.5(1) 10.0(1) 9.2(2) 8.6(1) 10.0(2) 32.2(2) 10.0(3) 12.0(3)
8 2 4 every spring 1.0(1) 10.0(1) 10.0(2) 10.0(1) 10.0(2) 33.8(2) 10.0(3) 9.5(3)
9 4 6 0.0(1) 10.0(1) 9.0(2) 7.1(1) 10.0(2) 29.0(2) 10.0(3) 9.0(3) S
10 1 2 Twice yearly 2.0(2) 0.0(3) 1.0(5) 4.4(2) 8.8(4) 31.6(4) 3.8(6) 5.4(6)
11 2 4 spring and 0.8(2) 0.0(3) 0.2(5) 3.2(2) 8.0(4) 32.8(4) 2.0(6) 2.3(6) .
12 4 6 fall 0.0(2) 0.0(3) 0.0(5) 0.3(2) 8.0(4) 17.8(4) 2.0(6) 0.0(6)


LSD (5% level) 1.2 0.7 1.2 2.0 0.6 NSD 1.0 4.4
(1%i level) 1.; 1.0 1.6 2.8 0.8 1.3 5.8


The simazine treatments 2 through 6 were applie'l in October 1957 on the minrc l soil and in April 1ll7 on the organic soil.
"** Tolerance ratings were based on 0 to 10 scale, as follows: 0 = no growth or complete kill of crop; 5 = -svere stunting or stand reduction; 10 =
normal stand and development.
Yield data are mean pounds per harvested plot.
Numerals in parentheses indicate the number of simazine applications made prior to observation date.

CO








10 Florida Agricultural Experiment Stations

more pounds of simazine the previous fall. At the end of one
year, only the 16 pound rate left active residues sufficient to
produce stunting. By the fall of 1959, after a lapse of two
years, all plots which had received the initial incorporated treat-
ments produced normal cabbage plants.
The initial treatments on the peat soil were made in the
spring, followed by six months of warm weather. At the end
of this time, there was some evidence of simazine activity with
all treatments involving 4 or more pounds per acre. However,
only the 32 pound treatment significantly reduced cabbage plant
growth. Even this activity disappeared during the next year.
Residual activity from the annual pre-emergent spray treat-
ments also was of short duration. Only once was there a clear
cut effect of a spring spray application on cabbage growth the
following fall. This occurred during the first year following the
6 pound spray treatment on the organic soil. On both soils,
Treatments 10, 11, and 12 resulted in measurable reductions
in cabbage growth as a result of fresh chemical applications.
Growth ratings and yield data for oats are given in Table 3.
By the fall of 1958, oats were no longer affected by single in-
corporation treatments on either soil. As with cabbage, after
the first year none of the sprays scheduled each spring (Treat-
ments 7, 8, and 9) had any effect on the next crop. Each of these
treatments involved applications up to 4 pounds on sand and 6
pounds on peat. In almost all cases, the yield data are in agree-
ment with the growth ratings.
Data similar to those given for cabbage and oats were col-
lected for several other crops during the course of these studies.
To facilitate presentation of the results, crop injury indices were
calculated by averaging all the tolerance ratings of the sensi-
tive crops (corn excluded) and subtracting this mean from ten.
This gave an injury index based on a 0 to 10 scale, with 0 in-
dicating no crop injury and 10 indicating complete crop kill.
These crop injury data, along with equivalent weed control
ratings for each season, are summarized for the two soil types
in Tables 4 and 5. Together these indices provide a concise
measure of simazine activity in the soil.
For graphical presentation, a further consolidation of the
data has been used, consisting of an average of the above de-
scribed weed control and crop injury indices. This resulted in
a total activity index, taking the observed responses of all sima-
zine-sensitive plants into consideration.






TABLE 3.-TOLERANCE OF OATS TO TREATMENTS USED IN STUDIES ON THE ACTIVITY OF SIMAZINE RESIDUES IN TWO CENTRAL FLORIDA SOILS.

