Soilless root substrates (substrates) are commonly used in the production of containerized greenhouse crops. These substrates may be composed of a single material such as rockwool or block-cut peat, but in most cases, they are composites formulated by the blending of two or more components such as peat, composted bark, perlite, whole rice hulls, or vermiculite (Hanan, 1998; Nelson, 2003). Substrates are designed to have appropriate physical properties for specific crops and growing conditions. The components used to formulate the substrate and the proportions of the components may be altered to change the physical properties of the substrate as desired (Bunt, 1988). Total pore space, air-filled pore space, waterholding capacity, available water content, easily available water, and bulk density are typically the physical properties of greatest interest for substrates to be used in greenhouse crop production.
Peat is one of the most common components used in the formulation of root substrates (Cattivello, 1991). Environmental concerns (Barkham, 1993; Buckland, 1993; Robertson, 1993) in the European Union and cost in markets such as Japan that are far from commercial peat sources have generated significant interest in the development of new substrate components that could serve as alternatives to peat in substrates.
Most research on the development of new substrate components that could serve the same purpose in the root substrate as peat has been focused on agricultural, industrial, and municipal waste products. Among these products were coconut (Cocos nucifera) coir (Evans and Stamps, 1996), cotton (Gossypium hirsutum) gin waste (Wang, 1991), waste paper products (Chong and Cline, 1993; Raymond et al., 1998), composted rice hulls (Laiche and Nash, 1990), kenaf (Hibiscus cannabinus) (Wang, 1994), feather fiber (Evans, 2004), municipal sewage sludge (Klock-Moore, 1999, 2001), composted yard waste (Beeson, 1996), and various composted animal manures (Tyler et al., 1993). Some of these materials were not produced in large enough quantities to impact the market, whereas others were too expensive for their intended use. Some of these materials have proven to be unsuitable because of their high degree of variability and their likelihood of containing contaminants such as metal fragments, glass, lead, and mercury, whereas others have been successfully used locally, regionally, or in niche markets.
Rice hulls are a byproduct of the rice milling industry. Rice is produced over large areas of Asia, and in the United States, it is produced primarily in Arkansas, Texas, Louisiana, Mississippi, and California. In Europe, most rice is produced in Italy's Po River Valley. The yield of rice hulls was reported to be ≈20% by weight. Because 350,000 mg of rice was produced in Italy in 2003 (Ente Nazionale Risi, 2006), the resulting rice hulls would be ≈87,500 mg. Rice production in the United States was estimated to be ≈7.5 million tons [U.S. Department of Agriculture (USDA), 2007], and worldwide production of rice was estimated to be 474 million tons per year. This would translate into 1.89 million and 118 million tons of rice hulls per year in the United States and worldwide, respectively. Therefore, rice hulls are an abundant and readily available material throughout much of the world.
Evans and Gachukia (2007) demonstrated that whole fresh rice hulls, because of their relatively large particle size, could be used in substrates to provide drainage and air-filled pore space. They reported that no nitrogen depletion occurred as a result of the incorporation of the fresh rice hulls and that plant growth was comparable in substrates where perlite was replaced with an equivalent amount of fresh rice hulls (Evans and Gachukia, 2004). Calderon (2001) reported that fresh rice hulls were commonly used for hydroponic cultivation in South America.
The objective of this research was to evaluate the physical properties of different sizes of fresh rice hull particles produced by grinding and screening and to determine if the ground rice hulls had similar properties to peat and if they would be suitable as an alternative to peat in substrates to be used for greenhouse crop production.
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