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Techniques for determining the effects of tillage on the distribution of sicklepod (Cassia obtusifolia L.) seed in the soil

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Title:
Techniques for determining the effects of tillage on the distribution of sicklepod (Cassia obtusifolia L.) seed in the soil
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Cassia obtusifolia
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Hacker, Larry Augustus, 1956-
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English
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vi, 98 leaves : ill. ; 28 cm.

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Subjects / Keywords:
Artificial seeds ( jstor )
Buried seeds ( jstor )
Infestation ( jstor )
No tillage ( jstor )
Plowing ( jstor )
Seeds ( jstor )
Soil samples ( jstor )
Soils ( jstor )
Tillage ( jstor )
Weeds ( jstor )
Agronomy thesis Ph. D
Dissertations, Academic -- Agronomy -- UF
Sicklepod ( lcsh )
Tillage ( lcsh )
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1986.
Bibliography:
Bibliography: leaves 90-96.
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Typescript.
General Note:
Vita.
Statement of Responsibility:
by Larry Augustus Hacker.

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University of Florida
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Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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TECHNIQUES FOR DETERMINING THE EFFECTS OF TILLAGE ON THE
DISTRIBUTION OF SICKLEPOD (Cassia obtusifolia L.) SEED
IN THE SOIL








By


LARRY AUGUSTUS HACKER


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


May 1986




TECHNIQUES FOR DETERMINING THE EFFECTS OF TILLAGE ON THE
DISTRIBUTION OF SICKLEPOD (Cassia obtusifolia L.) SEED
IN THE SOIL
By
LARRY AUGUSTUS HACKER
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
May 1986


ACKNOWLEDGMENTS
I would like to acknowledge my very true sincere
appreciation to Dr. Wayne Currey, Dr. David Hall, Dr. Jerald
Milanich, Dr. Sherlie West, and Dr. David Teem. I would
especially like to acknowledge Dr. David Teem and Dr. Wayne
Currey for their continuous support and guidance throughout
my graduate program. I thank them most for understanding
the problems encountered during the research and helping me
to overcome them.
Thanks are also extended to Dr. Larry Fitzgerald,
Dr. Charles Beatty, and Lester Burch for their help in the
preparation of the tools and loan of equipment necessary for
the completion of this work.
Finally, I would like to dedicate this work to my wife,
Barbara and my twin daughters Lauren and Lindsey for the
many sacrificies that they made during the course of this
study and preparation of this manuscript. Without their
support, love, and strength it could not have been achieved.
li


TABLE OF CONTENTS
PAGE
ACKNOWLEDGMENTS ii
ABSTRACT V
CHAPTERS
ILITERATURE REVIEW 1
Techniques of Seed Recovery 1
Estimation of Seed Number 5
Longevity 6
Number of Seeds Produced 9
Cropping Systems Effects 10
Effects of Chemical Weed Control on
Seedbanks 18
Distribution of Seedbank 21
Predicting Weed Problems 25
IITECHNIQUES FOR DETERMINING SEED DISTRIBUTION... 28
Introduction 28
Methods and Materials 30
Results and Discussion 40
IIICOMPARISON OF TECHNIQUES FOR DETERMINING SEED
DISTRIBUTION 53
Introduction 53
Methods and Materials 55
iii


Results and Discussion 59
IV COMPARISON OF ACTUAL AND PREDICTED EMERGENCE
GENERATED FROM DISTRIBUTION PATTERNS 71
Introduction 71
Methods and Materials 72
Results and Discussion 76
V SUMMARY AND CONCLUSIONS 85
Techniques 85
Tillage Systems 86
Predicting Infestations 88
Future Studies 88
LITERATURE CITED 90
BIOGRAPHICAL SKETCH 97
iv


Abstract of Dissertation Presented to the Graduate
School of the University of Florida in Partial
Fulfillment of the Requirements for the
Degree of Doctor of Philosophy
Techniques for Determining the Effects of Tillage on the
Distribution of Sicklepod (Cassia obtusifolia L.) Seed
in the Soil
By
Larry Augustus Hacker
May, 1986
Chairman: Dr. David H. Teem
Major Department: Agronomy
Field experiments were conducted from 1982 to 1984 at
the University of Florida to examine new techniques for
determining the distribution of sicklepod (Cassia
obtusifolia L.) seed in the soil following conventional,
minimum, and no-tillage operations.
Techniques involved were the development of a new
sampling device, synthetic seeds, and analysis by x-ray.
The sampling device and synthetic seeds were mainly
developed to assist in the use of x-ray for seed recovery.
Comparisons were made between the conventional and new
techniques to determine their effectiveness.
The results indicated that the new methods were
accurate tools for the study of seed distribution. The
synthetic seeds were effective in terms of providing
v


recapture with relatively no degradation. Two load
materials were used to load the synthetic seeds. Barium
sulfate was the best in that it allowed for better visual
separation on the radiographs. The x-ray technique was
highly effective when compared to an older wash method in
that it was faster and more accurate.
Conventional tillage was shown to result in a
distribution of approximately 75-80% of the recovered
synthetic seeds below the 10 cm level following a single
operation. However, following the second operation, the
distribution became more uniform throughout the working
depth. Significant differences were obtained between the
single and double tillage treatments with increases in the
top 6 cm and reductions in lower levels of the profile.
Minimum and no-tillage had similar distribution patterns for
all sample times with 80-90% of the synthetic seeds
recovered in the top 6 cm.
Infestation levels were found to vary between tillage
systems. Means of 10-14, 50-57, and 44-58 percent emergence
of the initial dispersed viable seeds were recorded for the
conventional, minimum, and no-tillage plots, respectively.
Thus a definite relationship exists between the resulting
distribution and later infestation levels. The conventional
tillage plots resulted in the lowest infestation numbers,
yet it was at the cost of an increase to the seedbank in the
soil.
vi


CHAPTER I
LITERATURE REVIEW
Techniques of Seed Recovery
The study of weed seeds in the soil requires that
efficient techniques of seed recovery be developed. Most of
the early research was conducted in an attempt to examine
the different weed flora of various locations. Some early
work, reported to be the first, was that conducted by
Putensen (34) in which soil samples were removed at three
soil depths. The samples were spread in trays and placed in
the greenhouse, and the number of emerging seedlings was
recorded to determine the number of viable seeds. He also
collected the remaining seeds by wet-sieving the soil
material after germination had ceased. Snell (58) utilized
this technique in his study in which he sampled the soil
seedbank in 10 cm increments to a depth of 120 cm and
monitored the germination at each increment. Later,
Brenchley and Warington (3,4,5) conducted experiments
utilizing a sampler that removed a 3 x 4 x 6 in deep plug of
soil. They incorporated this sampling method in studies
examining weed seed populations in fallow systems,
cultivation programs, and re-establishment of weed species
following various programs. Many researchers have utilized
1


2
variations of both these sampling methods and analysis of
cores removed for viable seed estimations.
An early problem was noted in respect to soil sampling
for estimation of seedbank size in that statistical analysis
often varied between sampler size and number of samples
removed (6,37). Roberts (37) tested two different sampler
sizes in an attempt to better understand this problem. He
collected data from a small sampler (2.3 cm internal
diameter) and a large sampler (4.4 cm internal diameter).
Three samples were extracted from each plot with the larger
device and eight samples from each plot with the smaller
device. All samples were removed to a depth of 15 cm.
Results indicated that by utilizing the smaller sampler,
removing a greater number of samples increased precision.
This probably minimized the chance of variation resulting
from collecting larger samples in areas where seeds
aggregated. This effect was also reported by Champness (6).
Robinson (53) compared the effect of the number of samples
removed with the same device. In this study, two large
samples of 154 plugs each or four small samples of 19 plugs
each were taken with a sampler of approximately 1 in
internal diameter. The data collected showed that the
degree of varability was less with the smaller sample size.
This was supported by the fact that the smaller sample size
gave much higher estimations of seed number and therefore
could explain very high populations reported in past
studies. In order to better analyze the samples, Robinson


3
handled each method in a different manner. The large
samples were washed using sieves of various sizes. Then the
collected material was subjected to the germination test.
The smaller samples were handled by spreading the collected
soil in trays, placing them in favorable conditions for
germination of viable seeds, and counting emerging
seedlings. Although the methods were different, the
importance of sample number is clearly shown.
Most of the early work involved the estimation of weed
seed population entirely on the emergence test in which the
samples were placed in containers, subjected to a favorable
controlled environment and monitoring emergence. This
method allows for good estimations of viable weed seeds.
However, it typically requires large amounts of greenhouse
space and long time frames to allow total germination of
viable seeds to occur. This time period also involves
careful monitoring of emergence since some species die
rapidly following emergence. Kropac (24) discussed the work
of various researches and their many modifications of a new
flotation method of separation. It was felt that this new
method was somewhat costly, time consuming, and possibly
could be detrimental to seed viability if careful selection
of density altering chemicals was not utilized. Malone et
al. (27) utilized a flotation method of seed separation in
which seed viability was not altered. Separation was
obtained rapidly and effectively by mixing a solution of
sodium hexametaphosphate, sodium bicarbonate and magnesium


4
sulfate. The first two chemicals aided in the breakdown of
soil aggregates and enhanced the separation while the
magnesium sulfate allowed for the flotation of the seeds.
This material was then decanted off and wet-sieved after
which seeds were separated by hand with the aid of a
stereomicroscope.
Following the development of synthetic screen materials
new methods were developed for seed extraction from the
soil. Fay and Olson (16) utilized mesh bags to remove seeds
from soil samples. Samples were placed in nylon bags which
were then placed in a washing device that gently agitated
the sample with the aid of an electric motor. The bags were
then removed and seeds obtained. Thorsen and Crabtree (64)
also used a mechanical washing device to wash away the soil
in polyester bags. Their device was motorized such that a
container tumbled and was flushed with a continuous water
flow. They also stated that the device could be modified
for unbagged samples by placing a mesh material on the
bottom of the container. Following the washing of free
samples, they suggested that the collected material could
then be separated by using sieves of various sizes.
The problem with the flotation method, or any method
involving wet sieving, is the subsequent separation of the
collected seeds. Several solutions have been offered in
this regard. The obvious is hand separation, which is
extremely laborious especially in situations with large
numbers of seeds represented by various species. Although


