| Passive Hydroponic Units
The Wick System
The principle underlying this type of passive system is that a
length of 3/8- to 5/8-inch thick braided nylon rope, used as a
wick, will draw enough nourishment from a reservoir filled with a
water/nutrient solution to keep a growing medium moist. The
container, which holds a rooting medium, has wicks running along
the bottom and dropping through small, tight-fitting holes to the
reservoir. Keeping the holes small makes it difficult for roots to
penetrate to the reservoir. By increasing the number or length of
the wicks, or their thickness, you can increase the amount of
water delivered to the medium. A three-gallon container should
have two wicks; a five-gallon container, three wicks. The wick
system is completely self-regulating: the amount of water
delivered depends on the amount lost through evaporation or
transpiration.
The Medium
A 1-1-1 combination of vermiculite, perlite, and Styrofoam
makes a convenient medium, because the components are lightweight
and readily available. Vermiculite alone sometimes develops too
air-free an environment and becomes compacted, so that a tall
plant might eventually tip over. Perlite, which doesn't compress,
keeps the medium loose and airy. Styrofoam beads hold no water,
and therefore help keep the medium drier. Pea-sized chopped
polyurethane foam, gravel, sand, and lava can also be used in a
medium. No matter which materials you use, the bottom inch of the
container should be filled only with vermiculite, which is very
absorbent, so that the wicks have an optimum medium for moisture
transfer. Keep in mind that each medium has a maximum saturation
level - beyond that point, an increase in the number of wicks will
not increase the level of moisture.
Construction
Cut four holes, about 1/2" in diameter, in the bottom of a
three-gallon container. Run the wicks through the holes so that
each end extends about three inches outside the container. Unbraid
the wicks to aid absorption. Put two bricks in the bottom of a
deep tray (an oil drip pan will do fine), into which you've poured
the water/nutrient solution, then place the container on the
bricks so that the wicks are immersed in the solution. Replenish
the solution as it is absorbed.
A variation on this system can be constructed by using an
additional outer container rather than a tray. With this method,
less water is lost through evaporation. To make sure that the
containers fit together and come apart easily, place the bricks in
the bottom of the outer container with the water/nutrient solution
until it comes to just below the bottom of the inner container.
The Reservoir System
Even simpler to use than the wick system, you need only fill the
bottom two or three inches of a 12-inch-deep container with a
coarse, porous, inert medium such as lava, ceramic beads, or
chopped pottery, and then pour in the nutrient/water solution.
Variations on this method include a plastic flower pot or plastic
growing bag placed directly in a tray or pail of nutrient/water
solution.
Watering
All passive systems should be watered from the top down, so
that any surface buildup of nutrient caused by evaporation will be
washed back to the bottom.
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