| COMPARING THREE GROWING MEDIA FOR PHYSICAL
CHARACTERISTICS AND TOMATO YIELD POTENTIAL Merle H. Jensen and Patricia A. Rorabaugh Introduction In the early 1970's, hydroponic/soilless culture became popular in North America, especially in the Southwestern regions of the United States. Unfortunately, escalating oil prices, starting in 1973, substantially increased the cost of greenhouse heating and cooling by one or two orders of magnitude. This caused many bankruptcies and a decreasing interest in hydroponics, especially in the United States. In recent years, intensive research and development programs in North America and Europe have greatly improved the technology of the greenhouse vegetable industry. Today, these new technologies, especially from Holland, are being successfully transferred to the U.S. Rockwool is the most common growing medium. While this medium provides good plant growth, it is expensive and presents a problem of disposal. Another medium which is becoming popular is perlite. It is much less expensive and can be reused for ornamental crops. A disadvantage is the perlite dust during the time of bagging. Full face respirators must be worn by the workers, as the dust is hazardous to the lungs and eyes. Several years ago a new growing medium made of urethane foam with the trade name Agri-Lite was introduced to the horticultural industry. Preliminary results were so encouraging that the University of Arizona initiated trials in 1997, in comparing Agri-Lite to rockwool and perlite. The following are the results of the 1998-99 trial. Methods and Materials The greenhouse tomato cultivar, Trust, was seeded August 7, 1998 to rockwool cubes. The seedlings were transferred to rockwool blocks on August 22 and placed on the growing slabs/bags, September 3. The research greenhouse is located at the University of Arizona, Campus Agricultural Center in Tucson, Arizona. The research consisted of the following three treatments: 1. Agri-Lite - 3 plants/slab 2. Perlite - 3 plants/bag 3. Rockwool - 3 plants/slab The volume of medium in the perlite bag was approximately 30 liters. The rockwool slabs measured 7.5 cm x 15.0 cm x 90 cm or a volume of slightly more than 10 liters, while the Agri-Lite slabs measured 5 cm x 15 cm x 97.5 cm, or approximately 7 liters. Drainage ports were cut on the bottom edge of the rockwool slabs, and with perlite and Agri-Lite, they were placed 2.5 cm from the bottom. Each treatment was replicated three times, with 18 plants per replication. Therefore, each treatment had a total of 54 plants. The first flowers appeared September 20. The first two clusters were pollinated with the battery operated pollinators and with bees thereafter. The crop was harvested approximately every two days. The fertilizer formula and the environmental control, including CO2, was close to that used in commercial production. What was different was the height of the greenhouse to the gutters, which was only 2.8 meters. Results and Discussion The plant growth response to the different media appeared the same. While the total daily irrigation requirements were similar, the Agri-Lite received twice the number of irrigations per day, with 50 ml of nutrient solution, rather than the 100 ml per irrigation used on the rockwool and perlite treatments. The fertilizer concentration and amounts provided to each treatment were identical. In this trial, the Agri-Lite was not pre-treated with a wetting agent. Since the drainage holes were 2.5 cm off the bottom of the slab, the urethane foam gradually took up the nutrient solution. In later trials at the University of Arizona, the initial watering of the Agri-Lite slabs contained a wetting agent of 2 ml of Amway spray adjuvant per 1000 ml of nutrient solution, with 4-5 liters applied to each slab and allowed to sit overnight. On the next day, several liters of regular nutrient solution were added to the slabs to reduce any possibility of phytotoxicity from the wetting agent although no toxicity was ever experienced when not leaching the slabs the next day. Statistically, there were no significant differences in yield between treatments. The first harvest was November 17, 1998 (Figure 1). For a harvest period of 6.5 months (November 17, 1998 - May 29, 1999), yields exceeded 15 kg of marketable beefsteak tomatoes per plant (Table 1). For the entire growing period, Agri-Lite consistently produced the largest fruit, averaging 224.7 grams per tomato fruit (Figure 2). The average kilograms of fruit harvested per meter2 per week increased as light levels increased in early spring (Figure 3). Due to improper thinning of fruit per cluster, weekly yields dropped in March, April and May. Due to the poor light conditions in the double-walled acrylic greenhouse, higher weekly yields were not obtained as one would expect in a glass greenhouse. Fruit quality - firmness, color and flavor from plants growing in Agri-Lite equaled that fruit from plants grown in rockwool and perlite. Trials with growers in British Columbia and Arizona show equally excellent results. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - Figure 1. Appearance of tomato crop growing on Agri-Lite just before harvest on November 17, 1998. |
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| Agri-Lite proved to be no different than other
media with regard to mineral nutrition and daily water needs. It has
excellent water-holding capacity and has the highest air porosity of any
growing media used in the greenhouse industry (Table 2). This is
especially important when solution temperatures increase during summer and
early fall. Generative and vegetable plant growth responses are
easily managed in Agri-Lite foam. In North America, it is becoming increasingly expensive to dispose of rockwool. Dump charges can be costly if buried in public landfills. With no provision to reconstitute rockwool, as in Europe and Canada, it is proposed to grind the used growing slabs into potting soil. Unfortunately, when introduced to planting beds, the rockwool, as well as perlite, is not degradable. Agri-Lite, when ground and mixed with other amendments such as peat moss, is photo degradable when exposed to the surface of the growing beds. It is proving to be an ideal medium for bedding and pot plants. A huge advantage of Agri-Lite is its consistency. Within each growing media, whether it be peat, perlite, rockwool, sand or coconut coir, the physical and chemical characteristics can differ due to differences in manufacturing and mining locations. Table 2. Physical Properties of Growing Media. Growing Media Physical Properties Water Holding % Air Porosity % Rockwool 86.85 10.30 Perlite 19.63 41.10 Agri-Lite 66.00 97.00 Conclusions Scientists at the University of Arizona find Agri-Lite environmentally compatible, economical, completely free of soluble salts and soil-borne diseases and excellent in production of high quality horticultural products. It is undoubtedly a prime candidate to be the next growing media for the upcoming millennium. Acknowledgement Paper # R-224780-33-04. Supported by the Youth Now Foundation and published here through the generosity of the CEAC, Controlled Environment Agriculture Center, College of Agriculture and Life Science, The University of Arizona. |
| Figure
1. Appearance of tomato crop growing on Agri-Lite just before
harvest on November 17, 1998. (Not Shown Online)
Table 1. Tomato Yield Comparison. Growing Media Yield Size (bag culture) kg/plt kg/m2 gms/fruit Agri-Lite 15.2 38.0 224.7 Perlite 15.0 37.5 217.2 Rockwool 15.3 38.2 219.5 *The yields between growing media were not statistically different. |
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