The water culture method of hydroponics is the simplest to set up on
a small scale. In this system the plant roots are totally immersed in a
nutrient solution. The major disadvantages of this system are the large
amount of water required per plant and the need to aerate the solution continuously.
The actual design of the system is limited only by the imagination of the
builder. The system must provide means to (1) support the plant above the
solution, (2) aerate the solution, and (3) prevent light from reaching the
solution (to prevent the growth of algae).
A standard tray or tank is shown in Figure 1. The tray may be made of concrete
or of plastic-lined or asphalt-sealed wood. If you use asphalt to seal the
tank, be sure that it does not contain creosote or tars. Do not use asphalt
that leaves an oil film on the surface of the water. A typical size is 6
to 12 inches deep, 2 to 3 feet wide, and as long as is convenient. The plants
can be supported by inserting them through holes drilled in a plywood top
or through holes punched in a l-inch-thick Styrofoam sheet that floats on
the surface of the solution.
You can make a small system from a child's wading pool, a plastic pail,
a fish tank, or a drinking tumbler. A large tomato plant should be grown
in a container that holds at least 2 gallons as the solution in a smaller
container will be used up too quickly. Lettuce plants, on the other hand,
may be grown in smaller containers.
Short plants such as lettuce and spinach will usually support themselves.
Drill a 1-inch hole in the Styrofoam or wooden cover and insert a transplant.
The plant may be held in place by packing a flexible material such as cotton
into the hole around the stem. A plant started in sand, perlite, or vermiculite
can be transplanted easily to the water culture system because these materials
can be washed from the roots readily.
Vining plants such as cucumbers and tomatoes must be supported by string.
When pruned to a single stem they can be wrapped around a loosely hung string
as they grow (Figure 2).
Aerate the solution continuously by pumping air through a perforated hose
or pipe immersed in the solution. For small systems an aquarium pump and
porous stone will work. Do not bubble the solution too vigorously because
excessive movement may damage the tender roots and impair plant growth.
Change the nutrient solution every two weeks when the plants are small and
once a week as they begin to mature. Add water daily to keep the solution
level constant.
Growing plants in aggregates such as sand or gravel is often preferred
to the water culture method since the aggregate helps to support the roots.
The aggregate is held in the same type of tank as is used for a water culture
system. The nutrient solution is held in a separate tank and pumped into
the aggregate tank to moisten the roots as needed. After the aggregate has
been flooded it is drained to provide aeration. Enough water and nutrients
cling to the aggregate and roots to supply the plant until the next flooding
(Figure 3).
The solution is generally pumped to within 1 inch of the surface and then
allowed to drain. If the top surface of the bed is kept dry, the growth
of algae will be minimal. To allow rapid drainage, the aggregate must be
coarse. Use sand with particles of at least 1/16-inch diameter or gravel
of about 1/4- to 3/8-inch diameter. The best aggregates are silica gravel,
granite, basalt, or smooth river-bottom rock of the inert type that contains
no calcium. Larger aggregates will require more frequent flooding, whereas
smaller aggregates will not drain properly. In small, experimental units
you may use any of several different substances. Perlite, Styrofoam, and
crushed marbles have all been used successfully by hobbyists.
The aggregate should be flooded for about 10 minutes and allowed to drain
for no longer than 30 minutes. Variations of the tray and tank arrangement
are shown in Figures 4 through 6.
The adventurous hobbyist may wish to try an even more exotic method of
growing plants. In the aeroponic system the roots of the plant grow in a
closed container. A misting system bathes the roots in a film of nutrient
solution and keeps them near 100 percent relative humidity to prevent drying.
The container may be of almost any design as long as it is moisture proof
and dark. Tomatoes may be grown in tall, narrow containers lined with plastic.
Lettuce and strawberries have been grown in A-frame containers (Figure 7)
to make the best use of available space and light.
Position the spray nozzles so that at least a portion of each plant's roots
are sprayed directly. You may leave the nozzles on at low pressure continuously
or operate them intermittently, on for 20 seconds and off for 40 seconds.
A fungicide may be added to the solution to avoid root rot pathogens.
Most commercial hydroponic systems direct a continuous flow of nutrient
solution over the plant roots. One continuous flow system uses polyvinyl
chloride (PVC) pipe of the type commonly used for household waste plumbing.
A 2-inch pipe for lettuce or a 4- to 6-inch pipe for tomatoes may be set
up with a slight gradient to allow for flow of the solution. Holes of 1-
to 1 1/2-inch diameter are drilled in the pipe, and the plants are inserted
into the holes. Lettuce plants will support themselves if they have been
started in growing cubes. Tomato plants must be supported with wire or string.
The nutrient solution is held in a large tank and pumped or allowed to flow
by gravity to the growing pipes. The continuously flowing nutrient solution
bathes the roots and then returns to the holding tank. The solution aerates
itself as it flows back into the tank.
Major problems with using PVC pipe are its relatively high initial cost
and the need for cleaning. After a crop has been grown in the pipe, it should
be thoroughly cleaned with a 0.5 to 1.0 percent sodium hypochlorite solution
(made by mixing one part of household bleach with nine parts of water) to
prevent contamination from disease organisms.
With the nutrient film technique (NFT) the same methods but less expensive
materials are used. A flexible plastic tube supported by a wooden tray is
used in place of rigid PVC pipe. The tube is made of black plastic film
(much like the plastic film mulch use for gardens) with holes punched at
specified intervals. The plants are started in root cubes and then placed
in the tube where they are bathed in a continuous flow of nutrient solution.
A variation of the continuous flow system is marketed as the Pipe Dream.
This system uses 2-inch corrugated plastic drainage pipe placed vertically
in a 6-inch drainage pipe for tomatoes or a 2-inch pipe for lettuce (Figure
8). A plastic mesh tube filled with peat moss is placed in the vertical
tube and allowed to hang into the horizontal pipe. A nutrient solution flowing
in the horizontal pipe supplies water and fertilizer, which move up into
the peat moss and thus to the plant roots. Although seeds can be planted
directly in the peat moss, it is best to start with transplants.
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