Efficiency Factors for Bark Substrates: Biostability, Aeration, or Phytotoxicity

In Quebec (Canada), approximately 3.5 million tons of bark are produced annually, most of which are burned or buried without being used or recycled, whereas they could be used as basic components in growing media. However, growing media made of fresh bark often inhibits plant growth due to high concentrations of phenols;, terpenes, organic acids, and heavy metals or by creating physical barriers to gas exchange. Therefore, the objectives of this study were: first, to evaluate the phytotoxicity of barks from seven different tree species on lettuce [Lactuca sativa L. 'Grand Rapids') germination and tomato (Lycopersicon esculentum Mill. 'Trust') growth; and second, to identify the possible physical (aeration, water availability), chemical, or biochemical (heavy metal, phenol, terpene, and sugar concentrations) factors causing those reductions. Results show that bark origin affected both lettuce germination and tomato dry matter production. Best results were obtained with raw paper birch (Betula papyrifera Marsh.; PB) bark, which outperformed the control (rockwool). Among bark substrate properties, air-filled porosity (θ_a) was significantly correlated to shoot dry weight (SDW) and germination index (GI). Plant growth parameters were correlated most strongly to biological stability, and then to θ_a, likely reflecting microbial competition for oxygen during the decomposition of organic matter. No relationships were found with terpenes, organic acids, or nutrient elements. These findings seemed to indicate that the apparent phytotoxicity of some barks could be explained by insufficient aeration in the substrate.