Effect of Different Types of Fertilizer Run-Off on the Reproduction of
Duckweed (Lemna minor)
Submitted by: __________________
Submitted to: ___________________
Date(s) of investigation: ______________________________
Due Date: ______________________
Course: ______________________
Course Code: ________________
Name of School: Pierre Elliot Trudeau High School
Abstract
Duckweed (Lemna minor) is a common aquatic plant that thrives when given a rich
supply of nutrients. Fertilizer run-off can cause the overgrowth of duckweed, leading to
disruptions in the growth of other plants in the aquatic ecosystem. The purpose of this
experiment was to discover which fertilizers had the greatest impact on the reproduction rate of
the duckweed fronds. Five commonly used fertilizers were selected. These included two
chemical fertilizers and five organic fertilizers. The chemical fertilizers were Schultz Orchid/
Plant food (19-31-17) and All Purpose Plant Food (20-20-20). The organic fertilizers were
Blood Meal (12-0-0), Miracle Gro Organic Choice (7-1-2) and Roses N Bloom (3-1-5). Each
fertilizer was placed into a separate container with a mixture of tap/pond water and five
duckweed fronds. The containers were placed in a location with indirect sunlight and
fluorescent light. The room temperature was monitored and remained between 20 oC - 25oC.
The fronds were counted daily using a magnifying glass.
At the end of ten days, the duckweed fronds in organic fertilizers showed the greatest
increase, with Blood Meal leading this group by tripling the number of fronds. The chemical
fertilizers showed the least impact on frond growth. The fronds in the All Purpose Plant Food
died on the seventh day.
Introduction
Clean water and healthy freshwater ecosystems are increasingly coming under attack
from pollutants in our waste water systems. These pollutants are changing the ecosystems and
impacting the life cycles of plants and animals.
Duckweed (Lemnar minor) is a small, rapidly reproducing aquatic flowering plant found in
slow-moving or still fresh water all over the world [7]. It has a simple structure, consisting of one
broad oval leaf with a long fibrous root for stability [7]. Duckweed reproduces vegetatively, that
is, by growing an identical daughter frond that eventually breaks off the parent [7].
The optimum living conditions for duckweed fronds are temperatures between 15 oC and
25oC, although they can tolerate temperatures between 6oC and 33oC. They thrive in areas with
high organic decomposition due to the rich and constant supply of nutrients. The fronds require
pH levels between 5 and 8 [7]. They grow in sunlight in open areas but research indicates that
blue light, as seen on the light ROYGBIV spectrum, is the most beneficial for stimulating growth.
Compact fluorescent light bulbs produce more blue light than incandescent bulbs. For optimal
growth, fronds require 16 hours of light. Limited hours of sunlight during the winter require the
use of artificial light and compact fluorescent lights have been proven to give the largest
increase in vegetative growth [7].
Duckweed is a major food source for animals and fish. It is also useful as a cleaning
solution for sewage or other water industries. However, when duckweed grows too rapidly, it
forms an opaque blanket of green that covers the water surface, blocking out sunlight to other
aquatic plants. This can lead to serious repercussions for aquatic ecosystems and the
economies/businesses that depend on them.
Duckweed growth is stimulated by the nitrates and phosphates that are found in
fertilizers. On a regular basis, fertilizers are used by farmers and gardeners to provide nutrients
to depleted soil. Different fertilizers contain different ratios of nutrients (shown clearly by the
three large numbers on the fertilizer package, nitrogen, phosphorus and potassium). The
perfect ratio depends on the species that the grower or gardener wishes to grow. Often,
fertilizers also include secondary nutrients, such as calcium, magnesium, or sulfur, which can
sometimes have the largest effect on the reproduction rate of plants. When rainfall washes
excess fertilizer into the sewage system and into surrounding ponds and freshwater
ecosystems, it impacts the growth and reproduction of duckweed.
In this experiment, the growth rate of duckweed was observed in the presence of five
different commonly used fertilizers. There was a wide variety of fertilizers to choose from for
this experiment. Local gardening stores and gardening departments provide many options for
buyers [6]. To ensure that the experiment was relevant, I wanted to select fertilizers that might
realistically be found in local bodies of freshwater. To narrow the selection, I consulted three
avid Markham gardeners about their preferred fertilizers. Since fertilizers can be expensive and
only minute amounts (5 ml) of each fertilizer were needed for this experiment, fertilizer samples
were obtained free of charge from the gardeners.
