Physical and hydraulic properties of commercial pine-bark substrate products used in production of containerized crops

Authors: Altland, James; OWEN, JAMES - Virginia Polytechnic Institution & State University; JACKSON, BRIAN - North Carolina State University; FIELDS, JEB - Louisiana State University

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2018
Publication Date: 12/27/2018
Citation: Altland, J.E., Owen, J., Jackson, B., Fields, J. 2018. Physical and hydraulic properties of commercial pine-bark substrate products used in production of containerized crops. HortScience. 53(12):1883-1890. https://doi.org/10.21273/HORTSCI13497-18.
DOI: https://doi.org/10.21273/HORTSCI13497-18

Interpretive Summary: Pine bark is the primary potting component for containerized nursery stock in the eastern United States. Physical properties of pine bark vary as a function of supplier and how bark piles are managed. Two of the most important management factors include the period of time the bark is allowed to remain in a pile and decompose, in a process called aging, and amendment with sand. Little is known about how pine bark properties vary by supplier, duration of aging, or amendment with sand. The objective of this research was to survey multiple bark suppliers to determine how supplier and management practices affect pine bark physical properties. Our data show that bark particle size reduction from pine bark aging or inclusion of sand will cause a decrease in air space and an increase in container capacity (water holding capacity) and bulk density. Variances observed between and among the six pine bark sources characterized in this study are likely indicative of bark suppliers across the entire southeastern U.S. With no regulatory body providing guidelines, recommendations, or best-management-practices for pine bark management (handling, processing, aging) there will likely continue to be broad ranges of product properties, quality, and consistency among suppliers in the future.

Technical Abstract: Pine bark is the primary constituent of nursery container media (i.e. soilless substrate) in the eastern United States. Pine bark physical and hydraulic properties vary depending on the supplier due to source (e.g. lumber mill type) or methods of additional processing or aging. Pine bark can be processed via hammer milling or grinding before or after being aged from =1 month (fresh) to =6 month (aged). Additionally, bark is commonly amended with sand to alter physical properties and increase bulk density (Db). Information is limited on physical or hydraulic differences of bark between varying sources or the effect of sand amendments. Pine bark physical and hydraulic properties from six commercial sources were compared as a function of age and amendment with sand. Aging bark, alone, had little effect on total porosity (TP), which remained at ˜ 80.5% (by vol.). However, aging pine bark from =1 to =6 months shifted particle size from the coarse (>2 mm) to fine fraction (<0.5 mm), which increased container capacity (CC) 21.4% and decreased air space (AS) 17.2% (by vol.) regardless of source. The addition of sand to the substrate had a similar effect on particle size distribution to that of aging, increasing CC and Db while decreasing AS. Total porosity decreased with the addition of sand. The magnitude of change in TP, AS, CC, and Db from a non-amended pine bark substrate was greater with fine versus coarse sand and varied by bark source. When comparing hydrological properties across three pine bark sources, readily available water content was unaffected; however, moisture characteristic curves (MCC) differed due to particle size distribution affecting the residual water content and subsequent shift from gravitational to either capillary or hygroscopic water. Similarly, hydraulic conductivity (i.e., ability to transfer water within the container) decreased with increasing particle size.