1
The Effect of Nutrient Solution pH on Growth and Germination of Hydroponically
Grown Lettuce
Research Question:
How does altering nutrient solution pH (5-9) affect the germination and growth of lettuce
in a nutrient film hydroponic system?
Subject: ​Biology
Supervisor: ​Mrs. Kathleen Franzen
Exam Session:
Word Count: ​4000
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Abstract
In this experiment, I will look into the effects of nutrient solution pH on the
germination and growth of hydroponically grown lettuce. The Research Question is:
How does altering nutrient solution pH (5-9) affect the germination and growth of lettuce
in a nutrient film hydroponic system?
In this experiment, I set up 5 hydroponic systems each capable of holding 10
plants. All trials were run using 3 gallons of the same nutrient solution with adjusted
pH’s done using hydroponic specific pH up and down solutions. Other conditions such
as temperature, humidity and lighting conditions were kept constant. I first ran two, five
day germination trials, in which one hundred lettuce plants were germinated in rockwool
cubes per trial, 20 at each pH. Each day I recorded whether or not the plants had
germinated. I then ran the 25 day growth trial with 45 plants starting the plants at a pH
of 7 for germination and after the first 5 days, lowering or raising the pH and allowing
the plants to grow for the next 20 days. I then removed the plants from the system,
patted them dry and massed both the leaves and the roots.
I concluded that more acidic conditions at a pH of 5 helped to speed up the
germination process but hurt the overall health of the plant, as determined by biomass,
during the growth stage. For the growth trial, a slightly acidic pH solution between 6 and
7 was beneficial to an increase in plant biomass.
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Table of Contents
1. Introduction 4
2. Materials and Methods 12
3. Results 15
4. Analysis 20
5. Conclusion and Evaluation 23
6. Works Cited 25
7. Appendix 27
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1. Introduction
Hydroponics is an increasingly common method of growing plants for both
hobbyists and large scale farms in areas with limited or poor quality farmland. There are
a number of factors that influence success of hydroponics, one of the most significant
being pH of the nutrient solution. In a study conducted by Kane, Jasoni, Peffley,
Thompson, Green, Pare and Tissue “Nutrient Solution and Solution pH Influences on
Onion Growth and Mineral Content”, effects of both nutrient solution concentration and
pH were tested. Their results showed that the largest biomass was achieved at a pH of
6.5 rather than the more acidic 5.8 (Kane et al.). However, while this experiment
demonstrated that changing the pH greatly affects plant biomass it didn't go into depth
about the effects of pH on the germination process and was limited to testing only two
acidic pHs. I chose to continue to study the field of hydroponics due to its wide use and
my interest in plants and technology, two things which hydroponics combines. This
investigation will further study how hydroponic plants are affected by a wider range of
pHs during the germination and growth stages of the plants, specifically lettuce.
1.1 Background information
1.1.1 Lettuce and Hydroponics
Lettuce is one of the most commonly eaten hydroponically grown vegetables in
the world and for this reason, I have chosen it as my object of study. Lettuce is also a
good source of Vitamins, Minerals, and other nutrients, and along with other
hydroponically grown plants is usually grown through a nutrient film technique (NFT) or
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a method of floating rafts. Both methods are effective for large-scale lettuce production
through the winter and in areas with limited or poor quality soil (Kaiser).
The nutrient film technique uses a series of pipes or gutters that carry a small
“film” of nutrient solution along the bottom of the pipe. Plants are suspended in net cups
above the solution which allows the roots to grow through the base of the cup and into
the water. The water is constantly recycled using a pump into a reservoir under the
tubes and away from the grow lights to limit algae growth. The water in the reservoir is
regularly adjusted for pH and nutrient concentration (Kaiser). The floating raft technique
is another common method used in mass production of lettuce and other hydroponically
grown plants. It uses large floating rafts, in which the lettuce seedlings are placed two
weeks after germination. These rafts with the germinated plants are placed in controlled
nutrient solution ponds and the roots grow through the raft and into the nutrient rich
solution feeding the plant. As the plants get bigger they are transferred to different rafts
and ponds according to size and are eventually harvested by workers at the end of their
growth cycle (Kaiser).