The first gardener, a retired professional grower and greenhouse owner, uses chemical
fertilizers including All Purpose Plant Food and Schultz Orchid Plant food for his flowerbeds. All
Purpose Plant Food is very high in nitrates (20%). Nitrates promote strong stem and leaf growth
and improve plant colour [6]. Orchid food is marketed for indoor use, but can be used for
outdoor flowers as well. This was confirmed by a second Markham gardener who was glad to
share his “secret” for growing beautiful flowers. The orchid food is extremely high in phosphate,
which stimulates greater flower production and larger fruit crops [6].
Chemical Fertilizers
All Purpose
Orchid Food
Total nitrogen (N)
20%
19%
Available phosphoric acid (P205)
20%
31%
Soluble potash
20%
17%
Iron (Fe) actual chelated
0.14%
0.33%
Zinc (Zn) actual chelated
0.06%
0.07%
Manganese Mn actual chelated
0.06%
0.05%
Copper Cu actual chelated
0.06%
0.07%
Boron (B) actual
0.03%
0.02%
Chelating agent
1.5%
Molybdenum(Mo) actual
0.001%
0.0005%
The third gardener is a member of the Markham Horticultural society who only uses organic
fertilizer. He provided me with the three organic fertilizers that are listed below. His gardening
focus is to create attractive landscaping using native plants and bushes that are more drought
resistant to prevent overuse of water during hot summer months. Flower and fruit production in
his landscaping plan is limited.
Organic Fertilizers
Blood Meal
Organic Choice
Roses N Bloom
Total nitrogen (N)
12%
7%
3%
Available phosphate (P205)
0%
1%
1%
Soluble potash (K20)
0%
2%
5%
Organic Matter
85%
60%
91%
These three gardeners reflect a range of perspectives on landscaping that could
influence the purchase of fertilizers. Consequently, the fertilizers selected represent both
organic and chemical choices designed for different plant and flower growth. All these will be
naturally present in bodies of fresh water in Markham as they are carried away from gardens by
water runoff.
To assess the effect of fertilizers on the reproduction of duckweed, the experiment was
set up with one control and five experimental conditions. The growth of duckweed fronds was
observed in a control container with no fertilizer. An equal number of duckweed fronds was
added to five additional containers, each containing a small amount of fertilizer. The following
fertilizers were used: Schultz Orchid/ Plant food (19-31-17), All Purpose Plant Food (20-20-20),
Blood Meal (12-0-0), Miracle Gro Organic Choice (7-1-2) and Roses N Bloom (3-1-5). To
measure the impact of each fertilizer, the fronds were counted daily to observe the reproduction
rate over a period of ten days. By choosing five different fertilizers of different ntenutrient ratios
and types (organic or chemical), we could observe which fertilizers have the greatest impact on
the growth and reproduction of the duckweed.
Purpose
The purpose of the experiment was to discover whether or not fertilizers have an effect
on the reproduction rate of duckweed, and if so, which type of fertilizer and what mineral ratio
results in the highest reproduction rates.
Hypothesis/Prediction
The duckweed fronds that are in water contaminated by fertilizer should reproduce at a
rate significantly higher than that of the control group, because they have more nutrients,
provided by the contents of the fertilizer. Duckweed reproduction is stimulated by the presence
of nitrates and phosphates. [5] Consequently there should be more growth in the containers with
Orchid Food and All Purpose plant food since these fertilizers contain more nitrates and
phosphates.
