The most common components of soilless container media used by the nursery industry in the United States are bark from loblolly pine (Pinus taeda L.) and douglas fir [Pseudotsuga menziesii (Mirb.) Franco]. Loblolly PB is widely used by growers on the East Coast, in the Midwest, and in the southern regions of the United States and douglas fir bark is commonly used on the West Coast. Within the last decade, the nursery industry has faced a steady decline in the availability of PB as well as higher costs because of an increase in demand for alternative uses (e.g., heating fuel), a decline in log harvest, and an increase in freight costs (Haynes, 2003; Lu et al., 2006). A greater shortage and inferior quality of PB are expected as a result of the increasing demand for wood-based materials to be used as biofuels (Day, 2009). The industry is interested in alternative, economical, and sustainable container substrates that are able to provide adequate growing conditions for nursery production.
To address this issue, a long list of bark alternatives has been evaluated, including but not limited to pine trees/wood (Boyer et al., 2008; Fain et al., 2008; Jackson, 2008; Jackson et al., 2009; Wright and Browder, 2005; Wright et al., 2006), recycled paper (Craig and Cole, 2000), composted turkey litter (Tyler et al., 1993), cotton gin waste (Cole et al., 2005; Jackson et al., 2005; Owings, 1993), and sewage sludge (Guerrero et al., 2002). Other new materials currently being evaluated as substitutes for PB in nursery production include some fast-growing herbaceous crops such as switchgrass, willow, corn, and bamboo (Boyer et al., 2010).
Another possible alternative is rice (Oryza sativa L.) hull. Rice hulls are a relatively underused and sustainable container substrate, which are normally considered a waste byproduct of the rice milling and processing industry (Lovelace and Kuczmarski, 1992). Large quantities of rice hulls are produced annually in the United States, especially in the southern and western states. Numerous studies have been conducted evaluating different forms of rice hulls as alternative substrates in propagation, greenhouse, and nursery production. Rice hulls are available in a variety of forms, including fresh, aged, carbonized, composted, burnt, and parboiled (Buck, 2008). Fresh rice hulls are typically avoided as container substrates because of residual rice and/or weed seed.
Parboiled rice hulls are produced by steaming and drying rice hulls after the milling process. This results in a lightweight and consistent product that is free of viable weed and/or rice seed (Evans and Gachukia, 2004). Another advantage in using PBH as a horticultural substrate amendment is the low decomposition rate during the typical production cycle of nursery crops. Despite being an organic compound, rice hulls consist mainly of lignin, cutin, and insoluble silica, providing a slow breakdown of particles and therefore making PBH an appropriate substrate for long-term crop production (Juliano et al., 1987).
Einert and Guidry (1975) published some of the earliest work on the use of fresh and composted rice hulls as an amendment for the soil-based container production of woody ornamentals. Although statistical analyses were not included, either form of rice hulls appeared to be a suitable media amendment based on mortality and growth data for Pfitzer juniper [Juniperus ×pfitzeriana (L.) Späth]. Laiche and Nash (1990) evaluated the effect of composted rice hulls on the growth of three woody plants (Rhododendron indicum L., Ilex crenata Thunb., and Juniperus horizontalis Moench.) in containers. Their results demonstrated plant growth in organic components of 100% composted rice hulls or 50% composted rice hulls:50% bark compared favorably with the growth obtained using 100% PB.
Lovelace and Kuczmarski (1992) reported that aged rice hulls compared favorably to 100% PB in cost and performance when used as a component of a blend including PB, rice hulls, and sand (2:2:1 by volume) for a variety of woody ornamentals. Baiyeri (2005) demonstrated that when using composted rice hulls amended with poultry manure (3:1 by volume), sucker plantlets from five banana genotypes generally resulted in more vigorous suckers than when sawdust and poultry manure (3:1 by volume) or rice hulls, sawdust, and poultry manure (1.5:1.5:1 by volume) were used at the nursery stage of production.
Fresh (Einert, 1972; Papafotiou et al., 2001; Sambo et al., 2008), carbonized (Kämpf and Jung, 1991; Tatum and Winter, 1997), parboiled (Evans and Gachukia, 2007), and ground parboiled (Buck and Evans, 2010) rice hulls have been evaluated as substrates on a number of greenhouse crops. Dueitt and Newman (1994) determined that fresh rice hulls hold more water than aged rice hulls in greenhouse media for seedlings of Tagetes erecta L. and Limonium suworowii (Reg.) Kuntze. Evans and Gachukia (2007) reported that the large particles of PBH provide adequate drainage and aeration in peat-based substrates. More recently, Buck and Evans (2010) revealed that given its physical properties, ground PBH can be used as a suitable replacement for up to 40% peatmoss to grow greenhouse crops.