5
this method is slow, the early research on seed recovery
typically involved the germination test in which the
collected soil samples were spread out in greenhouses for
germination, identification, and counts. This method
required extended time frames, often greater than three
years, in a controlled environment. In order to decrease
the time requirements for such studies, other methods have
been developed. Standifer (60) utilized air velocity to
separate different species of seeds from collected residue
and found it to be an effective method; however, this method
is somewhat less precise than desired. Based on these
observations, it appears that researchers should focus on a
particular species to reduce the time requirements. Fay and
Olson (16) demonstrated that the time required could be
greatly reduced by focusing on one species. They examined
the distribution of wild oat (Avena fatua L.) in a study and
found that the subsequent separation was rather easy due to
the elimination of the other species in the samples
collected.
From this discussion it is evident that several
techniques have been utilized to determine the actual seed
number, and that the importance of new techniques is
important to future research on the study of soil seedbanks.
Estimation of Seed Number
All of the methods described above have been utilized
to estimate the number of seeds in various soils.
Researchers estimated seed numbers in the cultivated soil


6
2
layers to range from 10,000 to 100,000 seeds/m (2,7,
11,22,27,28,29). Many factors are interrelated in regard to
seed bank populations some of which include longevity,
number of seeds produced, cropping practices and herbicide
applications.
Longevity
Many studies have examined the longevity of weed seeds
in the soil (14,15,23,26,67). Probably the best known study
was initiated by Beal who placed seeds of 20 different
species mixed with sand in pint bottles. The uncorked
bottles were then buried about 46 cm below the surface.
They were tilted to prevent accumulation of water in the
containers. Since the initiation of the study, bottles were
exhumed and the seeds subjected to germination tests.
Nearly half of the species had lost all viability by the
fifth year; however, eleven species were shown to be viable
after 20 years, two species were viable after 70 years, and
three species were found to be viable after 100 years
(14,23). A similar study was initiated by Duvel and
reported by Toole et al. (66) in which 107 species were
placed in sterilized soil buried in flower pots covered with
a porous saucer. These studies indicated the importance of
seed longevity in respect to potential problems from buried
weed seed. Apart from longevity of buried seeds in
undisturbed soil, these studies do not demonstrate the
effects of disturbances to the soil profile in respect to
longevity.


7
Another aspect of seed longevity is that of dormancy
mechanisms. Roberts (36) suggested that weed seeds which
are buried in the soil are subjected to some form of
dormancy if they are to survive. Virtually all seeds are at
some time during their burial subjected to adequate moisture
conditions for germination. The degree in which soil
moisture affects seed longevity has been examined in terms
of degeneration of seeds (25,67). Villiers (67) suggested
that weed seeds often have a repair mechanism that allows a
reduction in the rate of degeneration, enabling the seeds to
persist for longer periods. However, as shown in earlier
research, once soil cores are subjected to optimum
germination conditions rapid emergence follows. Therefore,
the rapidity of this response indicates that before these
seeds were disturbed, they were held in check by "enforced"
dormancy mechanisms. Enforced dormancy as described by
Harper (19) is a condition when viable seeds do not
germinate because of some limitation in their environment.
This type of dormancy is typically used to describe the
dormancy that is found on seeds buried in the soil, due to
the fact that germination occurs following extraction from
the soil and placement in conditions conducive to
germination. Although "enforced" dormancy is important, it
is not the only mechanism at work on buried seeds. Possibly
all three types of dormancy as described by Harper (19) are
found in some species. A better understanding of dormancy
in relation to seeds in the soil seedbank is needed.


8
One common dormancy mechanism in relation to weed seeds
is seedcoat hardness. Lueschen and Andersen (26) have an
ongoing study involving velvetleaf (Abutiln theophrasti
Medic.) in which they found seedcoat to be important in
terms of hardness. Along with data from other studies, much
work has shown the importance of seedcoat hardness as a
dormancy mechanism or survival mechanism. Stoller and Wax
(62) discussed the importance of seedcoat hardness of three
species: common ragweed (Ambrosia artimisiifolia L.),
ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.], and
velvetleaf. They stated that this one characteristic was
possibly the most important in relation to survival of these
species in the soil. They also reported that depth of
burial was important from a standpoint of seed decay.
Results indicated that decay was faster at the 2.5 cm depth
than at the 10.2 cm depth.
Creel et al. (12) examined the seedcoat of sicklepod
(Cassia obtusifolia L.) and found it had a hard wax-like
covering that was approximately 0.1 mm thick. The embryo
itself was surrounded by a dense hard material. He also
subjected unscarified seeds to burial for 12 months at 23 C
in moist soil while germination was monitored. Results
demonstrated that only 15% of the buried seeds germinated,
and 5% of these did so after only two days. The other 10%
germinated over a thirty day period. Also working with
sicklepod and coffee senna (Cassia occidentalis L.), Teem et
al. (63) found that both species germinated under a wide


9
range of temperatures from 24 to 36 C with optimum seedling
growth from 30 to 36 C. They also found the seeds of both
species to respond to various types of scarification and
that both species can emerge from a depth of 12.7 cm. The
percent emergence was low being only 25% for sicklepod and
50% for coffee senna. Both species still demonstrated the
ability to emerge from these depths which greatly increases
their survival chances. In relation to the rate of
emergence, sicklepod was shown to obtain 63% emergence after
only three days from a depth of 2.5 cm. This rapid flush of
seedlings in a field situation would create a large problem
in terms of weed control and crop competition, which in turn
could possibly generate new seed sources for the soil
seedbank.
These studies indicate the importance of seed longevity
in terms of burial in the soil and also demonstrate the
potential problems that can occur in terms of weed
infestation. Yet, apart from the longevity of buried seeds
and some discussion of dormancy mechanisms which affect
survival, they do not offer explanations of longevity in
relation to soil disturbance and to the soil profile.
Number of Seeds Produced
In general, weeds are extremely dynamic in terms of
seed production. Weeds typically can produce vast numbers
of viable seeds per plant under most conditions. Stevens
(61) reported how prolific many species are in terms of
production with numbers ranging from approximately 2,000 to


10
over one million seeds produced by a single plant. Anderson
(1) reported that seed number is clearly related to seed
size and weight. Thus weeds with extremely small seeds tend
to have higher numbers produced per plant. Recent research
conducted by Jordan (21) offered new information about the
number of seeds produced per plant. In studies involving
sicklepod, numbers of seeds produced per plant were recorded
in a study comparing soybean [Glycine max (L.) Merr.] row
spacing and sicklepod density with yield. At a row spacing
2
of 75 cm and a sicklepod density of 0.5 plants/m a two
year average of 12.98 million seeds was produced per
hectare. Thus he obtained data showing plants in these
conditions produced approximately 2600 seeds/plant. The
greatest production reported was 168.15 million seeds/ha,
which occurred in the second year of the study at a density
2
of 5.0 plants/m and a soybean row spacing of 75 cm. This
represents approximately 3363 seeds/plant. Creel et
al. (12) stated that sicklepod seed yields of greater than
900 kg/ha have been calculated from cotton fields which were
heavily infested with sicklepod. These studies indicate the
importance of seed production in terms of replenishment of
the soil seed bank and survival of the species.
Cropping Systems Effects
Early studies of effects of cultivation on seed
longevity were typically conducted by artificial means.
Chepil (9) initiated studies in which metal trays in the
field acted as plots; each tray contained soil and seeds of


11
numerous species. Over a six year period, the trays were
subjected to shallow cultivation by hand. In a second
study, Chepil (10) again utilized small metal trays;
however, this time the focus was on five weed species and
evaluated over a shorter time frame. He subjected the trays
to various types of cultivation and concluded that the
number of viable seeds decreased more under cultivation
systems when compared to the undisturbed trays.
Roberts (41) initiated four experiments in which seeds
of eleven weed species were mixed in a 7.5 cm layer of soil
in pots which were buried in the ground. Three studies were
continued for approximately 5.5 years. On five or six
occasions during each year, the soil in the pots was
thoroughly mixed by hand. From these studies he concluded
that there was a progressive decline in the number of
seedlings emerging. Also, the rate of decline for each
species was found to be approximately constant, although
some species declined faster than others. From an earlier
study, Roberts (38) reported a decline of viable seeds of
approximately 50% per year in field situations involving
cultivation. Since in this study the rate of decline was
determined by emerging seedlings, he could only approximate
the decline of viable seeds in this later experiment. When
all species were combined however, the results were shown to
fit that of the earlier test in terms of rate of decline.
One additional observation was that after the five-year
period seeds were still emerging although quite few in


12
number, thus indicating the effectiveness of some species in
terms of survival in cultivated soils.
Similar studies were conducted by Roberts and Feast
(46,47) in which they subjected seeds buried in earthenware
cylinders to different cultivation regimes by hand
disturbance. In one study, the soil was either disturbed to
a depth of 15 cm three to five times a year, or it was left
undisturbed. Seedling emergence was monitored for six years
and each year a calculation was made of viable seeds
remaining. The mean numbers of viable seeds remaining after
completion of the sixth year was 5.9% of those initially
added in the disturbed cylinders, and 27.5% where the soil
was left undisturbed (47) .
In an earlier study, Roberts and Feast (46)
incorporated depth into the test. Seeds were buried at
depths of 2.5, 7.5, and 15 cm. The depth of soil
disturbance was also tested at these various depths while
the checks were left undisturbed following burial. The data
collected from both the disturbed and undisturbed cylinders
demonstrated the effect of depth of seeds in the soil in
respect to depletion. After a five year period, the
percentages of viable seeds remaining at depths of 2.5, 7.5,
and 15 cm were 2.3, 4.0, and 7.7% respectively under the
disturbed conditions, and 6.8, 16.5, and 31.6 for the
undisturbed areas,respectively. All numbers were expressed
as percents of initial seeds buried. An interesting point
was that no two species responded in exactly the same way;


13
thus Roberts and Feast (46) stated that research should be
initiated on individual species in order to obtain a better
understanding of underlying differences in the mechanisms of
survival of buried seeds.
Although these studies involved artificial tillage
systems, and other conditions different from actual field
situations, they did demonstrate that seeds in the soil are
affected by tillage and that the existing seeds tend to
decrease exponentially with time in soils that are
undergoing cultivation.
Early work by Brenchley and Warington (2,3,4,5)
compared various types of cultural systems on existing weed
seed in the soil under natural conditions. They conducted
tests on areas consisting of both fallow and cultivation
systems, and later examined the re-establishment of weed
flora to these areas in relation to the soil seed bank. In
their earliest study involving different fallowing systems,
they found that by subjecting the plots to four year
fallowing periods the weed seed population could be greatly
reduced.
Roberts conducted a series of tests involving vegetable
cropping systems (37,38,39,40,51) which resulted in five
papers. The results obtained in the second paper of the
series best summarize the effects of six years of cropping
on weeds in the soil (38). The field in which these studies
were conducted was shown to have a high initial population
of viable seeds. Samples taken in 1953 by Roberts (37)


