Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2008
Publication Date: 6/17/2009
Citation: Altland, J.E., Magdalena, Z., Owen, J. 2009. Effect of Peat Moss and Pumice on Douglas Fir Bark based Soilless Substrate Physical and Hydraulic Properties. HortScience. 44(3):874-878.
Interpretive Summary: Douglas fir bark is the primary component used by west coast nursery growers. Bark is typically amended to some extent with either peat moss or pumice. Peat moss is amended because it is assumed to increase water holding capacity of nursery substrates, while pumice is assumed to increase drainage. However, the effect of these amendments have been studied very little at the application rates typically used by nursery growers. Considering the increasing cost of these amendments, and the increasing energy costs for mixing, the objective of this research was to determine if peat moss and pumice have any affect on substrate physical or hydrological properties. A secondary objective was to determine if physical properties could be predicted from the known properties of the components (bark, peat, and pumice). Adding pumice to bark decreased porosity and water holding capacity, but increased bulk density. Adding peat moss to bark increased porosity and water holding capacity, but decreased bulk density. This research does not suggest one amendment is superior to the other. No single substrate is universally suitable to all plant species. Plants will respond more favorably to substrates that best mimic conditions of their native habitats. The utility of this research is to more clearly demonstrate how Douglas fir bark responds to the most common amendments used along the west coast of the U.S. Nursery growers can utilize these results to make better decisions about peat moss and pumice amendment rates, and how they might interact with production or irrigation regimes.
Technical Abstract: Douglas fir [Pseudotsuga menziesii Mirb.(Franco)] bark (DFB), sphagnum peat moss, and pumice are the most common substrate components used in the Oregon nursery industry. The objective of this study was to document the effect of peat and pumice addition on the physical and hydrological properties of Douglas fir bark soilless substrates. A secondary objective was to determine if measured properties of mixed soilless substrates can be accurately predicted from the known properties of the individual components. Treatment design was a 3 x 3 factorial with three rates each of sphagnum peat moss and pumice (0%, 15%, and 30% by vol.) added to DFB. The resulting nine substrates were measured for total porosity, air space, container capacity and bulk density using porometers. Moisture characteristic curves were generated by measuring water content along a continuous column. Adding pumice to DFB decreased total porosity, container capacity, available water and water buffering capacity, but increased bulk density. Adding peat moss to DFB increased total porosity, container capacity and available water but decrease air space and bulk density. Comparison of predicted values against measured values indicated that bulk density could be predicted reliably, however, all other physical properties could not be accurately predicted.