Floating Hydroponics:
A Guide to Student Experiments Growing
Plants without Soil
Teacher’s Guide
by Melissa Brechner, CSIP Graduate Student Fellow, Cornell University
Overview
This long-term project allows students to grow plants in a
soilless environment and learn about the parameters necessary for plant growth. You can either have your students design
their own experiments in a deep-water hydroponic system, or specify a variable for
them to investigate.
Subject
Botany, Life Science, Earth Science
Audience
Middle School or High School
Time Required
The initial project introduction and setup requires one 40-minute
class. Depending on the crop chosen, the
project will require an average of one 20-minute data collection session every
week for four or more weeks.
Mini-lessons on nutrient cycling (water, carbon, nitrogen), plant growth
requirements, plant responses to various stimuli (water, gravity, light) and
even chemistry (pH, salt precipitation) can be used to fill the remainder of
these class periods.
Potential Timeline:
Week 1
Day 1:
Introduce hydroponics as a concept and give survey entitled “Growing Plants”
Day 2: Set
up lights, mix nutrient solution, plant seeds.
No handouts.
Day 3:
(optional) Begin lab write-up. Handout:
“Lab Write-Up”
Week 2
Day 1: Take
height and weight data or make observations.
Perform plant maintenance
tasks. 20 minutes. No handouts.
Week 3
Day 1: Take
height and weight data or make observations.
Perform plant maintenance
tasks. 20 minutes. No handouts.
Week 4
Day 1: Take
height and weight data or make observations. Perform plant maintenance
tasks. 20 minutes.
No handouts.
Continue for as many weeks as necessary until plants have
reached their final size. Leafy crops
such as lettuce and spinach will probably be harvested by week 4. Fruiting crops may take many months.
Background
Definition
Hydroponics
refers to the practice of growing plants in nutrient solutions. This can be
done either in liquid systems or in aggregate systems in which the plants are
planted in a soilless media consisting of substances such as vermiculite,
peralite, sand, coconut coir, expanded rock, gravel, rockwool or peat.
History of Hydroponics
Hydroponics is not a new idea. The hanging gardens of Babylon were thought to be a hydroponic
system. The Aztecs grew all their
vegetables in a hydroponic system because the area in which they lived was a
swamp and unable to support field agriculture.
They scraped soil out of the swamp and placed it on top of floating wooden
rafts. The Aztecs planted their plants
in this soil and allowed the roots to grow down through the raft into the water
below.
Before a more scientific approach could be taken to
hydroponics, many discoveries had to be made about how and why plants grow, and
how they make use of various chemicals. In
1860, Julius Von Sachs published the first recipe for a nutrient solution in
which to grow plants, and he called the growing system ‘nutriculture’. In the 1920s, Dr. William Gerizke from the
University of California coined the phrase ‘hydroponics’ from the Greek words hydro meaning water and ponic meaning work, implying that the
water does the work of providing the necessary nutrients to the plants for
successful growth. In the same decade,
one of the most famous recipes for the nutrient solution that is essential for
plant growth in a hydroponic system was published by Hoagland. Called
Hoagland’s solution, it is still in use today.
Since the 1920s, much formal research has been performed on hydroponic systems,
and many systems were developed that we are familiar with today (aeroponics,
ebb and flood, deep flow or pond culture, drip or bucket culture, nutrient film
technique or NFT and wick). For a more detailed timeline of hydroponics see:
Advantages of hydroponics over growing plants in soil:
- Plant density may be greatly increased per unit of growing area compared to field production, allowing more product to be grown in a smaller amount of space.
- Yield per plant is often increased. (An important point if you want to sell the crop!)
- Nutrient solution can be re-used, so less fertilizer can be used. (Good for the environment.)
- Using artificial lights, hydroponic systems may be stacked vertically, further increasing the plant yield per unit of floor space.
- Growing plants indoors allows greater control of temperature, light intensity, light quality (wavelengths of the spectrum that are used), light duration, nutrient composition and concentration, humidity, and gasses supplied to the roots.