14
determined which would be used in the later studies. During
the first of four years of cropping there was a progressive
fall in total weed seed population when compared to 1953
data. Also with the absence of seed replenishment, results
indicated that the number of viable seeds in any one year
was approximately half that of the previous year. During
the fifth year of the study, wet soil conditions at the time
of cultivation resulted in somewhat less effective control
than in the previous years resulting in extensive
replenishing of the soil seedbank. In the sixth year the
population was again reduced as in the previous years.
In the third paper (39) Roberts examined four
different primary cultivations on the weed seeds in the
soil. The comparisons consisted of deep plowing (36-41 cm),
shallow plowing (15-18 cm), shallow plowing (15-18 cm) plus
subsoiling (41-46 cm), and rotary cultivation (15-18 cm).
Over the six year period, the three plowing systems had
substantially the same effects on both the number of viable
seeds and on subsequent infestation levels. However,
following occassions in which seed production occurred, seed
number was shown to be greater in the top 15 cm on plots
receiving shallow plowings when compared to those deeply
worked. The rotary cultivation method, to the same depth as
the shallow plowing, was found to have greater numbers of
viable seeds of certain species when these two were
compared. Roberts also reported that both the deep plowing
and shallow plowing plus subsoiling methods decreased the


15
numbers above 15 cm while increasing numbers below 15 cm.
Uniform distribution patterns were also found in plots
receiving continuous deep plowing methods. In general, with
the absence of seed production, all methods of cultivation
reduced the population of viable seeds in the soil. Yet, if
plants were allowed to produce seeds the rotary cultivation
method allowed for build-up of seeds in the upper soil
strata.
Roberts (40) reported on cultivation methods with some
additional procedures added to the earlier study. Results
revealed that numbers of weed seeds and degree of
infestation was essentially the same for plowing (18-23 cm),
with and without subsoiling, deep plowing (30 cm) or hand
digging to 18-23 cm. Yet with shallow plowing (7.5-10 cm),
plots were extremely weedy which was thought to be due to
inadequate burial of seeds. As with the previous paper
(39), the plots with the highest seed number and largest
infestation levels were those receiving rotary cultivation
particulary those with shallow rotary cultivation (7.5-10
cm). Roberts and Stokes (51) reported that the primary
cultivation methods utilized followed the results found in
the earlier years of the study in respect to seed
populations and infestation levels.
Roberts and Dawkins (45) conducted a six-year study on
the effects of cultivation on weed seeds in the soil.
During the course of the study, no crops were grown on the
test area. Plots were treated in the following methods:


16
dug to a depth of 23 cm each year in mid-March, mid-June,
mid-September, and mid-December; dug to a depth of 23 cm
each year in mid-March and mid-December; or left
undisturbed each year. Initial populations were determined
by sampling methods described earlier for determination of
population changes. Under all methods, the number of seeds
in the 23 cm profile decreased exponentially from year to
year; however, the degree of seed depletion occurred at
different rates for each cultivation system. On the
undisturbed soil, losses were found to be 22% per year, with
the plots disturbed four times a year and two times a year
resulting in depletion rates of 36 and 30%, respectively.
These depletions were related in terms of infestation levels
with an annual decrease in infestation of 9, 7, and 3% from
the most to the least disturbed plots. In this study, as in
others, the decline of individual species was typically at
different rates, yet generally the decline was exponential.
Schwerzel and Thomas (56) also examined the effects of
cultivation frequency on certain weed species; however, they
introduced a species into an area that was essentially void
of the introduced species. This approach allowed them to
follow seed depletion without the need of initial population
estimation. Seeds of five species were mixed into the soil
to a depth of 22.5 cm in each plot. Plots were then
subjected to no cultivation, one cultivation, four
cultivations, or twelve cultivations. Cultivations were
done by hand-hoeing to a depth of 22.5 cm. They reported


17
that percent emergence of the original population over a ten
year period was as follows: no cultivation 20.0%, one
cultivation 34.2%, four cultivations 29.7%, and twelve
cultivations 41.8%. Although this does not agree with
reports by Roberts (38), it still demonstrates the
importance of soil disturbance in respect to depletion of
soil seed reserves.
An ongoing experiment involving seven cultural
practices on an area heavily infested with velvetleaf seed
is presently being conducted by Lueschen and Andersen (26).
They are attempting to determine the time period required
for total depletion of the soil seed bank. The first four
years of this study showed that the population of velvetleaf
seeds declined rapidly before leveling off, and this decline
was increased by tillage operations. The most intensive
tillage system, two plowings with continuous fallowing,
resulted in the fastest decline over this four-year period.
However, even though extreme care was taken to prevent
plants from producing new seeds, the population under this
intensive system was still estimated at 1300 viable
2
seeds/m to a depth of 23 cm. Although the initial
population was 130 million, this illustrates the difficulty
of totally eradicating such a vast number of viable seeds.
Pollard and Cussans (31,32) measured the effect of
different tillage systems on the weed flora by monitoring
emergence of seedlings under each tillage regime. Tillage
regimes were moldboard plowing, shallow or deep tine


18
cultivation, and direct drilling. They reported that small
seeded species were favored by reduced cultivation or direct
drilling, while larger seeded species were favored by
moldboard plowing and deep tine cultivation.
Froud-Williams et al. (17) also investigated the
effects of different cultivation regimes on weed floras of
arable soils. The cultivation treatments common to all five
locations were no cultivation and plowing to a depth of 20
cm. Typically they found that the total number of weeds
emerging in the count areas varied with cultivation
treatments depending on the dominant species at each
location. Total weed numbers were higher on plowed plots
than on the undisturbed plots at two sites where grasses
were not the predominant species. However, at the other
locations the reverse was true due to higher frequencies of
grass species. Their results indicate the importance of
understanding the effects of tillage on monocotyledonous and
dicotyledonous species and that different responses can be
related to seed size. Reports of reductions in the density
of dicotyledonous weeds and increases in the frequency of
annual grass weeds with the implementation of reduced
tillage operations were found by several researchers
(13,31,33).
Effects of Chemical Weed Control on Seedbanks
In an attempt to better understand how the addition of
chemical weed control affects the weed seed reserves in the
soil, Chancellor (8) examined the rate of increase of seeds


19
in the soil in the absence of chemical control. Plots were
either not sprayed or they were sprayed with a herbicide
mixture of bromoxynil (3, 5-dibromo-3-sec-butyl-6-methylura-
cil), ioxinyl (4-hydroxy-3,5- diiodobenzonitrile), and
dichlorprop [2-(2,4-dichlorophenoxy)propionic acid]. After
a two year period, both treatments were shown to have 21
species making up the soil seedbank; thus the chemical
treatment did not change the seedbank composition. However,
as expected the number of seedlings emerging had changed in
regard to treatment. The total number of emerged seedlings
was 686 on the sprayed plots and 1402 on the untreated
plots. It would appear from these observations that the
unsprayed plots had doubled after a two year period; yet
when these figures were compared with those collected at the
time of initiation of the experiment, he reported that a 10%
decline occurred on the untreated area, and a 52% decline
occurred in the treated area. He suggested that some of
these variations were possibly due to differing temperatures
during seed set periods thus altering the seed rain to the
soil surface or the effect of cultivation timing in terms of
soil moisture and weather conditions following these
cultivations.
A twelve year study was conducted by Hurle (20) in
which the long-term effects of weed control on viable weed
seeds in the soil were examined. Seven treatments were
utilized and included the following: (1) no weed control,
(2) MCPA ([(4-chloro-o-tolyl)oxy]acetic acid] at 1.5 kg/ha,


20
(3) 2,4-D [(2,4-dichlorophenoxy)acetic acid] at 1.5
kg/ha,(4) DNOC (4,6-dinitro-o-cresol) at 4.0 kg/ha,
(5) Calcium cyanide at 143 kg/ha, (6) a rotation treatment
which consisted of MCPA, DNOC, Calcium cyanide, and
harrowing, and (7) a harrowing alone treatment. He found
that all weed control measures reduced the number of viable
seeds in the soil over that of no weed control. The DNOC
treatment resulted in the greatest reduction at 60% with the
other three herbicide programs offering approximately 36-44%
reduction. The rotation treatment and harrowing alone were
the least effective overall.
Roberts and Neilson (48) determined the effect of
long-term chemical control on four monoculture systems in
respect to soil seedbanks. They compared spring barley
treated with triallate [S-(2,3,3-trichloroallyl) diisopro-
pylthiocarbamate] at 1.68 kg/ha, spring barley treated with
MCPA at 1.68 kg/ha, maize treated with simazine [2-chloro-
4,6-bis(ethylamino)-s- triazine at 1.68 kg/ha, and carrots
treated with linuron [3-(3,4-dichlorophenyl)-1-methoxy-l-
methylurea] at 0.84-1.68 kg/ha either as a postemergent or
in split applications. The reduction in number of viable
seeds was expressed by the comparison of seeds recovered to
those numbers found in 1963. Initially, in the second year
of the study, they reported an appreciable reduction of
viable seeds in all four monoculture systems. This decline
also continued throughout the next five years of the
experiment. However, changes in actual composition of the


21
seedbank were directly related to the cropping system and
herbicide used. There were shifts in response to these
treatments, and although reductions occurred, little
qualitative effects on species composition were reported.
Thurston (65) reported on a seven year study where
continuous winter wheat was grown. Various compounds, no
weed control, and a continuous fallow situation were
compared. He found that the number of species had not
changed; yet the densities had changed greatly due to
treatments imposed. Basically the addition of herbicides to
the study of seedbank depletion has demonstrated the effect
of various compounds on existing weed seeds. One thing that
should always be kept in mind is the possibility of
population shifts due to poor weed control of a certain
species with a certain compound (65).
Distribution of Seedbank
In order to better understand the importance of seed
movement in regards to cultural practices, many researchers
have attempted to examine distribution in relation to these
operations (52,54,56,59). Robinson and Kust (52) analyzed
the soil profile to determine the distribution of witchweed
[Striga asiatica (L.) Kuntze] seeds in the soil. Cores,
each 3.8 cm in diameter, were taken to a depth of 152.5 cm
and divided into 15.25 cm sections. Each section was then
separated by a flotation technique and seeds recovered.
They reported that the vertical distribution was not uniform
and that although the greatest numbers were found near the


22
soil surface, the cultural practices that formed this
distribution were not known.
In an attempt to better understand these distribution
patterns in regard to tillage systems, Soriano et al. (59)
hand dispersed seeds of two crop species on the soil surface
2
at a density of 4000 seeds/100m The area was then plowed
to a depth of 20 cm and disked several times. Samples were
taken by randomly removing 30 blocks of soil within the
sample area. Each 30 X 10 X 20 cm sample was extracted with
the aid of a metal frame which was hammered in the ground,
soil was removed from around it, and the block was then
lifted out of the field. In order to examine vertical
distribution, the block was then sliced into 4 cm thick
layers. Each layer was then wet sieved with the aid of
running water. They reported that although the
characteristics of the two seeds in terms of geometry and
seedcoat were very different, the distribution was the same.
A very characteristic vertical distribution was found with
both species increasing in frequency from the surface to a
depth of 20 cm. Although crop seeds were used in this
study, they were selected due to similarities in regard to
weed seed characteristics of both size and seedcoat.
Kellman (22) made observations of both horizontal and
vertical distribution of seeds in two cropping situations.
In both a corn field and pasture, aim baseline was
established and contiguous cores removed along this baseline
to a depth of 10 cm. Each core was then sliced into the


