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.