Plant Development and Harvest Yields of Greenhouse Tomatoes in Six Organic Growing Systems

Problems concerning the environmental impact of agriculture have led to a reassessment of the conventional practices used since the 1940s, and a renewed interest by growers in organic farming began during the 1980s. Consumer interest has also increased dramatically, both for health and environmental reasons (Greene, 2000; Lampkin, 1990; Uri, 1999). Since 1996, surveys of natural food wholesalers and retailers indicated that organic produce sales had increased ≈20% yearly (Dimitri and Richman, 2000; Greene, 2001; Greene et al., 2001). Certified organic produce typically commands a higher price than conventionally grown produce. A recent survey of North Carolina-based natural food retailers and wholesalers indicated that buyers were willing to pay up to 25% more for organically grown vegetables (Estes et al., 1999). As a result of all these factors, an increasing number of growers are considering the use of, or are already employing, organic production methods. Barriers to organic certification are particularly high for greenhouse vegetable growers because virtually no technical information is available. Practices used by an experienced greenhouse organic grower to grow plants in soil have been documented (Grubinger, 1999), but specific practices for injecting fertilizer in the drip irrigation lines were not adequately described. Several grower-formulated mixtures for organic transplant production have been described (Smith, 1994), but there is little or no documentation on how to prepare organic mixes for recirculating hydroponic systems or how to add composts to soilless media used for crop production. Compost teas can be used for drip irrigation in certified organic production. However, in a manual describing the use of compost teas, Ingram and Alms (1999) describe work on compost teas as being in its infancy. The beneficial effects of compost teas were demonstrated, but most testing was conducted by trial and error. This is also true of studies using organic composts as ingredients in potting mixes and in many cases, media and waste characteristics were not reported in detail. In Germany, effluent from a fish farming unit was brought into a hydroponic cucumber (Cucumis sativus L.) and tomato greenhouse as a substitute for some of the normal fertilization (Drews and Rennert, 1992). In this study, the amount of fertilizer added to the hydroponic system was reduced by 16% for N, 14% for P, and 12% for K. Although a few examples of the successful use of organic materials in greenhouse vegetable production have been documented, it is hard to extrapolate findings to other crops and media. Broiler litter has been tested as a potting component for lettuce (Lactuca sativa L.) (Flynn et al., 1995). There are also examples where system components were well described. Tyler et al. (1993a, 1993b), Kraus et al. (2000), and Kraus and Warren (2000) conducted a series of studies of the effects of composted poultry litter on growth of containerized nursery crops. These studies described the chemical and physical characteristics of litter when combined with a pine bark substrate. However, as nursery crops grow more slowly than tomato crops, their results cannot be directly applied to a greenhouse tomato production system. Since there are no generally accepted recommendations or guidelines specifically for organic production of greenhouse tomatoes, this study was designed as a systems research project. As such, management practices for each growing season utilized information gathered from the previous growing season(s) to optimize practices for each system. The objectives of this study were to discover methods to produce healthy and productive tomato plants using flowable organic fertilizers, determine formulations for such fertilizers, and develop an organic growing medium that promotes healthy and productive tomato plants. The goal of this research project was to develop a certifiable organic regimen for growing greenhouse tomatoes that would be comparable with those grown conventionally with regard to production methods, as well as nutritional status in leaf tissue, plant development, and harvest yields.