23
following sections: 0-1, 1-3, 3-6, and 6-10 cm. Sections
were then wet sieved using a 0.25 mm mesh and the sieved
material collected for germination test. Results indicated
that the pasture had many more species in the 0-3 cm layers
than the corn field; yet the corn field had a much greater
seed number in these surface layers. He proposed that the
vertical pattern in the corn field was probably a
combination of older, more evenly distributed seeds buried
by prolonged tillage, and very recent seeds produced by the
existing weed vegetation being concentrated at the surface.
Fay and Olson (16) monitored the distribution of wild
oat seeds in the soil profile under two tillage systems of
moldboard plowing or chisel plowing. Results showed that
seeds were more evenly distributed throughout the 18 cm soil
profile by annual moldboard plowing than by chisel plowing.
Over 60% of the wild oat seeds recovered occurred in the top
2.5 cm with none recovered below 10.2 cm in the soil
receiving the chisel plow treatment. Therefore, they
suggested that seeds would tend to concentrate near the soil
surface with continued chisel plowing as opposed to a more
uniform distribution with moldboard plowing.
Froud-Williams et al. (18) conducted a study in which
seed distribution as influenced by cultivation regime was
compared over a three year period. Using sampling
techniques discussed earlier and subsequent analysis of
cores with the emergence test, they effectively determined
vertical seed distribution. Distribution was determined in


24
5 cm layers to a depth of 20 cm. The two cultivation
regimes involved were direct drilling in uncultivated plots
or conventional seeding in plots plowed to 20 cm. They
found that the distribution of seeds in the soil profile was
not uniform, and fluctuated with each annual plowing of the
plots at all locations. Seed distribution on plowed plots
tends to reverse each year in terms of greatest number of
seeds. After the first years plowing, at two locations the
concentrated layers appeared at 10-15 cm. Following the
second year's plowing, distribution was relocated in terms
of major seed bands. It is clear from this work, that
plowing as a cultivation method results in severe relocation
of fallen seeds. In the no cultivation plots, very little
change in concentration occurred with the exception of one
site which possibly had undetected seed production. They
also stated that the rate of seed decline is directly
associated with soil disturbance.
A new method of examing seed distribution was
introduced by Rottele and Koch (54). Part of their study
involved the use of plastic pearls in an attempt to simulate
the seed of cleavers (Galium arparine L.). Both true seeds
2
and plastic pearls were distributed on an area 25 m and
replicated four times. Samples were taken with a soil probe
to a depth of 25 cm. All samples were sliced into 5 cm
sized pieces and stored in plastic bags until analysis.
Both seeds and plastic pearls were then extracted from the
soil by washing the sample sections in a sieve (1 mm) until


25
all the soil was removed. Both the seeds and pearls
remaining were counted and recorded. The results indicated
not only that the plastic pearls and seed demonstrated
similar distribution patterns, but also that the
distribution of both responded the same to each additional
tillage operation. After the first plowing (~ 25 cm)
vertical distribution of both viable seeds and plastic
pearls were shown to have approximately 36% and 35% of each
recovered in the 15-20 cm zone. In the case of the plastic
pearls, 79% of the recovery was below the 10 cm depth with
85% of the viable seeds recovered below this depth.
Following the second plowing, both the seeds and plastic
pearls were returned to the upper layers. Recovery numbers
for seeds and pearls from 0-15 cm were 71% and 82%,
respectively. In terms of an indicator, the plastic pearls
were shown to mimic the actual seed well, and this allowed
for analysis of seed response to tillage.
Predicting Weed Problems
According to Radosevich and Holt (35), the importance
of research involving the determination of soil seed banks
and early observations of seedling establishment from these
reserves offers the ability to predict potential weed
infestations. By inputing the parameters of seed production
and dispersal, soil seed bank size, rates of seedling
recruitment, and expected mortality into a demographic
model, it should be possible to determine both the dominant
species and its predicted density at a particular site.


26
Sagar and Mortimer (55) have developed a "working
model" which can be used to study population dynamics of
weeds. The model uses a known population of plants and
predicts the expected infestation levels for the next year.
Mortimer (30) examined four weed species with the aid of
similar models although they were simplified to examine the
effects of various levels of disturbance and the exclusion
of animals from the study area. Attempts were made to
compare the treatments in regard to seeds becoming buried,
eaten by predators, germinating, and forming of healthy
seedlings.
Roberts and Potter (49) studied the emergence patterns
of weed species in relation to cultivation and rainfall.
The study was conducted over a four year period, with a
cultivation every two weeks. The number of seedlings
emerging was counted weekly and showed the effects of
cultivation and rainfall on seedling emergence. They found
a flush in the spring of each year, and that subsequent
flushes on both disturbed and undisturbed soil were directly
related to rainfall. They estimated 90% of the seedling
emergence following disturbance occurred within ten weeks
following the early spring cultivation, and three weeks
after cultivation in the summer. A better understanding of
the influence of cultivation on germination and the
relationship to the soil seedbank may allow predictions of
weed emergence. Roberts and Ricketts (50) conducted a study
in which this association was examined. By comparing


27
seedling emergence in respect to cultivation with existing
viable seeds in the soil, they examined the possibilities of
a quantitative prediction of the weed flora. They found
that when soil moisture was adequate for seed germination,
only 3-6% of the seeds in the soil to a depth of 10 cm
germinated. They felt that this type of data between actual
and potential weed flora can offer predictive values with
additional study in regard to cultural practices and time.


CHAPTER II
TECHNIQUES FOR DETERMINING SEED DISTRIBUTION
Introduction
Weed seeds in the soil have been studied in an attempt
to better understand their behavor and to obtain an increase
in their control. Initially researchers studied the ability
of certain species to survive for long periods of burial in
the soil (14,23,66). Longevity is possibly the most
important aspect involved in the typical formation of large
reserves in the soil, called soil seedbanks. The importance
of these studies is that they were the first to demonstrate
the ability of weed seeds to remain viable in the soil for
extended periods.
In order to study the soil seedbanks in their natural
state, methods of seed recovery are of importance.
Researchers have utilized various soil core sampling tools
in studies to recover weed seeds in the soil (4,10,42).
Studies have also been conducted on the size and number of
cores to be removed for the least variation in population
estimations (6,37,53). The development of sampling methods
lead to the study of separation techniques which allowed for
seeds to be removed from the soil more efficiently and in
less time. This showed that the seedling emergence method
28


29
was both time consuming and required a controlled
environment to be most effective.
The next step was to study these buried weed seeds in
relation to actual cropping systems. Roberts and co-workers
(41,43,44,45,46,47,48) conducted an array of studies
offering information about the decline of populations under
normal cropping practices and the effects of these practices
on the many aspects of seeds in the soil. In order to more
clearly understand the effects of tillage on weed seeds in
the soil, Rottele et al. (54) introduced a new method to
examine seed distribution with the aid of plastic pearls.
He utilized the plastic pearls to simulate the seed of
cleavers and subjected them both to cultural practices. By
sampling the area receiving the artificial seeds he studied
the changes in distribution with each additional tillage
operation. This novel approach with modification allows for
closer examination of weed seeds in the soil and of those
factors which influence their survival or demise.
The objective of this study was to examine these
techniques more closely, and possibly develop additional
techniques for the study of weed seeds in the soil. Primary
interest was the effects of tillage operations on the
distribution of sicklepod seeds in the soil, and new methods
of determination of the soil seedbank distribution.


30
Methods and Materials
General
During the fall of 1982, an area of land with a seven
year history of pasture grass breeding was located. This
area was chosen due to its history of low broadleaf weed
infestation. The area was randomly sampled in twenty
places. Each sample consisted of taking approximately one
cubic foot of soil by shovel. The samples were then taken
to the greenhouse, spread out in trays and watered.
Seedling emergence was monitored, with the main goal to
determine presence or absence of sicklepod. This method
indicated that sicklepod seeds were not present; however,
some of the samples were also screened through sieves to
collect existing seeds in the soil. No sicklepod was found
in the screenings. Since no sicklepod was present, any
sicklepod emerging would be from those dispersed at
initiation of the study.
In the winter of 1982, a study was established at the
University of Florida Agronomy Farm on an Arredondo fine
sand soil (siliceous, well drained hypothermic, Grossarenic
Paleudult). A split-plot experiment with a randomized
complete block design and six replications was used. The
main plots were tillage systems. The subplots were areas
receiving year one viable sicklepod seed, year two viable
sicklepod seed, year one and year two synthetic seed, and an
unseeded check. The main plots measured 8.5 X 12.2 m, and
the subplots measured 4.25 X 6.1 m with four subplots per


31
main plot. All tillage treatments were maintained in the
same plots for two years. Both tillage systems and seed
dispersal in subplots will be discussed individually.
Viable Seed Collection
In the fall of 1982 and 1983, sicklepod seeds were
collected from a local field on the University of Florida
Agronomy Farm in which high infestation levels were found.
Seeds were collected by removing mature dry plants from the
2
field and hand threshing them over a 1.8 m cloth. Seeds
were then gathered and hand cleaned with the aid of various
sieves and compressed air. Care was taken throughout the
collection and cleaning to reduce the chances of seedcoat
damage. After cleaning, the seeds were stored at room
temperature in cloth bags.
Production of Synthetic Seeds
In order to develop a synthetic seed a random 100 g
sample was taken from the viable seeds collected in 1982.
This sample was used to determine the average diameter,
length, and density.
Synthetic seeds were produced with help from the
Material Science Department at the University of Florida.
Seeds were produced for both years, 1982 and 1983, from a
branched form of the polymer, polyethelene. The form used
3
had a low density of approximately 0.93 g/cm Due to the
low density of the polymer, a load material was required
to increase the density to the 1.25 g/cm^ of sicklepod.


