Soil texture is an intrinsic attribute defined as the distribution and proportion of particles at a given size to compose a given soil (Hillel, 1998). Knowledge of texture can be used to describe characteristics of soil relating to water-holding capacity, tortuosity, and infiltration rate. In soil, decreasing particle size results in smaller pores and an increased number of pores. Changes in pore size and number can affect water retention characteristics of soil and soilless substrates (Handreck and Black, 2002). A finer texture soil will retain greater amounts of water at higher tensions than a coarser texture soil or substrate because of smaller pores (Handreck and Black, 2002). Texture of soilless substrate has a similar impact on substrate physical properties as it has on soil physical properties. Texture for soilless substrates can be defined as the distribution and proportions of particle sizes in a substrate resulting from grinding, processing, and decomposition of the parent materials and added components. Mechanical analysis (Dane and Topp, 2002) of substrate particle size distribution is routinely performed to make inferences on infiltration, pore size and distribution, and, subsequently, water-holding capacity (Argo, 1998; Bilderback et al., 2005). Drzal et al. (1999) separated soilless substrates into three size or texture classes; course (greater than 2.0 mm), medium (0.5 to 2.0 mm), and fine (less than 0.5 mm) as suggested by Puustjarvi and Robertson (1975).
Substrate texture directly influences the substrate matric potential; however, this force can result in increased water-holding capacity at a given container height by overcoming the gravitational potential of 0.1 kPa·cm−1 from the bottom of the container (Milks et al., 1989). A water gradient occurs from the top to the bottom of a substrate in a container as a result of opposing matric and gravitational potentials. Matric potential in a substrate is constant throughout the container (assuming particle size distribution is constant), whereas gravitation potential in a substrate decreases from the top to the bottom of a container. Changes in total water potential, as a result of the combined effect of matric and gravitation potentials, causes a gradient of increasing substrate moisture from the top to the bottom of a container. This substrate moisture gradient results in an inverse relationship between air space (AS) and container capacity (CC) where AS decreases and CC increases from the top to the bottom of a substrate in a container. Substrate texture alters this moisture gradient by affecting the number and distribution of capillary and noncapillary pores (Argo, 1998).
Bilderback and Fonteno (1987) predicted the moisture gradient in a 17-cm tall container at 2-cm intervals using simulation models based on moisture characteristic curves of a 3 bark:1 sand (by vol.) substrate. The predicted gradient had a 37% (110 mL) increase in substrate moisture content from the top (32% CC) to bottom (69% CC) of a trade 1-gal container. This predicted moisture gradient within a container could greatly affect water management practices for ornamental plant growth throughout the production process. Direct measurement of the moisture gradient would be ideal to more assuredly determine moisture conditions throughout the profile of a soilless substrate. Kritz and Khaled (1995) developed a direct method to measure AS in relation to pot depth with peat-based substrates that used a drained, frozen column of substrate separated using detachable cylindrical rings from a unique, engineered device.
Douglas fir bark (DFB) used in soilless substrates in the northwest United States is commonly categorized as fine or medium bark when passing thru a 0.9-cm or 2.2-cm sieve, respectively (Buamscha, 2006). The objectives of this study were to 1) quantitatively determine the effect of container height on the physical properties (CC, AS, Db) of fine and medium DFB, 2) determine the effect of DFB particle size distribution on physical properties, and 3) directly measure water distribution in DFB in a 2.7-L container (#1 container).
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Buamscha, M.G. 2006 Chemical and physical properties of Douglas Fir bark relevant for the production of container crops in Oregon Oregon State Univ Corvallis, OR MS Thesis.
Cooke, A., Bilderback, T. & Lorscheider, M. 2004 Physical property measurements in container substrates: A field quantification study Proc. Southern Nurs. Assn. Res. Conf. 49 102 104
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
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Ownley, B.H., Benson, D.M. & Bilderback, T.E. 1990 Physical properties of container substrate and relation to severity of Phytophthora root rot of rhododendron J. Amer. Soc. Hort. Sci. 115 564 570
Puustjarvi, V. & Robertson, R.A. 1975 Physical and chemical properties 23 38 Robinson, D.W. & Lamb, J.G.D. Peat in horticulture Academic Press London
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Yeager, T., Bilderback, T., Fare, D., Gilliam, C., Lea-Cox, J., Niemiera, A., Ruter, J., Tilt, K.M., Warren, S.L., Whitwell, T. & Wright, R. 2006 Best management practices: Guide for producing nursery crops 2nd ed Southern Nurs. Assn Atlanta