In production of greenhouse crops, including most all bedding, green/foliage, and potted flowering plants, managing container substrate pH is a major nutritional challenge (Argo and Fisher, 2002; Nelson, 2012). Recent increases in the production of controlled environment container crops, including vegetables, leafy greens, soft fruits, and cannabis (Cannabis sativa), have furthered the need for soilless substrates (Caplan, 2018; Jackson, 2018; Kingston et al., 2017). Incorporating limestone into horticultural substrates is common practice of substrate manufacturers and growers to adjust substrate pH to the recommended pH 5.4 to 6.4, for most all bedding plants (Nelson, 2012). When determining how much limestone to incorporate into substrates, other variables, including chemical composition, particle size, and hardness (Argo and Fisher, 2002), should be considered.
Increased interest in using substrates containing pine wood components has led to many unanswered questions about their performance during crop production. Among these unknown issues are those relating to the requirements of limestone addition to substrates for pH adjustment for optimal plant growth. In previous studies, substrates containing wood components were evaluated; however, no indication of initial (pre-plant) substrate pH testing or changes to lime applications to the substrates were reported (Bohne, 2004; Starr et al., 2011). Boyer et al. (2008) incorporated 5 lb/yard3 dolomitic limestone to substrates produced from clean chip residuals (forestry materials left over from in-field chipping operations) and reported substrate pH of 6.7 (34 d after planting) to be above the optimal pH range (5.5 to 6.0) for ‘Blue Hawaii’ ageratum (Ageratum houstonianum).
In other studies, initial testing of substrates indicated the need to amend substrates with limestone; however, the rates of limestone addition resulted in pH of substrates above the recommended range for greenhouse crops. Fain et al. (2008a) processed loblolly pine (Pinus taeda), slash pine (Pinus elliottii), and longleaf pine trees (Pinus palustris) to produce three pine wood substrates (one from each species) and reported initial substrate pH of 5.3, 4.5, and 4.6, respectively. Substrates were amended with 3 lb/yard3 dolomitic limestone to adjust the pH. Substrate testing (30 d after planting) indicated pH of substrates were 7.1, 6.9, and 7.2, respectively, above the recommended pH range (5.4 to 6.2) for growing annual vinca (Catharanthus roseus). In addition, Fain et al. (2008b) amended a pine wood substrate [WholeTree, containing bark, wood, limbs, and needles (Fain et al., 2006)] with dolomitic limestone at a rate of 1.78 kg·m−3 and reported pH of all substrates to be higher than the recommended pH range (5.4 to 6.2) for petunia (Petunia ×hybrida), with the exception of the peat-lite (control), which was at the upper limit of the recommended pH range.
Some authors have reported that no lime addition was needed for 100% wood substrates because substrate pH was inherently within the range of 5.5 to 6.4 (Gruda and Schnitzler, 2006). Saunders et al. (2005) reported no advantage of amending 100% pine tree substrate (PTS) with limestone for african marigold (Tagetes erecta) growth. They found the addition of peatmoss or pine bark to PTS would require limestone incorporation as a result of the acidic nature of those materials.
Jackson et al. (2009) evaluated the effect of limestone addition of 0, 1.78, 3.56, 5.35, or 7.12 kg·m−3 on substrate pH and growth of ‘Inca Gold’ african marigold and ‘Rocky Mountain White’ zonal geranium (Pelargonium ×hortorum), two pH-sensitive greenhouse crops grown in 100% PTS and PTS amended with 25% or 50% peatmoss. They reported substrate pH of marigold and geranium to be highest for 100% PTS and pH generally decreased as the amount of peatmoss increased, regardless of lime rate. They also reported that pH increased with limestone additions, but the increase in pH was less responsive as the amount of peatmoss in the substrate increased from 25% to 50% (Jackson et al., 2009). In the study, 100% PTS did not require limestone for growth of marigold (pH 5.1), but they found as the percentage of peatmoss amended to PTS increased, there was an increased need for limestone to increase substrate pH and optimize plant growth. For zonal geranium, there were no growth differences when plants were grown in any substrate amended with 0 kg·m−3; however, the addition of lime increased plant dry weight when plants were grown in substrates amended with peatmoss compared with 100% PTS.
No information is available regarding lime requirements for sphagnum peat-based substrates amended with PWC aggregates as an alternative for perlite. Therefore, the objectives of this study were to determine lime requirements for pre-plant substrate pH modification and optimal plant growth in sphagnum peat-based substrates amended with aggregates of PWC or perlite.
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