Graduate Thesis Or Dissertation
 

The effect of physical and hydraulic properties of peat moss and pumice on Douglas-fir bark based soil-less substrates

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  • Douglas-fir [Pseudotsuga menziesii (Mirbel) Franco] bark (DFB), sphagnum peat moss, and pumice are the most common substrate components used in the Oregon nursery industry. Despite the widespread use of these three components, little information is available on the effect of physical and hydraulic properties of peat moss and pumice on DFB based soil-less substrates used in container production. Therefore, two studies were conducted in 2007 and 2008. The objectives of the first study were to (1) document the effect of peat and pumice addition on the physical and hydrological properties of Douglas-fir bark soil-less substrates; (2) determine if measured properties of mixed soil-less substrates can be accurately predicted from the known properties of the individual components. The second study was a continuation of the research. The objectives of this second study was to (3) compare volumetric and gravimetric method to determine particle size distribution of soil-less substrates composed of varying components; and (4) determine if existing model of Haverkamp and Parlange can be used to predict the moisture characteristic curve (MCC) of mixed substrates with known particle size distribution. In the first study, 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 decreased 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. The second study focused on comparing methods to measure particle size distribution for soil-less substrates and using those methods to predict moisture characteristic curve using soil based models. 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. Particle size distribution of the nine substrates was determined using volumetric and gravimetric methods. The particle size distributions of each substrate were used to determine if the Haverkamp and Parlange (1986) model could be used to accurately estimate a moisture characteristic curve for each substrate. There were statistical differences in particle size distribution between volume and weight based method. This resulted in shifts in the particle size summation curve (weight- or volume-based), however both methods remained strongly correlated providing relatively equivalent information. Regardless of substrates composition there was no similarity in measured particle size summation curve (weight- or volume-based) and measured moisture characteristic curve. Therefore, the Haverkamp and Parlange model was unable to be used to predict the moisture characteristic curve any of the soil-less substrates included in the study.
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