Thursday, November 16, 2006 - 10:10 AM

Apparent Phytotoxicity of Several Bark Substrates on Two Common Horticultural Species.

Rémi Naasz1, Jean Caron1, André Pichette2, and Dominic Dufour2. (1) Univ Laval, Dépt des Sols et Génie agroalimentaire, Centre de Recherche en Horticulture, 2480 Blv Hochelaga, Sainte-Foy, Québec, QC G1K 7P4, Canada, (2) Univ du Québec à Chicoutimi, Dépt des Sciences Fondamentales, Université du Québec à Chicoutimi,, Chicoutimi, QC G7H 2B1, Canada

In Québec (Canada), approximately 3.5 million tons of barks are produced annually, of which 10 % are buried or burned without use or recycling. The horticultural industry could certainly use such by-products as growing media. Previous work has already shown the possible use of composted bark as an additive or as a main component of substrate mixes. However, growing media made of fresh bark often show plant growth reduction. Risk of phytotoxicity could come from high concentration of organic molecules often found in fresh barks, such as phenolic, terpenes or organic acids, or from too high heavy metals concentrations. Some studies have reported, for bark based substrates, that large particles size increased air-filled porosity by increasing the proportion of drainable macropores, but their geometry (shape, size) created barriers to gaseous exchange and bioavailability in the porous medium, with corresponding yield decreases. Thus, these gas exchange physical barriers could also cause the observed plant growth reduction. Therefore, the objectives of this study were: to evaluate the phytotoxicity of barks of seven different tree species on the germination of lettuce and on tomato growth and to identify the possible factors of physical (aeration, water availability) or chemical (heavy metals, phenol concentrations, etc,) origin causing those reductions. With respect to germination of lettuce and dry matter production of tomato, bark origin affected both plant performance indicators. For both, best results were obtained with raw paper birch bark, and this treatment was superior to performances obtained with the control (rockwool), a commonly used product by growers. Chemical and physical properties were correlated to plant growth parameters. Among these properties, air-filled porosity was significantly correlated to shoot dry weight and germination index. These preliminary results indicate that the apparent phytotoxicity of some barks could be explained by a lack of aeration in the substrate.