Comparison of Pine Bark from Multiple Suppliers: Variation in Substrate Physical Properties

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 differences in physical properties among different pine bark suppliers and materials. To compare any differences amongst pine bark substrates, fresh and aged pine barks from four different sources were obtained and tested. The physical properties of the substrates were tested using the porometer method. Results showed that there was great variability amongst the different sources. The bark materials tested all had total porosities around 80%. The container capacity ranged from a high of 52% to a low of 19% among the bark sources. Aged pine bark had higher container capacities and less air space than freshly processed pine bark. The air space of the materials varied greatly from 30% to 45%. The recommended BMP guidelines for nursery substrates is 20-30% (used to be minimum 10% but that has been discounted in recent years as it was way too low) air space and 45-65% water holding capacities. The variation in these properties is directly related to the particle size of the bark, which is a direct result of the processing of the materials. The range of particle sizes (and shapes) in bark fit together in a matrix and helps create the porosity (air and water) of the bark substrate. Processors can screen bark to create any particle size combination they wish, or what is requested by a grower depending on the crop to be grown or irrigation practices preferred (more or less water). 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.