The production of container-grown plants requires substrates that provide adequate chemical and physical properties. Traditional substrate mixes are formulated on a volume basis of peatmoss, vermiculite, perlite, and/or pine bark [PB (Nelson, 2012)]. In the United States, peatmoss is the primary base component and perlite is the primary aggregate used in greenhouse substrates; however, substrate manufacturers and growers are looking to decrease the use of traditional components that have high (or increasing) costs. Transportation and occasional peat/bark shortages in the past decade are the primary reasons for the increased costs (Jackson et al., 2008a), thus a higher demand for regional and lower cost alternatives.
Recent investigations of fresh wood as a substrate component for plant production have proven successful as an alternative for peatmoss (Boyer et al., 2008; Gerber et al., 1999; Jackson et al., 2008a; Wright et al., 2008). In contrast to peat-lite- (PL) and PB-based substrates, plant growth in substrates composed of large portions of wood have a tendency to become N deficient as a result of the high degree of N immobilization (Handreck, 1991, 1993; McKenzie, 1958). Nelson (2012) described the desirable properties of substrates for greenhouse crops, noted the importance of organic matter stability and the C:N ratio as it relates to N immobilization affecting plant growth during production. The high C:N ratio of wood substrates can result in the tie-up of N from microbial immobilization, and wood substrate stability (decomposition) over time have been major concerns of researchers and growers (Boyer et al., 2012; Jackson et al., 2008b, 2009).
Wright and Browder (2005) demonstrated ‘Inca Gold’ marigold could be grown in substrates made from processed loblolly pine (Pinus taeda) chips (PC) at 100% PC, and 75% PC: 25% PB (by volume) if additional nutrients were provided, with minimum growth differences when compared with plants grown in 100% PB substrates. Jackson et al. (2008a) reported ‘Prestige’ poinsettia (Euphorbia pulcherrima) growth response to be similar when grown in pine tree substrates (PTS) compared with PL or PB substrates at increasing fertility concentrations. They reported substrate solution EC values increased with increasing fertilizer concentrations and were higher in PL or PB than in PTS, thus demonstrating higher fertilizer concentrations were required to achieve comparable substrate solution EC levels for PTS compared with PL or PB (Jackson et al., 2008a, 2008b; Wright et al., 2008). In addition to N immobilization, nutrient leaching of wood-based substrates (dependent on processing of wood material and resultant particle size) has been proposed as a possible reason for lower EC and nutrient levels of PTS compared with PL or PB substrates during plant production (Jackson et al., 2009a; Wright and Browder, 2005; Wright et al., 2008).
Jackson and Wright (2009) found shoot dry weight of marigold plants grown in 100% PTS to increase as peat amendment increased when fertilized with 50, 100, and 200 mg·L−1 N. When plants were fertilized with 300 mg·L−1 N, shoot dry weight of plants grown in at least 40% PTS (by volume) with the remainder being peat, did not increase with increasing peat amendment. Without additional peat amendment, plants and substrate microbes were supplied with sufficient N, thus minimizing effects of microbial N immobilization. Fain et al. (2008) showed similar results with petunia (Petunia ×hybrid) grown in various percentages of a pine wood substrates and peatmoss. Petunia growth was larger in substrate treatments containing peatmoss compared with 100% pine wood. Petunia grown in substrates containing wood increased in shoot mass when additional fertilizer was supplied. Decreased plant growth in wood-based substrates is generally only a concern when fertility levels (primarily N) are below optimal recommended levels for growth and development (Hicklenton, 1983). Wright et al. (2008) also found plant growth of ‘Baton Rouge’ chrysanthemum (Chrysanthemum ×grandiflora) grown in 100% PTS, required an additional 100 mg·L−1 of N fertilizer to obtain comparable growth to a PL substrate composed of 45% peat, 15% perlite, 15% vermiculite, and 25% bark (by volume).
No information is available regarding fertility requirements for peat-based substrates amended with aggregates of PWCs, which is a different material (size, structure, particle shape, etc.) than previous wood components tested. Therefore, the objective of this study was to determine N recommendations for optimal plant growth in peat-based substrates amended with perlite or PWC aggregates at various ratios.
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