Jim Wills, Gary Honea, Sam Ray, Mike Buschermohle, Allen Straw, Carl Sams

Two thirty-foot by ninety-six foot plastic greenhouses were constructed on the Plant Sciences Unit of the Knoxville Experiment Station in the Spring and Summer of 2002. The houses were equipped with trellis support systems for growth of indeterminate varieties of greenhouse tomatoes. In 2003 a crop of Trust variety tomatoes was grown in the Spring and again in the Fall to evaluate the production system and to evaluate a poly tube air-recirculation system for moving relatively warmer and drier air from the upper portion of the house down into the plant canopy. The goal was to reduce energy costs and to increase yields during cooling and heating periods of growth during the season. One house was used as a control house and the second house was used as a treatment house which contained the air-recirculation system. All tomatoes were harvested, weighed, graded and counted from each house to evaluate yield, quality, size, taste (informal evaluations), and marketability. Energy costs for each house were recorded and compared at the end of the growing season to determine costs for each house. Analysis of data from the Fall crop has not yet been completed.

Estimates are that over 600 plastic greenhouses are located in East Tennessee. For many years, some these houses were used for tobacco transplant production. With the decline in tobacco and tobacco transplant production the last few years, many of theses houses have been idle. Owners of many of these houses have been searching for viable crops to put the houses back into profitable production. Greenhouse tomatoes have the potential to be one of the more profitable crops for greenhouse production and have adequate market potential at a time when field tomatoes are out of season. Reduction of energy costs for tomato production can potentially increase profit. Major expenses in greenhouse production during the colder months of the year are electricity and heating fuel. Plastic houses require significant heating during cold periods due to the low insulating value of the plastic covers. Therefore, the effectiveness of a supplemental internal air distribution system for closed greenhouses with dense plant canopies was evaluated.

A trellis system to support the tomato crop was installed in each house prior to planting crops. The trellis system was constructed from treated 4" x 4" posts and high tensile wire. Five rows of posts were installed lengthwise in each house to create rows 85' long and five feet apart. Each greenhouse contained ten rows of tomato plants in double rows along each row of posts and wire.

Trust variety of tomatoes were planted in each greenhouse for both Spring and Fall crops. 700 plants were put in each house in three-gallon black plastic bags filled with approximately 0.4 cubic feet of perlite. A spray stake was inserted in each bag to supply water and dissolved nutrients to each plant. Nutrients pH modifiers were mixed with water in 55 gallon plastic barrels and injected into the irrigation system. Timing of irrigation was controlled by a solar controller that measured incident sunlight on plants and injected water and nutrients based on a preselected amount of sunlight each day. A solar sensor connected to the controller and mounted in the top of the greenhouse above the plant canopy measured incident sunlight and activated the controller to regulate plant feeding.

Nutrient mixes are determined by several factors including plant variety, plant size and minerals contained in the water supply. The pH of the water supply used was 7.2 and was adjusted to a pH of around 5.8 to 6.2, which is best for tomato growth and flavor. Nitric acid was used to lower the pH to the desired level. Acid was mixed with water in a 55 gallon plastic barrel and one Dosatron injector was used to meter this mixture into the irrigation water going to each plant. The final pH was measured at the spray stake at the bag to assure a constant pH of about 5.8 to 6.2.

A second Dosatron injector was used to meter water from a 55 gallon plastic barrel containing magnesium sulfate and 4-18-38 tomato fertilizer. The amount of water with plant nutrients supplied to each plant on a daily basis varied from one liter at transplant to 2 to 2-1/2 liters at full growth.

The houses were instrumented with thermocouples to measure temperature gradient vertically from the 2 ft level to the 10 ft level as well as horizontally and longitudinally. Relative humidity sensors and carbon dioxide sensors also were positioned to determine gradients inside the greenhouse.

The air-recirculation system pulls warmer air from the upper regions of the greenhouse and circulates the air along each row via poly tubes running lengthwise down the rows horizontally about 36 inches above the ground. Poly tubes have holes punched in the tube sized and spaced to achieve a uniform discharge and match the air volume output of the blowers used on each row. Tubes are mounted in the center of the double rows and are supported by eye bolts or spikes in the side of the posts supporting a high tensile wire running the length of the row and secured to the end post on each double-row. Blowers are mounted on the end posts of each row with plywood mounts and U-bolts clamped around the post.

A Spring crop of tomatoes was grown in two adjacent greenhouses at the Plant Sciences Unit of the Knoxville Experiment Station in the Spring of 2003. One house was equipped with the air- recirculation system and one house was not equipped with the system. The main purpose of the research was to evaluate the air-recirculation system with respect to possible increased yields and quality of tomatoes and possible reduction in overall energy costs to produce a tomato crop. Each house was heated with two 150,000 btu, propane dual combustion chamber heaters. Each house was cooled with two 42 inch fans, one 24 inch fan and a five foot by twenty four foot Carolina Cooler which circulated water through paper media via a recirculating pump.

During the spring season, the bulk of environmental differences between the houses occurred at night. Vertical and longitudinal thermal gradients were significantly less in the treatment house during the first half of the season. Reduced vertical and longitudinal relative humidity gradients were also realized in the treatment house during nighttime for most of the season. The north side of the control house experienced relative humidity levels from 95 to 100% from March to the end of the season, while the treatment house generally remained drier, at 90 to 95% relative humidity. However, the relative humidity gradients were shown to be primarily a result of the temperature gradients and not stratification in the humidity ratio. An elevated carbon dioxide concentration was found in the treatment house during nighttime hours, which is hypothesized to be due to higher respiration rates. The fuel consumption in the treatment house was reduced by 9%, resulting in a fuel savings of $140. The treatment house used 3,550 kWh ($230) more that the control house, primarily used to supply power to the supplemental air distribution fans. The treatment house yielded 14.5% more marketable fruit that the control, which was a difference of 140 pounds. Assuming a price of $1 per pound, the difference in yield returns was $1140 per year (assuming one crop per year). Capital cost of the system was estimated to be $178 pr year, which was amortized over 5 years at 10%. The net benefit of the system was estimated to be $896 pr year.

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This research represents one season's data and does not constitute recommendations.  After sufficient data is collected over the appropriate number of seasons, final recommendations will be made through research and extension publications.