Greenhouse Production Systems
Substrate Culture Systems
Trough Culture System
Trough culture systems involve growing plants in sloped troughs on benches filled with either gravel or sand (See Figure 14.11). Almost trough culture systems use a subirrigation system. That is, water is pumped into raised troughs or benches above the floor, which floods them within several inches of the surface, then drains back to the nutrient reservoir. This is similar to the ebb-and-flood system. Such a system is termed closed or recycled since the same nutrient solution is used each pump cycle. Trickle irrigation systems are used too but not nearly as popular as subirrigation systems.
Advantages and Disadvantages
Some of the advantages of trough culture systems is that the gravel and sand can be re-used over and over. Gravel or sand also has the advantage of being quite inexpensive and carries no risk of becoming waterlogged. Additionally, they are also one of the easiest mediums to keep clean. However, one of its drawbacks is that gravel or sand are extremely heavy to work with, making them less suitable for commercial production facilities.
Bed Construction
With subirrigation systems, plant beds can be built of wood and lined with 20-mil vinyl. The beds should be constructed using compacted river sand as the foundation. Once the proper configuration and slope have been compacted, the beds are lined with a vinyl. The beds must be designed to provide rapid filling and drainage. Connect the drain pipes from all beds to a main pipe at one end, which collects the nutrient solution and reroutes it the storage tank for reuse.
Gravel and Sand Substrate
Pea gravel and sand must be thoroughly washed before use to remove particles of soil or other material that might clog the drain line. Care should be used in selecting a gravel supply that is free from pathogenic organisms. Treatment of the gravel by steam sterilization or an appropriate fungicide is a wise practice when condition of the material is uncertain. The best choice of gravel for a subirrigation system is crushed granite of irregular shape, free of fine particles less than 1.6 mm in diameter and coarse particles no more than 1.9 mm in diameter. The particles should not be of calcareous material in order to avoid pH shifts.
Frequency of Irrigation
Gravel and sand particles retain very little water and nutrients compared to other soilless media, so the system must recirculate solution from the supply tank to the beds several times a day by means of a time clock and submersible pump. The frequency and duration of irrigation cycles is important to the success of the system. Each irrigation cycle must provide adequate water, nutrients, and aeration to the plant roots.
Sterilization of Gravel and Sand between Crops
The sterilization of gravel or sand between crops can easily be done with household bleach (calcium or sodium hypochlorite) or hydrochloric (muriatic) acid used for swimming pools. A 10,000 ppm of available chlorine waste solution is made up in the nutrient tank, and the beds are flooded several times for 20 minutes each time. The chlorine solution is then pumped to waste, and the beds rinsed several times with clean water until all bleach residues are eliminated.
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