A preferred container substrate should provide stable plant support, a reservoir for nutrients and water to the root system, and adequate gas exchange (Nelson, 2003). Bunt (1988) stated that the most important physical properties of containerized substrates are dry bulk density (DBD; g·cm−3), air-filled pore space (AS; %), water-holding capacity (WHC; %), and total porosity (TP; %). Particle size distribution is also considered a fundamental characteristic in the physical properties of substrates because the shape, size, and density of the individual particles as well as the proportion of the different particle sizes will determine the proportion of air and water in a substrate (Handreck and Black, 2002).
Physical properties of substrates in containers will change over time as a result of the decomposition of the components caused by physical and biological degradation. The breakdown of the components generally results in lower AS, higher water retention, and increased weight of solids in the container (Ingram et al., 2003). Substrate shrinkage is also an important factor to consider when looking at the changes in physical properties over time. Nash and Pokorny (1990) defined shrinkage as the loss of bulk volume of substrates in containers, which can be attributed to the settling of fine particles into the macropores located between coarse particles. Organic matter decomposition as well as physical breakdown will also cause the substrate to shrink as the particles become smaller and fit closer together (Ingram et al., 2003). Erosion can also affect substrate shrinkage in that the substrate particles can be washed out of the container after intense rainfalls and/or irrigation depending on the particle sizes (Ingram et al., 2003).
Recommended ranges for the commercial production of nursery crops include: DBD (0.19 to 0.7 g·cm−3), TP (50% to 85%), AS (10% to 30%), and WHC (45% to 65%) (Yeager et al., 2007). Plant production management can be expected to be less intensive if substrates are maintained within these suggested physical property ranges (Bilderback et al., 2005); however, growers' production techniques can influence the outcomes.
Research is needed on the effect of PBH as an amendment for PB-based container substrates in the long-term production of ornamental plants. Therefore, the objectives of the study were to compare the changes in physical properties as well as the plant growth responses for the PB substrates amended with various ratios of PBH and to characterize how the amount of PBH affected physical properties for production of container-grown shrubs over long-term crop cycles.
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
Argo, W.R. & Biernbaum, J.A. 1994 Irrigation requirements, root medium pH, and nutrient concentrations of Easter lilies grown in five peat-based media with and without an evaporation barrier J. Amer. Soc. Hort. Sci. 119 1151 1156
Baiyeri, K.P. 2005 Response of Musa species to macro-propagation. II: The effects of genotype, initiation and weaning media on sucker growth and quality in the nursery Afr. J. Biotechnol. 4 229 234
Boyer, C., Owen, J.S. Jr., & Altland, J. 2010 Development of sustainable and alternative substrates for nursery container crops Proc. Southern Nursery Assoc. Research Conf. 55 410 412
Boyer, C.R., Fain, G.B., Gilliam, C.H., Gallagher, T.V., Torbert, H.A. & Sibley, J.L. 2008 Clean chip residual: A substrate component for growing annuals HortTechnology 18 423 432
Buck, J.S. 2008 The use of ground parboiled fresh rice hulls as an alternative horticultural root substrate component for containerized greenhouse crop production PhD diss., University of Arkansas Fayetteville, AR
Buck, J.S. & Evans, M.R. 2010 Physical properties of ground parboiled fresh rice hulls used as a horticultural root substrate HortScience 45 643 649
Bunt, A.C. 1988 Media and mixes for container-grown plants: A manual on the preparation and use of growing pot plants 2nd Ed Unwin Hyman Ltd. London, UK
Cole, D.M., Sibley, J.L., Blythe, E.K., Eakes, D.J. & Tilt, K.M. 2005 Effect of cotton gin compost on substrate properties and growth of Azalea under differing irrigation regimes in a greenhouse setting HortTechnology 15 145 148
Day, M. 2009 Mulch producers tune into biofuel boom Soil Mulch Producers News 3 1 3, 16 2 Sept. 2010 <http://soilandmulchproducernews.com/archives/50-januaryfebruary-2009/117-mulch-producers-tune-into-biofuel-boom>.
