Container nurseries in Oregon use fresh and aged douglas fir bark (DFB). Although there is no general agreement as to what constitutes fresh, aged, or composted bark, the terms are used frequently in the nursery industry. For clarity, we offer the following descriptions of the three aforementioned bark types. Fresh bark refers to material sold soon after debarking from a tree, grinding, and screening to an appropriate particle size. Aged bark refers to material that goes through the same preparation process, but then also sits in undisturbed piles several months before use. Although processes may differ throughout the country, aged bark piles are often very large (up to 10 m high), exposed to ambient climate, and receive no additional inputs such as fertilizer, irrigation, or aeration. Composted bark refers to materials that have been processed to an appropriate particle size, arranged in piles (often less than 2 m high) suitable for turning and aerating, and often amended with supplemental fertilizers to promote decomposition. Composted bark is rarely used in Oregon container nurseries due to the additional costs associated with its preparation.
Physical and chemical properties of DFB as they pertain to use in nursery container substrates have only recently been studied. It was documented that fresh and aged DFB had different chemical properties; aged having lower pH and higher levels of extractable phosphorous (P), calcium (Ca), magnesium (Mg), boron (B), and iron (Fe) compared with fresh (Buamscha et al., 2007b). A concurrent study used annual vinca [Catharanthus roseus ‘Peppermint Cooler’] to evaluate micronutrient availability in DFB and found that fresh and aged DFB provided sufficient micronutrients for vinca without supplemental micronutrient fertilizers (Buamscha et al., 2007a). However, vinca growing in aged bark had higher stem biomass and foliar nitrogen (N) than those in fresh bark after 8 weeks; differences were attributed to N availability.
Comparisons of fresh and aged pine (Pinus spp.) bark on plant growth have been made with several crops. Japanese holly (Ilex crenata ‘Rotundifolia’) had similar size and quality when grown under a standard fertilization program in fresh or aged pine bark and with or without preplant N (Pokorny, 1979). Cobb and Keever (1984) compared fresh and aged pine bark amended with 1 kg·m−3 N using a controlled release N fertilizer and at four levels of supplemental N [0, 100, 200, and 300 mg·L−1 ammonium nitrate (NH4NO3)]; growth of dwarf japanese euonymus (Euonymus japonica ‘Microphilla’) and japanese holly (I. crenata ‘Compacta’) in fresh bark equaled or exceeded that in aged bark at all levels of supplemental N. Harrelson et al. (2004) compared fresh and aged pine bark and three rates of controlled release N fertilizer (11.2, 22.2, or 33.3 g/pot N); bearberry cotoneaster (Cotoneaster dammeri ‘Skogholm’) grown in aged pine bark were larger than cotoneaster grown in fresh pine bark.
There is no distinction in the fertility programs used for production in fresh or aged DFB. Most nursery producers base their fertility rates on instructions provided by the fertilizer manufacturer written on the fertilizer bag. These rate guidelines are based solely on container size and plant requirement for “low,” “medium,” or “high” fertility. Based on research with pine bark, and related research conducted thus far with DFB (Buamscha et al., 2007b), there is reason to believe that N availability in fresh and aged DFB differs. Therefore, the objective of this study was to determine if there are differences in plant growth in fresh and aged DFB, and then to document N immobilization and decomposition rates of fresh and aged DFB to determine if growth differences can be attributed to microbial N competition.
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