Materials
6 clear plastic containers
garden gloves
3 pairs of chopsticks
safety glasses
1 large plastic bucket
Roses n bloom fertilizer (1ml)
tap water
All purpose plant food fertilizer (1ml)
pond water
Organic growth fertilizer (1ml)
duckweed (35 fronds)
Orchid food fertilizer (1ml)
1 lid of a large plastic tupperware container
Blood meal fertilizer (1ml)
1 lamp (with fluorescent light bulb)
magnifying glass
1 tray
scissors
1 syringe
pyrex measuring cup
¼ tsp measuring spoon
microwave
6 plastic 12ml test tubes with lids
thermometer
litmus paper (12 pieces)
heater
parchment paper
pipette
permanent marker
Procedure
Safety Precautions:
1. The kitchen-stove exhaust fan was turned on to create a ventilated area (but not so it was
blowing the fertilizer around)/
2. Gloves and safety glasses were worn to prevent eye
irritation or skin reaction/infection by the fertilizers
3. Hands were thoroughly washed after completion.
4. The measuring spoon was carefully rinsed before
measuring a new fertilizer to prevent chemical reactions
Safety glasses and garden gloves
(Safety Precautions step 2)
between and contamination of the fertilizers.
Pre-Lab:
1. A bucket was filled with tap water and left for 24 hours to allow the
chlorine to evaporate [5]
Bucket of chlorine-free water
(Pre-Lab step 1)
2. All equipment that was planned to be use was put in the dishwasher (with no detergent) to be
sterilized.
3. The work area was cleared and set up. All equipment needed for the actual procedure
(containers, tap and pond water, syringes, test tubes, permanent marker, duckweed, litmus
paper, fertilizers, measuring spoon) were gathered.
4. The test-tubes were labeled appropriately and placed next to their corresponding fertilizer.
5. The parchment paper was labeled, then placed on the tray. A container was placed on top of
each label.
Lab Procedure (evening of December 21st 2012):
1. Safety glasses and gardening gloves were worn.
2. Fertilizers were opened (if not already) with scissors.
Fertilizers in appropriate test tubes (step 3)
from left to right : “Roses N Bloom”, “Blood Meal”, “Organic Choice”, “All Purpose”, “Orchid Food”
3. 1ml of one brand of fertilizer was put into the appropriate test tube.
4. Chlorine free water was heated up in a (sterilized) Pyrex measuring cup in a microwave for 1
minute at power level 8 (to increase the dissolving rate of the fertilizer).
Chlorine-free water in pyrex dish in the
microwave (step 4)
12ml of Hot chlorine-free water in test
tube along with fertilizer (step 5).
5. Hot chlorine free water was poured into test tube until the tube was filled up to 12ml.
6. The lid was left off for approximately 15 seconds (so as not to build up pressure inside the
test tube) before the lid was put on.
7. The unused chlorine free water was poured down the sink.
8. Steps 3-7 were repeated for each brand of fertilizer.
9. After ensuring that the fertilizers had efficiently diffused into the
water, litmus paper was used to measure the ph level [7]. If the ph
level was not between 6 and 8, the mixture was diluted accordingly
(“All Purpose Plant Food” needed to be considerably diluted because
its original pH level was 4).
Measuring Ph levels using litmus paper(step 9).
From left to right : Control group, “Roses N Bloom”, “Blood Meal”, “All Purpose”, “Organic Choice”, “Orchid Food”
Labeled Test tubes with diffused
fertilizers (step 9).
Six containers were filled, each with 150 ml of chlorine free water and 30
ml of pond water (as to provide sufficient nutrition).
10. The contents of the test tubes were added into their corresponding
Removing pond water from
duckweed using a syringe. (step
10)
containers.
11. Duckweed fronds were carefully placed into the containers (5
fronds per container) using the pipette.
12. The tray was brought to the observation area (next to a window,
and with a lamp using a florescent light bulb [1], a thermometer
Moving duckweed fronds
using the pipette (step 12).
[7]).
13. Chopsticks were separated (total of 6 sticks) and labeled for
each fertilizer, and one for the “controlled” group (to be used
during observation).
14. The thermometer was read several times a day, ensure that the
Observation Area (step 13)
temperature was between 18 and 22 C consistently throughout the observation period (to
create optimum growing conditions) [7].
Clean up and Disposal
1. Test tubes, pipette, and syringe were cleaned thoroughly
2. Unused fertilizer was sealed (each package had either a zip lock or a lid) and put into the
plastic container for storage.
Fertilizer sealed and returned to
container (step 2).
