Recent concerns over peatmoss availability, cost, and harvest restrictions have created the need for alternative substrates for potted production of ornamental greenhouse and nursery crops (Jackson et al., 2008; Kuack, 2014; Taylor et al., 2012; Wright and Browder, 2005). Chipped wood from softwood trees has shown excellent potential to supplement peatmoss supplies with few changes in production practices (Boyer et al., 2012a; Murphy et al., 2011). Pine wood substrate studies on ornamental crops have revealed limitations of the wood use, particularly reductions in substrate water and nutrient holding capacities with increased potential for nutrient leaching losses (Fain et al., 2008; Gaches et al., 2011; Jackson et al., 2009; Murphy et al., 2011; Witcher et al., 2014; Wright and Browder, 2005; Wright et al., 2008), and reductions in leaf N, P, and K concentrations (Jackson et al., 2008; Wright et al., 2006).
Comparatively little data are available on the use of hardwood chips in greenhouse substrates. Pettersen (1984) reported ranges in wood lignin concentrations (% of dry weight) of 20% to 26% in 49 U.S. hardwood species and 26% to 31% in 35 U.S. softwood species. Lower lignin in hardwood presents a higher proportion of relatively rapidly degradable C at the expense of substrate fertility due to higher collateral nutrient demands of microorganisms. Hardwood chips may thus be less satisfactory for substrates than softwood chips because of their decomposition rate and nutrient immobilization (Nelson, 2012). Murphy et al. (2011) compared effects of hardwood and softwood chips in potting substrates on growth of several herbaceous ornamental species and their findings supported the unfavorable prognosis for use of hardwood chips.
In Fall 2012, we completed a pilot study at two nearby commercial greenhouse and nursery production sites using various rates of pecan wood chips in the substrate, prepared as described below for the present investigation. A pecan wood chip incorporation rate as low as 25% (peatmoss or pine bark substitution by volume) reduced total plant dried biomass of ‘Step’ pot chrysanthemum by 49% after 2.5 months, and by 21% in a hybrid lantana (Lantana L. ‘New Gold’) and a hybrid trailing rose (Rosa L. ‘Red Cascade’) after 6 months. These plants were subjected to only one fertilization regime, opening the possibility for further study to address the effects of fertilizer rate on crop performance. Reports on pine wood substrate have shown that increasing fertilizer rate is necessary to overcome growth-related suppressions of the wood (Jackson et al., 2008; Wright and Browder; 2005; Wright et al., 2006, 2008). Still et al. (1972) reported that the visual appearance of pot chrysanthemum shoots on a substrate amended with aged sawdust from white oak trees (Quercus alba L.) (33% to 50% by volume) compared favorably to that on a conventional peat-only substrate, provided that soluble fertilizer rates were increased accordingly. However, similar data are lacking to support or disqualify the use of hardwood chips in greenhouse potting substrates, particularly in view of what appear to be small differences in lignin concentrations between hardwoods and softwoods noted previously.
New Mexico pecan acreage has increased rapidly (U.S. Department of Agriculture, 2015a) and the state recently ranked second in the United States in total pecan production and economic value (U.S. Department of Agriculture–NASS, 2015b). Doña Ana County has the highest concentration of pecan farms in New Mexico and thus has an important impact on U.S. pecan production (Lillywhite et al., 2007). As pecan trees mature, pruning becomes essential to maintain crop productivity and quality (Picchioni et al., 2000a). Over the last decade, annual hedge pruning has become common in New Mexico pecan orchards with increased availability of equipment and positive effects on annual productivity (R.J. Heerema, personal communication). Kallestad et al. (2008) determined that as many as ≈38,000 t of dry prunings are generated annually in the Mesilla Valley of Doña Ana County where there are also greenhouse and nursery producers. A similar scenario applies throughout the entire southern United States, where pecan production is regionally aligned with significant greenhouse and nursery production.
In the Mesilla Valley, pecan prunings have traditionally been disposed of by burning, but a relatively new practice is mechanical shredding for orchard mulch (Kallestad et al., 2008) or tillage to a shallow soil depth (Tahboub et al., 2007). The tillage does not appear to immobilize soil nutrients and may improve soil tilth and aggregation (Tahboub et al., 2007, 2008). Mulching and tillage have provided an environmentally acceptable means of branch disposal in view of recent burning restrictions, but add to pecan production costs.
Recycling hedged pecan wood in potting substrates could provide a value-added revenue stream for pecan growers while aiding their greenhouse and nursery production neighbors seeking alternative substrate components. This practice would be free of the problems associated with widespread adoption of substrate alternatives, such as lack of local availability, high transportation costs, irregular and nonrenewable supplies, and inconsistent quality (Fain et al., 2008; Wright et al., 2008).
Research is needed to determine growth and performance of potted plants in response to pecan wood-amended substrates, particularly for how fertilization practices should be modified to address potential nutrient limitations. In this study, we selected garden chrysanthemum as a vegetative growth model as it represents a traditional greenhouse crop with high nutrient demands (Cavins et al., 2000; Nau, 2011; Wright et al., 2008). In light of nutrient limitations reported in pine-based substrates and our preliminary findings on pecan wood, we hypothesized: 1) garden chrysanthemum would express significant increases in growth and leaf nutrient concentrations with increasing WSF rate when grown in a substrate-containing pecan wood chips, whereas such responses to WSF would be more limited in the absence of pecan wood, and 2) growth and leaf nutrient suppressions would increase with increasing amounts of pecan wood in the substrate, especially at a low WSF rate.
Our objectives were to evaluate substrate electrical conductivity (EC) and pH, vegetative growth, and leaf nutrient composition of garden chrysanthemum on a commercial peat-based substrate amended with various amounts of pecan wood chips, and subjected to varying rates of WSF. Here, we provide preliminary information to guide in developing sustainable practices that could advance the implementation of pecan wood substrates in greenhouse pot culture.
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