Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2011
Publication Date: 10/20/2011
Citation: Altland, J.E. 2011. Influence of pumice and plant roots on substrate physical properties over time. HortTechnology. 21:554-557.
Interpretive Summary: Douglas fir bark is the primary component used in potting mixes throughout the Pacific Northwest U.S. Nursery producers often mix pumice into the bark because it is thought that pumice maintains air space and prevents shrinkage of the potting mix throughout the production cycle of the crop. Pumice is expensive, and previous research by the authors found that it decreased total porosity and water holding capacity when added to Douglas fir bark. This research was conducted to determine if there are beneficial long-term effects of pumice, such as maintaining air space and preventing substrate shrinkage. We found that the change in air space over time decreased with increasing pumice rate. Thus the notion that pumice maintains air space in nursery substrates seems to be valid. There was little shrinkage in any of the substrates we tested, thus the hypothesis that pumice prevents shrinkage in substrate over time could not be adequately tested in our experiment.
Technical Abstract: An experiment was conducted to test the hypothesis that either pumice or plant roots maintain air space and porosity over time, or renders substrates more resistant to shrinkage. Treatment design was a 3×2 factorial with three substrate types and either presence or absence of a plant. The three substrates were composed of Douglas fir bark alone or amended with 15% or 30% (by vol.) pumice. Substrates were packed in aluminum cores to facilitate measurement of physical properties with NCSU Porometers at the conclusion of the experiment. Half of the cores with each of the three substrate types were packed with a single plug of coreopsis (Coreopsis 'Autumn Blush' PPAF PPV) (Experiment 1) or 'Blue Prince' holly (Ilex ×meserveae) (Experiment 2). The remaining cores were maintained in the same production environment, but without a plant. Substrate physical properties were measured before the experiment and after 41 d for coreopsis plants and 382 d for holly. Both experiments had relatively similar responses despite using different crops and production times. Summarizing over all treatments, air space decreased, container capacity and total porosity increased, and bulk density remained constant over time. The presence of a plant in the container tended to exacerbate the decrease in air space and the increase in container capacity. Shrinkage was affected by the presence of a plant, but only minimally.