Pounds Simazine Mineral Soil Organic Soil *
per Acre per
Application Fall Spring Fall Fall Spring Fall
Treat-- Applica- 1958 1960 1960- 1958 1960 19601
ment Mineral Organic tion --
No. soil soil Schedule* Rating Rating Yield Rating Yield Rating Yield Rating Yield Rating Yield
1 0 0 (Check) 10.0 8.0 1.7 10.0 3.4 10.0 11.2 10.0 0.7 10.0 3.4
2 1 2 Single 10.0 9.5 2.4 10.0 2.3 10.0 8.1 9.5 0.7 10.0 4.0
3 2 4 initial 10.0 9.8 2.2 10.0 2.8 10.0 11.5 9.0 0.3 10.0 3.0
4 4 8 treatment 10.0 9.0 1.8 10.0 3.0 10.0 9.1 9.0 0.3 10.0 4.2
5 8 16 only 10.0 9.5 2.4 10.0 3.2 10.0 6.6 8.0 0.4 10.0 2.8
( 16 32 10.0 9.5 2.6 10.0 3.4 10.0 8.4 9.2 0.4 10.0 2.6
7 1 2 Once 10.0(1) 8.0(3) 1.6(3) 10.0(3) 3.5(3) 10.0(2) 7.9(2) 9.0(4) 0.4(4) 10.0(4) 3.6(4)
8 2 4 yearly 10.0(1) 7.2(3) 1.1(3) 10.0(3) 3.0(3) 10.0(2) 7.0(2) 7.2(4) 0.4(4) 10.0(4) 4.1(4)
9 4 6 every 10.0(1) 1.5(3) 0.0(3) 10.0(3) 2.6(3) 10.0(2) 7.2(2) 6.0(4) 0.1(4) 10.0(4) 3.8(4)
spring
10 1 2 Twice 0.0(3) 6.8(6) 0.6(6) 10.0(6) 3.3(6) 9.5(4) 9.1(4) 7.8(7) 0.1(7) 10.0(7) 2.7(7)
11 2 4 yearly 0.0(3) 1.5(6) 0.0(6) 10.0(6) 3.0(6) 9.2(4) 4.4(4) 4.5(7) 0.2(7) 10.0(7) 3.8(7)
12 4 6 spring and 0.0(3) 0.2(6) 0.0(6) 10.0(6) 2.8(6) 10.0(4) 3.2(4) 4.2(7) 0.1(7) 10.0(7) 3.9(7)
fall


LSD (5% level) 0.1 2.4 1.1 ND NSD NSD NSD 1.8 0.4 ND NSD
(1% level) 0.2 3.2 1.4 2.4 NSD


"* The simazine treatments 2 through 6 were applied in October 1957 on the mineral soil and in April 1957 on the organic soil.
** Tolerance ratings were based on 0 to 10 scale, as follows: 0 = no growth or complete kill of crop; 5 = severe stunting or stand reduction; 10 = normal stand and
development.
Yield data are mean pounds per harvested plot.
Numerals in parentheses indicate the number of simazine applications made prior to observation date.
t During fall of 1960, oats were grown as a uniform test crop, and the usual simazine treatments were omitted.












TABLE 4.-SUMMARY OF WEED AND CROP RESPONSES TO SIMAZINE TREATMENTS ON LEON FINE SAND SOIL AT SANFORD.

Treatment 1957 1958 1959 1960
Pounds A Fall Spring Fall Spring Fall Spring Fall
simazine Application -
per acre schedule WC* CI** WC CI WC CI WC CI WC CI WC CI WC CI
0 (Check) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.1 0.0
1 Single 9.5 8.0 0.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.9 0.0 .
2 initial 10.0 10.0 1.6 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.2 0.0 (
4 treatment 9.8 10.0 2.5 2.5 0.0 0.0 0.2 0.0 1.5 0.2 2.3 0.0
8 only, Oct. 10.0 10.0 5.8 3.0 0.0 0.0 1.0 0.0 1.5 0.2 1.9 0.0
16 1957 10.0 10.0 8.6 5.0 0.0 2.0 2.0 1.0 2.5 0.1 0.8 v 0.0
1 1 Once yearly, 0.0 0.0 5.6 6.5 0.0 0.0 10.0 10.0 0.5 0.3 "o 2.8 0.0
2 every 0.0 0.0 6.0 9.0 0.0 0.0 10.0 10.0 0.0 0.0 4.6 0.0
4 spring 0.0 0.0 7.0 10.0 0.0 0.0 10.0 10.0 0.2 0.3 0 7.8 0.0
1 Twice yearly, 9.8 6.0 6.4 8.0 10.0 10.0 10.0 10.0 10.0 8.3 3.8 0.0
2 spring and 10.0 10.0 7.1 9.2 10.0 10.0 10.0 10.0 10.0 8.6 6.8 0.0
4 fall 10.0 10.0 8.9 10.0 10.0 10.0 10.0 10.0 10.0 9.1 9.6 0.0


