44: The Fish'll Keep the Plants Alive
Associated Lesson Topics:
- Plant requirements
Planting the seed....
What do you think plants need to grow? (water, carbon dioxide,
light, oxygen, nutrients) What do you think they use each of these
for? Where do plants "normally" get their nutrients? Can you
grow plants without soil? How might these plants meet their need for support?
Nutrients? Has anyone heard of hydroponics? If so, what is it? If not,
what might this word mean? What do you think "hydro" refers to?
Hydroponics involves growing plants without soil. Although this concept
may be new to your students, the technique is actually quite old. In fact,
humans have been growing food hydroponically for hundreds of years. By
taking an interdisciplinary approach in planning your hydroponics unit,
you can easily incorporate an exploration into Aztec culture. The Aztecs
were, after all, the first known humans to grow food using hydroponics,
sometime in the 1400s!
Hydroponically grown plants get their required minerals from a nutrient
solution rather than from the soil. In order for the plants to grow successfully,
the nutrient solution must contain several elements, including nitrogen,
potassium, phosphorus, iron, manganese, and sulfur. In aquaponics,
the nutrient solution is water containing fish excrement. Aquaponics is
the integration of hydroponics and aquaculture (the cultivation of the
natural produce-like fish or shellfish-of water). Live fish are raised
in a traditional fish tank. The fish excrete their waste into the surrounding
water, which is used to supply nutrients to the growing plants positioned
above the tank. Bacteria living in the water and on the growth medium
eat the fish wastes and unlock nutrients for the plants. The plants absorb
the nutrients, and the filtered water is returned to the fish tank. As
you have probably guessed, students will grow plants aquaponically in
- Rockwool propagation cubes. Rockwool is molten lava rock that has
been processed into a fibrous, cotton candy-like texture. These cubes
provide the anchor for the growing plant. They can be purchased from
any of the suppliers listed below.
- Plant seeds. We recommend basil, chives, dill, lettuce, nasturtiums,
oregano, pansies, parsley, petunias, spinach, thyme, and tomatoes.
- Two rectangular, shallow, black plastic planting trays. One of these
trays must be able to sit on top of your fish tank without falling in.
These can be purchased at any garden supply store.
- Fluorescent grow lights.
- Fish tank, fish, and accessories (such as pebbles). Any fish will
do, but if you stock the tank with the tropical fish Tilapia,
students can feed the fish the hydroponically grown plant material to
complete the cycle. These fish consume plant matter as a regular part
of their diet.
- Four feet of clear, flexible rubber tubing (1 inch in diameter), cut
into two pieces.
- Submersible pump.
- Two small sections of screen. Window screen will work fine. You need
only enough to cover one end of both tubes. The tubing, submersible
pump, and screen can be purchased from a hardware store.
- Growing medium. We recommend pea gravel, vermiculite, or untreated
- Inexpensive light timer from the hardware store.
- Electroconductivity meter. This can be purchased from one of the hydroponic
suppliers listed below.
- A pH meter or test strips.
- Soak the rockwool cubes in water overnight.
- Plant seeds in the rockwool cubes. Do not separate the rockwool cubes
from one another. Seeds should be planted as deep as they are long (at
least 1/4 inch deep). Push the rockwool fibers together to cover the
- Place the rockwool cubes in one planting tray and place on a windowsill.
Until the first true leaves appear, seeds will not require supplemental
lighting and can be watered with plain tap water. Pour water into the
bottom of the planting tray. The amount of water you will need depends
on how many rockwool cubes you are using. The cubes should be kept moist,
but not drenched.
- When the first true leaves appear, place the entire setup under fluorescent
lights. The lights should be suspended approximately 1 foot above the
seedlings. Begin fertilizing with a dilute plant fertilizer.
- Meanwhile, set up the aquaponic system. Follow the directions that
you received from the aquarium or pet shop to set up the fish tank.
- Cut one hole in each short side of the second planting tray. This
is the tray that will be placed on top of the fish tank. Fit the plastic
tubing in both holes. You may need to seal each hole with epoxy to prevent
leaking. One tube will be attached to the submersible pump that will
supply water to the plants. The other tube will be used for drainage.
Once each tube has been pushed through the holes, cover the ends that
are inside the planting tray with screening. This will prevent the growing
medium from escaping into the fish tank.
