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Most commercial hydroponic systems use a method of directing a continuous flow of nutrient solution over the roots of the plants. One type of continuous flow system uses PVC pipe of the type commonly used in household plumbing. A 2-inch pipe for growing lettuce or a 4- to 6-inch pipe for growing tomatoes might be set up with a slight gradient to allow for the flow of the nutrient solution. Holes of 1- to 1 1/2-inch diameter are drilled into the pipe, and the plants are the gently inserted into those holes. Lettuce plants will usually support themselves if they have been started in growing medium cubes. Tomato plants should always be supported with wire or string hanging from above the plant.
The solution of nutrients is held in a large tank and is then pumped or allowed to flow by using gravity into the growing pipes. The continuously flowing nutrient solution then bathes the roots and returns to the solution holding tank. The great part of this hydroponic system is that the solution aerates itself as it flows back into the tank.
The main problems with using the PVC pipe is the relatively high initial cost and also the need for regular cleaning. After a crop has been grown in the PVC pipe, it should then be thoroughly cleaned with a 0.5 to 1.0 percent sodium hypochlorite solution. This solution can be made by mixing one part of household bleach with nine parts of water. This will prevent contamination from disease organisms.
The nutrient film technique (NFT) employs the same methods but is less expensive as cheaper materials are used. A flexible plastic tube supported by a wooden tray is generally used in place of the rigid PVC pipe. The tube is made from black plastic film (simialr to the plastic film mulch you can use for weed protection in your garden) with holes punched in it at regular intervals. The plants are usually started in root cubes and are then placed into the tube where they are the soaked in a continuous flow of the solution.
A variation of this is known as the Pipe Dream. This system uses a 2-inch corrugated plastic drainage pipe that is actually placed vertically in a 6-inch drainage pipe (when used for growing tomatoes) or a 2-inch pipe (when used for growing lettuce). A plastic mesh tube that is filled with peat moss is then placed inside the vertical tube and is allowed to hang into the pipe that uis horizontal. A nutrient solution that is flowing in the horizontal pipe supplies the water and fertilizer, which then moves up into the peat moss and therefore into the plant roots also. Although seeds can be also be planted directly in this peat moss, it is generally best to start with transplanted seedlings. | 