For my study, I will be using a nutrient film based system similar to those seen in
many industrial greenhouses as well as personal gardens. I chose to go with the
nutrient film technique as it was more suited to smaller scale growing compared to a full
raft system which is more difficult to set up and less suited for the scale of my
experiment and observation methods. In addition, the method of choice shouldn’t
greatly affect my results as lettuce plants have been seen to grow successfully in either
style of system.
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Lettuce, like other plants, requires a set group of nutrients which are delivered
through the nutrient solution, as well as light, and a controlled temperature and
environment. In order to get a consistent concentration of nutrients, I will be diluting a
pre-made hydroponics specific nutrient solution or powder available for purchase on the
internet. I chose this method to enable correct and consistent concentration of nutrients
to support healthy plant growth which would affect my data (Kaiser). By using a
pre-made powder I can be sure the plants are receiving the correct nutrients and that all
plants receive the same amount. For lighting, I will be using two fluorescent grow lamps
from Aquabrite with a wattage of 54 and a kelvin rating of 6400 in a room without extra
artificial or natural light. This will ensure that all trials and plants receive the same
amount of light and that their growth will not be affected by outside light pollution.
1.1.2 pH and Growth / Germination
pH is a commonly used logarithmic scale which measures the acidity or alkalinity
of a solution where 7 is neutral and lower is more acidic while higher is more basic. The
scale is based on the negative log of the hydrogen ion concentration of the solutions
measured (Simply Hydroponics).
In hydroponics, as well as typical soil gardening, the pH of the nutrient solution
can have significant effects on a plant's ability to grow for a number of different reasons.
At extreme phs, the nutrient solution may become too corrosive for plant survival as the
roots may be damaged. This would be seen as browning or lack of growth of a plant's
root system. Also, as the pH becomes extremely acidic or basic, nutrients such as iron
and nitrogen become increasingly unavailable becoming insoluble as you move out of a
7
range of pH from 5-7.5 (Simply Hydroponics), causing nutrient deficiency and eventual
death of the plant and as pH becomes increasingly acid toxic concentrations of
aluminum, Iron and Magnesium occur (SUNY-ESF Office of Communications). Nutrient
deficiency is a plants, either hydroponically or soil grown, reponse to the lack of some
general nutrients including nitrogen, phosphorus, potassium, calcium, magnesium,
sulfur, iron, manganese, and boron. Some common responses to a lack of nutrients
include: dropping, discolored (yellow or brown), or small leaves, weak or brittle stalks
and other general unhealthiness or lack of growth (Schmidt). pH can also affect the rate
and success of plant germination, as more acidic conditions are generally more
favorable to and increased germination rate (Shoemaker et al.).
1.1.3 The relationship between plant health and root mass and branching
In both hydroponic and soil grown plants, root health and development is
extremely important to the growth and survival of the plant. An unhealthy root system
prevents the uptake of nutrients slowing growth, eventually causing the death of the
plant (Morgan). There are a number of factors that influence root growth, including the
availability of oxygen, nutrients, and water. When there is a lack of oxygen, plants will
exhibit oxytropism (roots grow towards a source of oxygen), as roots will not grow in
deoxygenated areas (Morgan). In hydroponics there are a number of ways oxygen
availability can be controlled, such as porous growth medium, aerating the nutrient
solution, and decreasing the water level in the system (Morgan). Though I will not be
aerating the solution, turbulence created by water circulation, low water levels, and
correct growth medium should supply adequate amounts of oxygen to the roots.
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Another factor is space and unlike soil grown plants, hydroponically grown plants are
extremely limited in the amount of space available for root growth as they are typically
grown in grow tubes with extremely high plant densities. This can hurt plant growth
because if continual root growth is restricted by neighboring plants, so is nutrient
uptake. Also, this confined space results in unusually high root density and large
amounts of root branching as the roots are unable to spread out (Morgan). This
restricted the number of plants I was able to grow per tube to avoid running into space
limitations.
