In the eastern United States, nurseries use either loblolly pine (Pinus taeda L.) or longleaf pine (Pinus palustris Mill.) bark as the primary organic component in soilless substrates. Pine bark was initially used as a growing substrate in the 1970s, with increasing acceptance due to its availability, favorable physical properties, and lack of detrimental chemical constituents when used to grow container crops. The harvesting, dilution or contamination with wood from other species, lumber mill and associated equipment, processing (e.g., milling, grinding, and/or screening) at the bark supplier, season, and age at each stage in the process affect physiochemical properties of bark products from different suppliers (Bilderback, 1987, 2000; Bilderback et al., 2005; Pokorny, 1975). Despite these many differences, pine bark has been used for decades, and following very general guidelines (Bilderback et al., 2013), has been used so successfully that it is now regarded as a vital component for container nursery production. Although pine bark processing and storage practices can ultimately affect its physical properties and performance in containers, pine bark age, source, and type of sand for utilization as an amendment are among the most important and controllable factors affecting physical properties (Bilderback et al., 2005).
Pine bark age is classified by two seemingly binary terms: fresh or aged. However, both terms can include a continuous spectrum of age with no universally accepted definition of fresh vs. aged bark. Fresh pine bark is an identifiable, undecomposed organic matter that remains “relatively stable” due to high lignin content (Mauseth, 1988). Fresh pine bark continually changes physically and chemically and thus is not completely stable. A recent study concluded that physical properties will stabilize (degree of change lessened considerably) after 6 months of aging (Kaderabek, 2017). Buamscha et al. (2007) described fresh bark as material sold soon after tree debarking, grinding, and screening to size, whereas aged bark is material that goes through the same preparation process but sits in undisturbed piles (7 to 12 m tall) for an average of 7 months before use. Pokorny (1975) described the aging process as open-air stockpiling and weathering of bark, with no additions of fertilizer or lime to adjust pH and no effort to control moisture level. Pokorny (1975) furthermore suggested this process requires 3 to 18 months; thus, any bark that sits in these piles for less than 3 months would be considered fresh bark and anything longer than 3 months would be aged bark.
Limited research documents the physical or hydrological properties of, or differences in, pine bark as a function of age. Harrelson et al. (2004) grew cotoneaster (Cotoneaster dammeri C.K. Schneid. ‘Skogholm’) in fresh or aged (1 year) pine bark amended with three fertilizer rates. Cotoneaster were smaller in fresh pine bark, and differences in growth were attributed to lower water holding capacity in fresh bark compared with aged bark. In contrast, Cobb and Keever (1984) compared fresh and aged pine bark amended with a controlled-release nitrogen (N) fertilizer, supplemented at four levels of N, and found that dwarf Japanese euonymus (Euonymus japonicus Thunb. ‘Microphylla’) and Japanese holly (Ilex crenata Thunb. ‘Compacta’) growth in fresh bark equaled or exceeded that in aged bark at all levels of supplemental N. The authors did not attribute measured plant responses to differences in substrate physical properties. Contrasting results in plant growth response to substrate physical properties are not surprising. No single substrate is suitable for all plant species (Lea-Cox and Smith, 1997). With respect to physical properties, plants respond more favorably to substrates that best mimic conditions (e.g., air and water availability) of their natural habitats.
Sand is the primary inorganic component added to pine bark in the southeast United States because of its low cost and availability. Coarse, sharp sands (0.25–2 mm) are recommended to be added to pine bark to increase Db (Reed, 1996), although the literature is not clear about the effects of sand on most physical properties. Jenkins and Jarrell (1989) developed models to predict physical properties of white fir bark [Abies concolor (Gordon & Glend.) Lindl. ex Hildebr.] and fine sand. The resulting model predicted that AS would decrease and CC increase with increasing proportions of sand. They also report poor correlation between actual and predicted values for these two parameters. In contrast, Brown and Pokorny (1975) showed decreasing water percolation through pine bark and sand substrates with increasing proportion of sand. Decreased percolation in this context suggests that fine sand particles nested within the larger pores created by pine bark decreased AS while increasing CC and tortuosity. Bilderback et al. (2005) also reported, without statistics, that adding 20% (by volume) sand to pine bark decreases AS and increases CC and Db.
While pine bark physical properties have been addressed, physical properties are generally reported from a single source or only as background information on the treatments applied. The objective of this research was to provide an overview of pine bark physical properties from multiple commercial sources and describe these properties as a function of age or amendment with sand. Additionally, hydrological properties from three of the bark sources were compared to make greater inferences on container production.
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