- There is a shorter growing time (plants grow faster) compared to field grown plants.
- There is a smaller weed problem than in field grown plants.
- Plants do not need to have soil washed off, so they are ready to eat right away!
Disadvantages of hydroponics compared with plants grown
in soil:
- Price – There are higher set-up costs than field grown or conventionally grown greenhouse plants.
- Time – You don’t have the buffer of soil to provide adequate moisture and nutrients and temperature control if the plants need to be left alone for a long time. Different hydroponic systems are associated with different amounts of risk in terms of massive crop failure if something goes wrong (e.g. the pump breaks or the temperature gets too high or low, or the tank springs a leak).
- Resources – plants grown in the winter must be provided with light and heat, which typically are obtained indirectly through fossil fuels.
Learning and
Behavioral Objectives
General Learning Objectives
·
Students will learn the following vocabulary
associated with hydroponics: Hydroponics, nutrient solution, pond, media, pH,
Electrical Conductivity (EC).
·
Students will identify the differences between
the cultural tasks and environmental differences associated with growing plants
outside versus indoors.
·
Students will be able to explain the need for
the following aspects of scientific experiments: control group, replicates,
data table, photo and/or sketched observations. They will design an experiment
using these concepts and will be able to identify each of these aspects in
their project.
·
Students will work in small groups and
demonstrate the ability to cooperate in the coordination of periodic plant maintenance
tasks.
·
Students will create a visual display of their
data in the forms of graphs and charts.
·
Students will analyze their data and reach a
conclusion about the effect of the variable that was investigated.
·
Students will communicate their results by writing
a paragraph on the variable they manipulated, explaining why it is important to
plants and what their experimental results showed.
Variable-Specific Learning Objectives
·
Development of roots, shoots and leaves –
students will observe the transformation of a seed to a mature plant and
document the speed of formation of roots, shoots, and leaves. Sketches will be completed and length
measurements made on a weekly basis.
·
Germination time – students will compare the
length of time it takes for various types of plants to germinate and be able to
make suggestions (playing the role of agricultural consultant) regarding the
type of seed to plant if germination is required in a specific amount of time.
·
pH - Students will understand and be able to explain
the concept of pH and the effect it has on nutrient availability to the plants.
·
Electrical Conductivity – students will be able
to explain why an EC meter is used, why the amount of salts present is
important, and how to modify the strength of a nutrient solution.
·
Comparison between two different cultivars in
one species, or between two different species – students will be able to
explain the developmental differences between the two plant types in terms of
germination time, growth habit, speed of growth, and time to final
harvest. Students should be able to
write a recommendation as to which crop would be the better choice in a
commercial setting (which would make the most money the fastest).
·
Competition among cultivars/species – students
will be able to explain why farmers tend to grow monocultures, and to form
ideas about commercial implications of growing a baby lettuce mix.
·
Plant Density – students will evaluate the
effect of plant density on plant size.
·
Allelopathy – students will understand that some
plants make chemicals that are detrimental to other plants. For example, they may learn to identify
allelopathy as the reason that walnut trees don’t make good choices for a shade
garden.
·
Photoperiod – students will understand that some
plants react to long days and some react to short days.
·
Light Quality – students will understand that
plants use only specific types of light and learn what it means when we see
certain colors. Students will be able to
predict what colored light bulbs would be ‘good’ or ‘bad’ to use to stimulate plant
growth.
·
Effect of mineral and nutrient
excesses/deficiencies – students will understand that plant growth depends on
availability of key nutrients.
National Science
Education Standards
Students can use this as an exercise in designing and
performing an experiment, collecting data, and writing a lab report. If the
project is expanded to explore various aspects of the system design (light type
and/or level, water level, nutrient solution composition or concentration,
plant species), students can gain scientific inquiry skills:
·
identify
questions and concepts that guide scientific investigations
·
design
and conduct scientific investigations
·
use
technology and mathematics to improve investigations and communications
·
formulate
and revise scientific explanations and models using logic and evidence
·
recognize
and analyze alternative explanations and models
·
communicate
and defend a scientific argument.