LSD (5% level) 1.0 1.2 1.2 1.5 0.7 0.6 0.3 1.4 0.9 2.7 ND
(1% level) 1.3 1.6 1.6 2.0 1.0 0.8 0.4 1.9 1.2 3.6

Weed control index: Mean of all weed control ratings, based on 0 to 10 scale, as follows: 0 = no weed control or injury; 5 = approximately 50%
reduction in weed stand or growth; 10 = complete weed kill.
** Crop injury index: Mean of all tolerance ratings observed for simazine-sensitive crops, subtracted from 10, giving 0 to 10 scale as follows: 0 =
no crop injury; 5 = approximately 50% reduction in crop stand or growth; 10 = complete crop kill.










TABLE 5.-SUIMMARY OF WEED AND CROP RESPONSES TO SIMAZINE TREATMENTS ON EVERGLADES MUCKY PEAT SOIL AT ZELLWOOD.

Treatment 1957 1958 1959 1960
Pounds Spring Fall Spring Fall Spring Fall Spring Fall
simazine Application --
per acre schedule WC* CI WC CI WC CI WC CI WC CI WC CI WC CI WC CI
0 (Check) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0
2 Single 1.1 5.8 1.9 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0
4 initial 5.8 7.8 0.8 1. 0.3 0.0 0.0 0.0 0.0 0 0.0 0.0 0.4 0.0
8 treatment 8.3 8.0 2.8 3.0 1.1 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.0
16 only, April 9.7 8.6 2.3 3.0 2.8 0.0 0.0 0.0 0.0 0.0 0.0 1.7 0.0
32 1957 10.0 % 9.4 6.1 5.4 3.0 0.0 0.0 0.0 0.0 a 0.3 0.0 0.; 0.0
2 Once yearly, 6.4 p 8.0 1.2 9.4 2.2 0.0 0.0 9.5 9.0 0.0 9.0 1.5 0.0
4 every 8.6 c 5.8 0.6 10.0 5.6 0.0 0.0 10.0 9.5 0.0 9.0 2.9 0.0
6 spring 9.4 8.3 1.2 10.0 7.2 0.0 0.03 10.0 10.0 0.0 9.5 4.7 z 0.0
2 Twice yearly, 7.6 o 10.0 2.8 9.7 3.9 9.2 4.2 9.5 9.0 7.4 9.5 2.2 0.0
4 every 8.7 Z 8.8 4.3 10.0 7.8 9.5 5.3 10.0 9.5 1 8.4 9.5 4.7 0.0
6 fall 8.9 10.0 6.5 10.0 10.0 9.8 5.5 10.0 10.0 8.4 9.5 5.9 0.0


LSD (5% level) 1.5 2.4 2.2 1.8 1.4 0.3 0.6 0.6 1.0 0.6 1.2 2.2 ND
(1% level) 2.0 3.2 2.9 2.4 2.0 0.4 0.8 0.8 1.3 0.9 1.6 2.9

Weed control index: Mean of all weed control ratings, based on 0 to 10 scale, as follows: 0 no weed control or injury; 5 = approximately 500, reduction in wee!l
stand or growth; 10 = complete weed kill.
"** Crop injury index: Mean of all tolerance ratings observed for simazine-sensitive crops, subtracted from 10, giving 0 to 10 scale as follows: 0 = no crop injury;
5 = approximately 50(,' reduction in crop stand or growth; 10 = complete crop kill.