By controlling all of these factors and ensuring that are the same for each tube, I
will be able to measure the root mass in order to determine the pH most favorable for
the root and plant growth of hydroponic lettuce.
1.1.4 The relationship between plant health and biomass
Another method for quantifying the growth and health of a plant is through
mass, as well as the allocation of mass in the plant. For a general summary, it is
believed that increased mass allows for increased uptake of nutrients and light.
Therefore plant mass is a measure of the growth rate, and therefore health, of a plant
as it determines the amount of nutrients that can be absorbed (Poorter et al.). There are
two different methods for estimating a plant growth rate based on the mass of specific
plant organs.
The first is based on the assumption that photosynthesis occurs in the leaves only and
includes factors such as leaf surface area, leaf mass in relation to surface area, and leaf
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mass compared to total plant mass. When these three factors are combined, one can
estimate the relative growth rate of the plant because all of these factors are linked to
the rate and efficiency of photosynthesis and gas exchange. The second method uses
an overall relationship between the masses of respective plant organs (Poorter et al.).
Overall, plants must effectively maintain a ratio between root, stem and leaf
masses which best matches their required functions as well as the needs of the plant for
the specific situation (Atwell et al.). Accurately measuring the allocation of mass within a
plant can pose a number of issues that must be addressed to get accurate results. One
issue is that when measuring the allocation of mass, a number of factors other that pH
may determine the plant's behavior. These factors include light availability, sufficient
water, and sufficient nutrients all of which may result in an overall decrease in plant
mass if the necessary amounts of each are not supplied (Poorter et al.). I will attempt to
isolate those variables by ensuring an equal amount of light and nutrients given to the
plants therefore isolating pH as the sole variable that would affect plant growth and
health.
As I discussed above, plants will respond to the environment in which they are
growing by allocating resources to different plant organs to meet the needs of the plant.
One way that allocation of plant resources can be measured is through root to shoot
ratio which is an effect of the plant attempting to balance the availability of resources
acquired by the shoots (CO2 and light) and nutrients acquired by the roots (Agren). So,
for example, for a plant in strong light, root growth would be emphasized, while for a
plant in low light, shoot growth would be emphasized. The same logic applies for the
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roots of a plant, where root growth would be emphasized in low nutrient conditions
compared to high nutrient conditions where it would not (Agren). For each tube, I should
be able to compare the different root : shoot ratios by mass and therefore determine the
availability of nutrients for each independent system and trial.
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1.2 Hypothesis
I believe that plants in the tube with a pH 7 nutrient solution will show the
greatest success and speed of germination over the course of the 5 day trial. This is
because the younger plants will be more susceptible to the damaging effects of the
higher and lower phs and damage to them would reduce the percent germination rates.
However, after germination, the plants in the pH’s 5 and 6 tubes will be healthier than
those in the basic and neutral tubes as shown by their having a greater biomass by the
end of the 25 day growth trial. This is because the most nutrients are still in solution
between these two pH’s and as the pH moves away from this range nutrients begin to
fall out of solution resulting in nutrient deficiency and a decreased biomass. In addition,
more basic pH’s will damage the plant's root systems and inhibit growth resulting in a
much lower overall biomass.
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2. Materials and Methods
2.1 Materials
-
-
-
-
-
5 Hydroponics tubes, 5ft long with ten spots for plants per tube. The water levels
should be 0.5 in above the base of the net cup for germination and at the base of
the cup for the growth.
5, 8-gallon per hour pumps for each system
5, 3-gallon reservoir labeled with water level of 3 gallons and pH (must either be
opaque or covered from light to prevent algae growth)
50 x 2.5in net cups (which suspend in the drilled holes in NFT hydroponic
system)
Grow lights (I used 2 high output fluorescent lights from AquaBrite with a wattage
of 54W each and a kelvin rating of 6400 K)
Nutrient solution mixture (I used a powder from General Hydroponics called
Maxigrow though any premixed nutrient solution of powder would function) (The
nutrient is a 10- 5 -14 blend)
Ph measuring solution that came with pH adjusting kit to measure and adjust the
pH of the nutrient solution.
