Greenhouse Production Systems
Hydroponic Systems
Floating Raft System
The floating raft system consists of horizontal, rectangular-shaped tanks 6 to 12 inches (15–30cm) in depth, holding a large volume of nutrient solution (Figures 15.3 & 15.4). Plants are grown on floating polystyrene sheets on a nutrient solution where the plants are placed in the holes. The plant roots are partially or totally submerged in several inches of nutrient solution. Raft systems have traditionally used 4 × 8 feet (1.2 × 2.4m) polystyrene sheets with thicknesses of 0.75 to 1.5 inches (1.9–3.8cm).
Aeration
While pH, EC, and water pathogens are managed similarly to NFT systems, the floating raft system requires aeration to maintain adequate dissolved oxygen concentrations. Commonly, floating raft systems have plumbing to circulate water (so that throughout the pond, there are uniform pH, nutrient, oxygen, and temperature conditions).
Nutrient Solution
The nutrient solution from the beds is recirculated through a nutrient tank of 1,000 to 1,250 U.S. gallons (4,000–5,000L). There, the solution is aerated by an air pump, chilled with a refrigeration unit, and then pumped back to the far ends of each bed. The solution in the beds is static, with a circulation of 0.5 to 0.8 gallons per minute (GPM) (2–3L/min). It is standard practice when growing lettuce in a raft culture system to use a water chiller in the nutrient tank to maintain the temperature of the nutrient solution between 65 and 75 degrees F (18 & 23°C).
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