Aged Acacia melanoxylon bark as an organic peat replacement in container media

Abstract

The continuous search for peat substitution by local available organic materials, to reduce horticulture industry environmental foot-print, is growing worldwide. In Mediterranean region, Acacia melanoxylon bark is an abundant waste material from non-native tree species control actions, lacking suitable disposal. This study explores the potential use of A. melanoxylon bark as an alternative material for container media. Bark anatomy was characterized, the effect of different bark sieve sizes (4, 6, 8, and 10 mm) and the ageing treatment among successive periods of time (at 0, 4 and 8 weeks) were evaluated regarding substrate physical and chemical performance, as well as phytotoxic effect on tested seeds. Fresh bark was phytotoxic (cress root index < 22%) due to phenolic and extractives presence in bark material. Ageing bark during 8 weeks might eliminate those toxic elements which promoted cress roots growth equal to peat (root index > 99%). Ageing process may motivate Nitrogen immobilization which raises pH (up to 6.2) and dropped electrical conductivity (minimum of 14 mS m−1), however initial substrate amendment is required prior to potting, thus providing enough nutrients according to microbial-plant needs. Physical substrate performance was strongly correlated with particle size distribution: coarse bark (10 mm) increase air filled porosity, enhancing substrate aeration, while fine bark (4 mm) retain equal water content as commercial peat. Aged A. melanoxylon bark can be blended up to 50% with peat and produce plants as great as in commercial peat-based substrates.