- Fill the bottom of the planting tray with 1 ½ to 2 inches of growing
- Place the tray on top of the fish tank. Attach one tube to the submersible
pump located on the bottom of the fish tank. Place a piece of wood under
this side of the tray to raise it slightly higher than the other side.
Insert the other tube inside the fish tank in the water. You are now
ready for transplanting.
- When seedlings reach 3 to 4 inches in height, transplant them to the
aquaponic system. Bury the rockwool cubes at least 1/4 inch deep in
growing medium. Transfer the fluorescent lights and keep them suspended
12 to 18 inches above tops of plants.
- Set the timer to turn on the submersible pump three times a day for
10 to 20 minutes each session.
- Flush the system out with plain water every six to eight weeks to
prevent silt buildup.
- Enjoy watching your plants grow!
*Note to teachers...
It is likely that the fish tank water will contain enough nutrients for
the plants to grow successfully. However, it is still a good idea for
you and your students to test the amount of nutrients in the water. This
can be done using an electroconductivity meter. Electroconductivity meters
measure the amount of dissolved solids in a solution-the more dissolved
solids in a solution, the higher the amount of electroconductivity of
that solution. Values between 1200 and 1300 parts per million (ppm) are
ideal. If your values are significantly off, you can add fish to (raise
the ppm) or remove fish from (lower the ppm) the system. In addition,
the pH of your water should be maintained at 6.0 to 6.8. Students can
use a pH meter to monitor the pH. If the pH is not within this range,
consult your local fish tank supplier about manipulating the pH.
If you are looking for a less complicated hydroponic system, we refer
you to the Growing Ideas article linked to this activity.
Harvesting the Crop...
With hydroponics, as with most discoveries, some people are strong supporters
and others are adamantly opposed to it. Students can compile a list of
advantages and disadvantages associated with hydroponics and conduct a
debate. Divide the class in half. Assign one group to be pro-hydroponics
and the other to be anti-hydroponics.
Students in grades K-4 should compile their lists based on their hydroponic
experiences in the classroom. What were some good things about hydroponics?
What didn't work out so well?
Students in grades 5-8 should conduct library research to compile their
list of advantages and disadvantages to fuel the debate. Below we have
included some of the common advantages (+) and disadvantages (-) that
have been reported by the educational and scientific community.
(+) Reduced pest problems, resulting in reduced need for pesticides.
(+) Less contaminated food.
(+) Increased yields because:
- plants can invest less in roots (anchor is provided) and more in aboveground
- more plants can be grown in a given space because there is no competition
for limited water or nutrients.
(+) Water is recycled.
(+) All of the necessary nutrients are provided (soils cannot always meet
the necessary plant requirements).
(+) Hydroponics can be adapted for use in every climate, even where environmental
conditions are unfavorable for plant growth.
(+) There are almost no weeds in the system.
(+) Less reliance on already overstressed agricultural lands.
(-) Some opponents claim that yields are not increased. Ask students
how they might determine who is right. They can conduct an experiment
in the classroom-by planting and growing the same seeds "normally" at
the same time as they are being grown hydroponically-to determine the
(-) Requires high energy and labor inputs to set up and monitor the system.
(-) Hydroponics may diminish our concern for the soil and for ecologically
benign agricultural practices.
(-) System can be expensive to set up.
(-) Power failures could be disastrous. This is not so much a concern,
perhaps, in a small system such as yours, but imagine a hydroponic farm
(-) If the water in the system becomes contaminated, the entire system
- Beginning Hydroponics: Soilless Gardening,
by Richard E. Nicholls, Running Press Book Publishers, 1990, Philadelphia,
- Grow with the Flow, by Philson A. Warner
et al., Cornell Cooperative Extension Publication #141M7, 1993, Ithaca,
- Home Hydroponics...and How to Do It!,
by Lem Jones et al., Crown Publishers, 1990, New York, NY.
- Hydroponics Home Food Gardens, by Howard M.
Resh, Woodbridge Press, 1990, Santa Barbara, CA.
- Hydroponics supplies (rockwool, electroconductivity meter)
Worm's Way Garden Supply 1-800-274-9676
Eco Enterprises 1-800-426-6937
New Earth Indoor/Outdoor Garden Center 1-800-462-5953
Science Kit & Boreal Laboratories 1-800-828-7777
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