Composted organic wastes have the potential to substitute for peat and bark as components of the growth substrates in containerized plant production systems (Carlile, 2008; Clark and Cavigelli, 2005; Estévez-Schwarz et al., 2009; Fitzpatrick, 2001; Gouin, 1995; Moore, 2005; Raviv, 2005; Sterrett, 2001). Most of the feedstocks used for composting are readily available from local sources. Sourcing locally available feedstocks for conversion to container substrates eliminates the need for long-distance transport thereby reducing both the environmental and economic costs of the substrates. The container plant industry is an obvious choice for using some of these recycled wastes because of its constant need for plant growth substrates. As plants are sold, the containers and the media inside are sold with them, resulting in the demand for more.
Growers must have a reliable source of high-quality growth substrates that are consistent over time. From a grower’s perspective, one of the major impediments to using composts for container substrates is the variation in physical and chemical characteristics between different types of compost, different sources of compost, and even between different batches of the same compost from the same source (Bettineski, 1996; Fitzpatrick, 2001; Sterrett, 2001; Tyler, 1996). Compost producers must manufacture their products to meet the needs of container growers (Moore, 2005; Raviv, 1998; Sterrett, 2001).
Previous research at Washington State University, Puyallup, has shown that a blend of Tagro mix, produced by the City of Tacoma, WA, and bark is equal to or superior than standard peat-based substrates for growing chrysanthemum (Chrysanthemum ×morifolium) in greenhouse (Krucker et al., 2010). Tagro mix is a garden amendment made from Class A biosolids from the City of Tacoma (50%), screened sand (25%), and sawdust (25%). Biosolids are a residual product of wastewater treatment (sewage sludge) that has been treated to meet U.S. Environmental Protection Agency (USEPA) quality standards for land application (Sullivan et al., 2007). Class A biosolids undergo an enhanced pathogen removal step to make them suitable for home and garden use. Biosolids from a given source tend to have a more consistent composition than other recycled organic materials, which is an advantage for container substrate production. The City of Tacoma now produces Tagro Potting Soil from Tagro Mix and bark and sells it locally as the most profitable product of their biosolids stream (City of Tacoma, 2011). It has become accepted by users as a quality potting substrate and garden amendment. The city produces about 4000 yard3 of Tagro Potting Soil annually, using 25% of their biosolids stream.
Biosolids composts have the potential to be a major ingredient in locally produced container substrates (Bugbee, 2002; Fitzpatrick, 2001; Klock-Moore, 1999; Krucker et al., 2010; Sterrett, 2001). Class B biosolids are suitable for farm or forestry use, but they must go through another pathogen reduction step (such as composting) to become a Class A material suitable for use in gardens, parks, and greenhouses. Carbon-rich recyclable materials such as woody construction debris, woody storm debris, and horse manure are abundant around metropolitan areas (Frear et al., 2005), and these could be composted with Class B biosolids to make a Class A product that could potentially be used in container substrates.
Use of locally available organic waste materials and biosolids as substrates would enhance the sustainability of container plant production systems. Results of a consumer preference survey for ornamentals, vegetable transplants, and herbs indicated consumers were interested in obtaining plants produced locally and sustainably (Yue et al., 2011). The objective of this research was to determine the suitability of composts made with biosolids and different locally produced urban organic carbon sources for use as container substrates, based on physical and chemical properties and plant growth response at two nitrogen rates under greenhouse conditions.
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