Abstract:
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.