3. The plastic lid used as the work surface was cleaned and returned to its box.
Work surface (lid)
cleaned (step 3)
4. The pond water were chlorine free water were emptied into the garden.
5. The kitchen counter and sink were carefully wiped down and disinfected before further use.
Results
Over a period of ten days (from December 21st to December 31st), the number of duckweed
fronds in the six containers were counted. Figure 1 shows the containers on Day 1. Figure 2 shows
the containers on Day 10 at the end of the experiment.
Figure 1: Each of the containers have 5 duckweed fronds
on Day 1.
Figure 2: The containers on Day 10
From left to right: Control, All-purpose plant food
From left to right: Orchid Food, Roses N Bloom
From left to right: Organic Choice, Blood Meal
Table 1 summarizes the observations while Graph 1 compares the number of fronds in the
different environments from days 1 to 10. With the exception of all purpose plant food, all of the
fertilizers improved the growth and reproduction of duckweed fronds. Blood meal improved the
growth of duckweed the most while all purpose plant food resulted in the death of duckweed. In
addition, some algal growth was observed in the control group on Day 4, and in the orchid food
container on Day 7.
Table 1: The growth of duckweed fronds in the presence of different fertilizers from days 1 to 10
# of fronds
Additional observations
Control
(water)
Blood
meal
Organic
choice
Roses n
Bloom
Orchid
food
All
purpose
plant
food
1
5
5
5
5
5
5
2
5
5
6
5
6
5
3
6
7
6
5
6
7
4
7
11
6
6
8
7
5
7
13
6
7
8
7
6
7
13
7
10
8
6
Day
7
7
13
9
11
8
5
8
7
14
10
12
9
0
9
8
14
11
12
9
0
10
8
15
12
13
10
0
Average #
of fronds
grown per
day
0.3
1
0.7
0.8
0.5
-0.5
Evidence of algae growth in the
control. Algae did not appear in any
other group.
The fronds in “All purpose” fertilizer
became more yellow and translucent.
Substantial algae growth in the
“Orchid Food”. No other groups
showed evidence of algae growth.
Graph 1: The growth of duckweed fronds in the presence of different fertilizers from days 1 to 10
Data Analysis
Control Group Trends: As seen in Clustered Column: Duck
Weed Growth Rates the control group grew less then 4 other
groups (“Blood Meal”, “Roses N Bloom”, “Orchid Food” and
“Organic Choice”) but more then the “All Purpose” group.
Contrary to the predictions made by James W Cross (Duck
Weed Specialist), the fronds in the control group did not double their number in a week [6]. As seen
in Duck Weed Growth Rate: CONTROL no new fronds were produced during a period of 5 days,
which is very irregular.
“Blood Meal” Trends: As seen in Clustered Column: Duck Weed
Growth Rates this group showed the largest growth of fronds
compared to the other groups. The final total of fronds was 15;
tripling it’s original number in 10 days. Although it grew considerably
more then all the other groups, it still had times (day 5-7) where no
new production of fronds were seen (see Duck Weed Growth Rate:
BLOOD MEAL).
“Organic Choice” Trends: This group (as seen in Clustered
Column: Duck Weed Growth Rates) “Organic Choice” produced
the third most fronds. Looking at Duck Weed Growth Rate:
ORGANIC CHOICE we can see that although these fronds had no
growth at the beginning (days 2-5 showed no change in the
number of fronds), they eventually had a consistent growth pattern (producing approximately one
additional frond a day) The final total was 12 fronds. Note that this group more then doubled its
population in 10 days (what was predicted for a perfect control group by James W Cross).
“Roses N Bloom” Trends: similarly to the “Organic Choice” group,
the “Roses N Bloom: group had a slow beginning, not reproducing
for the first 3 days of the observations (see Duck Weed Growth
Rate: ROSES N BLOOM). After the first three days, however, the
fronds reproduced at a steady pace, finishing with 13 fronds (more
then doubling it’s population in the time predicted)
“Orchid Food” Trends: This group doubled its original population
(see Clustered Column: Duck Weed Growth Rates), perfectly in the
estimated time (10 days according to James W. Cross). Although
this is true, there was inconsistent reproduction, as no new fronds
appeared over a period of 4 days (day 4-7) (see Duck Weed
Growth Rate: ORCHID FOOD).
