Hydroponic Lettuce Production (Part 3)
The previous article “Hydroponic Culture of Lettuce II” described the narrow channel NFT system for growing lettuce. This article describes the raft or floating system, which is especially suited for warmer climates.
B. Raft, Raceway or Floating System
Raft culture is a water culture system that uses large beds contains large volumes of nutrient solution. Styrofoam “boards” or “rafts” float on top of the solution supporting the plants. The raceways may be constructed in a number of ways. Firstly, the floor must be leveled or for shorter beds may be sloped 1 to 2 percent. If not using a concrete floor, fill with about 3 to 4 inches of sand, moisten lightly and pack with a heavy roller or packer machine. This would be the final grade so be sure no pockets or depressions are present. The floor must be level across the beds, but can be slightly sloped along the length of the beds.
Beds can be constructed of lumber sides and lined with 20-mil vinyl. The sides can be 2” x 10” pressure treated wood. Paint it with marine paint to further preserve the wood. Styrofoam sheets come as 4 ft. by 8 ft., so make the beds so that they have at least ½ – inch extra width to permit the liner and the boards to fit. If you want to insulate the side, use 1-inch thick Styrofoam, but allow for this thickness in the bed design. Then, you would make the bed width 8 ft.–2 ½ inches. The vinyl liner must be welded with a special heat welder for such use. In this way, leaks will be prevented. Vinyl cement is not suitable as joints cannot be sealed properly. In a greenhouse that is 30 ft. wide make the beds oriented perpendicular to the gutters of the greenhouse. You will need a 4 ft. wide aisle on one side and a 2-ft. wide one on the other side of the greenhouse under the gutters. Allow 1-ft. aisles between beds. Therefore, the beds would measure 8 ft. x 24 ft. oriented across the width of the greenhouse.
On one end of the beds locate the inlet header and pipes and on the other the drain return line to the cistern. With these smaller beds, circulation through the beds should permit up to 3 to 4 exchanges of solution per day. The solution is aerated as it falls back into the cistern from the return line. A course screen filter on the return end will collect any particulate matter before the solution falls into the cistern. As the solution is pumped back to the beds it must pass through a UV sterilizer and/or filtration of 50 microns followed by one of 5 microns to remove fungal spores, particularly the zoospores of Pythium.
In a larger commercial range of greenhouses, after leveling the floor, pour concrete (Photo 1).
Molded plastic beds 24 inches (60 cm) wide by 8 inches (20 cm) deep in 10 ft. (3 meter) sections are glued together in situ to obtain the needed length (Photo 2).
A 3-inch diameter return line to the nutrient cistern is located in a trench at the end of the beds together with 2-inch diameter waste line (Photo 3).
A drainpipe from the bottom of each bed joins into the return line nearest the beds. A second drainpipe from the bed attaches to the waste line. Ball valves installed on the drainpipes allow the sterilization water, a 10% bleach solution, to drain into the waste line during cleaning of the beds between crops. The waste solution drains to a septic field.
Beds in production have their return pipes opening into the main return line re-cycling the solution back to the cistern. An overflow pipe into the return regulates the solution level. With this design the beds may be cleaned with a push broom and bleach solution (Photo 4).
Precaution is necessary to not brush neighboring plants with the bleach solution or allow fumes to accumulate in the greenhouse. Keep the greenhouse well ventilated during the sterilization process. Rinse the beds with raw water after sterilizing.
After sterilizing the beds are filled with water and the nutrient solution is made up (Photo 5).
If the nutrient solution is prepared for each bed separately after cleaning, the overall solution will stay in balance better than if water is added to the tank and solution adjusted. The nutrient tank is only about 1000 gallons (3800 liters) in volume, while each bed contains 900 gallons (3600 liters). Beds should not exceed 100 ft. (30 m.) in length as oxygen deficit can occur.
In a greenhouse of one-acre (0.4 hectare) with dimensions of 110 ft. (33.5 m) by 400 ft. (122 m), the 2 ft. wide beds are laid out in sections of 10 beds with a 24 inch (60 cm) aisle between sections to allow access. About 85% of the greenhouse floor area may be utilized with the raft system. An acre of greenhouses could produce about 112,000 head of lettuce per crop.
A pump in the cistern circulates the nutrient solution through a ultraviolet (UV) sterilizer, ozone sterilizer and hot-water sterilizer as was described for the NFT system. In this operation only a UV sterilizer was used (Photo 6).
Alternatively, since the most serious disease is Pythium fungus, a series of filters (50 and 5 microns) downstream from the pump can protect against this disease organism. The UV sterilizer is more effective on bacteria than on any fungal resting spores. An ozone sterilizer oxidizes the chelated forms of iron, zinc and manganese, so with such a system check these levels of micronutrients and add them as necessary downstream from the sterilizer. You can also use the sulfate forms of zinc and manganese, but not iron, as it does not stay in solution.
The solution is aerated in the cistern with an air pump and air stones. In hot climates a refrigeration chiller unit partially submerged in the tank may cool the nutrient solution as shown in Photo 7 of the Hydroponic Farm at Cuisinart Golf Resort & Spa.
A water chiller unit of one horsepower is capable of cooling 1000 gallons of solution below 75 F (24 C), which delays bolting of the lettuce and slows the growth of Pythium. In fact, we have found here in the tropics where daytime temperatures exceed 95 F (35 C), it is advantageous to chill the nutrient solution to 65 F (18 C) or slightly less to decrease bolting and Pythium.
Experimental research work by Thompson, H. C. et al. (1998) substantiates reports that root temperatures influence lettuce growth in raft culture. They demonstrated the importance of optimizing root and air temperatures in lettuce production. By using 24 C (75 F) root temperature in hydroponic water culture systems of lettuce the crop growth was maximized under elevated temperatures. This led to the conclusion that lettuce production could be grown in warmer geographic areas.
