83 Greenhouse Tomato Plant Development under Organic Growing Conditions: a Case Study of Six Organic Soils V. Gravel a , C. Ménard, M. Dorais b and S. Pepin Horticultural Research Centre Agriculture and Agri-Food Canada Envirotron, Laval University Québec City, QC G1V 0A6 Canada Keywords: containers, Lycopersicon esculentum, organic farming, fruit yield, CO 2 effluxes, irrigation systems, nutrient availability, soil biological activity, organic matter mineralization Abstract Fertilization in greenhouse organic tomato production relies on solid amendments and on the mineralization rate of nutrients. Soil biological activity and its potential effect on mineralization is therefore an important aspect to consider when trying to coordinate nutrient availability with plant nutritional needs. The objective of this experiment was to evaluate the growth and development of tomato plants in six organic soils under organic greenhouse tomato production with water recirculation conditions. The experiment, conducted at Université Laval (Québec City, Canada), consisted of 36 experimental units (1.5 m 3 containers). The organic soils used were: 1) loam, 2) sandy loam, 3) sandy soil, 4) muck soil, 5) reconstituted organic soil with 40% air porosity and 6) peat soil amended with sawdust. Ten grafted tomato plants (Lycopersicon esculentum Mill ‘Clarance’ grafted on ‘Beaufort’) were cultivated in each growing container from February to December 2009. The crop was fertilized using certified organic compost, crab meal and seaweed extract. Irrigation was regulated for each soil according to the measured matric potential. Effluents and irrigation solution were continuously collected and analyzed. Soil CO 2 effluxes, an indicator of the soil biological activity, were evaluated every month. Plant biomass accumulation and fruit yield were evaluated throughout the production period. Yield and biomass accumulation were higher in the muck soil compared to the other soils. However, the highest microbial activity was observed in the peat and sawdust mixture. INTRODUCTION Tomato growers are now looking to improve the sustainability of greenhouse growing systems. Irrigation management and the stimulation of soil biological activity are important aspects to consider in organic greenhouse cropping systems in order to optimize fertilization management and minimise the release of pollutants into the environment. Excessive irrigation, often based on solar radiation and soilless irrigation management practices, has been shown to lower biological activity within soils and be detrimental to plant growth in organic systems (Dorais et al., 2007). This can affect nutrient availability as it is directly linked with organic matter mineralization (Tu et al., 2006; Kaya et al., 2008). Achieving high quality organic greenhouse tomato production is therefore dependent on the effect of fertilization sources on microbial population activity (Peet et al., 2004; Rippy et al., 2004; Tüzel et al., 2004). A more efficient irrigation system, located directly in the root zone, could potentially improve the overall biological activity. The overall objective of this study was to evaluate plant growth development and biological activity in six organic soils with different characteristics under two irrigation a Valerie.Gravel@agr.gc.ca b Martine.Dorais@agr.gc.ca Proc. First IC on Organic Greenhouse Hort. Eds.: M. Dorais and S.D. Bishop Acta Hort. 915, ISHS 2011