“All Purpose” Trends: The results of the “All Purpose” group were
very surprising, as they all died off before the experiment was over
(on day 8, see Clustered Column: Duck Weed Growth Rates). The
highest frond number was 7; This total was not double the original
population (see Duck Weed Growth Rate: ALL PURPOSE). The
fronds died by day 8 of the experiment.
To calculate the average # of fronds grown per day, the following formula was used:
average # of fronds grown per day
=
# of fronds on day 10 - # of fronds on day 1
10 days
For example, to calculate the average # of fronds grown per day for the control group:
average # of fronds grown per day
=
# of fronds on day 10 - # of fronds on day 1
10 days
average # of fronds grown per day
=
8 fronds – 5 fronds
10 days
average # of fronds grown per day
=
0.3 fronds / day
Discussion
Evaluating the Hypothesis
In some ways the results reflected my hypothesis. I expected the control group to rank
lowest since the control fronds didn’t receive extra nutrition. The growth rate of the control group
from 5 to 8 fronds shows healthy, but not accelerated growth.
I expected the duckweed fronds in chemical fertilizers to show accelerated growth since
these chemical fertilizers are extremely high in phosphates (PO4 3-) and nitrates (NO3 1-). All
Purpose plant food had nitrogen levels of 20%. Schultz Orchid/ Plant food had nitrogen levels of
19 %. This was 7% higher than the leading organic fertilizer.
All Purpose plant foods has phosphate(PO4 3-) levels at 20%. Orchid food has an
astounding 31% of phosphate while the organic fertilizers contained 1%. Yet, the fronds in the
Orchid Food showed an increase of only 5 fronds over the 10 day period and the fronds in the
All Purpose Plant food died on the seventh day of the experiment.
In comparison, the duckweeds in the organic fertilizer actually showed a higher growth
rate than the fronds in the chemical fertilizer. The duckweed in the blood meal showed that
largest growth with an increase of 10 fronds over the 10 days. The duckweed in the Roses N
Bloom showed an increase of 8 fronds over the seven days. The duckweed in the organic
choice showed an increase on 7 fronds over the 10 days.
Questions posed by the experiment
These results raise a number of questions. Why did the duckweed fronds in the All
Purpose fertilizer die? Why did the duckweeds in the organic fertilizers thrive?
Why did the fronds in the All Purpose Plant Food die? Initially the fronds showed an increase
growing from 5 to 7 fronds in the first five days.
were dead by day 7.
By day 6 fronds were becoming yellow and all
The first possible explanation was that water evaporation caused an
increase in salt levels. Duckweed fronds require fresh water and although they can tolerate
some salt they are sensitive to increases in salinity.
The increase in the salt level caused
stress in the tiny plants. However, evaporation also occurred in the Orchid food. Orchid Food
has a similar chemical composition (see appendix). Consequently, I would expect that the
same increase in water salinity would occur in the water sample with Orchid food, however, the
fronds in Orchid food did not die.
By comparing the chemical fertilizers to the organic
fertilizers, I can suggest an alternate explanation.
All the organic fertilizers contained organic matter. Blood meal contains 85% organic
matter, Organic Choice contains 60% organic matter, and Roses and Bloom 90% organic matter
mostly Kelp and Alfalfa according to the packaging. More specific information about the
composition of the organic matter was not on the packaging or available on the product
websites. This raised the question, could the organic matter contain minerals, substances, and
chemicals that are essential to frond growth? One example of an essential plant nutrient is
chelated iron. Plants use chelated iron during the process of photosynthesis. [9] Professor
James Cross notes on his website that chelated iron is also important for frond growth [4].
While fronds can benefit from excess amounts of nitrates and phosphates, they can only
process these if they have sufficient chelated iron for photosynthesis. All the organic fertilizers
have high amounts of organic matter.
Organic matter is a source of chelated iron. [9]
Chelated iron was found in both chemical fertilizers. All Purpose fertilizer contains
0.14% and Orchid Food contains 0.33%. I would suggest that the levels of chelated iron were
too low in the water sample with All Purpose fertilizer. Initially the fronds showed growth
increasing from 5 to 7 fronds but as the chelated iron was used up the fronds could no longer
multiply.