14 Florida Agricultural Experiment Stations

In mineral soil, 1 pound of simazine applied in the fall
showed no activity when the next crops were planted the fol-
lowing spring. Applications of 2, 4, and 8 pounds affected
the spring plantings, but, after an intervening summer season,
showed no residual activity towards crops and weeds one year
after the simazine was incorporated into the soil. The 16 pound
treatment continued to influence weed growth in the plots dur-
ing the fifth crop season, two years after the October 1957 ap-
plications. Figure 1 shows the relative responses to the 1 and

10

9 MINERAL SOIL SINGLE APPLICATION

8

x 7
w
z 6

S5
S--*-- I POUND / ACRE
4
S- \ 16 POUNDS /ACRE







0I
FALL SPRING FALL SPRING FALL SPRING FALL
1957 1958 1959 1960

Figure 1.-Activity of simazine over a three-year period following in-
corporation of 1 and 16 pound per acre applications in Leon fine sand in
October 1957.

16 pound treatments. In the spring of 1960, growth was erratic
in several of the plots. The apparent crop injury at the 1 pound
rate was not statistically significant.
Simazine activity persisted in organic soil only through the
third crop season, ending 11 years after the single incorporated
applications were made. (See Table 5 and Figure 2.) Even
the 32 pound treatment applied in April 1957 left no evidence
of toxic residues by the fall of 1958. Figure 3 shows one of









10

9- ORGANIC SOIL SINGLE APPLICATION

"8- ---- 2 POUNDS /ACRE
\ --A-- 32 POUNDS /ACRE
7-
x



/ \
S6-\

S5- \




4 / \
2 //
3-
/ \ \


-\ -
,-,--- -----4" --- ~- ~,'-

SPRING FALL SPRING FALL SPRING FALL SPRING FALL
1957 1958 1959 1960
Figure 2.-Activity of simazine over a three and one-half year period
following incorporation of 2 and 32 pound per acre applications in Ever-
glades peaty muck in April 1957.



























Figure 3.-All six vegetable crops in plot A6 made normal growth
during the fall 1959 crop season. This plot on organic soil at Zellwood
received a 32 pound per acre simazine treatment in April 1957. A fresh
pre-emergent application of 4 pounds per acre of simazine was applied to
the adjacent plot in the background. Garden peas in the righthand row
was the only crop displaying tolerance to this treatment.








16 Florida Agricultural Experiment Stations

these plots the following year, during the third successive crop
season after growth returned to normal.
A comparison of the rates of simazine depletion or inactiva-
tion reveals little difference between these two soils. Figures
4 and 5, giving curves for the 2 and 16 pound per acre treat-
ments, portray the decreasing activity index in these soils over
a three-year period. Where only 2 pounds were applied, evi-
dence of remaining activity appeared only with one succeeding
crop on the sand and two such crops on the peat. With the
16 pound treatment, simazine activity in the organic soil had
completely dissipated by the time the third succeeding crop was
planted, 1 years after application. A low level of activity
from the 16 pound treatment on the sand persisted much longer,
until the spring of 1960, 21/ years after the treatment date. The
uniform response by oats throughout both experimental fields
in the fall of 1960 verified that the dissipation of chemical had
proceeded to subtoxic levels in all plots. The persistence of sim-
azine activity in the two soils from each of the single applica-
tion treatments is graphically presented in Figure 6.
As mentioned previously, this greater inactivation of herbi-
cides by fine textured and high organic matter soils has been
observed by several workers. This effect may be due partially
to greater colloidal absorption, as proposed by Ogle and War-
ren (11). In many cases, however, the depletion from the soil
is believed to be due mainly to the action of soil organisms which
break the chemical down into inactive products (4). Evidence
of this microbial activity, commonly much greater in fine tex-
tured soils, especially those high in organic matter, was ob-
tained in the laboratory by Sheets (16). He found that ethyl-
N,N-di-n-propylthiolcarbamate (EPTC) was inactivated in au-
toclaved soil only one-third as fast as in similar untreated soil.
Also, Ragab and McCollum (12) observed that there was very
little decomposition of C14-labeled simazine in sterilized soil in
contract to unsterilized soil. Recently, Burnside, Schmidt, and
Behrens (2) observed that simazine in soil was deactivated un-
der conditions favorable to microorganisms. Using non-sterile
Waukegan silt loam incubated at field moisture capacity and
85 degrees F, the phytotoxicity of 4 ppmw of simazine was
reduced 90 percent during 10 months. In this experiment, no
deactivation occurred in sterile soil.
Further evidence in support of this microbiological inacti-
vation of simazine in the soil is provided by the influence of ex-
ternal environmental factors. Working with 2-chloro-N,N-di-









10

9 SINGLE APPLICATION 2 POUNDS PER ACRE

8 -


x 7 --- MINERAL SOIL

z 6 ---- ORGANIC SOIL

5

-4
3 A


2 /


0\
; ^ '^ ^- ^,....._

S 1/2 I 1/2 2 2/2 3 3

YEARS AFTER TREATMENT APPLIED

Figure 4.-Activity of simazine in mineral and organic soils following
incorporation of single 2 pound per acre applications at the beginning of
the experiments.

