Peatmoss is one of the most commonly used substrate components in the greenhouse and nursery industry and has been used since the 1960s (Li et al., 2009; Shober et al., 2010). Currently, the environmental impacts of harvesting peatmoss are a concern since draining peatlands accelerates the decomposition process and results in the release of stored carbon to the atmosphere as carbon dioxide (Li et al., 2009). Wetlands, including peatlands, contain a vast pool of organic carbon, and currently hold about one-third of the global soil carbon stock (Freeman et al., 2004). Because of these environmental concerns, the restricted use of peat in some countries, rising cost (Sterrett, 2001), and peat scarcity issues during years of excessive precipitation (Jackson and Fonteno, 2013), a number of alternatives have been tested that include: coconut coir (Evans and Stamps, 1996), rice hulls (Buck and Evans, 2010), corn tassels (Vaughn et al., 2011), poultry feather fiber (Evans, 2004), tree-based products (Fain et al., 2008; Jackson et al., 2009), and different types of compost.
Compost has been used as a horticultural substrate additive since the 1970s and is a viable replacement for some components used in commercial substrates, specifically peatmoss (Benson, 1996; Bugbee, 1996). However, compost has not become a staple component of horticultural substrates used in the industry due to problems such as phytotoxicity, high concentration of heavy metals, chemical carry over, high salts, high pH, and inconsistency between batches (Hummel et al., 2014; Stoffella and Kahn, 2001). Since there are endless ingredients for making compost, the efficacy and rates for using each of these in greenhouse production are not well-understood (Murray and Anderson, 2004). Many of these problems can be avoided by using appropriate, high-quality, and consistent feed stocks, uniform production methods, followed by quality control testing. There are many positive impacts of using compost since it is created from recycled materials and places them back into the production stream; compost is typically locally produced, can provide supplemental nutrition, may suppress disease causing organisms, and can be used as a limestone substitute for pH establishment (Carrión et al., 2008; López-Lópeza and López-Fabal, 2013; Taylor, 2011; Wong et al., 1998).
Peatmoss has a low pH of 3.0–5.0 (Martinez et al., 1988) and typically needs 8 to 20 kg of lime per m3 to raise the pH to an acceptable level for most crops (Nelson, 2012). Compost pH, however, can range from 5.0 (Hue, 1992) to over 8.0 (Carrión et al., 2008), but is typically 7.0 or above. Because of the high pH of most composts, limestone rates can be reduced or even eliminated when compost is used as a component of substrate or as a peatmoss replacement (Bugbee, 2002; Taylor and Nelson, 2007). Many studies have shown that when compost is added to a peat-based substrate, the resulting pH is greater (Bugbee, 2002; DeKalb et al., 2014; Dolores Perez-Murcia et al., 2005; Jeong et al., 2011; Lopez et al., 1998; Taylor and Nelson, 2007; Wilson et al., 2001).
The pH buffering capacity of growing substrates may be affected when different materials, such as lime and/or compost, are used to establish substrate pH. Titration studies have been performed on pure compost with elemental sulfur and sulfuric acid to lower the high pH of the composts to make them more suitable for containerized crops and field-grown blueberries (Carrión et al., 2008; Costello and Sullivan, 2011b). However, these studies evaluated acidification of the compost directly, rather than after it was incorporated into a final blend. The direct impact on pH buffering capacity when compost is used to establish pH compared with limestone in horticulture substrates has not been determined. Determining the pH buffering capacity of substrates produced with compost is important for pH control and high-quality crop production. The objectives of this study were to: 1) determine the resulting substrate pH when using a range of compost and limestone rates and 2) compare the pH buffing capacity of substrates that had the pH established by the addition of compost, limestone, or a combination of both.
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