Prior to disappearing completely the leaves turned yellow and translucent which is a
sign in plants that are experiencing iron deficiency or chlorosis. [9]
With 0.33% of chelated iron in the orchid food, which is more than double the iron levels
in All Purpose fertilizer, we could expect that the fronds would thrive in the water sample that
contained Orchid Food. Why did the growth rate lag behind the other organic fertilizers? With
very high amount of nitrates and phosphates in the Orchid Food, I would expect the duckweed
in Orchid food to thrive and have the most impressive growth rates. The iron content in the
Orchid food may still be too low to produce a large growth rate in the duckweed fronds. This
raises another question. How much iron is necessary for duckweed growth? Did the duckweed
fronds thrive in the organic fertilizers because the iron contents were significantly higher? Since
it is difficult to evaluate the iron content of the organic matter in the organic fertilizer where there
may be much higher amounts of chelated iron, there is no definitive answer in this experiment.
There was on additional observation that might explain the disappointing growth rates of
the duckweed in Orchid food. The Orchid Food when compared to All Purpose fertilizer has
very high volumes of nitrates and phosphate. It contains 31% phosphate, which is a
significantly higher level. Phosphates contribute to the growth of algae [2] and the orchid food
was the only container where there was a significant amount of algae growth. Since algae was
present in the pond water, the algal spores were probably attached to the duckweed fronds.
The excessive growth of algae causes eutrophication, a reduction of the oxygen supply. [3]This
algae growth could have robbed the Duckweed of important nutrients and oxygen limiting the
growth rate of the orchid food and causing a lower growth rate than I anticipated It is
interesting that the fronds in the orchid food multiplied faster in the first four days than the fronds
in either Roses N Bloom or Organic Choice. After day four the growth rate dropped
considerably. The increase in the amount of algae in the container could explain the change in
growth rate.
The blood meal showed the highest growth rate of the five fertilizers. This surprised me
since the blood meal contains no phosphates. The nitrate level is only 12%, which is lower
than both of the chemical fertilizers. The Duckweed thrives with both nitrates and phosphates
and is now being used in special facilities for cleaning human sewage.[10] In this experiment
duckweed could thrive without phosphates.
Duckweed in the Roses N Bloom had the second highest growth rate. The frond number
increased from 5 to 13 fronds over the 10 day period. Yet Roses N Bloom has the lowest
amounts of nitrate 3% and phosphate 1%.
The organic matter of the Roses N Bloom also
includes large amounts of alfalfa and kelp. Kelp is a very good source of iron. Alfalfa also
contains iron. Both kelp and alfalfa are used as herbal supplements to improve iron levels for
people with iron deficiency. It is difficult to confirm if this iron was in a form that could be used
by the duckweed but this may have contributed to high growth rates of the duckweed.
Duckweed in The Organic Choice showed a growth rate that was only slightly less than
the growth rate of the roses N Bloom. The Duckweed in organic choice fertilizer grew from 5 to
12 fronds. This was only one frond less than the Roses N Bloom. During the first two days of
the experiment Duckweed in the organic choice sample seemed to be growing faster but by day
three the Roses N bloom showed a greater growth rate than organic choice.
Organic choice
has 7% Nitrates, and 1% phosphates. The nitrate level in organic choice is higher than Roses
N Bloom. Once again this may suggest that another nutrient beside nitrate is impacting the
growth of the fronds. The phosphate levels in both fertilizers are identical.
Evaluating the experiment and suggesting changes to experimental design
To begin the experiment, I obtained a bucket of pond water with Duckweed fronds. I had
obtained the pond water sample in the hopes of isolating algae but the large amount of
duckweed sparked my interest in duckweed. I continued my research on both duckweed and
algae for two weeks to decide on a hypothesis for my experiment. During this time, the fronds
had been thriving and multiplying for two weeks in the bucket. The pond water had large
amounts of organic matter. This convinced me that the pond water was sufficient to promote
the growth of the fronds. I decided to add the pond water to each sample environment in hope
that the pond water would provide sufficient organic material to keep the fronds alive and
observe the impact of the fertilizers. By filtering the pond water through a sieve I was able to
remove the fronds but obtain enough pond water for my experiment. I decided to add 30 ml of
pond water to each of the sample environments of tap water to provide nutrients (and hopefully
sufficient chelated iron) to support the fronds. If I had chosen to increase the proportion of
pond and fresh water I might have seen a greater increase in frond growth in the sample of All
Purpose Plant food. If I were to repeat the experiment I would exclusively use pond water for
the water samples. This might lead to increase growth in the duckweed fronds in the chemical
fertilizer because there would be a higher amount of chelated iron.
