| COMPARING FIVE GROWING MEDIA FOR PHYSICAL
CHARACTERISTICS AND TOMATO YIELD POTENTIAL Merle H. Jensen1 Patricia A. Rorabaugh2 Marcos Garcia A.3 Abstract: The growth of the greenhouse tomato industry is rapidly on the increase in North America. Rockwool and perlite are the most popular growing media. Rockwool is expensive and substitute media containing peat moss are also expensive, especially if transported long distances to the southwestern part of the United States and to Mexico. Coconut coir is abundant in Mexico and of low cost. There is evidence that this medium could be a substitute for peat. This study was to investigate the physical characteristics of five media and the yield response of tomatoes produced in a greenhouse. The water holding capacity and air porosity varies a great deal between media but there was no significant difference in marketable tomato yields. The cost difference between media is great, with coconut coir and perlite far less costly than rockwool and any media containing peat moss. Keywords: Coconut coir, rockwool, perlite, soilless media Introduction In the early 1970's, hydroponic/soilless culture systems became popular in the United States especially in Arizona. That occurred in part through research at the University of Arizona (Jensen, 1975) and through private investments. In private practice, however, failures far outnumbered successes due to management inexperience and engineering support. In recent years, intensive research and development programs both in the U.S. and Europe have vastly improved hydroponic/soil culture systems. Today, these new technologies, especially from Holland, are being successfully transferred to the southwestern desert region of 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. Peat-vermiculite mixtures have long been used. While this medium is also expensive, it can be reused for growing container plants such as ornamentals or for bedding plants. Another medium that is becoming increasingly popular is perlite. It grows excellent greenhouse tomato and cucumber crops plus it is much less expensive and can be reused for ornamental crops. This study was to investigate other media for use in greenhouse vegetable production. One of special interest is coconut coir. This material is presently a waste product in the Mexican coconut industry. While the fiber from the coconut husk is used for various products, the coir dust is mostly discarded and is available in abundance. Horticultural trials suggest this may be a suitable organic component in soilless media (Argo and Biernbaum, 1996, Evan et. al., 1996). Unpublished results of trials by the above authors show the coir to be an excellent substitute for peat moss in the production of bedding plants. In the southwestern United States and Mexico, coir dust is very inexpensive. Peat moss is expensive due to high freight costs in transporting the medium from Canada. Substituting coconut coir for peat moss seems to be an excellent choice to lower the cost of growing media for greenhouse vegetable production. Methods and Materials The greenhouse tomato cultivar, Trust, was seeded August 3, 1996, to rockwool cubes. The seedlings were transferred to rockwool blocks on August 17 and placed on the growing slabs, August 31. The research greenhouse was located at the University of Arizona, Campus Agricultural Center in Tucson, Arizona. The research consisted of the six following treatments: 1. Rockwool - 3 plants/slab 2. Peat-lite - 3 plants/bag 3. Coconut Coir - 3 plants/bag 4. Coconut Coir/Perlite - 3 plants/bag 5. Perlite - 3 plants/bag 6. Perlite - 6 plants/bag The volume of medium in each 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. Each treatment had 18 plants and was replicated twice. With each irrigation, water soluble fertilizer was injected into the water. On bright sunny days, mature plants would be fertigated over 30 times per day. The first flowers appeared September 11 and were bee pollinated. The first harvest was November 16, 1996. The crop was harvested approximately every two days. The marketable and total yield was recorded along with fruit size of the marketable product. The tomato crop was terminated March 29, 1997. Results and Discussion The plant growth response to all the different media appeared the same. The irrigation requirements were similar as were the fertilizer needs. The perlite bags containing six plants needed to be irrigated considerably more than those bags having three plants. For a four-month harvest period (November 16 - March 29) the yield was quite good for winter production. The fruit size was largest during the first two months of harvest. The marketable tomato yield is reported in Table 1. Table 1. Tomato Yield Comparison. Growing Media Yield Size (bag culture) kg/plt kg/m2 gms/fruit Coconut Coir 10.63 26.58 196 Perlite (3 plts/bag) 10.27 25.69 195 Peat-lite 9.92 24.81 193 Coir/Perlite 9.71 24.27 192 Rockwool 9.61 24.02 185 Perlite (6 plts/bag) 9.36 23.40 192 While there appeared to be a difference in marketable yield and fruit size between growing media, the differences were not statistically different. There was a great difference in the physical properties of the growing media (Table 2). The water holding capacity and air porosity is vastly different. There is speculation that the combination of high water holding capacity and low air porosity can be detrimental during periods of high air temperature, which can influence higher nutrient temperature in the bags, therefore possibly lower oxygen content in the solution. Table 2. Physical Properties of Growing Media. Growing Media Physical Properties Water Holding% Air Porosity% Coconut Coir 88.35 23.5 Perlite 19.63 41.1 Peat-lite 84.78 20.0 Coir/Perlite 57.40 35.0 Rockwool 86.85 10.3 Perlite is opposite of rockwool in that it has low water holding capacity but high air porosity. In order to maintain a large fruit size and high yield, growers using rockwool intercrop a second tomato crop. Growers using perlite have no need to intercrop. Fruit number and size does not seem to be affected by producing from the same plant the entire growing season when growing in perlite. It is thought that high temperature solution during the spring months will be more of a problem in rockwool versus perlite. Since this is speculation, this hypothesis needs to be tested. To intercrop with a second crop is costly. Growers not having to intercrop certainly have an economic edge since the process of intercropping by planting a new crop in with an existing crop, which is later removed, is time consuming and expensive. In the United States, most growers will discard the rockwool in a landfill after one year’s use. Normally, perlite is also not reused, but taken away by growers of ornamentals or it can be sterilized, rebagged and used again for greenhouse vegetable production. The peat in the peat-lite mix was quite decomposed while the coconut coir did not decompose as readily as peat. It appears that the coir could be reused. Whether it should be sterilized before reuse, needs to be investigated. Conclusions There were no significant differences in yield and fruit size among the five different growing media. There is a real difference between the physical properties, of rockwool and perlite. Coconut coir and peat-lite had a similar water holding capacity to rockwool but had twice the air porosity. In the southwestern part of the United States and northern Mexico, rockwool and peat-lite are far more expensive than perlite and coconut coir. Perlite is undoubtedly suitable for greenhouse vegetable production. Coconut coir also appears suitable but should be put to further tests, along with other new media coming on the market, such as urethane foam Literature Cited Argo, W.R., J.A. Biernbaum. 1996. Component comparisons: coconut coir. Grower Talks, February: 62-66. Evans, M.R., S. Konduru, and R.H. Stamps. 1996. Source variation in physical and chemical properties of coconut coir dust. Hort Science 31:965-967. Jensen, M.H. 1975. Arizona research in controlled-environment agriculture. Proc. Tennessee Valley Greenhouse Vegetable Workshop, Tennessee Valley Authority. p. 13-22. Acknowledgement Paper # R-224000-32-04. 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. |
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