32
Two separate load materials were used for 1982 and 1983
seeds to allow for identification and separation following
analysis. The two load materials utilized were iron powder
and barium sulphate for 1982 and 1983 respectively. The
materials for each of the two years were blended with the
polymer and extruded from an Extruder (Mini Max) at 175 C to
produce a rod with an average diameter of 3.0 mm. Following
production of the rod material, they were then chopped into
average lengths of 4.5 mm with the aid of a Pelletizer
(Custom Scientific Ins.). All of the procedures described
above resulted in the production of synthetic seeds that
best mimicked the true sicklepod seed collected locally in
terms of size, shape, and density. Both year one and year
two synthetic seeds were subjected to density checks and
were shown to be approximately 1.25 g/cm^.
Counting Method
Both the viable seeds and the synthetic seeds were
counted with an Electronic Counter into lots of 2200
seeds/bag. This number was based on previous research
conducted at the University of Florida. Jordan (21)
reported the number of sicklepod seeds produced per plant at
2
a density of 0.5 plants/m and a soybean row spacing of 75
cm to be approximately 2100 in 1981 and 3100 in 1982. From
these data and personal discussion, a figure of 2200
seeds/plant was utilized. Although the subplot areas were
to receive 28,600 seeds each, observations were made during


33
the counting procedure that demonstrated less error in
smaller lot sizes; thus lots of 2200 seeds/bag were used.
Seed Dispersal
The test area was established in the winter of 1982,
and the dispersal of synthetic or viable seeds on the soil
surface of the subplot areas was accomplished by hand
spreading. Distribution was carefully executed to produce
as uniform seed distribution as possible. All areas in
which seed were dispersed were alloted 28,600 seeds which
2
resulted in an initial population of 1100 seeds/m .
Immediately after dispersal the entire area was disked twice
in the same direction with a tandem disk, and wheat
(Triticum aestivum L.) 'Florida 301' was planted. Second
year seeds of both types were handled in the same manner;
however, winter wheat was not sown after the two diskings,
and replication five received no seeds of either type. This
area was to be excavated later to examine the distribution
of year one synthetic seeds.
Tillage Operations
In the spring of 1983, three tillage systems were
randomly applied to each replication following the removal
of the wheat to a 15 cm stubble. The three systems were
(1) Conventional Tillagemoldboard plowing (24 cm),
followed by two diskings (7.5 cm); (2) Minimum Tillagetwo
diskings (7.5 cm); and (3) No-tillagedirect seeding into
stubble. Following the completion of tillage operations,


34
"Bragg" soybeans were planted with a row width of 75 cm.
Early in the summer of 1984, all procedures were repeated as
in 1983; however, no soybeans were planted in the second
year and replication five was left undisturbed for
excavation and analysis of year one tillage effects. In
both years, shortly after the completion of the tillage
operations, oryzalin (3,5-dinitro-N,N-dipropylsulfanilamide)
was applied at 0.68 kg/ha over the entire test area for
annual grass control.
Seed Sampling Technique
Soil samples were extracted with the aid of a new
device developed for this and additional studies (Figure 1).
The soil sampling device was composed of three components.
The first of these was the sampler which was constructed
from 0.32 cm steel sheet in a manner that allowed the second
component to be inserted. On the bottom of the sampler were
two tooled blades approximately 5 cm high which aided in the
penetration of the sampler into the soil profile (Figure 2).
The second components, soil collection trays, were
constructed from 0.32 cm sheet aluminum to specifications
that allowed insertion into the sampler (Figure 2).
Originally the collection trays were constructed from
polyvinal cloride; however, it proved to be unstable and
trays were not strong enough to allow repeated use. These
were replaced with the trays constructed from aluminum. The
collection trays served as the container for each sample
following extraction. A sliding impact hammer was utilized
to drive the sampling device into the soil (Figure 2).


35
Figure 1. Soil sampling device utilized for studing the
distribution of synthetic seeds in the soil
profile. Designed to remove a vertical slice
with dimensions 39.25 X 3 X 24 cm. Top:
Insertion of collection tray into sampler
Below: Collection tray with intact soil slice


36
Figure 2. Schematic drawing of soil sampler illustrated
by components. Main component diagram: (A)the
sampler. Additional components: (B)collection
trays and (C)impact hammer.


37
Once positioned in the sampler, the inside of the tray
was flush with the bottom opening which allowed the sampler
to be impacted into the ground and the sampler filled with
undisturbed soil. Following removal of the sampler from the
soil, it was positioned such that the lid of the aluminum
tray was on top. The lid was then removed to provide more
freedom for the tray to be removed. Before the tray was
taken out a stainless steel blade was inserted in the gap
between the lip and the bottom of the tray. This separated
the sample from fill soil contained in the 5 cm extension
below the tray bottom. The tray with an intact vertical
slice of soil was then easily removed.
Following removal of the trays, samples were then
sealed by placing the lid back on the tray and taping the
bottom opening with duct tape. Two large rubber bands were
used to help hold the tray closed during transport, although
they were not necessary.
Following the tillage operation and after the soil had
been weathered to some degree, all subplots receiving the
synthetic seeds were sampled as described above. Subplots
were sampled 10 times in a 1 X 3 m area within the subplot.
Samples were then sealed, labeled, and stored for analysis
at a later time. Methods were repeated following year two
tillage treatment. No samples were removed from
replications five and six.


38
Excavation Methods
Efficiency and accuracy of the sampling device was
accomplished by excavation of replications five and six at
randomly designated sites within the subplot area.
Replication five received only first year synthetic seeds
and only one tillage operation on the main plot areas.
However, replication six received both first and second year
synthetic seeds and two tillage operations. These were
excavated by removing a randomly selected block of soil
measuring 60 X 80 cm at 2 cm depth intervals. Each layer
was carefully measured, the soil hand removed with the aid
of a small garden spade, and samples placed in labeled bags.
The conventional tillage subplots were excavated to a depth
of 24 cm; while both the minimum tillage and no-tillage
subplots were sampled to a depth of 16 cm. The two reduced
tillage systems were never found to have seeds below 14 cm,
thus a depth of 16 cm proved to be adequate. These bagged
layers of soil were then analyzed by the wash method
utilized for all other soil samples. Seeds were recorded,
and the data collected were utilized for comparisons with
other recovery methods.
Analysis of Soil Samples
All samples collected were subjected to a washing
technique which involved a sieve apparatus constructed of
two 39.25 X 25 cm perforated steel plates containing 5.5
2
perforations/cm with a diameter of 2.5 mm (Figure 3).
These two plates were cut such that when they were placed in


39
Figure 3. Drawing of washing device utilized to remove soil
from samples for determination of synthetic seed
recovery. Designed to allow for high pressure
wash with reduced seed loss.


40
the metal box frame they were slightly offset. This allowed
for the rapid removal of soil while decreasing the chance of
seed loss. The metal box frame was 40 X 25 cm with sides 15
cm high and was placed in a stand made from angle iron.
Each sample was unsealed and marked into 2 cm sections from
the top (0 cm) to the bottom (24 cm). Each section was then
sliced, placed in the washing apparatus, flushed with
running water until all soil was removed, and the remaining
synthetic seeds were recorded. Care was taken to check the
organic matter in each sample to assure accurate seed
recovery. This technique allowed for the comparison of seed
distribution by depth and year in relation to the three
tillage systems implemented.
Results and Discussion
Comparison of Wash and Excavation
New techniques utilized
This study was initiated to evaluate the effectiveness
of two new techniques for studying the distribution of seeds
under three separate tillage regimes. The first of these, a
new sampling device, was tested for both effectiveness in
the removal of undisturbed slices of the soil profile and
accuracy in the determination of the seed distribution
within these slices of the profile. The second technique of
this study examined was the use of synthetic seeds as
mimickers of the actual weed seed. In order to determine
the degree of accuracy of seed recovery of the sampling


41
device, an excavation treatment was implemented as a control
for examing seed distribution.
After the tillage treatments the test area received
both the sampling and later parts were excavated. A
comparison of these two methods of seed recovery was made
first. Overall, there was little difference between the two
methods. Also there appeared to be no compaction with the
sampling device. There were some differences in the percent
seed recovered. These differences typically occurred in the
reduced tillage systems in the upper soil layers. Possibly
a large portion of these differences was due to the
difficulty in excavating the upper layers of soil. When the
excavation was initiated, the soil surface was generally
uneven and therefore made the initial layers difficult to
remove exactly 2 cm at a time. Yet, even with these
differences, the two methods were very much the same in
terms of predicting the actual distribution of seeds under
the three tillage systems.
Conventional tillage
The wash and excavation techniques resulted in
essentially the same seed distribution pattern (Figure 4).
These data show the means for the three sample times at each
depth interval. The same data also show that the wash
method is not significantly different from the excavation
method, representing the control. Results indicate that the
conventional tillage caused the movement of the largest
portion of seeds below the 10 cm depth interval following


HSD VALUES SEED RECOVERED (?5)
42
Figure
0-2 4-6 8-10 12-14 16-18 20-22
DEPTH INTERVALS (CM)
4. Mean distribution of synthetic seeds following
conventional tillage with comparisons between
wash and excavation methods. #=excavationD=
wash HSD value determined by Tukey's at Alpha=
.05 level.


43
one tillage operation. Approximately 78% and 83% of the
seeds were recovered below this depth during the wash and
excavation methods respectively. This trend was shown to be
consistent in all cases after the first conventional tillage
operation.
Minimum tillage
Data from the minimum tillage plots shows that
distribution was less uniform throughout the working depth
when compared to the conventional tillage treatment (Figure
5). In the reduced tillage system, approximately 88 and 87%
of the seeds recovered were in the top 6 cm for both the
wash and excavation methods, respectively. The maximum
recovery depth throughout the study was typically at 12-14
cm. There was a greater chance for error between the two
sampling methods due to the increase in number of seeds in
the upper layers.
In the reduced tillage systems accurate recovery was
more difficult to achieve with the excavation method;
however only one significant difference exists between the
two methods at the 2-4 cm depth interval (Figure 5).
Distribution patterns were generally the same for both
methods with a decrease in seeds as depth increased and no
seeds recovered below the 12-14 cm interval. This same
pattern was found in all cases including the sequential
treatment in year two.
No-tillage
The no-tillage system like the minimum tillage
treatment showed a simple decrease in seeds as depth


HSD VALUES SEED RECOVERED (5S)
44
45
40
35
30
25
20
15
10
5
0
0-2 2-4 4-6 6-8 8-10 10-12 12-14
Figure 5. Mean distribution of seeds following minimum
tillage with comparisons between wash and
excavation methods. #=excavation D=wash
HSD values determined by Tukey's at Alpha=.05
level.


