Influence of White Wood on Physical Properties of Pine Bark Substrates

Bark has been an important component in horticultural substrates for years (and is rapidly increasing in greenhouse professional mixes and in retail potting soil mixes), and in the nursery industry bark is the most common substrate component used in the United States. Bark is obtained as a byproduct of the timber industry (pulp mills, saw mills, pole peeling operations, etc.) when it is striped off logs after harvest. The objective of this work was to evaluate the influence of white wood on the physical properties of pine bark substrates. To test these differences, fresh and aged pine barks from four different sources were obtained and tested. White wood was removed (by hand) from each pine bark material until enough volume of material was obtained to analyze physical properties using the NCSU porometer method. All substrates were wet up as needed to have moisture contents between 50-60% to prevent substrate shrinkage/swellage when tested. The amount of air decreased and container capacity increased with the removal of white wood for the majority of bark samples tested. The amount of change in air and water percentages that occurred correlated with the size of the white wood particles in each substrate. Those with larger white wood pieces (from the fresh bark sources) showed a greater change in the amount of air and water percentages when comparing with and without white wood, than those with smaller pieces (from aged bark sources) of wood. Larger white wood particles naturally create more air space than smaller ones. As an example of the variation, 10-15% white wood in a bark substrate may not effect physical properties at all if the particle size of the white wood pieces is similar to the bark, but on the other hand, as little as 5% white wood with a large (length or diameter) particle size can greatly change the physical properties of a bark. The variability in pine bark supplies can be likely influenced by many factors including species of pine tree, thickness of bark at time of removal, tree harvest season, method of bark removal from logs, variations in hammer milling, variations in bark handling and screening, and variations in bark pile management and aging. These variability’s can exist between/among different suppliers, but also it can (and very often does) vary from season to season at the same supplier.