This experiment included many independent variables that were difficult to isolate so it
raises many questions. The experiment was designed to reflect the impact of fertilizer runoff
from gardens on the growth of duckweed in local ponds and still moving water. This required
that I obtain fertilizers that were popular with local gardeners. (See appendix for details on
fertilizer selection) The chemical balance in each fertilizer varies greatly so it is difficult to
isolate the variables. This is especially true for the unidentified “organic materials” in all the
organic fertilizers. It would be interesting to isolate nitrates and phosphates. The results of the
duckweed production in the Blood Meal suggest that duckweed can thrive without phosphates.
I could design an experiment where duckweed reproduction could be observed in varying
amounts of nitrates and /or phosphates.
In my research on duckweed prior to starting the experiment, I learned that duckweed
thrives in environments that have high nitrogen and phosphate levels. I expected the
duckweed growing in chemical fertilizers with high nitrogen and phosphate levels to have the
greatest growth rate. As I discussed earlier, the largest growth rates did not appear in the
fertilizers that contained large amounts of phosphate.
This leads me to speculate that another
nutrient was also impacting the growth of the fronds. Possibly this nutrient is chelated iron. The
amount of chelated iron in the organic fertilizers was unknown. It would be interesting to find
pure samples of chelated iron and add the iron in different amounts to sample environments
while monitoring the growth of the duckweed.
Conclusion
I expected that duckweed fronds in water contaminated by fertilizer would reproduce at a
rate significantly higher than that of the control group, because they have more nutrients,
provided by the contents of the fertilizer. Duckweed reproduction is stimulated by the presence
of nitrates and phosphates. Consequently, I expected more growth in the containers with
fertilizers that have higher nitrate and phosphate levels. Significantly higher nitrate and
phosphate levels were found in the chemical fertilizers, so I expected these to yield higher frond
growth. The duckweed fronds in the water containing organic fertilizers actually had a higher
growth rate.
This leads me to suggest that the organic matter in the organic fertilizers contains
something that is essential to duckweed growth that works in combination with the nitrates and
phosphates. This ingredient in the organic matter is likely chelated iron, which is essential for
the process of photosynthesis. Future experiments on duckweed growth could include an
experiment that tests the impact of varying levels of chelated iron on frond reproduction.
In addition, future experiments on duckweed growth could include separate studies on the
impact of chemical and organic fertilizers. The greatest advantage to chemical fertilizers is the
packaging provides a very specific breakdown of the chemicals and nutrients in the fertilizers. There
are different quantities of organic matter in organic fertilizers, but the content is not specific and this
results in variables that are difficult to isolate.
Duckweed can be either a threat to or salvation of an ecosystem. In areas where water is
contaminated by human sewage, or other forms of waste, the duckweed can be used to remove
pollutants. Alternatively, duckweed can damage or destroy a healthy ecosystem when it grows
too rapidly and completely covers the water surface.
The results of this experiment illustrate the dangers of extensive fertilizer use since
fertilizers can eventually wash into fresh water ecosystems producing a large increase in
nitrates and phosphates and stimulating duckweed production. Although organic fertilizers are
more environmentally friendly than chemical fertilizers, they can still negatively impact an
ecosystem by flooding an ecosystem with nutrients and creating an ecological imbalance.
Gardeners and growers need to be environmentally responsible with the application and use of
all fertilizers.
Limiting the use of fertilizers will help to keep our fresh water ecosystems
healthy.
References
1. Arizona master gardener manual- mg manual reference: Environmental factors that affect plant
growth. (Master's thesis)Retrieved from http://ag.arizona.edu /pubs/
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