There has been a considerable amount of interest in alternative substrates for both nursery and greenhouse crop production in recent years. This is due primarily to a decrease in domestic production of pine bark (PB), from which the primary substrate in the eastern United States is derived (Lu et al., 2006). Another limiting factor has been a change in forest harvesting practices from one that produces PB as a waste product at the mill to one where PB is left in the plantation by whole-tree in-field harvesting operations. PB is a material not readily used by most forest product industries. In fact, it was considered a waste product until the 1950s when alternative uses for PB were developed (Davidson et al., 2000). Today PB is used as a source of fuel, charcoal, wood-based building materials, mulch, soil amendment, and as a container-grown plant substrate (Harkin and Rowe, 1971).
Competition for PB coupled with a decrease in collection of residual PB has led to a steady decline in the availability of PB for horticultural uses. A letter to PB customers from D. Phillips (Phillips Bark Processing; Brookhaven, MS) dated Feb. 2010 stated that, due to the U.S. Federal Biomass Crop Assistance program which had driven the price of raw materials up by a substantial margin, there would be a shortage and possible unavailability of PB for horticultural and landscape uses (D. Phillips, personal communication). Other growers confirmed the statements from their PB suppliers that they would not be able to fill orders in the coming years (D. Marteney, personal communication). Decreased availability has resulted in price increases for PB, which could affect economic profitability of many growers. In fact, Abt et al. (2009) projected continued price increases and decreased inventory for several timber products through 2030. Current PB prices (delivered) range from $12 to $25 per cubic yard depending on the level of handling (raw, screened, aged, or composted) and freight costs for delivery to far-reaching geographic locations (J. Phillips, personal communications). Additionally, though the “PB crisis” of 2004–10 seems to have passed at this time (J. Phillips, personal communication), the fluctuating market necessitates the continued evaluation of alternative nursery crop substrates.
The history of alternative substrates in horticultural production is extensive (Boyer, 2008; Lu et al., 2006), yet PB remains the primary source of substrate material for nursery crop production in the last 60 years (eastern and central United States). Industry dogma has been that to use unaged (green) PB or PB with wood fragments is detrimental to plant growth, attributed to significant nitrogen (N) immobilization in these materials. Laiche and Nash (1986) reported that plants grew largest when grown in PB compared with wood chips or PB with wood chips. Investigating these types of substrates was not revisited in the United States until 2005 (Wright and Browder, 2005), though several wood-based substrates have been evaluated in Europe (Gruda and Schnitzler, 2001; Muro et al., 2005). Whole pine trees (either with bark, limbs, and needles or with bark only) have been successfully and extensively evaluated as stable substrate components in recent years (Fain et al., 2008a, 2008b; Jackson et al., 2008, 2009a, 2009b). In substrates composed of 100% pine tree substrate (PTS) a higher rate of supplemental N (≈2.4 kg·m−3) was required for ‘Compacta’ japanese holly (Ilex crenata) and ‘Delaware Valley’ azalea (Rhododendron obtusum) to achieve shoot growth in PTS comparable to shoot growth in PB (Jackson et al., 2008). Blends of PTS and peat required less limestone than 100% PTS to achieve ideal production pH (Jackson et al., 2009b). Whole pine tree materials can be obtained by harvesting trees in plantation salvage situations or in a nursery-owned plantation harvesting operation. A third wood-based substrate material is CCR (Boyer et al., 2007, 2008a, 2008b, 2009). This material is derived as a byproduct of forest thinning operations and is generally left in the plantation or sold to pulp mills for fuel. CCR has higher bark content than whole PTSs, but it has not yet been determined what that percentage is and how it can affect plant growth.
Pine plantation management is an important industry in the southeastern United States, the primary region for loblolly pine [Pinus taeda (Little, 1971)]. Plantations are intensively cultivated to produce large trees resulting in products such as sawtimber, utility poles, and paper (Wahlenberg, 1960). Plantations are thinned on a regular schedule to make room for the remaining trees to grow larger for the aforementioned wood products. Trees that are harvested in the thinning process are generally used for making paper and as fuel. Clean chips used in the pulping process may contain only 1% PB, or the finished paper products will be marred. Residual materials from the harvesting of pine plantations are the limbs, tops, and cull portions of the merchantable and nonmerchantable trees. These residues are woody biomass components not recovered by the harvesting system and are generally sold for fuel or left in the field (Stokes et al., 1989).
CCR has been evaluated as a growth substrate for annuals, perennials, and woody crops. Boyer et al. (2008a) demonstrated that ‘Blue Hawaii’ ageratum (Ageratum houstonianum) and ‘Vista Purple’ salvia (Salvia superba) grown in CCR or combinations of CCR and peat produced similar-sized plants when compared with the traditional PB substrate. Later, Boyer et al. (2008b) evaluated nine perennial species: ‘Pink Delight’ butterfly bush (Buddleja davidii), ‘Siskiyou Pink’ gaura (Gaura lindheimeri), ‘Early Sunrise’ coreopsis (Coreopsis grandiflora), ‘Sweet Dreams’ coreopsis (Coreopsis rosea), ‘Homestead Purple’ verbena (Verbena canadensis), ‘Butterfly Blue’ scabiosa (Scabiosa columbaria), ‘Firewitch’ dianthus (Dianthus gratianopolitanus), ‘Irene’ rosemary (Rosemarinus officinalis), and ‘Black and Blue’ salvia (Salvia guaranitica) in CCR and reported similar results among most substrate blends. Substrates with plants having less growth were primarily the result of substrate physical properties as 100% CCR had high air space and low water-holding capacity. Addition of peat increased water-holding capacity. All treatments resulted in acceptable growth for perennial species evaluated. A further study indicated that use of supplemental nitrogen was not necessary for growth of ‘Pink Delight’ butterfly bush (Boyer et al., 2007). Five woody crops such as loropetalum (Loropetalum chinensis var. rubrum), ‘Black Knight’ butterfly bush (B. davidii), ‘Hopi’ crapemyrtle (Lagerstroemia indica), ‘Natchez’ crapemyrtle (Lagerstroemia fauriei), and ‘Mrs. G.G. Gerbing’ azalea (Rhododendron indicum) were evaluated for growth in CCR over the course of 1 year (Boyer et al., 2009). Results for woody species were similar to growth responses of annual and perennial crops, but few differences in various growth parameters were also observed. All plants were of acceptable size and quality at the conclusion of the study. CCR has the potential to replace PB and possibly peatmoss as primary nursery and greenhouse crop substrates in several regions of the United States with few changes in crop production strategies as a result of the higher wood content in CCR.
The objectives of this study were to describe the availability, quantity, content, and consistency of substrate materials derived from forest thinning operations. Describing the types of operations from which CCR can be obtained (equipment used, plantation characteristics, etc.) is useful for illustrating to growers what they can expect from a product marketed as “clean chip residual.”
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