10

9- SINGLE APPLICATION 16 POUNDS PER. ACRE

8-

x 7-
"m- MINERAL SOIL
z 6 --"-- ORGANIC SOIL

>5
I-

0
4 \

\\
3

2
S\




0 1/2 I I1'2 2 21/2 3 31/2
YEARS AFTER TREATMENT APPLIED

Figure 5.-Activity of simazine in mineral and organic soils following
incorporation of single 16 pound per acre applications at the beginning
of the experiments.








18 Florida Agricultuvral Experiment Stations

allylacetamide (CDAA) and 2-chloroally diethyldithiocarbamate
(CDEC), Gantz and Slife (7) noted faster disappearance of toxic
residues in soil at 80 to 100 degrees F than when the tempera-
ture was held at 40 to 60 degrees. Also, toxicity decreased more
rapidly in moist than in dry soils. Likewise, Switzer and Rau-
ser (18) observed that simazine dissipation increased with soil
temperature, rainfall, and aeration. Whereas 2 pounds per acre
of simazine lost its effectiveness in eight weeks under warm
moist summer conditions, its residues in dry soil continued to
damage oats after 12 months. The influence of aeration was
also pointed out by Rahn and Baynard (13), who observed that
the disappearance of monuron from asparagus soils was accel-
erated by cultivation.
SINGLE APPLICATION


SjMINERAL SOIL
ORGANIC SOIL
2 MAXIMUM LIMIT OF EFFECT.

Wu STATISTICALLY SIGNIFICANT
"o EFFECT.
4-


I
z




N 16





I I I
32-



S 1/2 I 1 /2 2 21/2 3 31/2
YEARS AFTER TREATMENT APPLIED
Figure 6.-Relative persistence of simazine activity in mineral and
organic soils resulting from single initial applications ranging from 1 to
32 pounds per acre.

Favorable environmental conditions keep microbiological ac-
tivity in Florida organic soils at a high level. It is especially
high throughout the warm part of the year. This is partially
due to the large moisture holding capacity of these soils, which







Simazine in Florida Mineral and Organic Soils 19

approaches 200 percent, approximately 10 times that of the
local mineral soils, making them more resistant to drought.
These factors may well explain the more rapid rate of decrease
of simazine activity observed in the organic soil in these experi-
ments. Also, the high rainfall and temperatures which are
normal for this area may help explain the early dissipation of
simazine residues in contrast to that observed in cooler or drier
climates. At Sanford, the mean annual rainfall over the period
of this experiment was 52.9 inches, and the mean annual tem-
perature was 72.2 degrees.
Simazine treatment appears to have no adverse effects on
the high level of microbiological activity in Florida soils. This
has been demonstrated by Eno (6) using soil from the 0, 1,
and 16 pound sand soil plots described here. He studied samples
taken while the chemical activity was still high, five months after
the simazine was applied. Working with the soil flora, he ob-
served that there was no reduction in the numbers of fungi and
bacteria, or in the production of carbon dioxide and nitrate ni-
trogen, as a result of the treatments. Burnside, Schmidt, and
Behrens (2) have reported similar results.
Treatments 7 through 12 in each of these experiments were
designed to study the effects of repeat annual and semiannual
applications which might be made for the production of com-
mercial crops. A gradual increase in phytotoxicity from the
treatments would indicate that simazine residues were accumu-
lating in the soil. The total amount of chemical for each treat-
ment applied during the course of the experiments is given in
Table 1, and the calculated weed control and crop injury in-
dices are listed in Tables 4 and 5. Residual activity, measur-
able in the plots of Treatments 7, 8, and 9 each fall, was observed
only during the first year on the organic soil. These plots
showed no evidence of residual simazine activity thereafter. By
the spring of 1960, the highest level treatment on the sand had
received a total of 24 pounds per acre of simazine. The com-
parable peat soil plots had been treated with a total of 42 pounds.
When the regular fall sprays were omitted and oats were planted
on all of the plots during the fall of 1960, it was impossible to
detect toxicity from any of the treatments. Figure 7 shows the
levels of simazine activity during each of the crop seasons for
the 4 pound rate. Except for the response observed on the
organic soil during the fall of 1957, in all cases simazine activity
from the spring applications was reduced to innocuous levels
within six months.








