Soilless substrates used for producing of floriculture crops may be composed of one or a mix of either peatmoss, aged pine bark, coir, or wood fiber, each varying in initial pH. It is a common practice to incorporate limestone into substrates to neutralize substrate acidity, thereby raising substrate pH to a range of 5.4 to 6.5 for most greenhouse crops (Nelson, 2012). To neutralize substrate acidity, three types of horticultural lime are commonly incorporated at different proportions: calcitic (CaCO3), DL (CaCO3·MgCO3), and HL [Ca(OH)2·MgO; Owen, 2013]. These liming materials differ in their reactivity, residual effect, chemical composition, and particle size distribution. Reactivity is the magnitude of pH change (ΔpH) over time and is primarily a function of lime particle size, lime chemistry, acid naturalizing value, and initial substrate pH (Fisher et al., 2006). Residual lime is the proportion of unreacted lime remaining after neutralization of substrate pH. In addition, the effectiveness of a lime material to neutralize the acidity of substrates depends on its neutralizing capacity, fineness of grinding (particle size), chemical composition [providing calcium (Ca) and/or magnesium (Mg)], and mineralogy (Barber, 1984). Particle size of the liming material directly influences the dissolution rate and its effectiveness in neutralizing substrate acidity (Huang et al., 2007). Therefore, substrate manufacturers and greenhouse growers must select the appropriate liming material(s) and proportion(s) for immediate reactivity to initially raise the substrate pH and some residual effect to maintain an optimal pH range throughout the crop cycle.
During a crop production cycle, substrate pH can drift above or below the recommended range (5.4 to 6.5), affecting plant growth and nutrient availability and uptake, potentially inducing nutritional disorders. Substrate pH drives the chemical reactions determining whether nutrients are either soluble and available for root uptake or insoluble and unavailable (Argo and Fisher, 2002). Phosphorous (P) and most micronutrients, such as manganese (Mn), copper (Cu), iron (Fe), zinc (Zn), and boron (B), are affected by substrate pH. For example, in a study determining the optimal substrate pH of five sedum (Sedum sp.) species, Zheng and Clark (2013) reported species-specific effects for plant growth and leaf tissue P, potassium (K), Mg, Ca, Mn, Cu, Fe, Zn, and B concentration of plants grown in peat-based substrates amended with increasing concentrations of HL (pH 4.4–8.2). In another study, Dickson et al. (2016) reported Fe deficiency sensitivity of 24 genotypes of calibrachoa (Calibrachoa ×hybrida Cerv.) grown in peat-based substrates amended with 1.1 (pH 5.4) or 2.0 (pH 7.1) kg·m–3 HL. Whereas these studies report the influence of lime/substrate pH on leaf tissue nutrient concentrations, Andrews and Hammer (2006) reported the influence of increasing limestone incorporation concentrations from 0 to 11.9 kg·m–3 DL or 11.9 kg·m–3 DL + 5.9, 8.3, or 10.7 kg·m–3 HL (pH 4.3–7.8) on substrate pH, growth, and leaf color of zonal geraniums (Pelargonium ×hortorum L.H. Bailey ‘Candy Lavender’, ‘Fireball’, and ‘Patriot Red’) and ivy geraniums [P. pelatum (L.) L’Hér. Ex Aiton ‘Global Deep Lilac’, ‘Global Salmon Rose’, and ‘Global Soft Pink’]. Similarly, Owen (2013) determined the influence of increasing limestone incorporation concentrations from 0 to 7.1 kg·m–3 DL (pH 3.7–6.1) and 0–8.9 kg·m–3 DL (pH 4.0–6.4) on substrate solution pH and EC, plant growth, and observed nutrient disorders of garden mum (Chrysanthemum ×morifolium Ramat. ‘Mildred Yellow’) and African marigold (Tagetes erecta L. ‘Moonsong Deep Orange’), respectively.
While previous research investigated the effects of lime on substrate pH and growth of popular floriculture bedding plant species, little attention has been given to florist-quality potted flowering crops, which generate $1.8 billion USD in sales annually (U.S. Department of Agriculture, 2015). Calceolaria (Calceolaria ×herbeohybrida Voss.) frequently exhibit high pH-induced Fe deficiency (personal observation). Furthermore, growers using previous literature as recommendations found their crops to exhibit Fe deficiency because optimal substrate pH ranges for calceolaria production were previously reported to be between 6.0 and 6.5 (White, 1975), then 5.7 and 6.5 (Erwin, 1994), and most recently ranges from 5.8 to 6.2 (Currey, 2017). Therefore, it is unclear the optimal substrate pH to produce high-quality flowering calceolarias. Therefore, the objectives of this research were to determine 1) the optimal incorporation concentration(s) of dolomitic and/or hydrated lime to adjust substrate pH; 2) the influence of the liming material on substrate pH, plant growth, and leaf tissue nutrient concentrations; and 3) the optimal substrate pH to grow and maintain during calceolaria production.
I gratefully acknowledge Darby Anderson, Melanie Connors, and Kyle Martin for greenhouse and laboratory assistance and the Americana Foundation and Michigan State University College of Agriculture and Natural Resources for greenhouse space and maintenance. I thank Ball Horticultural Co., Inc., for plant material; Dr. Brian E. Whipker for liming materials; and J.R. Peters, Inc., for fertilizer.
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