The Effect of Hammer Mill Processing on Pine Bark Substrate Wettability

The potential of a substrate component (organic or inorganic) to capture and retain water is integral for contributing to a materials water holding capacity and improving plant growth.  Like most organic materials pine bark becomes hydrophobic at low moisture levels.  However some research has shown that the variation in size and structure of milled pine bark particles may contribute to water holding.  The goal of this study was to examine the wettability of two different bark materials that were each processed, handled and aged in different ways. One bark material was obtained from a local supplier and the second bark material was processed (from whole bark nuggets) at the Substrate Processing and Research Center at NC State University (NCSU).  The two pine bark materials were tested at 50% and 25% initial moisture content (MC) by weight. Both barks were sieved to x-large, large, medium, and fine particle fractions and also tested at 50% and 25% MC in order to determine their wettability. Both bark materials (processed commercially and at NCSU) hydrated significantly different at both 50% and 25% MC even at the same particle size distribution. The commercially processed pine bark and the pine bark processed and NCSU were reengineered in order to have the same particle size distribution. At 50% MC both bark materials reached maximum hydration after 2 hydration events. Bark processed at NCSU held significantly more water throughout the hydration testing with 63.6% after 10 hydration events, while the commercial bark only held 47.9% water after testing. At 25% MC, the NCSU bark initially wet up more than commercial bark at the same PSD with 23.9% and 17.9% respectively. The commercially processed bark wet up more slowly than NCSU bark, however both materials retained similar values after 10 hydration events. Particle size distribution alone does not provide enough characterization of an organic substrate in order to determine its ability to capture and retain water.