Growing substrates constitute one of the largest costs to growers in the greenhouse and nursery industries. Peatmoss is the primary component of many of these substrates, although it is associated with substantial transportation costs and is a nonreadily renewable resource. In the last few decades, as interest in recycling and waste use has increased, researchers have studied a wide range of potential peat alternatives, including many agricultural, industrial, and consumer waste byproducts. A number of these materials have demonstrated the potential to replace peatmoss or serve as substrate amendments. These include substrate components made from tree or wood residues (Conover and Poole, 1983; Fain et al., 2006, 2008; Gruda and Schnitzler, 2001; Kenna and Whitcomb, 1985; Wright and Browder, 2005), cotton gin compost (Cole et al., 2005; Jackson et al., 2005; Owings, 1993), vermicompost (Bachman and Metzger, 1998; Hidalgo et al., 2006), municipal waste compost (Bugbee and Frink, 1989; Chong, 2005), and many other waste byproducts.
Pulp mill boiler ash is a widely available industrial waste byproduct produced when the paper industry burns tree residues and other materials to fuel paper mill boilers. The ash has been shown to have a high pH and the ability to raise soil alkalinity (Demeyer et al., 2001). Muse and Mitchell (1995) reported boiler ash from 19 mills in Alabama had an average pH of 9.9, electrical conductivity (EC) of 3.1 dS·m−1, 0.45% total nitrogen, 0.3% total phosphorus (P), 1.3% potassium (K), and 12.0% calcium (Ca) and over 50% of a given sample passed through a 0.106-mm sieve. Currently, the majority of this boiler ash is put into landfills, whereas some is applied to forest and agricultural land. Previous land application studies have shown that boiler ash can increase field soil pH (Chirenji and Ma, 2002; Muse and Mitchell, 1995); increase extractable soil P, K, Ca, and magnesium (Mg) concentrations (Muse and Mitchell, 1995); increase EC (Chirenji and Ma, 2002); and increase waterholding capacity and reduce soil bulk density of a fine sand (Chirenji and Ma, 2002). Although some studies have demonstrated that ash applications have negative or no effects on plant growth (Demeyer et al., 2001), most field studies indicated that the application of boiler and other wood ash to soil can improve plant growth, yield, or both (Demeyer et al., 2001; Muse and Mitchell, 1995; Myers and Kopecky, 1998; Rakala and Jozefek, 1990).
In the United States, more than 80% of boiler ash is disposed of in landfills (Vance, 1996). However, landfill disposal costs are increasing and it is becoming more difficult to acquire new sites for disposal (Demeyer et al., 2001). It is possible that boiler ash can be used as a substrate amendment in greenhouse and nursery production while reducing substrate costs and alleviating some problems pulp mill operators confront when using current methods of disposal. However, peat-based greenhouse and nursery substrates differ significantly from field soils, and plants may respond differently to ash applications. Using marigold as a test plant, the objective of this study was to evaluate the potential of using pulp mill boiler ash as an alternative substrate component for greenhouse production.
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