The pH and EC of the nutrient solution are monitored and the solution adjusted by use of an injector system with stock solutions from tanks.
The nutrient solution is pumped to the opposite end of the beds via a 2-inch diameter PVC pipe inlet header with a 1-inch diameter inlet pipe to each bed (Photo 8).
A plastic ball valve regulates the flow rate to about 3 liters per minute that exchanges the solution in all the beds every 24 hours. The solution flows through the beds and out the return pipe at the tank end to the main return line to the cistern.
The “rafts” are of a high density “Roofmate” Styrofoam commonly used in the construction of houses. One-inch thick boards for this raceway system are cut to measure 6 inches (15 cm) wide by 2 ft. (60 cm) long to fit into the beds. Four holes 7/8 inch (22 mm) in diameter are cut 3 inches (7.5 cm) from each end and at 6-inch (15 cm) centers along the centerline of the board. The holes must be of exact diameter to permit the rockwool or Oasis cubes to fit tightly so they will not fall through the boards. The rafts support the plants and insulate the underlying solution.
The rafts (boards) are cleaned with a hose and then soaked in a 10% bleach solution for about an hour in a vat. They are air dried to remove any residual chlorine before re-using them.
Three to four wire hooks attached to a nylon string are secured to the same number of boards along the bed length, one every 25 ft. (7.5 m) (Photo 9).
The hooks attached to the string are used to pull the entire bed of rafts toward the harvesting end of the bed. A small boat winch that is secured at the end of the bed in a piece of metal pipe is used to wind in the string pulling the rafts as harvesting proceeds (photo 10).
The rafts float freely if the return level pipe is closed to allow the solution level to rise about 1/2 inch. Normally, the solution level is maintained about 1 inch below the rim of the beds. During transplanting 3 to 4 rafts at a time are placed at the harvesting end of the full bed. Seedlings are placed in them before pushing them downstream and adding more rafts.
The lettuce is seeded in rockwool or Oasis cubes of 1” x 1” x 1 ½” dimensions as described earlier for the NFT system. After 14 to 18 days the seedlings are transplanted by simply pushing their cubes into the boards sufficiently so that they extend about 1/8-inch (3 mm) below the bottom of the boards into the solution underneath. This is important as the cubes must initially touch the solution below or they will dry out and the plants will die.
Lettuce can be harvested at night or early morning when temperatures are cooler and plants are fully turgid. They are packaged in plastic bags and put into cardboard cases of 24 per case. The plants are removed by hand and roots trimmed to a stub of about 1 inch (2~3 cm), with the growing cube left attached, which is speculated to give longer shelf life. However, leaving on the roots and growing cube can present other problems if the plants are packaged in plastic bags. The moisture of the cube causes decay of the lower leaves of the lettuce. In addition, there is some risk of the cubes breaking apart and making the lettuce unclean. If not washed well before consumption, there is a possibility of getting pieces of the cube in your salad. In my opinion, the cubes can be kept on the plants during harvesting if they are marketed in rigid clam shell containers, otherwise, cut the plants at the crown when harvesting and keep no roots or cubes on the finished product.
Small raft culture systems may be made as ponds or beds. For example, use wood or concrete blocks or bricks to make the sides after leveling the floor as was described earlier. These ponds or beds need to be a multiple of 4 feet in dimensions so that the 4 ft. wide boards will fit into them. They can be lined with a 10-mil black polyethylene. As was seen in Peru, the ponds can be very simple without any pumps or circulation if they are small enough. Aeration can be done manually by beating the solution periodically with a wisk (Photo 11).
Hobbyists can build small ponds on the concrete floor of your basement in your house. Simply, frame the ponds with 2” x 8” boards. Put brackets in the corners to strengthen the joints so the frame will support the outward pressure of the solution. Place a 10-mil black polyethylene liner in it. Be sure that no rough edges are present that could puncture the liner. To aerate the solution use a small fish aquarium pump with an air stone. Make the pond a multiple of 4 ft. such as, 4 ft. x 4 ft., 4 ft. x 8 ft., 8 ft. x 8 ft., etc. Each board will measure 4 ft. x 4 ft. and contain 64 head of lettuce.
This type of ponds, but somewhat larger than for hobby purposes were constructed at Cuisinart Golf Resort & Spa Hydroponic Farm. One pond measures approximately 32 ft. x 20 ft. and the other 20 ft. x 16 ft. (Photo 12).
The larger pond contains 4600 gallons and the smaller one 2300 gallons of solution. The sides are constructed of concrete blocks and the bottom is concrete. The solution depth is 10 inches and the sides of the ponds are 12 inches. This allows for the 1-inch thick boards plus some freeboard space. Boards of 4 ft. x 4 ft. fit into the pond. The lettuce is ready to harvest in 26 days after transplanting or 44 days from seeding (Photo 13).
The main advantage of the raft culture system over the NFT gutter system is that in hot climates the large volume of solution in the beds stabilizes the solution temperature. With the addition of a chiller in the nutrient tank the solution temperature can be maintained at 75 F (24 C) or lower to prevent bolting.
The main disadvantages are the higher capital costs, higher maintenance, and greater use of fertilizers due to the large volume of nutrient solution in the system.
- Marlow, D.H. 1993. Greenhouse crops in North America: A practical guide to stonewool culture. Grodania A/S, Milton, ON, Canada.
- Portree, J. 1996. Greenhouse vegetable production guide for commercial growers. Province of British Columbia Ministry of Agriculture, Fisheries, and Food.
- Resh, H.M. 2013. Hydroponic Food Production, 7th edition. CRC Press, Boca Raton, FL, U.S.A.
Articles Written by Dr. Haward Resh