Depending on the ways in which this unit is used, concepts
from the following standards also may be addressed:
·
Chemical Reactions
·
Interactions of Energy and Matter
·
Matter, Energy, and Organization in Living Systems
·
Geochemical Cycles -- water cycle, nitrogen cycle,
carbon cycle
·
Science and Technology – Abilities
of Technological Design
Materials
Students can work in groups in order to reduce time and costs
involved in creating the hydroponics setups. Each group will need the following
materials:
·
A tub that will contain the hydroponic solution
and floating plants. This will be known as the ‘pond’. The size depends on the size of your light
source, but a plastic sweater box or dissection pan that is approximately 15” x
13” x 6.5” works well. If the pond is
transparent or translucent, it should be covered with butcher paper or aluminum
foil to keep light out and discourage growth of algae.
·
A floating plant holder, choose one (see photos
attached):
1.
An expanded polystyrene speedling tray that is filled with growing media
(soil mix), or
2.
A piece of polystyrene insulation (any thickness between ½” and 1”) cut
to fit the inside of the pond, and rockwool cubes to hold the plants.
Note - If you are using option 2,
you will have to cut holes in the insulation to create areas for the plants to
grow. This is generally accomplished
with a hole saw using either a hand-held drill or a drill press. The spacing of the holes will depend on the
type of plant that will be grown. The
holes should be cut to the diameter of the rockwool cubes that are being used.
·
Seeds – Any type you are interested in growing,
but the following have been proven to work well in this type of system: lettuce
(romaine, buttercrunch, mixed), spinach, herbs (basil red or green, coriander,
parsley), radish, carrot, beans, rice (for allelopathy experiments).
·
Aquarium air pump with air stone and air tube.
·
Light source such as fluorescent fixture – a 4’
utility light (sometimes known as a ‘shop light’) can be purchased for about $10-25
at a hardware store and will light two hydroponic sweater-box size ponds. Alternate light sources include metal halide
and high pressure sodium fixtures, or sunlight.
·
pH paper or pH meter.
·
Diluted Potassium Hydroxide (used to raise pH).
·
Diluted Hydrochloric Acid (used to lower pH).
·
Dibble – something to compress the soil
with. Can be a finger or the bottom of a
test tube or something hand-made as shown in the photo in the appendix.
·
Nutrient solution – The most commonly used
recipe is called Hoagland’s solution after the scientist who created it. Making this solution requires many different
types of salts, but an alternative is to purchase pre-mixed solution from a
science supplier such as Ward’s. (For the original Hoagland recipe, see
attached page). Alternately, Peter’s
company makes Hydrosol, a just-add-water hydroponic fertilizer mix that is
similar to Hoagland’s formulation.
Hydrosol does not contain calcium due to possible precipitation
problems, so it is necessary to add this nutrient separately in the form of
calcium nitrate.
·
Aluminum foil or butcher paper to cover the pond
if it is made out of a translucent material.
Tape to attach the foil or the paper to the pond.
·
Stick for stirring the nutrient solution. A yard stick works well for this purpose.
·
A 5- or 10-gallon bucket for mixing the nutrient
solution.
·
Tweezers for seeding. (optional)
·
Petri dish for holding the seeds while seeding.
(optional)
·
Rulers for measuring length of leaves, stems,
roots etc. (optional)
·
Camera to record experiment photographically.
(optional)
Procedure
Note: One of the
most important lessons in performing quality research is recordkeeping. Students should focus on keeping clear and
detailed records in a dedicated notebook or set of stapled pages for every step
in this experiment. Emphasis should be
placed on the importance of recording visual observations of plant growth along
with physical measurements of variables such as height and weight.
