HYDROPONIC SYSTEMS



Water Culture or Aquaculture

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.

Figure 1. Cutaway view of a typical tray for an aquaculture system.


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.

Figure 2. Cutaway view of a typical tray for an aquaculture system.

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.

Aggregate Culture

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).

Figure 3. Cross-section of plants growing in aggregate.


Figure 4. A manual gravity-feed system.


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.

Figure 5. A simple gravity-feed system. The solution flows from vat A into the aggregate material in the growing bed. When the growing bed is flooded, the solution is drained into vat B and then returned to vat A.


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.

Figure 6. A simple mechanical subirrigation system.

Aeroponics

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.

Figure 7. An A-Frame container for aeroponic culture of small plants.


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.

Continuous Flow Systems

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.

Figure 8. A commercially available type of continuous flow system.



Go on to next section
Return to Table of Contents