45
increased (Figure 6). Mean seed recoveries in the top 6 cm
for the sampling and excavation methods were only 85 and
78%, respectively. Although lower than minimum tillage,
this is basically the same trend in terms of distribution.
Comparison between the two methods showed several
significant differences; however, much of this is believed
to be human error due to the excavation technique. This
technique was difficult to perform accurately at 2 cm
intervals because the surface of the excavation area was
uneven and irregular.
Comparisons of Sequential Tillage Operations
Distributions following tillage treatments
This portion of the research was designed to allow a
detailed examination of the seed distribution for each year
as influenced by tillage. In the first year of the study,
seeds were dispersed and plots were subjected to their
respective tillage operations. Thus sampling after this
period was year one of the study with one tillage operation.
In the second year of the study, additional seeds were
dispersed and plots were again subjected to their respective
tillage operations. However, sampling after this date gave
data on year one seeds after two tillage operations, and
year two seeds after one tillage operation. These sample
times allowed comparisons of the effects of one tillage
operation on distribution of seeds and, also, how this
pattern changes with the introduction of a second tillage
operation. The graphs (Figures 7, 8, and 9) show the means


HSD VALUES SEED RECOVERED (%)
46
i i i i i
0-2 2-4 4-6 6-8 8-10 10-12 12-14
DEPTH INTERVALS (CM)
Figure 6. Mean distribution of seeds following no-till
tillage with comparisons between wash and
excavation methods. =excavationD=wash
HSD values determined by Tukey' s at Alpha=
.05 level.


47
of the wash and excavation methods by sample time, depth,
and tillage. The means of the two recovery methods were
utilized since there were insignificant differences in early
results.
Conventional tillage
Examination of the effects of conventional tillage
shows a similar pattern to those discussed (Figure 7). The
key differences are how the pattern changes with the
addition of the second tillage and how the single tillage in
the second year resulted in a slight shift. As with the
data discussed earlier, the conventional tillage resulted in
a bulk of the seeds being distributed below 10 cm in depth.
In both the first and second year when seeds received a
single tillage operation, 84 and 89% of the seeds recovered
were below 10 cm. The bulk of this recovery was in the zone
12-18 cm deep. Statistical comparisons between the one
tillage and two tillage operations revealed several
significant differences in terms of seed distributions; when
a second tillage was added seeds were moved in two
directions, deeper and towards the surface. This resulted
in significant differences in the numbers of synthetic seeds
recovered in the 0-2, 2-4, and 4-6 cm layers with an
increase in seeds recovered following the second tillage
operation. Significant differences also were shown at 12-14
and 14-16 and 18-20 cm and a significant increase occurred
in the 18-20 cm layer. When the movement of seeds in the
plots receiving two tillages was compared to the second year


48
single tillage plots, essentially the same results were
noted with the exception being the 18-20 cm depth layer in
which no significant difference occurred. The first and
second year tillage plots offered interesting results.
Between the two years, even though both involved dispersing
the seeds followed by a single tillage operation there was a
shift in synthetic seed recovery. It is believed that since
this area was carefully selected to be free of sicklepod,
and since its history involves the last seven years in
pasture, that some of this shift could be explained in terms
of the condition of the soil before tillage. In year one
the soil was tightly packed, hard and less friable. In year
two of the study it had been worked once the year before.
This may explain the significance occurring in depths of 8-
10, 12-14, and 14-16 cm in which an increase of recovered
seeds occurred in the second year of the study at the 14-16
cm depth while the number decreased significantly in the
8-10 and 12-14 cm depth. The complete downward shift is
readily noticed on the graph shown (Figure 7).
Minimum tillage
Data collected under the minimum tillage system offered
few differences from those discussed earlier. The patterns
were essentially the same for all sample times; however, a
significant change occurred in the 2-4 and 4-6 cm depths
with the addition of the second tillage operation (Figure
8). The change showed a movement of seeds from the 2-4 cm
layer into the 4-6 cm layer, demonstrating that each


HSD VALUES SEED RECOVERED (J5)
49
0-2 4-6 8-10 12-14 16-18
DEPTH INTERVALS (CM)
20-22
Figure 7. Distribution of synthetic seeds as influenced by
one or two conventional tillage operations. =
batch 1 with 1 tillage, =batch 1 with 2 tillage
operations, and *=batch 2 with 1 tillage. HSD
values determined by Tukey's at Alpha=.05 level.


50
additional tillage possibly would more uniformly distribute
seeds in the working depth of 6-8 cm. As with the
conventional tillage system, there were significant
differences in seeds recovered between the two single
tillage operations. More seeds remained in the 0-2 cm layer
in the second year tillage plots. Again, this could be
related to the condition of the soil in terms of friability
between the two years.
No-tillage
The no-tillage system followed the established pattern
showing a decrease in seeds as depth increased (Figure 9).
The pattern shifts were much the same as those observed in
the minimum tillage plots. A significant decrease again was
noted in the 0-2 cm layer after the second tillage
demonstrating that even though less tillage occurred the
trend towards uniform distribution in the worked depth of
0-8 cm possibly would occur with a continuous no-tillage
system. However, this difference was only statistically
significant when compared to the second run of the single
tillage in year two. Again as in the minimum tillage
treatment, an increase was observed in the 0-2 cm layer
which tends to support the conclusion of soil friability
following the first years tillage. It is possible that due
to less clod formation and less compaction, the soil tends
to have fewer cracks and crevices into which the seeds may
fall during the second years tillage.


HSD VALUES SEED RECOVERED (*)
51
0-2 2-4 4-6 6-a a-10 10-12 12-14
DEPTH INTERVALS (CM)
Figure 8. Distribution of synthetic seeds as influenced by
one or two minimum tillage operations. D=batch 1
with 1 tillage operation, =batch 1 with 2 till
age operations, and A=batch 2 with 1 tillage
operation. HSD values determined by Tukey's at
Alpha=.05 level.


HSD VALUES SEED RECOVERED (?5)
52
DEPTH INTERVALS (CM)
Figure 9. Distribution of synthetic seeds as influenced by
one or two no-tillage operations. D=batch 1
with 1 tillage operation, =batch 1 with 2
tillage operations, and *=batch 2 with 1 tillage
operation. HSD values determined by Tukey's at
Alpha=.05 level.


CHAPTER III
COMPARISON OF TECHNIQUES FOR DETERMINING SEED DISTRIBUTION
Introduction
An increase in interest for analyzing the distribution
of weed seeds in the soil produced an increase in the
development of new sampling and separation methods
(16,24,27,38). Most of the research conducted in these
areas was initiated to develop more effective methods to
separate seeds from the soil samples collected in order to
better understand the seedbank distribution. Originally,
distribution was determined by utilizing the emergence test
in which the soil samples were placed in a controlled
environment to allow viable seeds to emerge (2,3,4,9,10).
This method is not only very laborious, but also requires
long periods of time to assure accurate counts. It also
makes no estimation of nonviable seeds in the soil seedbank.
Putensen (34) realized that the nonviable seeds might
possibly offer some information so he introduced wet-sieving
to collect these following the completion of the emergence
test.
Following the development of new mesh materials,
separation methods in which washing the soil through various
sieve devices was typically utilized (27,53,60,64). The
53


54
main problem found with the sieve methods was that there was
still a time requirement for the separation of the sieved
material especially in studies of soil seedbanks containing
a large number of species with various seed sizes. Fay and
Olson (16) showed that the time required for the separation
of the material collected by sieve techniques was greatly
decreased when a single species was used. Mechanical
devices were constructed by several researchers to lower the
time requirement for the separation procedure (16,53,64).
Some researchers studied different flotation methods but
there was some question about damaging the viability of the
seeds collected (27,57).
The main purpose of the study reported here was to
develop and evaluate a new technique for the determination
of seed distribution under different tillage systems. The
method developed involves the use of synthetic seeds made
from polyethelene with parameters of size, shape, and
density made as close as possible to the seed being
mimicked. Due to the utilization of the synthetic seed, new
methods of separation from or detection in the soil samples
could be examined.
The introduction of the x-ray technique was
incorporated into this study to examine its accuracy.
Comparison of this new method against the older more widely
used method of washing would offer data which hopefully
would demonstrate its effectiveness. The sampling device
described in Chapter II was shown to be an effective device
for soil sampling and, it is hoped these samples obtained


55
would lend themselves to rapid analysis via x-ray. A
comparison was made between the two methods, and the
efficiency of the new method when compared to the older
washing technique was examined.
Materials and Methods
General
Methodology of the field experiment was discussed in
Chapter II. The experimental design of the study was a
split-plot experiment with a randomized complete block
design. Main plots were tillage systems, and the subplots
were treatments utilizing dispersal. The main focus of the
study was on the subplots receiving synthetic sicklepod
seeds; therefore, referal to the methods involving
synthetic seed production, dispersal, recovery, and analysis
may be helpfull. For simplicity, "Conventional tillage"
refers to moldboard plowing to a depth of 24 cm, followed by
two tillings to a depth of 7.5 cm with a tandem disk;
"Minimum tillage" refers to two tillings to a depth of 7.5
cm with a tandem disk; and "No-tillage" refers to direct
seeding with no soil disturbance.
Soil samples were collected using the device described
in Chapter II, with 10 samples being extracted per subplot
from an area 1 X 3 m centrally located within the subplot.
Analysis of Soil Samples
Wash method
Synthetic seeds were recovered by slicing each sample
into 12 sections each 2 cm in height. Each section was


56
washed individually to remove soil, leaving only small
debris and the synthetic seeds. The depth of the seeds in
the profile was noted and recorded (Refer to Chapter II).
X-ray method
One half of the samples collected from each subplot was
taken to Shands Teaching Hospital, University of Florida,
for analysis by x-ray. Due to the load materials utilized
in the production of synthetic seeds (Chapter II), both 1982
and 1983 synthetic seeds could be detected and
differentiated by this analytical technique.
Initial studies in developing the technique utilized
soil samples spiked with synthetic seed. These were
examined under a fluoroscope which allowed manipulation of
settings while viewing the samples on a monitor, thus
verifying that the seeds could be located. Next, the spiked
soil samples were subjected to various exposure levels and
durations of x-ray, from which a working technique was
developed (Table 1).
Analysis of radiographs was accomplished by using a
modified view screen that was divided into 2 cm sections
with 1.5 mm graphing tape. The radiographs of each sample
were then viewed. The synthetic seeds were counted and
recorded in each 2 cm band (Figure 10). Comparison of the
data collected from both recovery methods will be discussed
in terms of seed distribution, effectiveness of seed
recovery, and time requirements for each analysis method.


57
Radiograph shown on view screen illustrating the
location of synthetic seeds which allowed the
determination of actual numbers at each depth.
Circled areas show examples of synthetic seeds.
Figure 10.


58
TABLE 1. SETTINGS FOR THE X-RAY TECHNIQUES UTILIZED
Samples
were shot using:
a
1) .
Philips Super 100
Control and Generator
2) .
Table Top setting
at 100 cm from tube
3) .
Small Focus
4) .
55 KVP
5) .
130 mAs
6) .
Film type: Lanex
medium OH Kodak
7) .
Film size: 16 X 14"
a Note: settings may vary between equipment
KVP=kilovolt potential mAs=milliamps/sec.


