Soilless root substrates (substrates) are commonly used in the production of containerized greenhouse and nursery crops (Bunt, 1988; Nelson, 2003). Substrates are formulated from various organic and inorganic components to provide suitable physical and chemical properties as required by the specific crop and cultural conditions (Bunt, 1988). One of the most common materials used in the formulation of substrates is sphagnum peat (peat). However, environmental concerns (Barkham, 1993; Buckland, 1993; Robertson, 1993) in the European Union and cost in markets such as Japan that are far from commercial sphagnum peat sources have generated significant interest in the development of new substrate components that could serve as alternatives to peat used in substrates. Additionally, an increased emphasis on sustainability has increased interest in finding uses for agricultural and municipal byproducts and one of the potential areas in which such byproducts might be used is as horticultural substrate components.
Most research on the development of alternatives to peat in substrates has been focused on agricultural, industrial, and municipal waste products. Among these products were coconut coir (Evans and Stamps, 1996), cotton gin waste (Wang, 1991), waste paper products (Chong and Cline, 1993), composted rice hulls (Laiche and Nash, 1990), kenaf (Wang, 1994), composted yard waste (Beeson, 1996), ground feather fiber (Evans, 2004), ground pine tree wood (Jackson et al., 2008), and various composted animal manures (Tyler et al., 1993). Although some of these materials have been successfully used, some were not produced in large enough quantities to impact the market, some were too expensive for their intended use, some had a high degree of variability, and others had a high likelihood of containing contaminants such as metal fragments, glass, lead, and mercury.
Rice is produced in many areas of the world and in the United States extensively in Arkansas, California, Florida, Louisiana, Mississippi, Missouri, and Texas. Rice hulls are a byproduct of the rice milling industry and consist mainly of hemicellulose, lignin, and amorphous silica (Juliano et al., 1987). Kamath and Proctor (1998) estimated that 34 million tons of fresh rice hulls were produced annually in the United States. According to Bunt (1988) and Hanan (1998), fresh rice hulls had a bulk density of 0.10 g·cm−3, water-holding capacity of 20% (v/v), total pore space of 89% (v/v), and an air-filled pore space of 69% (v/v). Evans and Gachukia (2007) demonstrated that the large particle size of whole parboiled fresh rice hulls increased drainage and air-filled pore space in peat-based substrates without causing significant nitrogen immobilization. Evans and Gachukia (2004) also reported that shoot and root dry weights of impatiens (Impatiens wallerianana Hook), marigold (Tagetes patula L.), and pansy (Viola ×Wittrockiana Gams.) grown in perlite-containing substrates were not significantly different from those grown in parboiled fresh rice hull-containing substrates.
Sambo et al. (2008) reported the physical properties of various ground nonparboiled fresh rice hulls. No fresh ground rice hulls products passed through 1-, 2-, 4-, or 6-mm diameter screens had the same physical properties as peat. All ground fresh rice hull products had higher bulk density, lower total pore space, higher air-filled pore space, and lower water-holding capacity than sphagnum peat.
In most situations, the physical properties of substrates are largely determined by the size of the particles of the substrate components. Larger particles tend to result in the creation of larger pores. Larger pores tend to drain after irrigation and provide the air-filled pore space of the substrate (Bunt, 1988; Raviv and Lieth, 2008). As the particle sizes increase or the proportion of large particles in the substrate increases, the air-filled pore space decreases. In contrast, smaller particles tend to pack more closely together than larger particles. Closer packing results in a decrease in air-filled pore space and may result in a decrease in total pore space, an increase in water-holding capacity, or both.
Because substrate particle size influences pore size and pore size directly influences substrate physical properties, it should be possible to manipulate the physical properties of ground parboiled fresh rice hulls by altering the grind size and the sieve dimensions to produce a product with different particle sizes or different mixtures of particle sizes that have physical properties similar to sphagnum peat. Therefore, the objectives of this research were to develop a ground parboiled fresh rice hull product with physical properties similar to sphagnum peat and to evaluate how the inclusion of such a ground rice hull product as a complete or partial substitute for peat with different common aggregates affects the physical properties of the substrates.
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