Soilless substrates containing wood fiber are increasingly being used for the production of containerized floriculture crops in the United States. This is partially a result of increasing concerns regarding the environmental sustainability of sphagnum peat harvesting (Barrett et al., 2016; Maher et al., 2008), in addition to increasing costs to manufacture and transport peat and other substrate components. Substrates containing (by volume) 20% to 40% wood fiber as a substrate amendment or replacement for peatmoss, perlite, and other common soilless components have gained significant market share among floriculture operations in North America (Drotleff, 2018).
Wood fiber components used in soilless substrates have potential to immobilize fertilizer N and reduce N uptake by plants (Blok et al., 2008; Handreck, 1992a, 1992b, 1993; Jackson et al., 2009; Maher et al., 2008; Prasad, 1996a). Substrates with a carbon:N (C:N) ratio exceeding 30:1 tend to immobilize N due to microbial decomposition of the available C, a process that requires N (Bunt, 1988; Nelson, 2011). Wood components such as composted barks, hammer-milled wood materials, and sawdust can have C:N ratios of 300:1 or greater (Maher et al., 2008; Nelson, 2011; Prasad, 1996b), and have high potential to immobilize N from applied water-soluble fertilizers. Non-wood components that have high C:N ratios, such as coconut (Cocos sp.) coir fiber, can also immobilize N (Maher et al., 2008; Prasad, 1996a). In contrast, other organic materials such as peatmosses and organic composts tend to have a near zero N immobilization effect, or mineralize N (release additional N) during microbial decomposition (Bunt, 1988; Prasad, 1980; Raviv, 2005). Factors that affect microbial activity also tend to influence N immobilization, including substrate temperature, moisture, pH, oxygen level, N form, and C source (Geisseler et al., 2010; Handreck, 1993; Jackson et al., 2009; Schulten and Schnitzer, 1998).
Incorporating wood fiber components into peat-based soilless substrates can increase the amount of fertilizer N needed during production as a result of greater N immobilization (Gruda et al., 2000; Jackson et al., 2008; Maher et al., 2008; Prasad, 1996a). Previous substrate research with various wood components has shown that increasing the proportion of wood increases the amount of N required for optimal plant growth (Handreck, 1993; Jackson et al., 2008; Wright et al., 2008), and therefore similar results may be expected with increasing wood fiber components. Wood materials can differ in their potential to immobilize N (Handreck, 1993; Nelson, 2011; Prasad, 1996b), and also influence plant growth by affecting various substrate physical properties (Argo, 1998a, 1998b; Bunt, 1988; Maher et al., 2008). There is a need to evaluate peat-based substrates containing wood fiber components for effects on N immobilization and plant growth to establish updated best management practices for commercial floriculture operations.
The objective was to evaluate substrate effects on substrate and plant tissue nutrient level and plant growth, with an emphasis on evaluating N immobilization from wood fiber components. Two greenhouse experiments were conducted to evaluate plant growth and N level for peat substrate amended with (by volume) 30% expanded pine (Pinus sp.) wood fiber during a production and simulated consumer retail phase with container-grown petunia (Petunia ×hybrida). Substrates consisting of peat amended with either 30% hammer-milled pine wood or 30% coconut coir, in addition to 100% peat substrate, were included for comparison. In Expt. 2, the same substrate blends, but without plants, were evaluated in the laboratory for physical properties and potential to immobilize N using the Handreck (1992a) N drawdown index (NDI) method and the Association of German Agricultural Analytic and Research Institute’s (VDLUFA, 2007) procedure for testing N immobilization and mineralization in soilless substrates. We hypothesized that peat containing pine wood fiber and hammer-milled pine wood would increase N immobilization, reduce N uptake, and decrease plant growth.
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