Hydroponic pH up or down (I used a diluted liquid kit from General Hydroponics)
Distilled or purified water (for my experiment I used reverse osmosis water) (tap
water should not be used due to the possibility of excess chemicals such as
chlorine and minerals in the water that may affect results and damage the plants)
Rock Wool cubes with precut divots into which the seeds can be placed (the
base of the cut should not be lower than the water level)
Paper towels for patting drying the plant roots prior to weighing and for clean up
spills
Scale accurate to ±0.01 grams
Lettuce Seeds (I used the Simpson Elite variety from Jung Seed as it performed
best in my initial test trials for feasibility of the experiment) (all seeds should be of
the same strain and from the same provider)
pH neutral growth medium such as clay pellets
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2.2 Methodology
2.2.1 Germination
1. Create, or purchase, a hydroponics system with five, five foot independently run
tubes with ten plants per tube. Each tube should be run off of a 3 gallon reservoir
with an 8 gallon per hour pump. Also, each tube should have a variable water
level, one for germination where the base of the net cups touches the water, and
another where the water is approximately 0.5” away from the base of the net
cups allowing the roots to dangle in the solution while still having access to
oxygen. Photographs of the setup at the end of the growth trial are included in
the appendix as Figures 8 and 9.
2. Place all five tubes in temperature regulated room without light, either natural or
artificial. Arrange the lights 10 inches above the tubes so that each plant receives
an equal amount of light. A reflector made from tin foil can be added to both
block light from the tanks and make the lighting more efficient and evenly
distributed.
3. For each independent system create a nutrient solution of the same
concentration then adjust each pH respectively to 5, 6, 7, 8, and 9. Then begin
circulating the solution through the system. Allow the solution to circulate for a
day or two while getting familiar with your system and how to properly maintain
the pH and nutrient levels, follow instructions on the packaging for these. For
both of these use a pH meter, tape or solution indicator
4. place 100 lettuce seeds from the same strain and supplier which were purchased
at the same time into 50 pieces of rock wool with two seeds per each making
sure that the seeds rest in the center of the cut out and are not forced down into
the wool. Also ensure that when placed into the system the seeds are not
submerged.
5. Take each set of seeds and rockwool and place it into the 2.5 in netcup so that
the bottom of the rockwool is touching the water inside of the pipe. fill the
remaining space with a pH neutral hydroponic growth medium to stabilize the
rockwool in the center of the net cup (this may not be needed depending on the
size of the rock wool and net cups).
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6. Over the next 5 days record observations about health of the plant and
germination, or lack of, while also making sure to measure and adjust pH daily
and replenish nutrient solution according to the given instructions for your
solution.
7. Once 5 days has passed finish recording which plants germinated as well as any
observations about algae growth or plant health and remove the germinated
plants from the system.
2.2.2 Growth
1. Using the same setup as used for germination, prepare the 50 net cups with
three seeds per cup, as well as rock wool and a pH neutral growth medium
(again this may not be necessary).
2. For each independent system create three gallons of a nutrient solution of the
same concentration and a neutral pH (7) following the directions given for both
pH up and down, as well as for the nutrient solution.
3. Place the prepared net cups into the holes in the tops of the hydroponic tubes
with the nutrient solution circulating and at the highest water height where 0.5” of
the base of the net cup is submerged, but the seeds are not. Mark the time and
date at which the trial was started.
4. As the plants begin to germinate, check root length daily for each of the tubes by
carefully lifting out one of the net cups and observing root growth and length.
Make observations about the health and size of the roots and plants in each
tube.
5. Once the roots are beginning to poke out the bottom of the net cups reduce the
water level so that the tips of the growing roots are in the water but the rock wool,
growth medium, and net cups are not. Make sure to record the date and time that
the water level was reduced for the tubes.
6. Once the water level has been lowered adjust the pH of the circulating nutrient
solution to 5, 6.5, 7, 7.5, and 9 respectively and check daily. Also, check the
water level and nutrient concentration adjusting as needed.
