Response of Tomato Plants to Organic Nutrition in Soilless Culture S. Kaya, Y. Tuzel N. Okur E. Caturano, C. Leonardi Ege University, Faculty of Agriculture Dept. of Horticulture, Bornova/Izmir, Turkey Ege University, Faculty of Agriculture Dept. of Soil Science, Bornova/Izmir, Turkey Catania University, Faculty of Agriculture, Dept. of Horticulture and Food Technologies, Catania, Italy ABSTRACT Organic nutrition could reduce the environmental impact of soilless culture in particular in open systems. The objective of this study was to understand better the response of tomato plant fed with organic nutrient solution in substrate culture in relation to the possible role of the substrate and to provide more sustainable and efficient production by environmentally friendly nutrition method. For this purpose tomato plants were grown in the pots in a semi closed growing system. Plant response was studied in relation to: i) the nutrient solution (i.e.: organic vs mineral); ii) the substrate (i.e.: sand vs coconut fiber); iii) to the amount of amendment (i.e.: 40 vs 80 g/ pot). Plant response was highly affected by organic nutrition in part dependent on the uptake of nutrients and their content in plant tissues. Also, plants grown with the organic substrate showed a higher microbial activity and as a consequence organic matter mineralization. On the other hand, the amount of amendment added to the substrate did not influence significantly plant growth and fruit characteristics. It was concluded that to develop organic fertilization regimens for soilless cultivated tomato the aspects should be better deepen in future research concerns the right choice of organic fertilizers, the timing of nutrient solution preparation and the substrate which all could affect nutrient availability for the plants. Keywords: Organic, nutrient solution, nitrogen uptake, mineralization INTRODUCTION Soilless culture has been developed rapidly in many countries all over the world in particular as an alternative to methyl bromide. The major advantage of soilless systems is therefore the elimination of the need for soil sterilization, but also the more precise control of the application of nutrients and water (Maloupa, 2002). On the other hand, soilless culture is not without its drawbacks; the system does not forgive the cultural mistakes, requires specialized equipment and needs complete nutrient fertilizers during the crop cycle (Liedl et al., 2004). Besides, the wastes of the substrate, plastic materials and/or nutrient solution cause environmental pollution (Benoit and Ceustermans, 1995). Problems concerning the environmental impact of soilless culture could be reduced by the use of organic fertilizers, since they are considered environment friendlier when compared to the mineral ones (Böhme et. al., 2001; Popescu et al., 2004; Peet et al., 2004). There are limited researches on the use of organic fertilizers in greenhouse soilless vegetable production such as poultry litter compost (Flynn et al., 1995; Kraus and Warren, 2000; Liedl et al., 2004), organic soluble fertilizer and amendment to the media (Rippy et al., 2004) or different organic manure solutions (El-Shinawy et al., 1997). The main difference between the organic and inorganic fertilizers is the release timing of nutrients which is related to the nutrient solution characteristics and also to the substrate itself (Smith and Hadley, 1989; Jiang et al., 2004). Besides, the substrate in particular with higher organic matter content promotes microbial biomass and activity (Maslova and Sharkov, 1993). This difference could