1. The first step is to decide on the experimental design.
Students could choose from a range of potential variables, or you could assign
a variable and have them decide how to test it. Possible variables include:
- Germination time of different plants.
- How different pH levels affect plant growth.
- How different amounts of nutrient salts (created by changing the strength of nutrient solution) affect plant growth.
- The effect of plant density (or amount of plants per given area) changes plant yield.
- The effect of different amounts (quantity) or different lengths of daylight (photoperiod) on plant growth and development.
- The effect of artificial light instead of or in addition to sunlight.
- The effect of different colors of light on plants using theatre gels.
- Allelopathy – how one plant can chemically affect the growth of others. (advanced)
- Effects of nutrient deficiency on the growth and development of plants. (advanced)
2. The next step is to assemble the hydroponic system. Hang light fixtures and mix the nutrient
solution. Cover the sides of the pond to
prevent light from entering the nutrient solution. Attach the air stone to the air tube and then
to the aquarium air pump. Place the air
stone into the pond.
For an expanded
polystyrene plug tray:
- Fill with media.
- Scrape off excess media with a flat surface so that media fills each hole.
- Tap on the table to settle the media.
- Press the media in each hole down to a uniform depth.
- Place one seed per cell.
- Place more media on top of each seed, and press lightly.
- Place into a plastic bag for germination. The length of time in the bag will depend on the germination time, which is listed on the seed packet and may vary from three days to one week.
- When the seeds have germinated, place the entire flat into the pond filled with nutrient solution.
- Turn on the light and air pump.
For a
polystyrene insulation system:
- Rinse the rockwool to remove any chemicals that might interfere with pH control.
- Place the rockwool cubes into a plastic tray such as a plastic flat.
- Place one or two seeds into each rockwool cube.
- Place the tray in a plastic bag for germination. The length of time spent in the bag will depend on the germination time listed on the seed packet and may vary from 3 days to a week.
- When the seeds have germinated, place one rockwool cube into each hole in the polystyrene float. Place the float into the pond that has been filled with nutrient solution.
3. Make observations
on the same day each week. Possible
observations include plant height, number of leaves, size of leaves, relative
color of leaves if growing two different types of plants or if the experimental
condition involves inducing a color change within the plant. A small number of plants could be sacrificed
each week so that fresh weights can be taken and a growth curve produced.
4. Perform
maintenance tasks on the plants – the crop may need to be thinned to the
spacing recommended on the seed packet, and the hydroponic solution level
should be kept constant by adding either more nutrient solution or tap water.
5. At the conclusion
of the experiment, turn off the lights and unplug the air pumps. Used hydroponic solution may be disposed of
down the drain.
Teaching Tips
It is not necessary to tape the aluminum foil to the pond,
but students like to do this and one roll of tape per group of students is
helpful.
It may take a long time for the salts to dissolve when
mixing the nutrient solution. If
possible, use hot water to make the salts dissolve faster.
Students probably will try to put more than one or two seeds
per cell in a plug tray or per cube of rockwool. This should be highly discouraged. It’s a
good idea to keep a close eye on the seeding process because students tend to become
upset when ‘extra’ plants must be removed after germination. If the extra plants are not removed, then
they will crowd each other and not grow robustly. If many extra plants are seeded and not
removed, this may provide a dramatic lesson regarding the importance of
allowing plants adequate space in which to grow.
Middle school students do not seem to have the hand-eye
coordination required to seed with tweezers.
Therefore, even though it will take a long time, it might be less
frustrating if they use their fingers to seed.
Many students are tempted to ‘weed’ the seedlings for the
first week after germination and before the true leaves have emerged. A piece of plastic wrap placed in front of
the pond suspended from the light source is an effective deterrent, enabling
the seedlings to develop undisturbed.
Students like the challenge of trying to remember all the
materials they used for the experimental setup and all the steps required to
plant the seeds. Therefore, I think it
is better to lead the class through the experimental setup and allow students
to write their own materials list and procedure in a subsequent class rather
than hand them a written procedure.