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7. Continue with daily observations about plant health (leaf color and root system),
growth rate, and any other factors that may need to be recorded.
8. At the end of each tube's 25 day period, finish observations about apparent plant
health, remove the plants from the system and pat them dry. Then find total
mass, root mass, and leaf mass by cutting the plant in half between the root
system and the leaves.
3. Results:
3.1 Germination:
Table showing Number of Plants Germinated compared to pH for Trial 1
Day 1
pH
Plant Number
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
1
0
0
1
0
0
0
1
0
0
3
6
0
0
1
0
1
0
0
0
0
0
2
7
0
0
0
0
0
0
0
0
0
0
0
8
0
0
0
0
0
1
0
0
0
0
1
9
0
0
0
0
0
0
0
0
0
0
0
Day 2
pH
Plant Number
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
2
0
0
1
0
0
0
1
0
1
5
6
1
0
2
0
1
0
1
2
1
1
9
7
1
0
0
0
0
0
0
0
0
0
1
8
0
0
0
1
0
1
0
0
0
1
3
9
0
2
0
0
0
0
0
1
1
0
4
Day 3
Plant Number
16
pH
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
2
0
0
1
0
0
0
2
0
1
6
6
2
0
2
0
1
0
1
2
1
2
11
7
1
0
0
0
0
0
0
0
0
0
1
8
0
0
0
1
0
1
0
0
0
1
3
9
1
2
0
0
0
0
0
1
2
0
6
Day 4
pH
Plant Number
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
2
0
0
1
0
1
0
2
0
1
7
6
2
0
2
0
1
0
2
2
2
2
13
7
2
0
0
0
0
0
2
0
0
0
4
8
0
0
0
1
0
1
0
0
0
1
3
9
1
2
0
0
0
0
0
1
2
0
6
Day 5
pH
Plant Number
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
2
0
0
1
0
1
0
2
0
1
7
6
2
0
2
0
1
0
2
2
2
2
13
7
2
0
0
0
0
2
2
0
0
0
6
8
0
0
0
1
0
1
0
0
0
1
3
9
1
2
0
0
0
0
0
1
2
0
6
Figure 1
Example of Total Number Germinated Calculations:
1+2+0+0+0+0+0+1+2 = 6
Table showing Number of Plants Germinated compared to pH for Trial 2
Day 1
Plant Number
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pH
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
0
0
0
0
0
0
0
0
0
0
0
6
0
0
0
1
0
0
0
0
0
0
1
7
0
0
0
0
0
0
0
0
0
0
0
8
0
0
0
0
0
0
0
0
0
0
0
9
0
0
0
0
0
0
0
0
0
0
0
Day 2
pH
Plant Number
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
2
2
2
2
1
2
2
2
2
1
18
6
2
2
0
2
2
2
2
2
2
0
16
7
0
2
2
2
1
2
2
0
2
1
14
8
0
1
2
1
2
0
2
0
1
1
10
9
1
1
0
1
0
0
2
2
0
1
8
Day 3
pH
Plant Number
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
2
2
2
2
1
2
2
2
2
2
19
6
2
2
0
2
2
2
2
2
2
2
18
7
1
2
2
2
2
2
2
2
2
1
18
8
1
1
2
2
2
2
2
2
2
2
18
9
1
2
1
2
1
1
2
2
1
2
15
Day 4
pH
5
Plant Number
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
2
Number
10 Germinated
2
20
18
6
2
2
0
2
2
2
2
2
2
2
18
7
2
2
2
2
2
2
2
2
2
2
20
8
2
2
2
2
2
2
2
2
2
2
20
9
2
2
2
2
2
2
2
2
2
2
20
Day 5
Plant Number
pH
