Ornamental container crops in the Pacific Northwest are grown primarily in Douglas fir [Pseudotsuga menziesii (Mirbel) Franco] bark (DFB). Similar to 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, pers. obser.). Chemical and physical properties of DFB, as they pertain to use in container substrates, have recently been documented by Buamscha et al. (2007b). They reported nonamended DFB to have low pH (3.7 to 4.4); little or no available nitrogen (N); relatively high water-extractable phosphorus (P; 10 to 28 mg·L−1) and potassium (K; 78 to 162 mg·L−1); low levels of calcium (Ca), magnesium (Mg), and sulfur (S); and moderate levels of some micronutrients. In a separate study, Buamscha et al. (2007a) documented that DFB alone provides sufficient micronutrients for annual vinca [Catharanthus roseus (L.) G. Don ‘Peppermint Cooler’] grown at low pH (4.5 to 5.5). Macronutrient and micronutrient availability may not be sufficient to support plant growth when substrate pH is higher. Altland (2006) reported reduced growth of japanese maple (Acer palmatum var. atropurpureum Thunb.), hydrangea [Hydrangea macrophylla Thunb. (Ex J.A. Murr.) Ser. ‘Endless Summer’], and leucothoe [Leucothoe axillaris (Lam.) D. Don] caused by a pH-induced reduction of available N, P, and micronutrients in DFB. Similar observations of reduced plant growth with high substrate pH have been reported for crops grown in pine bark (Wright and Hinsley, 1991).
Little is known empirically about how pH affects nutrient availability in DFB substrates. Lucas and Davis (1961) determined the relationship between pH and nutrient availability in organic soils. They concluded that the ideal pH range (in terms of total nutrient availability) for wood-sedge soils to be between 5.5 and 5.8 and 5.0 for sphagnum peat soils. They further commented that this range is one to 1.5 points lower than what was considered ideal for mineral soils. This report formed the basis for future studies years later as the greenhouse industry switched from mineral soils to those composed primarily of peat or bark. Peterson (1980) documented the effect of substrate pH on macronutrient and micronutrient availability in a well-fertilized commercial greenhouse substrate (peat moss, perlite, vermiculite, granite sand, and composted pine bark; ratios not given). His study agreed with Lucas and Davis (1961) in that the optimum pH range was 5.2 to 5.5, which he characterized as being a whole pH unit or more below what is considered optimum for mineral soils. Peterson (1980) reported decreasing availability of P, iron (Fe), manganese (Mn), boron (B), zinc (Zn), and copper (Cu) with increasing pH. Argo (1998) reviewed the effects of pH on nutrient availability in soilless substrates citing numerous sources and generally agreed with conclusions from Peterson (1980).
In view of the widespread use of DFB in the Pacific Northwest, and the corresponding lack of information available on its chemical properties, two experiments were initiated to document the influence of pH on nutrient availability. The objectives of these experiments were to determine the influence of two lime sources on DFB pH as well as nutrient availability with respect to changes in substrate pH.
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