Groups of 3 or 4 seem to be optimal. Two groups can collaborate on an experiment, for
example with one group seeding the plants that will serve as the control
condition and the other group seeding the plants that will be subjected to the experimental
treatment.
Students enjoy using colored lights to experiment with impacts
of light quality on plant growth.
Fluorescent lights do not get hot and will not create fire hazards when
used with theatre gels to produce various colors.
Potential sources of
supplies:
Note: Cornell
University does not endorse
any of these retailers.
Ward’s Scientific (http://www.wardsci.com/)
·
Mineral Deficiency $85 – Much easier than mixing
this yourself!
·
Ebb and Flood Hydroponic system: pump, timer,
seed starter cubes, geolite, nutrient solution, pH test kit $180. This is a very small system and does not
allow for two identical setups, but a nice demonstration tool.
Home improvement stores
·
Fluorescent light fixtures also known as
‘utility lights’ or ‘shop lights’ $10-25
·
Styrofoam boards (4’x 8’) ~$10
Greenhouse supply companies such as Hummert (http://www.hummert.com) or Griffin (http://www.griffins.com)
·
fluorescent lights
·
pumps
·
Peter’s Hydrosol (by Scott’s) $43/25 lb. bag and
Calcium Nitrate $25/25 lb. bag
Roscoe (http://www.rosco.com/us/filters/supergel.asp) is a source for theatre gels (thin non-melting films that can be placed over fluorescent lights without creating a fire hazard)
Hydroponic Solution
Recipe
Easy Hoagland Solution
2.14 g/gal Peter’s Hydrosol
2.14 g/gal Calcium nitrate
Mix each component separately in ½ the final desired total
volume and then combine when the salts are fully dissolved.
Example: Obtain two vessels, one may be the final vessel
that will hold the solution and the second one should be able to hold at least
½ the final desired volume. A small grey
trash bucket that will hold 10 gallons will be final vessel for solution. A second identical bucket or a 5 gallon
bucket will be used for an intermediate step.
Mix 21.4 g Hydrosol with 5 gallons water in the large trash bucket. Mix 21.4 g Calcium nitrate with 5 gallons of
water into the 5 gallon bucket. When
both types of salts are completely dissolved in their respective vessels
combine both solutions into the final vessel, for this case pour the calcium
nitrate mixture into the Hydrosol mixture.
Mix one final time. Now you have
10 gallons of nutrient solution.
True Hoagland Solution
D.R. Hoagland and D.I. Arnon. The water-culture method of
growing plants without soil. Calif.
Agr. Expt. Sta. Circ. 347. 1950.
6 ml of 1 M potassium nitrate
4 ml of 1 M calcium nitrate
1 ml of 1 M monoammonium phosphate
2 ml of 1 M magnesium sulfate
1 ml of micronutrient stock
solution
1 to 5 ml of iron chelate stock
solution as for #1
Micronutrient stock solution per liter:
2.86 g boric acid
1.81 g manganese chloride - 4
hydrate
0.22 g zinc sulfate - 7 hydrate
0.08 g copper sulfate - 5 hydrate
0.02 g 85% molybdic acid
When diluted 1:1000, the micronutrient stock solution
provides the
following in mg/liter:
Boron 0.5
Manganese 0.5
Zinc 0.05
Copper 0.02
Molybdenum 0.01
Pond covered with
butcher paper. Speedling-type
polystyrene plug tray.
Polystyrene drilled
for rockwool. Rockwool
cubes.
Mixed seedlings in
rockwool in Be sure to
look at the roots from time to polystyrene.
time. This is a case of sick (tan) roots from lack
of oxygen (no air pump!!)
R – filled tray, L –
Scraped off. Pressing media with homemade wooden dibble.
Seeding spinach with
tweezers. Spinach seed
waiting to be covered.
Basil seedlings – 2
weeks old. Lettuce
plant density experiment. 3 weeks.
