Horticultural crops grown in containers require appropriate growing substrates (i.e., potting mixes) that comprise organic matter components such as sphagnum peat, bark fines, compost, and/or mineral components such as perlite and vermiculite. Relative to plant requirements, a suitable combination of physical and chemical properties of the potting mix is needed to ensure satisfactory plant performance (Clark and Zheng, 2015; Raviv et al., 2004). Species-specific growing substrates are currently mixed in-house by some greenhouse and nursery growers or customized for growers by professional growing substrate companies. However, many commercially available retail potting mix products provide home gardeners with a generalized ready-to-use product. The properties of such retail potting mix products vary with component availability, sustainability initiatives, and environmental factors (Riviere and Caron, 2001). Despite the changing composition of potting mixes, each mix can be characterized by its physical (i.e., bulk density, container capacity, air space, and total porosity) and chemical (i.e., pH, EC, nutrient concentrations, etc.) characteristics (Wiberg et al., 2005). Optimal pH and EC ranges that produce the majority of horticultural crops in growing substrates have been suggested to be 5.5 to 6.5 and 2.6 to 4.6 mS·cm−1, respectively (Cavins et al., 2000; Reed, 1996). Others have indicated that certain properties [i.e., pH, nitrogen (N) or phosphorus (P) concentrations, soluble salts, water-holding capacity, and/or air space] were correlated with plant growth or ensured appropriate growing conditions by selecting species-specific potting mixes (Allaire et al., 1996; Panj et al., 2014; Pittenger, 1986). A few studies have evaluated plant performance using commercial potting mixes available for general use (Peet et al., 2008; Pittenger, 1986; Wiberg et al., 2006), but no such study has been conducted in Canada.
Currently, a wide variety of potting mixes are commercially available for consumers (e.g., home gardeners) in Canada. In practice, only some commercially available consumer potting mixes perform well. Therefore, benchmarking research is needed to determine which properties characterize top-performing retail potting mixes in Canada. General purpose potting mixes can be used to grow a wide range of plants; however, petunia (Petunia ×hybrida), tomato (Solanum lycopersicum), and zonal geranium (Pelargonium ×hortorum) are among the plants most commonly grown by home gardeners (Froese, 2014). Tomato plants are considered “heavy feeders” and plant performance is primarily determined by plant growth and fruit yield; however, in comparison, petunia and zonal geranium are “medium feeders” with performance determined by plant growth and flower/inflorescence production (Hebbar et al., 2004; Kang and van Iersel, 2000). The authors are not aware of season-long, container-grown tomato or zonal geranium plant performance studies performed in Canada with a representative survey of retail potting mixes. However, tomato germination and petunia growth research has been presented for commercially available potting mixes in the United States (Peet et al., 2008; Pittenger, 1986; Wiberg et al., 2006). The goal of the current study was to evaluate and provide a benchmark for retail potting mixes that are commonly available to consumers in Canada.
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