Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2008
Publication Date: 10/1/2008
Citation: Altland, J.E., Buamscha, G., Sullivan, D., Horneck, D. 2008. Nitrogen Availability in Fresh and Aged Douglas Fir Bark. HortTechnology. 18(4):619-623.
Interpretive Summary: Douglas fir bark is a major component used in nursery crop potting mixes. Fresh and aged Douglas fir bark is routinely used, however, little is known about the fertility requirements of these two bark types. It is thought that fresh bark requires a greater amount of nitrogen (N) fertilizer to compensate for perceived differences in microbial N competition. Thus, the objective of this research was to document differences in N nutrition for container grown geraniums while concurrently documenting N disappearance from fresh and aged bark, and decomposition rates for fresh and aged fir bark. We found that geraniums grow larger and absorb more N when grown in aged bark compared to fresh bark. However, we also documented no differences in N disappearance from fresh or aged bark, and no differences in decomposition rates of the two bark types. It is speculated that differences in physical properties of the two substrates are most responsible for differences in plant growth and absorbed N. Researchers continue to explore new ways of reducing N fertilization and N waste in nursery crop production. This research dispels some misperceptions about the need for additional N in container nursery crops grown with fresh bark.
Technical Abstract: The objective of this study was to determine if there are growth differences in geranium (Pelargonium ×hortorum Bailey 'Maverick Red') produced in either fresh or aged Douglas fir [Pseudotsuga menziesii (Mirbel) Franco] bark (DFB). A second objective was to document nitrogen immobilization and decomposition rates of fresh and aged DFB to better understand the cause of growth differences. A series of experiments to measure plant response, N draw-down index (NDI), and percent cumulative carbon loss (C loss) were conducted on fresh and aged DFB. On 16 June 2005 and 19 June 2006, geranium plugs were transplanted to #1 containers filled with fresh or aged DFB. Treatments were arranged in a 2 x 3 factorial with two DFB ages (fresh and aged) and three N fertilizer rates (100, 200, and 300 mg'L-1 in 2005 and 200, 300, and 400 mg'L-1 in 2006). Plant growth was affected by DFB age; geranium stem growth, expressed as shoot dry weight, was smaller when grown in fresh DFB. Nitrogen draw-down analysis determined that a large fraction of N in solution was immobilized in fresh and aged DFB with little or no differences between the two bark types. Carbon loss, measured as a gauge of bark decomposition, was unresponsive to N rate and similar for both bark types. Similarities in C loss between fresh and aged DFB agree with the similar N immobilization potential (NDI) in the two materials. Some factor other than microbial N competition is responsible for differential geranium growth in fresh and aged DFB.