20 Florida Agricultural Experiment Stations

REPEATED PRE EMERGENCE TREATMENT
4 POUNDS PER ACRE PER APPLICATION

"MINERAL SOIL I
---- ORGANIC SOIL
MINERAL SOIL SPRING AND
--A-- ORGANIC SOILJ FALL
10

9-





2 6 \

5-
" I I
> 4 \
0 | I i \ \






0
SPRING FALL SPRING FALL SPRING FALL SPRING FALL
1957 1958 1959 1960
Figure 7.-Relative activity of simazine in mineral and organic soils
resulting from both annual and semiannual 4 pound spray applications
ending in the spring of 1960.

Instead of revealing accumulation, the data from these trials
suggest that the rate of dissipation of simazine activity from
soils may increase with repeat applications, possibly due to fur-
ther development or adaptation of soil microorganisms to break-
down the chemical. In the peat soil at Zellwood, activity of res-
idues from the spring 1957 applications persisted to injure the
succeeding fall crops, but this has not occurred since, even with
the high 6 pound per acre treatments. Reid (14) has stated
that many weed killers are used as a source of nutriment by
some soil bacteria. In most cases in his tests, the bacteria
adapted themselves to make use of the chemical as food. He
postulates that, given proper temperature, moisture, and air
supply, none of the herbicides can last in soil for a long period
of time. Additional studies to clarify this hypothesis with re-







Simazine in Florida Mineral and Organic Soils 21

gard to the role of soil microorganisms in determining the rates
of simazine breakdown, persistence, and accumulation in soil are
desirable.

SUMMARY AND CONCLUSIONS
The activity of simazine persisting in two Florida soils was
studied over the period from 1957 to 1960 using corn, cabbage,
oats, and several other triazine-sensitive indicator crops. In
order to determine the rate of dissipation of this residual ac-
tivity, 1, 2, 4, 8, and 16 pounds per acre of simazine were ap-
plied initially to Leon fine sand, and 2, 4, 8, 16, and 32 pounds
per acre were applied to Everglades mucky peat. Activity loss
and build-up were also studied using both annual and semiannual
applications of 1, 2, and 4 pounds per acre of simazine on the
sand and 2, 4, and 6 pounds on the peat. Crop plantings were
made twice each year. Crops and weeds were rated for stand
and development, and fresh plant weights were recorded for
several of the plantings to determine the chemical activity as
reflected by plant response.
Among the characteristic responses were the following:
1. Sweet corn on both soils was injured by 8 or more pounds
of simazine during the first crop season after application. Defi-
nite injury to corn attributable to simazine residues was re-
stricted to the organic soil and occurred only during the second
season six months after treatment.
2. Cabbage on sand grew .normally after six months where
1 and 2 pounds of simazine were applied, after one year where
rates up to 8 pounds were applied, and after two years where
16 pounds were used. On peat six months after treatment,
cabbage showed no significant visible evidence of simazine ac-
tivity from application rates up to 16 pounds per acre. After
11V. years, cabbage in the 32 pound organic soil plots no longer
showed toxicity.
3. The first time oats were planted during the fall of 1958,
they grew without evidence of injury in all plots, except those
freshly treated. This included simazine treatments on mineral
soil ranging from 1 to 16 pounds per acre incorporated one year
earlier and up to 4 pounds sprayed six months before. On or-
ganic soil there was no activity against oats from treatments
as high as 32 pounds incorporated 18 months before and up
to 6 pounds sprayed six months before.








