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.
Aendekerk, T.G. 1997 Decomposition of peat substrates in relation to physical properties and growth of Chamaecyparis Acta Hort. 450 191 198
Allaire-Leung, S.E., Caron, J. & Parent, L.E. 1999 Changes in physical properties of peat substrates during plant growth Can. J. Soil Sci. 79 137 139
Fonteno, W.C. & Bilderback, T.E. 1993 Impact of hydrogel on physical properties of coarse-structured horticultural substrates J. Amer. Soc. Hort. Sci. 118 217 222
Gabriel, M.Z., Altland, J. & Owen J. Jr 2009 Effect of peat moss and pumice on douglas fir bark based soilless substrate physical and hydraulic properties HortScience 44 874 878
Gizas, G. & Savvas, D. 2007 Particle size and hydraulic properties of pumice affect growth and yield of greenhouse crops in soilless culture HortScience 42 1280
Lenzi, A., Oggiano, N., Maletta, M., Bolaffi, A. & Tesi, R. 2001 Physical and chemical characteristics of substrates made of perlite, pumice and peat Italus Hortus 8 23 31
Nelson, P.V., Oh, Y.M. & Cassel, D.K. 2004 Changes in physical properties of coir dust substrates during crop production Acta Hort. 644 261 268