Container crops in the Pacific Northwest (PNW) are grown primarily in Douglas fir [Pseudotsuga menziesii (Mirbel) Franco] bark (DFB). Similar to pine (Pinus taeda L.) bark in the southeast U.S., DFB comprises the highest portion of most nursery substrates (60% to 80% of the substrate mix, personal observation) and is often incorporated to some extent with peatmoss, sand, compost, pumice, and other materials, including fertilizers.
Fresh and aged DFB are used in Oregon (OR) container nurseries. Fresh DFB refers to material sold soon after bark is removed from the tree, ground to smaller particle size, and screened; aged DFB refers to material that goes through the same process but then sits in undisturbed piles (7 to 12 m tall) for an average of 7 months before use. Based on personal conversations with companies that handle DFB, container nurseries are equally divided in their preference for fresh and aged DFB.
Little is known about the chemical and physical properties of DFB with respect to its use as a container substrate, and little is known about the effect of DFB age on its chemical properties. Most information in the literature refers to the chemical properties of soluble components that might be extracted for pulpwood or other industrial chemical purposes (Harkin and Rowe, 1971). Bollen (1969) described the chemical and physical properties of DFB with respect to how surplus bark supplies could be disposed of in an agricultural setting, but provides little information relevant to its use as a container substrate.
Bollen (1969) defined Douglas fir, ponderosa pine (Pinus ponderosa P. & C. Lawson), redwood [Sequoia sempervirens (Lamb ex D. Don) Endl], and red alder (Alnus rubra Bong) bark as materials with low initial fertility. However, research has shown that pine bark media contains sufficient micronutrients to produce woody plants. Niemiera (1992) extracted slightly lower levels of copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) from pine bark alone compared with pine bark amended with Micromax (The Scotts Co., Marysville, Ohio) or Ironite (Ironite Products Co., Scottsdale, Ariz.); Niemera speculated that such small differences would not be physiologically significant in terms of plant growth. Svenson and Witte (1992) showed that pine bark amended with 25% to 50% composted hardwood bark provided sufficient boron (B), Fe, Mn, and Zn for geranium (Pelargonium ×hortorum L.) growth.
Research on micronutrient additions to container media and its effect on plant growth have found contrasting results. Rose and Wang (1999) reported no improvement in rhododendron (Rhododendron L. × ‘Girards Scarlet’) growth when adding compost or micronutrient fertilizer to a 3.0 pine bark : 1.0 hardwood bark : 1.0 peat : 0.2 sand (by volume) medium compared with a nonamended control. In contrast, vinca [Catharanthus roseus (L.) G. Don] shoot length and dry weight were greatest in a peat-based media with sulfated micronutrients (pH not adjusted) or chelated micronutrients (pH adjusted to 5.5) compared with a nonamended control (Thomas and Latimer, 1995). Wright et al. (1999) analyzed the effect of micronutrient and lime addition on substrate pH and growth of nine container tree species in pine bark; micronutrient additions resulted in the best growth responses for all species, whereas lime depressed growth. Micronutrients increased growth when pH was higher than 5.2 and lime had been applied.
Douglas fir bark in the PNW is used similarly to pine bark in the southeast U.S. Both bark types are irrigated frequently, fertilized with similar products and rates, and mixed with similar components (sand, peatmoss, and so on). Despite similarities in these two resources, several chemical properties of DFB have been found to differ from other conifer barks. For example, bark pH, nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), and C/N ratio differ among Douglas fir, ponderosa pine, and redwood (Bollen, 1969). Research conducted on pine bark with respect to nursery container nutrition cannot be assumed applicable to DFB.
To accurately assess micronutrient status of DFB substrates, a reliable protocol must be used that provides values that are correlated to or predictive of plant micronutrient status. Most laboratories use water extraction for micronutrient analysis, although Warncke (1986) advocates the use of diethylenetriaminepentaacetic acid (DTPA) extraction primarily because it yields larger values.
The objectives of this study were: 1) to evaluate micronutrient availability in fresh and aged DFB; 2) to determine the effect of micronutrient amendments on substrate and foliar micronutrient levels; and 3) to compare water and DTPA extractions for measuring micronutrient availability in DFB substrates. Our initial hypothesis was that DFB alone provides sufficient micronutrients for annual vinca.
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