Number
10 Germinated
1
2
3
4
5
6
7
8
9
5
2
2
2
2
2
2
2
2
2
2
20
6
2
2
0
2
2
2
2
2
2
2
18
7
2
2
2
2
2
2
2
2
2
2
20
8
2
2
2
2
2
2
2
2
2
2
20
9
2
2
2
2
2
2
2
2
2
2
20
Figure 2
Example of Total Number Germinated Calculations:
2+2+2+2+2+2+2+2+2+2 = 20
3.2 Growth:
Table Showing Total Mass, Leaf Mass and Root Mass compared to pH
Total Mass
(g) ±0.01
Number
plant
pH
5
6
7
8
9
1
0.78
2.8
2.59
0.75
0.19
2
2.35
10.68
9.66
2.95
0.67
3
3.48
9.68
13.08
5.84
0.86
4
2.79
13.69
16.71
7.91
1.02
5
1.78
14.01
14.96
6.91
0.81
6
2.26
11.32
16.45
5.69
0.63
7
1.66
4.48
15.37
4.27
0.65
8
1.74
6.45
12.78
2.28
0.43
19
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Average:
0.42
2.36
4.51
1.06
0.21
1.92±0.01
8.39±0.01
11.79±0.01
4.1±0.018
0.61±0.01
Leaf Mass
(g) ±0.01
pH
Number
plant
5
6
7
8
9
1
0.64
2.4
2.1
0.68
0.18
2
1.72
8.69
7.85
2.45
0.55
3
2.91
6.94
10.95
3.91
0.74
4
2.27
10.69
13.29
5.65
0.79
5
1.43
10.29
11.56
5.65
0.64
6
1.79
9.27
11.43
4.33
0.52
7
1.18
3.69
11.77
3.04
0.56
8
1.48
4.83
10.56
1.69
0.37
9
0.38
2.17
2.53
1
0.18
1.53±0.01
6.55±0.01
9.12±0.01
3.16±0.01
0.50±0.01
Average:
Root Mass
(g) ±0.01
Number
plant
pH
5
6
7
8
9
1
0.14
0.4
0.49
0.07
0.01
2
0.63
1.99
1.81
0.5
0.12
3
0.57
2.74
2.13
1.93
0.12
4
0.52
3
3.42
2.26
0.23
5
0.35
3.72
3.4
1.26
0.17
6
0.47
2.05
5.02
1.36
0.11
7
0.48
0.79
3.6
1.23
0.09
8
0.26
1.62
2.22
0.59
0.06
20
9
Average:
0.04
0.19
1.98
0.06
0.03
0.38±0.01
1.83±0.01
2.67±0.01
1.03±0.01
0.10±0.01
Figure 3
Example of Average Calculations:
(0.14 + 0.63 + 0.57 + 0.52 + 0.35 + 0.47 + 0.48 + 0.26 + 0.04) / 9 = 0.38
4. Data Analysis:
Table Showing Total Number of Plants Germinated Per Day for Each pH
pH
Total (Day 1)
Total (Day 2)
Total (Day 3)
Total (Day 4)
Total (Day 5)
5
0
18
19
20
20
6
1
16
18
18
18
7
0
14
18
20
20
8
0
10
18
20
20
9
0
8
15
20
20
Figure 4
Example of Average Calculations:
2+2+2+2+2+2+2+2+2+2 = 20
Figure 5 (Graph showing pH compared to total average plant mass for plants 2-8 in
each tube. Error bars = ±1 standard deviation)
21
Figure 6 (Graph showing pH compared to total average leaf mass for plants 2-8 in each
tube. Error bars = ±1 standard deviation)
Figure 7 (Graph showing pH compared to total average root mass for plants 2-8 in
each tube. Error bars = ±1 standard deviation)
22
T-test:
Overlap in the standard deviation bars would suggest that there may not be a
statistically significant difference between the plants grown at pH 6 and 7 nutrient
solutions. To confirm this a t-test was done.
Null Hypothesis: ​No difference between the total biomasses of plants grow at pH 5-6.
6-7, 7-8, and 8-9.