59
Results and Conclusions
Comparison of Wash and X-ray Techniques
Conventional tillage
General trends for each of the tillage operations will
be examined using plots constructed from the means of each
recovery method under each tillage system. Examination of
the conventional tillage system shows very similar patterns
of distribution (Figure 11) .
The results show that the wash and x-ray methods are
very similar in terms of seed recovery per depth interval.
One significant increase in seeds recovered by the x-ray
method at the 8-10 cm depth interval was found (Figure 11).
Other than this difference, the two recovery curves were
similar. As in the first study, seed distribution was very
similar in terms of bulk recovery. The largest portion of
the seeds recovered was found below the 10 cm depth
interval with approximately 81 and 74% of the seeds
recovered in the wash and x-ray methods, respectively.
These curves were similar to those reported earlier and also
to those reported by other researchers following a single
tillage operation.
Minimum tillage and no-tillage
The comparison of both the minimum tillage and
no-tillage systems was virtually the same. There were few


HSD VALUES SEED RECOVERED (X)
60
0-2 4-6 8-10 12-14 16-18 20-22
DEPTH INTERVALS (CM)
Figure 11. Mean distribution of synthetic seeds recovered
following conventional tillage with comparisons
between wash and x-ray techniques. D=wash =
x-ray. HSD values determined by Tukey's at
Alpha= .05 level.


61
statistically significant differences between the two
methods. Both tillage systems resulted in a similar
patterns of a steady decline in seed recovery down through
the working depth. In both tillage systems, the bulk of the
seeds recovered were in the top 6 cm of the soil surface
which would be expected with a 7.5 cm average working depth
(Figures 12 and 13). In the minimum tillage treatments,
approximately 88% of the seeds recovered were found in the
top 6 cm with both recovery methods; while the no-tillage
treatments resulted in approximately 90 and 80% of the seeds
recovered in the top 6 cm by the wash and x-ray methods,
respectively. The only significant differences occurred in
the no-tillage system with an increase of seeds recovered in
the wash method over that of the x-ray method at 0-2 cm and
a decrease at 8-10 cm. However, when the curves are
examined, it is shown that some of these differences are due
to the actual handling of the sample necessary for the wash
method. This is concluded from the x-ray data. It appears
that seed shift in the physical slicing of the soil into 2
cm layers offers some displacement into adjacent layers,
whereas the x-ray method reduces this error.
Comparisons of Sequential Tillage Operations
Conventional tillage
This portion of the study compares distribution
patterns with additional tillage operations. As in Chapter
II, the plots represented are the various sample times which
actually represent the number of tillage operations and year


HSD VALUES SEED RECOVERY (X)
62
0-2 2-4 4-6 6-8 8-10 10-12 12-14
DEPTH INTERVALS (CM)
Figure 12. Mean distribution of synthetic seeds recovered
following minimum tillage with comparisons
between wash and x-ray techniques. D=wash
=x-ray HSD values determined by Tukey's at
Alpha=.05 level.


HSD VALUES SEED RECOVERED (5S)
63
20 -
10 -
T
T
0-2
2-4
4-6 6-3 8-10
DEPTH INTERVALS (CM)
1
10-12 12-14
Figure 13. Mean distribution of synthetic seeds recovered
following no-tillage with comparisons between
wash and x-ray techniques. D=wash #=x-ray HSD
values determined by Tukey's at Alpha=.05 level.


64
of study. The graphs were constructed from the means of the
wash and x-ray data because very little differences between
the two methods were found in the initial study. The first
tillage method to be discussed is the conventional tillage
treatment (Figure 14). As with the initial study, there
were interesting changes in seed distribution. Shifts
between the two single tillage operations and the double
tillage operation altered the pattern in comparison to both
single treatments. The two curves representing single
tillage operations in both year one and year two of the
study were very similar in terms of pattern; however the
shift in the second year is quite noticeable. Although
there was no statistically significant difference between
the two single operation curves, this shift raises questions
in terms of soil compaction changes between year one and
year two. The bulk of the seeds recovered following the one
conventional tillage operation were found below the 10 cm
depth interval, with the main concentration for the first
year being from 10-20 cm. While in the second year,
possibly due to previous tillage the concentrated area
shifted down to the 14-23 cm range. For both of the single
conventional tillage operations, approximately 87 and 88% of
the seeds were recovered below the 10 cm depth.
Results obtained by comparing the seeds subjected to
the two conventional tillage operations show that repeated
conventional tillage results in a uniform distribution
throughout the working depth. Following this second tillage


HSD VALUES SEED RECOVERED (55)
65
i 1 1 1 ; < i > i i
02 4-6 3-10 12-14 16-18 20-22
DEPTH INTERVALS (CM)
Figure 14. Distribution of synthetic seed as influenced by
one or two conventional tillage treatments.
Percents represent the means of the x-ray and
wash methods. d=batch 1 with 1 tillage
operation, =batch 1 with 2 tillage operations,
and A=batch 2 with 1 tillage operation. HSD
values determined by Tukey's at Alpha=.05 level.


66
operation, the distribution is such that this effect can be
easily seen. The high levels found below 10 cm from the two
single operations are redistributed both deeper and
shallower following the second tillage operation. This
redistribution generates a new pattern that becomes more
linear. The second tillage operation generates several
statistically significant increases in the upper layers
while the areas that had high numbers of seeds with one pass
decrease significantly. The 0-2, 2-4, and 4-6 cm layers all
increased significantly from both single conventional
passes. The two highest recovery zones from each single
pass were also significantly decreased. These zones were
the 16-18 cm level in year one and the 18-20 cm level in
year two. Other areas demonstrated decreases, yet they were
not found to be statistically significant.
Minimum tillage
The minimum tillage operation was very similar in
regard to all sample times and number of tillage operations
(Figure 15). There were no statistically significant
differences between any of the times of recovery.
Approximately 88% of the recovered seeds were found in the
upper 6 cm for all three sample times. The only visual
difference or trend was during the first year of the study
when one tillage pass was made. In the first year plot 74%
of the seeds were found in the top 4 cm while during the
second year, this recovery decreased to approximately 65%


HSD VALUES SEED RECOVERED (JS)
67
20 -
15 -
10 -
5 -
+
~T
0-2
T
T
2-4
4-6 6-8 8-10
DEPTH INTERVALS (CM)
10-12 12-14
Figure 15. Distribution of synthetic seed as influenced by
one or two minimum tillage operations. Percents
represent means of the x-ray and wash methods.
=batch 1 with 1 tillage operation, #=batch 1
with 2 tillage operations, and A=batch 2 with 1
tillage operation. HSD values determined by
Tukey's at Alpha=.05 level.


68
with the additional tillage. Again, this is believed to be
due to the original compaction of the soil.
No-tillage
The no-tillage system resulted in several statistically
significant differences between sample times and number and
time of tillage (Figure 16). It is important to note that
although this system involves no tillage at the time of
soybean planting, there still were two passes prior to the
winter wheat planting which contributed to the similarity to
the minimum tillage regime. The first year curve actually
represents distribution following two passes with a disk at
7.5 cm; the second year curve represents distribution
following four passes with a disc at 7.5 cm. Although this
area was treated as a no-tillage soybean program, the seeds
were subjected to some tillage following dispersal in land
preparation for winter wheat.
The results indicate (Figure 16) that the major
differences were those between the first year single tillage
and the second year single and double tillage operations.
The first year data showed that recovery was largely
obtained in the 0-2 cm layer which was approximately 53%.
Both the second year operations resulted in statistically
significant decreases to 31 and 35%, respectively. This
result again is most likely due to the soil conditions prior
to the start of the tillage operation. Both the 4-6 cm and
6-8 cm depths were found to be statistically significantly
with higher numbers of seeds recovered in the samples


HSD VALUES SEED RECOVERED (JC)
69
20 -
10 -
0 i 1 1 1 1 1
0-2 2-4 4-6 6-8 8-10 10-12 12-14
DEPTH INTERVALS (CM)
Figure 16. Distribution of synthetic seed as influenced by
one or two no-tillage operations. Percents
represent means of x-ray and wash methods. =
batch 1 with 1 tillage operation, =batch 1 with
2 tillage operations, and A=batch 2 with 1
tillage operation. HSD values determined by
Tukey's at Alpha=.05 level.


70
removed during the second year when compared to year one.
There was no comparable difference between the two
recoveries in the second year.


CHAPTER IV
COMPARISON OF ACTUAL AND PREDICTED EMERGENCE
GENERATED FROM DISTRIBUTION PATTERNS
Introduction
In order to better utilize the information obtained
from studies examining weed seed populations in the soil,
research has been directed towards prediction models.
Radosevich and Holt (35) examined the potential of weed
infestations by studing the soil seedbank. Some researchers
have developed working demographic models to aid in the
study of weed population dynamics (30,55).
Research has also been conducted on various forms of
cultivation and tillage regimes in respect to seedling
emergence. Roberts and Potter (49) studied the relationship
between cultivation and rainfall in terms of seedling
emergence. The importance of becoming more familar with
weed seed emergence in regard to cultivation will aid in the
ability to make predictions of future infestations.
Roberts and Ricketts (50) investigated the relationship
between cultivation and existing weed seeds in the soil.
They reported that such studies could possibly offer
information which would allow for predictions to be made on
weed infestations. They demonstrated how predictive values
71


72
can be obtained from actual and potential weed flora data
with additional work in regard to cultural practices.
This research was directed towards weed emergence and
distribution patterns. Attempts were made to predict weed
emergence with the aid of a model. Data collected on the
population in the soil seedbank was utilized through an
examination of distribution patterns under different tillage
systems, and actual emergence numbers collected from the
same study areas. The comparison between the actual and
predicted emergence was analyzed to answer questions
relating seed populations and their distribution in the
soil.
Methods and Materials
General
The establishment of the field experiment in terms of
the methodology applied was discussed in Chapter II. The
experimental design of the study was a split-plot experiment
with a randomized complete block design. Main plots were
tillage systems, and the subplots were treatments of seed
dispersal. The focus of this comparison will be on the
correlation between the distribution of the synthetic seeds
due to the tillage systems implemented and the effects of
this distribution on seedling emergence. A review of
methods involving viable seed collection, synthetic seed
production, seed dispersal, seed recovery, and analysis may
be helpful (Chapter II). For simplicity, "Conventional