Dueitt, S. & Newman, S.E. 1994 Physical analysis of fresh and aged rice hulls used as a peat moss substitute in greenhouse media Proc. Southern Nursery Assoc. Research Conf. 39 81 85
Evans, M.R. & Gachukia, M.M. 2004 Fresh parboiled rice hulls serve as an alternative to perlite in greenhouse crop substrates HortScience. 39 232 235
Evans, M.R. & Gachukia, M.M. 2007 Physical properties of sphagnum peat-based root substrates amended with perlite or parboiled fresh rice hulls HortTechnology 17 312 315
Fain, G.B., Gilliam, C.H., Sibley, J.L. & Boyer, C.R. 2008 Whole tree substrates derived from three species of pine in production of annual vinca HortTechnology 18 13 17
Fonteno, W.C., Hardin, C.T. & Brewster, J.P. 1995 Procedures for determining physical properties of horticultural substrates using the NCSU Porometer Horticultural Substrates Laboratory, North Carolina State University Raleigh, NC
Guerrero, L., Gascó, J.M. & Hernández-Apaolaza, L. 2002 Use of pine bark and sewage sludge compost as components of substrates for Pinus pinea and Cupressus arizonica production J. Plant Nutr. 25 129 141
Haynes, R.W. 2003 An analysis of the timber situation in the United States: 1952–2050. Gen. Tech. Rept. PNWGTR- 560 U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station Portland, OR
Ingram, D.L., Richard, W.H. & Thomas, H.Y. 2003 Growth media for container grown ornamental plants Florida Coop. Ext. Serv. University of Florida, Bul. 241
Jackson, B.E. 2008 Chemical, physical, and biological factors influencing nutrient availability and plant growth in a pine tree substrate PhD diss., Virginia Polytechnic Institute & State University Blacksburg, VA
Jackson, B.E., Wright, A.M., Cole, D.M. & Sibley, J.L. 2005 Cotton gin compost as a substrate component in container production or ornamental plants J. Environ. Hort. 23 118 122
Jackson, B.E., Wright, R.D. & Seiler, J.R. 2009 Changes in chemical and physical properties of pine tree substrate and pine bark during long-term nursery crop production HortScience 44 791 799
Laiche, A.J. Jr. & Nash, V.E. 1990 Evaluation of composted rice hulls and a lightweight clay aggregate as components of container-plant growth media J. Environ. Hort. 8 14 18
Lee, J.-W., Lee, B.-Y., Lee, Y.B. & Kim, K.-S. 2000 Growth and inorganic element contents of hot pepper seedlings in fresh and decomposed expanded rice hull-based substrates J. Kor. Soc. Hort. Sci. 41 147 151
Lovelace, W. & Kuczmarski, D. 1992 The use of composted rice hulls in rooting and potting media Comb. Proc. Intl. Plant Prop. Soc. 42 449 450
Lu, W., Sibley, J.L., Gilliam, C.H., Bannon, J.S. & Zhang, Y. 2006 Estimation of U.S. bark generation and implications for horticultural industries J. Environ. Hort. 24 29 34
Owings, A.D. 1993 Cotton gin trash as a medium component in production of ‘Golden Bedder' coleus Proc. Southern Nursery Assoc. Research Conf. 38 65 66
Papafotiou, M., Chronopoulos, J., Kargas, G., Voreakou, M., Leodaritis, N., Lagogiani, O. & Gazi, S. 2001 Cotton gin trash compost and rice hulls as growing medium components for ornamentals J. Hort. Sci. Biotechnol. 76 431 435
Sambo, P., Sannazzaro, F. & Evans, M.R. 2008 Physical properties of ground fresh rice hulls and sphagnum peat used for greenhouse root substrates HortTechnology 18 384 388
Tatum, D. & Winter, N. 1997 Rice Hull Ash as a potting substrate for bedding plants. Proc Southern Nursery Assoc. Research Conf. 42 121 122
Tyler, H.H., Warren, S.L., Bilderback, T.E. & Fonteno, W.C. 1993 Composted turkey litter: II. Effect on plant growth J. Environ. Hort. 11 137 141
Wright, R.D., Browder, J.F. & Jackson, B.E. 2006 Ground pine chips as a substrate for container-grown woody nursery crops J. Environ. Hort. 24 181 184
Yeager, T.H., Fare, D.C., Lea-Cox, J., Ruter, J., Bilderback, T.E., Gilliam, C.H., Niemiera, A.X., Warren, S.L., Whitwell, T.E., Wright, R.D. & Tilt, K.M. 2007 Best management practices: Guide for producing nursery crops 2nd Ed Southern Nurserymen's Assoc. Marietta, GA