Alternative Hypothesis: ​There is a difference between the total biomasses of plants
grow at pH 5-6. 6-7, 7-8, and 8-9.
pH Difference
t value
5-6
6-7
7-8
8-9
0.0024056332
0.1558062155
0.0020004901
0.0030778072
Therefore the null hypothesis can be rejected for the difference in biomass between
plants grown in a nutrient solution at pH 5-6, 7-8 and 8-9 and the alternate hypothesis is
accepted. When comparing the growth at these pH nutrient solutions, there is a
significant difference in biomass with a p<0.05. However, the null hypothesis cannot be
rejected in the case comparing pH 6 and 7 with a p>0.05. There was no significant
difference between biomass of lettuce grown at pH 6 versus 7.
23
5. Conclusion and Evaluation:
By using 5 independent nutrient film systems under the same conditions for the
germination and the growth trials, it was possible to observe the effects of different pH’s
on the biomass and germination of hydroponically grown Simpson Elite lettuce.
For the first germination trial, results were inconclusive due both to a large algae
bloom in the pH 7 reservoir as well as the incorrect positioning of seeds under the water
level which resulted in lower than expected percent germinations. However for the
second trial, these factors were removed through the addition of aluminum foil to
prevent light coming into contact with the water in the reservoirs and increasing the
height of the seeds by using a different method for insertion into the rockwool. This
meant that I was able to achieve more consistent results. The data for the second trial
suggests that a more acidic nutrient solution will result in slightly faster germination
times compared to neutral and basic pHs. This is evidenced by data in figure 4 which
shows the pH 5 tube reaching 18 germinated plants on day 2 compared to 8 germinated
plants in the pH 9 tube. While the plants with a pH of 6 nutrient solution did have a lower
percent germination rate, this was due to the seeds in one of the net cups being
submerged which prevented the germination of those two seeds and introduced random
error. Had this not been the case, I believe that pH 6 trial would have been the second
fastest germinating tube after pH 5, though a additional trials would need to be done to
establish a pattern of germination. This agrees with what was found during background
research which suggested that a more acidic condition would increase the germination
percentage and the rate of germination (Shoemaker).
For the 25 day growth trial, there was a statistically significant increase in total
biomass at pH 6-7 (p< 0.05) compared to the plants grown at pHs of 5, 8 and 9. This
was supported by data in figures 5,6 and 7 which show the average total mass, leaf
mass and root mass at each pH. My hypothesis was incorrect for this trial as I predicted
that the pH 5 to 6 would be the most effective as they do not restrict the presence of
metal ions in solution which plants use for nutrients (Simply Hydroponics). Instead, it
seemed that the acidic conditions of a pH of 5 damaged the plants making the neutral
pH the most effective growing condition.
One of the major limitations in this experiment was the depth of the seeds
preventing germination in the first 2 germination trials. This introduced a random error
which was extremely influential on the first trial, and while less so on the second, it
made it difficult to determine whether a pH 5 or 6 would be more effective for rapid
germination. Another issue was precision as for all of the plants within a certain pH
tube, the mass varied significantly based on the position down the tube as the middle
plants received more light than the end plant to the length of the fluorescent tubes used.
This effect was increased at pH 6 and 7 as light became a larger factor in determining
24
the plant's health at these pH’s. This meant that I had to remove plants in positions 1
and 9 for all tubes when doing data analysis to prevent systematic error and remove
outliers. Additional lights should have been added to reduce the effect of this and insure
that all positions in the tubes received adequate light. In addition, there was a
systematic error because the trial was only allowed to continue for 25 days rather than
the recommended growth period of 48 for the variety of lettuce used. This reduced the
size of all of the plants as they did not reach maturity however I believe that the same
results would have been achieved as it affected all plants equally.
Another improvement would have been a more controlled method for placing the
seeds into the rock wool as this would have reduced the chances that the seed didn’t
germinate due to it being submerged which resulted in a random error. While the
addition of more plants would have reduced the random error, I believe that the sample
sizes of 10 and 9 were sufficient to suggest a relationship between pH, germination, and
growth.
25
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27
Appendix ​- Images of Setup
Figure 8: Image showing the NFT tube setup 25 days into the growth trial.
28
Figure 9: Image showing the nutrient solution reservoirs for the pH 5,7, and 9 tubes.
The reservoirs, and reflector to block light from coming into contact with the solutions
and the tubes from the pumps to the pipes above can be seen.