Container crops in the Pacific Northwest (PNW) are grown primarily in Douglas fir [Pseudotsuga menziesii (Mirbel) Franco] bark (DFB). Similar to Loblolly pine (Pinus taeda L.) bark in the southeast United States, DFB comprises the highest portion of most nursery substrates (60% to 80% of the substrate mix, personal observation in the PNW). Douglas fir bark is often incorporated with peatmoss, sand, compost, pumice, or other materials. Despite its widespread use, little information is available on the chemical and physical properties of DFB as it pertains to use as a container substrate. Most literature on this subject refers to the chemical properties of soluble components extracted for pulpwood or other industrial chemical purposes (Bowyer et al., 2003; Harkin and Rowe, 1971).
Chemical properties of pine bark (based on water extractions) have been documented and summarized in a review by Ogden et al. (1987). Tucker (1995) reported for nonamended pine bark; low pH (3.4 to 4.5), high phosphorous (P; 11.5 to 23 mg·L−1) and potassium (K; 134 to 215 mg·L−1); sufficient manganese (Mn; 4.5 to 15 mg·L−1) and copper (Cu; 0.22 to 0.50 mg·L−1); and low calcium (Ca; 8.5% to 24% of cation exchange capacity [CEC]), magnesium (Mg; 4.5% to 6.2% of CEC), and zinc (Zn; 1.8 to 4.4 mg·L−1) when compared with established sufficiency ranges (Warncke, 1998). Niemiera (1992) reported pine bark alone provided 0.10 mg·L−1 Cu, 22.7 mg·L−1 iron (Fe), 9.7 mg·L−1 Mn, and 3.9 mg·L−1 Zn, just slightly lower than bark amended with Micromax (Scotts Co., Marysville, OH) and Ironite (Ironite Products Co., Scottsdale, AZ).
Fresh and aged DFB are used commonly in Oregon container nurseries. Fresh bark refers to material sold soon after tree debarking, grinding, and screening to size; aged bark refers to material that goes through the same preparation process but also sits in undisturbed piles (7 to 12 m tall) for an average of 7 months before use. Container nurseries are equally divided in their preference for fresh and aged bark (Jack Hoeck, Rexius Bark, Eugene, OR, personal communication). Some of those preferring fresh DFB often claim it is more consistent from batch to batch than aged DFB. Skogholm cotoneaster (Cotoneaster dammeri C.K.Schneid ‘Skogholm’) grown in aged pine bark was larger than cotoneaster grown in fresh pine bark (Harrelson et al., 2004). The authors attributed the reduction in growth in fresh bark to differences in physical properties. Container capacity and available water in fresh pine bark were significantly lower than in aged bark, in particular at the beginning of the study. In the same study, pine bark age had no effect on substrate pH or electrical conductivity (EC).
Nutrient content of bark differs not only between species, but also with tree age, environmental factors, and growing site (Bollen, 1969). Bollen also stated that DFB has almost no plant nutrient value in terms of nitrogen (N), P, K, Ca, and Mg. This statement is based on concentration of each nutrient on a dry matter basis. Buamscha and Altland (2005) contradict this notion in that they reported high levels of water-extractable P and sufficient levels of water-extractable K compared with established sufficiency ranges (Warncke, 1998; Yeager et al., 2000). Bollen (1969) also reported that bark of Douglas fir, ponderosa pine (Pinus ponderosa P. & C. Lawson), and redwood [Sequoia sempervirens (Lamb ex D. Don) Endl] differ in pH, carbon to nitrogen (C/N) ratio, and content of the mentioned nutrients. Considering the differences in chemical properties of DFB and other conifer barks, research conducted on pine bark with respect to nursery container nutrition cannot be assumed completely applicable to DFB.
Physical properties of a substrate must also be considered. Container substrates are often developed or chosen by nursery growers based primarily on their perceived physical properties. Most research on the physical and hydraulic properties of container substrates has been done with peatmoss or pine bark. Milled pine bark needs a range of both fine and coarse particle sizes to be suitable as a container substrate; as a general rule, 70% to 80% of the particles should be within a range of 0.6 to 9.5 mm in diameter and the remaining particles less than 0.6 mm (Pokorny, 1979). After irrigation and drainage, pine bark-based substrates should have 10% to 30% air space (AS), 45% to 65% container capacity (CC), 25% to 35% available water, 25% to 35% unavailable water, and 0.19 to 0.70 g·cm−3 bulk density (Db) (Yeager et al., 2000). Most of the available water in a pine bark substrate is held at tensions less than 2.5 kPa, whereas water held at tensions greater than 10 kPa is not readily available for plants (Ingram et al., 1993). In the PNW, substrates are compared with the aforementioned guidelines for pine bark.
Uniformity of DFB properties throughout the year has not been studied. Trees are harvested by lumber mills virtually year-round. Bark removal is easy during the spring when water flows readily through the xylem. However, during fall and winter, bark is more difficult to remove; thus, lumber mills scrape more wood off the tree in an effort to remove all the undesirable bark. Higher concentration of wood in bark supplies is one way that chemical and physical properties of bark may change throughout the year. Moisture can also impact the bark screening process; moisture causes small particles to stick to large particles, making the screening less precise.
The north Willamette Valley in Oregon receives ≈1.1 m precipitation annually, most of which occurs between November and March (Taylor, 2005). Consequently, time of the year relative to rainfall may affect particle size distribution and other properties of DFB (Scott Leavengood, Wood Products Extension Agent, Oregon State University, personal communication).
Douglas fir bark is widely accepted as an excellent substrate for container production among nursery producers, hence its widespread use in Oregon and other regions where Douglas fir constitutes a significant portion of the forest products industry. Despite its widespread use, little is known about DFB as it pertains to use as a container substrate. Therefore, the objectives of this study were: 1) to document baseline chemical and physical properties of DFB that have relevance to production of container plants; 2) to determine the effect of age on DFB chemical and physical properties; and 3) to document the consistency of those properties throughout the year.
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