Root substrates (substrates) are commonly used in the production of containerized greenhouse and nursery crops (Nelson, 2003). Substrates are formulated from various inorganic and organic components to provide suitable physical and chemical properties as required by the specific crop and growing conditions (Bunt, 1988). An important physical property of substrates is air-filled pore space. Air-filled pores allow for drainage and gas exchange between the root environment and the outside atmosphere (Bunt, 1988). Various materials are used to provide, at least in part, for air-filled pore space in substrates, with one of the most common being perlite (Boertje and Arnold Bik, 1975; Bunt, 1988).
Perlite is an inorganic, expanded aluminosilicate of volcanic origin (Nelson, 2003) and it is produced by mining the ore, grinding the crude ore to the desired particle size, and heating it to temperatures of up to 982 °C. Heating causes the ore to expand from four to 20 times its original volume, resulting in a lightweight white porous particle (Hanan, 1998). Because of the costs associated with mining, transportation, and heating, perlite is a relatively expensive substrate component. In addition to its cost, in its dry state perlite produces a siliceous dust that is classified as an eye and lung irritant. Substrate components that are lower in cost, do not have potential health issues, and could provide for air-filled pore space in the substrate would be beneficial to the nursery and greenhouse crops industries.
Some potential alternative components to perlite (e.g., shredded rubber, ground bovine bone) have undesirable chemical properties (Evans, 2004; Evans and Harkess, 1997) such as high pH, high electrical conductivity, or phytotoxic levels of one more mineral nutrients. Other materials evaluated as potential alternatives to perlite are either too expensive or have unacceptably high bulk densities (e.g., calcined clay aggregates, gravel), which resulted in unacceptably high shipping costs.
Parboiled fresh rice hulls (PBH) are a milling coproduct of the rice industry and comprise ≈20% of the rice grain at harvest. Parboiled fresh rice hulls are obtained as a result of a steaming process and are therefore free of viable weed seed. Evans and Gachukia (2004) demonstrated that PBH could be successfully used as an alternative to perlite in the root substrate for the production of several ornamental species. However, the physical properties of PBH-amended sphagnum peat-based substrates compared with those amended with perlite have not been reported. Additionally, how increasing amounts of PBH in the substrate affects the physical properties of the substrate has not been reported.
The objectives of this study were to determine and compare total pore space (% by volume), air-filled pore space (% by volume), water-holding capacity (% by volume and weight per weight), and bulk density (weight per volume) of sphagnum peat-based substrates amended with various amounts of PBH or perlite, and to determine how the amount of PBH or perlite affects these physical properties.
Bilderback, T.E. & Fonteno, W.C. 1993 Impact of hydrogel on physical properties of coarse-structured horticultural substrates J. Amer. Soc. Hort. Sci. 118 217 222
Evans, M.R. & Gachukia, M. 2004 Fresh parboiled rice hulls serve as an alternative to perlite in greenhouse crop substrates HortScience 39 232 235
Evans, M.R. & Harkess, R.L. 1997 Growth of Pelargonium × hortorum and Eurphorbia pulcherrima in rubber-containing substrates HortScience 32 874 877