73
tillage" refers to moldboard plowing to a depth of 24 cm,
followed by two tillings to a depth of 7.5 cm with a tandem
disk; "Minimum tillage" refers to two tillings to a depth
of 7.5 cm with a tandem disk; and "No-tillage" refers to
direct seeding with no soil disturbance.
Soil samples were collected using the device described
in Chapter II, with 10 samples being extracted per subplot
from an area 1 X 3 m centrally located within the subplot.
Both analysis methods discussed in Chapter II and Chapter
III were utilized in terms of synthetic seed recovery.
Seedling Emergence Counts
The population of sicklepod seedlings emerging
following the application of tillage treatments was obtained
by periodic counting of seedlings in a 1 X 3 m permanent
area within each subplot. All subplots receiving the viable
sicklepod seed dispersal treatments, which included either
1982 or 1983 viable seeds, were treated in this manner.
Counts were made until emergence had ceased in the subplots.
After counts were made, the seedlings were removed by hand
pulling; however, since they were typically in the
cotyledon to three leaf stage, no major soil disturbance
occured. Emergence numbers were recorded for analysis and
compared to distribution in the soil profile.
Calculating Predicted Emergences
The calculations utilized to generate the predicted
emergence values were constructed from data collected in a


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51 frl £1 31 U 01 6 9 L 9 9 i C Z 10
PREDICTED EMERGENCE


75
study by Teem et al. (63). This study involved the
capabilities of sicklepod to emerge from various depths.
The seeds were placed at depths from 2.5 to 15.2 cm, and
emergence was determined as a percentage of seeds planted.
The values were first transposed to an initial emergence of
55% which was a base starting point. This point is believed
to be closer than that of the acid scarified seeds in the
work by Teem et al. (63) to the unscarified seeds used in
this study. A multiple regression on values of emergence at
the various depths was run. This regression fit a curve so
that an equation could be found to allow the calculation of
predicted emergence percent at any depth within the range of
0 to 15.2 cm (Figure 17). From this equation the recovered
synthetic seeds within each depth interval could be
corrected to a value representing the predicted number
emerging from that depth.
Y = 36.89 + 5.92 X DEPTH 0.543 X DEPTH2
When the depths from 0 to 14 cm were handled in this
manner and the total recovery corrected from the equation,
the predicted values were then compared with the actual
values collected on the test areas. The depth of 14 cm was
utilized as the bottom depth since this is below the maximum
depth of reported sicklepod emergence.


76
Results and Discussion
Actual versus Predicted
Tillage
In order to compare the actual emergence numbers with
the predicted values, it was important to first examine the
main differences observed. The most obvious differences in
the actual emergence were shown in the conventional tillage
treatment in comparison to either the minimum or no-tillage
system. The reduction in the emergence following the one
tillage operation is approximately four fold lower than
either of the reduced tillage systems (Figure 18). The
conventional tillage resulted in approximately 14% of the
seeds dispersed emerging following a single tillage
application. This is a definite response of the
distribution pattern observed where approximately 85% of the
seeds were found below the 10 cm depth. Both the minimum
and no-tillage systems were found to have approximately 60%
emergence in comparison to the conventional tillage
treatment.
Seed Batch
Figure 18 shows the grouping patterns of each tillage
with the actual values plotted against the predicted values.
However, the predictions appear to be higher than expected
in some cases, reflecting higher recoveries. Since the
predicted emergence values are calculated from the recovery
values by depth, the relationship between seed batch and
recovery becomes more important than in the previous


ACTUAL EMERGENCE/M
77
900
800
700
600
500
400
300
200
100
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
(Thousands) 2
PREDICTED EMERGENCE/^*
Figure 18. Actual vs. predicted emergence by tillage.
^conventional, ^minimum, and A=no-till.


78
studies. An analysis by seed batch was run and linear
2
models fitted to both batch 1 and 2 (Figure 19). The R
values were better when the points were separated by seed
batches than when the linear model was fit to all points
(Figure 18).
Recovery Values
These questions brought about the need for closer
examination of the recoveries of seeds by the various
methods utilized. Analysis of the recovery data from both
the x-ray versus wash, and wash versus excavation studies
revealed similar findings. Comparisons of the wash and
excavation methods (Figure 20) demonstrated that the means
were higher than the predicted recovery, however they were
not statistically significantly different among the tillage
systems. Yet, when the seed batches were compared overall
by the means, the recoveries of the second seed batch was
significantly higher than either recovery of batch one from
year one or two (Figure 21).
Similar results were noted with the wash versus x-ray
recovery such that there was no difference in the mean
recoveries by tillage systems (Figure 22), yet again the
second seed batch was recovered in significantly higher
numbers over batch one (Figure 23).
These obsevations help to explain why the predictions
in some cases are higher than the expected numbers by at
least two fold. It appears also that the load material and
color are also important especially in methods involving


LOG OF ACTUAL EMERGENCE/M
79
Figure 19. Log plot of actual vs. predicted emergence of
sicklepod by seed batch. D=batch 1 *=batch 2


80
CON MIN NOT
TILLAGE TREATMENTS
Figure 20. Seed recovery means for wash and excavation by
tillage. CON=conventional, MIN=minimum, and NOT
=no-tillage. Predicted recovery shown by
horizontal line. HSD=500 at Alpha = .05


81
Figure 21. Seed recovery means for wash and excavation by
synthetic seed batch. BlTl=batch 1 tillage 1,
BlT2=batch 1 tillage 2, and B2Tl=batch 2 tillage
1. Predicted recovery shown by horizontal line.
HSD=340 at Alpha = .05


82
2.6-r
14-
12-
2.0-
i
\ 1.8-
Q
U
0^ 1.6-
Uj n
CON MIN NOT
TILUGE TREATMENTS
Figure 22. Seed recovery means for wash and x-ray by tillage.
CON=conventional, MIN=minimum, and NOT=no-tillage.
Predicted recovery shown by horizontal line.
HSD = 520 at Alpha = .05


83
16 7
14-
12-
10-
\ 1.8-
B1T1 B1T2 B2T1
SEED BATCH AND TILLAGE NUMBER
Figure 23. Seed recovery means for wash and x-ray by synthe
tic seed batch. BlTl=batch 1 tillage 1, BlT2=batch
1 tillage 2, and B2Tl=batch 2 tillage 1. Predicted
recovery shown by horizontal line. HSD=322 at
Alpha = .05


84
washing where visual separation from darker soil particles
and litter are involved. Although some of the predicted
values are high, it is clear that with an understanding of
distribution and population of seeds in the soil as well as
knowlege of the seeds capabilities and limitations,
acceptable prediction models can be developed to estimate
future weed infestation levels.


CHAPTER V
SUMMARY AND CONCLUSSIONS
Techniques
The techniques developed and utilized in these studies
were found to be efficient in terms of seed recovery and
determination of distribution patterns. The soil sampling
device was found to be effective in removing an intact
vertical slice of soil which was well suited to analysis by
both wash and x-ray. When compared to the excavation
method, or control, the data collected demonstrated that the
soil sampling device collected samples that were not
compacted eliminating shifts in seed location; the device
samples also allowed for excellent distribution analysis.
The synthetic seeds were found to be excellent in terms of
their ability for recapture following dispersal; they
should remain buried for extended periods of time without
degradation. The source of load material and color of the
synthetic seeds was found to be extremely important. The
barium sulfate load was better in terms of the utilization
of x-ray for analysis because of its higher resolution in
comparison to the iron powder. Barium sulphate also
resulted in a brillant white color that increased the ease
of separation from soil and litter in the wash method.
85


86
One of the greatest values of the techniques developed
was that they were found to be much faster than the other
methods. The wash method has been used for many years in
various ways, yet it still involves a large degree of hand
separation which is time consumming and laborious. The use
of x-ray was found to be at least five times as fast and
required no disturbance to the soil sample which means a
lower chance of seed movement during handling. Although the
cost of x-ray equipment is high, it was found to be readily
available for research purposes at a local hospital. The
major cost involved was the purchase of film for the actual
radiographs which was compensated for by the amount of time
saved. The samples are collected, exposed, and developed;
the sample requires no opening, slicing, or hand separation.
A larger number of soil samples could be handled at a much
faster rate with this method than with the wash and
germination method.
Tillage Systems
The results of these studies on three tillage systems
indicated that the conventional tillage system was the
system affecting distribution to the greatest degree. The
seeds were found to move deep into the soil profile with
approximately 80-85% found below the 10 cm depth. The
minimum and no-tillage systems resulted in little change
with all seeds being distributed in the top 7.5 cm.
Approximately 75-80% were found in the top 6 cm.


87
Another important factor was the number of tillage
operations, especially with the conventional tillage
treatment. The seeds were moved to different locations with
each tillage application, reflecting shifts in
concentrations. Following one tillage operation the bulk of
the seeds were found below 10 cm with high concentration
bands around 13-17 cm. Yet following the second tillage
operation, the distribution became more uniform throughout
the working depth. Even the seeds distributed in the second
year which received a single tillage operation resulted in a
somewhat different pattern. Although the second year seeds
were very similar in terms of actual patterns when compared
to the first year single tillage operation there was a key
difference. The concentrated bands shifted to deeper depths
in the profile. The most plausable explaination for this
was the difference in friability of the soil between the two
dispersals and tillage passes. Prior to the initiation of
the study, the field had been in grassland for approximately
seven years, thus the soil was tightly packed. In the
second year of the study it had been worked the previous
year and was in a more friable state.
Although few differences were noted between the two
reduced tillage systems, there was some statistically
significant difference in the upper 6 cm with additional
tillage applications.


88
Predicting Infestations
Understanding the distribution patterns and population
shifts among the seeds in the soil as influenced by tillage
are all important concerns when attempting to predict
emergence levels. However, of equal importance is a
complete understanding of the seed itself and its
capabilities and limitations in regard to both germination
and emergence. If these factors are known and if a good
method for analyzing the distribution and population within
the soil profile is available, then accurate predictions can
be made.
A better understanding of this area of weed science is
extremely important for achieving weed management. With a
complete knowledge of an adversary the battle can become
much easier.
Future Studies
With the introduction of the new techniques developed
in this work, there are many aspects of the soil seedbank
that could possibly be examined. The use of x-ray and
synthetic seeds offer two additional tools for a closer
examination of various soil seedbanks under many varying
conditions. The soil sampling device developed for these
studies was also found to be highly functional in relation
to both the wash method and the new x-ray techniques. The
advantages of a vertical slice as apposed to a core sample
are clearly seen when working with both of these methods.
The sampling device has immediate use in work involving soil


89
sampling for collection of data, because the area recovered
is greater and it lends itself to analysis better than a
core sampler. The study of seeds in the soil seedbank
becomes less of a problem when data can be collected in a
manner that requires less time and labor. The synthetic
seeds offer a new approach to questions of soil seedbank
structure. By loading synthetic seed with materials having
different atomic numbers various species could be examined
under different cropping practices and situations with the
ability to separate their individual responses. It is the
hope of the author, that this information and technology be
utilized in future studies. With these new tools, it is
hoped some new questions can be asked, and their answers found.


LITERATURE CITED
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30.Mortimer, A. M. 1976. Aspects of the seed population
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38.


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