22 Florida Agricultural Experiment Stations

4. Activity indices for simazine were computed by combin-
ing the responses of all sensitive plants, including the weeds.
These revealed that:
a. In fine sand soil, 1 pound per acre was completely
inactivated during the first crop season (within six months);
2, 4, and 8 pounds affected crops for two seasons (one year);
and 16 pounds showed activity against five successive crops
(21/2 years).
b. In peat soil, 2 and 4 pounds showed significant effect
for only two seasons (one year), and 8, 16, and 32 pounds lasted
three seasons (11 years).
5. Zero activity indices at the conclusion of the experiments
indicate that there were no accumulations of toxic simazine res-
idues in the soil from either annual of semiannual spray treat-
ments at rates up to 4 pounds per acre for three years on the
mineral soil or 6 pounds per acre for 31/2 years on the organic
soil. Based on the plant response observed, dissipation appeared
to be complete from these applications totaling 24 pounds and
42 pounds per acre on the two soils, respectively.
Whereas 16 pounds of simazine remained phytotoxic to sensi-
tive plants 212 years in the light sand soil, 32 pounds in peat
showed no activity after 18 months. This rapid rate of herbi-
cide breakdown found in fine textured and high organic matter
soils has been reported previously by several other workers who
have attributed it to a higher level of microbiological activity
than commonly occurs in light mineral soils. Reported effects
of soil temperature, soil moisture, and aeration support this,
since they influence the activity of the microorganisms. The
year-around warm and humid climatic conditions in Florida may
be responsible for the more rapid depletion of residual simazine
activity from both of these soils in comparison with others in
cooler or drier climates.

LITERATURE CITED
1. Burnside, O. C., and R. Behrens. Phytotoxicity of simazine. Weeds
9:145-157. 1961.
2. Burnside, O. C., E. L. Schmidt, and R. Behrens. Dissipation of sima-
zine from the soil. Weeds 9: 477-484. 1961.
3. Burschel, Peter, and V. H. Freed. The decomposition of herbicides in
soils. Weeds 7: 157-161. 1959.









Simazine in Florida Mineral and Organic Soils 23

4. Chandra, Purna, W. R. Furtick, and W. B. Bollen. The effects of four
herbicides on microorganisms in nine Oregon soils. Weeds 8: 589-
598. 1960.

5. Davis, D. E., H. H. Funderburk, Jr., and N. G. Sansing. The absorp-
tion and translocation of C"-labeled simazine by corn, cotton, and
cucumber. Weeds 7: 300-309. 1959.

6. Eno, C. F. 1958. Nitrification in Florida soils, effect of soil applica-
tions of simazine on the microflora. Fla. Agr. Expt. Sta. Ann.
Rept. 1958: 160.

7. Gantz, R. L., and F. W. Slife. Persistence and movement of CDAA
and CDEC in soil and the tolerance of corn seedlings to these herb-
icides. Weeds 8: 599-606. 1960.

8. Meadows, M. W., and Ora Smith. Effect of temperature, organic mat-
ter, pH, and rates of application on persistence of 2,4-D in soil.
Proc. Noeast. Weed Control Conf. 3: 24-29. 1949.

9. Montgomery, M., and V. H. Freed. The absorption, translocation, and
metabolism of triazine herbicides by corn. (Abstr.) Proc. Weed
Soc. Amer. 1960: 41.

10. The uptake, translocation, and metabolism of simazine
and atrazine by corn plants. Weeds 9: 231-237. 1961.

11. Ogle, R. E., and G. F. Warren. Fate and activity of herbicides in soils.
Weeds 3: 257-273. 1954.

12. Ragab, M. T. H., and J. P. McCollum. Degradation of C"-labeled sima-
zine by plants and soil microorganisms. Weeds 9: 72-84. 1961.

13. Rahn, E. M., and R. E. Baynard, Jr. Persistence and penetration of
monuron in asparagus soils. Weeds 6: 432-440. 1958.

14. Reid, J. J. Bacterial decomposition of herbicides. Proc. Noeast. Weed
Control Conf. 14: 19-30. 1960.

15. Sheets, T. J. The comparative toxicities of monuron and simazine in
soil. Weeds 7: 189-194. 1959.

16. -- Effects of soil type and time on the herbicidal activity of
CDAA, CDEC, and EPTC. Weeds 7: 442-448. 1959.

17. Uptake and distribution of simazine by oat and cotton
seedlings. Weeds 9: 1-13. 1961.

18. Switzer, C. M., and W. E. Rauser. Effectiveness and persistence of
certain herbicides in soil. Proc. Noeast. Weed Control Conf. 14:
329-335. 1960.





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