Soilless substrates are dynamic due to their predominantly organic nature, changing both physically and chemically over time. Substrate decomposition, settling, or a combination of the two can cause shrinkage in substrates. This shrinkage or reduction in substrate volume results in a change in physical properties that affect AS and CC. Aendekerk (1997) showed the relative decomposition and shrinkage of several peat sources as a function of substrate pH and sub-irrigation level. While pH and sub-irrigation level both influenced AS, pH as a function of peat source was more influential than irrigation factors. Allaire-Leung et al. (1999) showed that AS in peat substrates decreased and easily available water increased over a 14-month period, with a net effect of no change in TP. Settling and decomposition of peat-based substrates may be more common than in bark-based substrates. Nash and Laiche (1981) reported that increasing levels of peat relative to bark in substrates caused an increase in the amount of shrinkage. However, they also reported that a 4 bark:1 sand substrate had decreased hydraulic conductivity after 5 months, whereas several substrates with varying bark:peat ratios had increased hydraulic conductivity.
Nursery and greenhouse growers attempt to overcome substrate shrinkage by adding components that are not subject to change due to their inert, stable, inorganic composition. Nursery growers in the Pacific northwestern United States use pumice as the primary inorganic substrate component. Pumice is a porous igneous rock found primarily in volcanic regions of the world, including the Cascade Mountain Range in Oregon. The impact of pumice on crop growth and substrate physical properties has been studied throughout the world, as pumice from each volcanic region has unique properties (Gizas and Savvas, 2007; Gunnlaugsson and Adalsteinsson, 1995; Lenzi et al., 2001). Pumice is usually added to nursery substrates at rates of 10% to 20% (v/v) because it is perceived to increase aeration and drainage. Recent research contradicts these perceptions by showing additions of pumice to douglas fir bark (DFB) decreased TP, CC, available water, and water buffering capacity (water occurring between 5.0 and 9.9 kPa), but increased bulk density (Db) (Gabriel et al., 2009). In addition, it was hypothesized by growers that including a stable inorganic substrate component that cannot decompose would maintain structure in organic substrates and create more uniform physical properties throughout the growing season. It has also been speculated that plant roots form an effective scaffolding that maintains substrate structure and thus limit change to the physical properties of the substrate. Aendekerk (1997) supported this hypothesis by showing an average of 8% shrinkage after 31 weeks across four different peat substrates potted with ‘Rubinetta’ skimmia (Skimmia japonica), but over 18% shrinkage during the same time in containers without a plant. The objective of this research was to test the hypothesis that pumice maintains AS and porosity over time or renders substrates more resistant to shrinkage. A second objective of this research is to test the alternative hypothesis that plant roots alone maintain substrate structure thus preventing shrinkage.
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