Hydroponic system saves space and work
With Dr. Bernard A. (Bernie) Kratky’s floating hydroponic system in high tunnels, multiple crops of lettuce can be produced throughout most of the year.
What’s more, his suspended–pot, noncirculating method saves labor and space.
On sabbatical from his research position on the culture and management of vegetable crops at the University of Hawaii, Kratky experimented with his method at the Center for Plasticulture at Penn State University from June through October last year.
Innovative method, simple concepts
The system allows the crop to be grown with only the initial water and nutrient application. In addition, no electrical power or pumps are needed for aeration or circulation. Only minimal attention is required before harvest.
Kratky says, “The floating lettuce concept is cheaper to construct than built-up tanks.” The tanks of his system consist of simple frames sitting on the soil floor of the high tunnel. The crop grows on Styrofoam insulation boards that float on the lettuce nutrient solution. Kratky adds, “Space is utilized more efficiently because interior walk aisles are not needed. Operations are conducted from one end of the raceway. This saves labor.”
Yields and quality
Michael D. Orzolek, professor of vegetable crops and director of the Center for Plasticulture at Penn State, observes that the protection afforded in high tunnels results in greater yields and better quality than field-grown vegetables. Indeed, the lettuces in Kratky’s hydroponic system consistently produced high yields of outstanding quality.
In Pennsylvania, it was 35 days from transplanting to harvest in the August through September plantings. Heads typically weighed half a pound. Kratky advises growers to harvest that size or smaller, rather than producing overly large heads. In unusually warm weather, growers should harvest early to avoid bolting.
Orzolek says, “In areas where nighttime temperatures do not fall below 40 degrees, lettuce can be produced all year in high tunnels. Where temperatures fall consistently to 28 degrees in the autumn, eight to nine month’s production can be realized. With thermal blankets or row covers applied in late afternoon and removed the following morning, even those areas can produce lettuce in high tunnels [for] nine to 10 months. The water in the system also contributes to the retention of daytime heat.”
Orzolek points out that hydroponic production in a high tunnel on Styrofoam boards also results in less disease than field growing. Foliage is kept dry, and a sterile growing medium is used. In Dr. Kratky’s system trials, there were no instances of botrytis, rhizoctonia, sclerotinia, or of nematode damage.
Kratky advises starting seedlings in a location other than the seedling house. Also, he recommends speedy harvesting, because older plants are more likely to develop aphids, thrips and other insects. Some aphids appeared on the romaine in October at the Rock Springs, Pa., site.
For a short period of time, an unusually large grasshopper population devoured every crop available at Penn State. Screening the sides of the open tunnels controlled them.
Although not a problem at Penn State due to the dry, windy conditions in 2007, or in Hawaii because of the location’s cool, high elevation, Kratky cautioned that mosquitoes can breed in a noncirculating nutrient solution. They can be controlled by Bacillus thuriengiensis subspecies israelensis toxins or Pyronyl crop spray.
Patented hydroponic system
Kratky has been developing hydroponic plant-growing systems for over 10 years and holds several patents. Simplified methods characterize his research, which adapts to various scales of vegetable production. Numerous scientific journal articles give details on many of his systems and his research. Kratky is currently with the College of Tropical Agriculture and Human Resources at the University of Hawaii.
The Center for Plasticulture web site, http://plasticulture.cas.psu.edu, contains data on high tunnels, plus links to other information.
The author is a writer-researcher specializing in agriculture. She currently lives in central Pennsylvania.
Trials Show Method Potential
For the Penn State trials, two lumber-frame raceway tanks lined with 6 mil, greenhouse-grade plastic were constructed on a level surface in a high tunnel at the Center for Plasticulture’s Rock Springs, Pa., facility. When filled, each tank held 300 gallons. Each 5.5-inch-deep tank measured 48 inches wide and 24 feet long. The 36 by 17-foot tunnel used could accommodate three tanks of this size.
Two parallel lengths of plastic pipes were supported by the floor of the tanks and placed 3 feet apart. Kratky experimented with three pipe sizes, 3.2, 4.0 and 4.7 inches in diameter, to compare the effects of the different heights of humidified air space.
The boards, potted with lettuce, float on the nutrient solution when the solution depth is greater than the support pipe diameter. Rock-filled plastic bags placed inside the pipes prevent the pipes from floating. As evaporation and transpiration cause the solution to recede, the boards rest on the pipes, creating the humidified air space.
Twelve 1-inch-thick, 2-by-4-foot, blue Styrofoam boards held 12 plants each. Staggered holes were cut, spaced 8 to 12 inches apart.
The greenhouse-grown seedlings, which ranged from seven to 15 days old, were transplanted into 5 cm plastic net pots filled with peat-perlite growing media. Each tank of the pot-filled boards accommodated 144 plants.
Equal amounts of two stock nutrients were added—just once per crop—so that the electrical conductivity of the nutrient solution in the raceways ranged between 1.5 to 2 mS. One nutrient stock solution consisted of 1 pound of soluble greenhouse-grade calcium nitrate per gallon of water. The other stock solution was comprised of a mixture of .6 pounds of magnesium sulfate and 1 pound of Chem-Gro 8-15-36 Lettuce Formula per gallon of water. The Chem-Gro formulation, available from Hydro-Gardens (www.hydro-gardens.com), also contained micronutrients. The 25-gallon batches of stock solutions were stored in opaque plastic trash containers and mixed prior to use.
At harvest times, Kratky filled the raceway with solution to allow the boards to float. Then, from one end of the raceway, he simply pulled the attached line, lifted the boards onto the storage containers and cut off the lettuce heads. After discarding the net pots and washing the Styrofoam boards, the next crop could be started.
Adriana, Green Forest and Red Sails were the varieties used in the Penn State experiment. Jericho, Green Mignonette and Red Sails were grown in the Hawaii trials.
After the first crop, Kratky topped off the solution for the subsequent two of the six consecutive June through October crops in the Penn State trials. Although the results of the topping-off were favorable, he notes that at some point the tanks should be drained and refilled with fresh solution.